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(BQ) Part 2 book Guyton and hall: Textbook of medical physiology presents the following contents: General organization, the tactile and position senses; pain, headache, and thermal sensations; optics of vision; receptor and neural function of the retina,... Invite you to consult.
CHAPTER 8 The somatic senses are the nervous mechanisms that collect sensory information from all over the body These senses are in contradistinction to the special senses, which mean specifically vision, hearing, smell, taste, and equilibrium CLASSIFICATION OF SOMATIC SENSES The somatic senses can be classified into three physiologi cal types: (1) the mechanoreceptive somatic senses, which include both tactile and position sensations that are stim ulated by mechanical displacement of some tissue of the body; (2) the thermoreceptive senses, which detect heat and cold; and (3) the pain sense, which is activated by factors that damage the tissues This chapter deals with the mechanoreceptive tactile and position senses In Chapter 49 the thermoreceptive and pain senses are discussed The tactile senses include touch, pressure, vibration, and tickle senses, and the posi tion senses include static position and rate of movement senses Other Classifications of Somatic Sensations. Somatic sensations are also often grouped together in other classes, as follows: Exteroreceptive sensations are those from the surface of the body Proprioceptive sensations are those relating to the physical state of the body, including position sen sations, tendon and muscle sensations, pressure sensa tions from the bottom of the feet, and even the sensation of equilibrium (which is often considered a “special” sen sation rather than a somatic sensation) Visceral sensations are those from the viscera of the body; in using this term, one usually refers specifically to sensations from the internal organs Deep sensations are those that come from deep tissues, such as from fasciae, muscles, and bone These sensations include mainly “deep” pressure, pain, and vibration DETECTION AND TRANSMISSION OF TACTILE SENSATIONS Interrelations Among the Tactile Sensations of Touch, Pressure, and Vibration. Although touch, pressure, and vibration are frequently classified as sepa rate sensations, they are all detected by the same types of receptors There are three principal differences among them: (1) touch sensation generally results from stimula tion of tactile receptors in the skin or in tissues immedi ately beneath the skin; (2) pressure sensation generally results from deformation of deeper tissues; and (3) vibra tion sensation results from rapidly repetitive sensory signals, but some of the same types of receptors as those for touch and pressure are used Tactile Receptors. There are at least six entirely different types of tactile receptors, but many more similar to these also exist Some were shown in Figure 47-1 of the previous chapter; their special characteristics are the following First, some free nerve endings, which are found every where in the skin and in many other tissues, can detect touch and pressure For instance, even light contact with the cornea of the eye, which contains no other type of nerve ending besides free nerve endings, can nevertheless elicit touch and pressure sensations Second, a touch receptor with great sensitivity is the Meissner’s corpuscle (illustrated in Figure 47-1), an elon gated encapsulated nerve ending of a large (type Aβ) myelinated sensory nerve fiber Inside the capsulation are many branching terminal nerve filaments These corpuscles are present in the nonhairy parts of the skin and are particularly abundant in the fingertips, lips, and other areas of the skin where one’s ability to discern spatial locations of touch sensations is highly developed Meissner corpuscles adapt in a fraction of a second after they are stimulated, which means that they are particu larly sensitive to movement of objects over the surface of the skin, as well as to low-frequency vibration Third, the fingertips and other areas that contain large numbers of Meissner’s corpuscles usually also contain large numbers of expanded tip tactile receptors, one type of which is Merkel’s discs, shown in Figure 48-1 The hairy parts of the skin also contain moderate numbers of expanded tip receptors, even though they have almost no Meissner’s corpuscles These receptors differ from Meissner’s corpuscles in that they transmit an initially strong but partially adapting signal and then a continuing 607 UNIT IX Somatic Sensations: I General Organization, the Tactile and Position Senses Unit IX The Nervous System: A General Principles and Sensory Physiology Therefore, they are particularly important for detecting tissue vibration or other rapid changes in the mechanical state of the tissues E FF C CF A AA 10 mm Figure 48-1. An Iggo dome receptor Note the multiple numbers of Merkel discs connecting to a single large myelinated fiber (A) and abutting tightly the undersurface of the epithelium AA, nonmyelinated axon; C, capillary; CF, course bundles of collagen fibers; E, thickened epidermis of the touch corpuscle; FF, fine bundles of collagen fibers (From Iggo A, Muir AR: The structure and function of a slowly adapting touch corpuscle in hairy skin J Physiol 200:763, 1969.) weaker signal that adapts only slowly Therefore, they are responsible for giving steady-state signals that allow one to determine continuous touch of objects against the skin Merkel discs are often grouped together in a receptor organ called the Iggo dome receptor, which projects upward against the underside of the epithelium of the skin, as is also shown in Figure 48-1 This upward pro jection causes the epithelium at this point to protrude outward, thus creating a dome and constituting an extremely sensitive receptor Also note that the entire group of Merkel’s discs is innervated by a single large myelinated nerve fiber (type Aβ) These receptors, along with the Meissner’s corpuscles discussed earlier, play extremely important roles in localizing touch sensations to specific surface areas of the body and in determining the texture of what is felt Fourth, slight movement of any hair on the body stim ulates a nerve fiber entwining its base Thus, each hair and its basal nerve fiber, called the hair end-organ, are also touch receptors A receptor adapts readily and, like Meissner’s corpuscles, detects mainly (a) movement of objects on the surface of the body or (b) initial contact with the body Fifth, located in the deeper layers of the skin and also in still deeper internal tissues are many Ruffini’s endings, which are multibranched, encapsulated endings, as shown in Figure 47-1 These endings adapt very slowly and, therefore, are important for signaling continuous states of deformation of the tissues, such as heavy prolonged touch and pressure signals They are also found in joint capsules and help to signal the degree of joint rotation Sixth, Pacinian corpuscles, which were discussed in detail in Chapter 47, lie both immediately beneath the skin and deep in the fascial tissues of the body They are stimulated only by rapid local compression of the tissues because they adapt in a few hundredths of a second 608 Transmission of Tactile Signals in Peripheral Nerve Fibers. Almost all specialized sensory receptors, such as Meissner’s corpuscles, Iggo dome receptors, hair recep tors, Pacinian corpuscles, and Ruffini’s endings, transmit their signals in type Aβ nerve fibers that have transmis sion velocities ranging from 30 to 70 m/sec Conversely, free nerve ending tactile receptors transmit signals mainly by way of the small type Aδ myelinated fibers that conduct at velocities of only to 30 m/sec Some tactile free nerve endings transmit by way of type C unmyelinated fibers at velocities from a fraction of a meter up to 2 m/sec; these nerve endings send signals into the spinal cord and lower brain stem, probably sub serving mainly the sensation of tickle Thus, the more critical types of sensory signals—those that help to determine precise localization on the skin, minute gradations of intensity, or rapid changes in sensory signal intensity—are all transmitted in more rapidly con ducting types of sensory nerve fibers Conversely, the cruder types of signals, such as pressure, poorly localized touch, and especially tickle, are transmitted by way of much slower, very small nerve fibers that require much less space in the nerve bundle than the fast fibers Detection of Vibration. All tactile receptors are in volved in detection of vibration, although different recep tors detect different frequencies of vibration Pacinian corpuscles can detect signal vibrations from 30 to 800 cycles/sec because they respond extremely rapidly to minute and rapid deformations of the tissues They also transmit their signals over type Aβ nerve fibers, which can transmit as many as 1000 impulses per second Lowfrequency vibrations from up to 80 cycles per second, in contrast, stimulate other tactile receptors, especially Meissner’s corpuscles, which adapt less rapidly than Pacinian corpuscles Detection of Tickle and Itch by Mechanoreceptive Free Nerve Endings. Neurophysiological studies have demonstrated the existence of very sensitive, rapidly adapting mechanoreceptive free nerve endings that elicit only the tickle and itch sensations Furthermore, these endings are found almost exclusively in superficial layers of the skin, which is also the only tissue from which the tickle and itch sensations usually can be elicited These sensations are transmitted by very small type C, unmy elinated fibers similar to those that transmit the aching, slow type of pain The purpose of the itch sensation is presumably to call attention to mild surface stimuli such as a flea crawling on the skin or a fly about to bite, and the elicited signals then activate the scratch reflex or other maneuvers that rid the host of the irritant Itch can be relieved by Chapter 48 Somatic Sensations: I General Organization, the Tactile and Position Senses scratching if this action removes the irritant or if the scratch is strong enough to elicit pain The pain signals are believed to suppress the itch signals in the cord by lateral inhibition, as described in Chapter 49 Almost all sensory information from the somatic seg ments of the body enters the spinal cord through the dorsal roots of the spinal nerves However, from the entry point into the cord and then to the brain, the sensory signals are carried through one of two alternative sensory pathways: (1) the dorsal column–medial lemniscal system or (2) the anterolateral system These two systems come back together partially at the level of the thalamus The dorsal column–medial lemniscal system, as its name implies, carries signals upward to the medulla of the brain mainly in the dorsal columns of the cord Then, after the signals synapse and cross to the opposite side in the medulla, they continue upward through the brain stem to the thalamus by way of the medial lemniscus Conversely, signals in the anterolateral system, imme diately after entering the spinal cord from the dorsal spinal nerve roots, synapse in the dorsal horns of the spinal gray matter, then cross to the opposite side of the cord and ascend through the anterior and lateral white columns of the cord They terminate at all levels of the lower brain stem and in the thalamus The dorsal column–medial lemniscal system is com posed of large, myelinated nerve fibers that transmit signals to the brain at velocities of 30 to 110 m/sec, whereas the anterolateral system is composed of smaller myelinated fibers that transmit signals at velocities ranging from a few meters per second up to 40 m/sec Another difference between the two systems is that the dorsal column–medial lemniscal system has a high degree of spatial orientation of the nerve fibers with respect to their origin, whereas the anterolateral system has much less spatial orientation These differences immediately characterize the types of sensory information that can be transmitted by the two systems That is, sensory informa tion that must be transmitted rapidly with temporal and spatial fidelity is transmitted mainly in the dorsal column– medial lemniscal system; that which does not need to be transmitted rapidly or with great spatial fidelity is trans mitted mainly in the anterolateral system The anterolateral system has a special capability that the dorsal system does not have—that is, the ability to transmit a broad spectrum of sensory modalities, such as pain, warmth, cold, and crude tactile sensations Most of these sensory modalities are discussed in detail in Chapter 49 The dorsal system is limited to discrete types of mech anoreceptive sensations With this differentiation in mind, we can now list the types of sensations transmitted in the two systems Touch sensations requiring a high degree of localiza tion of the stimulus Touch sensations requiring transmission of fine gra dations of intensity Phasic sensations, such as vibratory sensations Sensations that signal movement against the skin Position sensations from the joints Pressure sensations related to fine degrees of judg ment of pressure intensity Anterolateral System Pain Thermal sensations, including both warmth and cold sensations Crude touch and pressure sensations capable only of crude localizing ability on the surface of the body Tickle and itch sensations Sexual sensations TRANSMISSION IN THE DORSAL COLUMN–MEDIAL LEMNISCAL SYSTEM ANATOMY OF THE DORSAL COLUMN–MEDIAL LEMNISCAL SYSTEM Upon entering the spinal cord through the spinal nerve dorsal roots, the large myelinated fibers from the special ized mechanoreceptors divide almost immediately to form a medial branch and a lateral branch, shown by the right-hand fiber entering through the spinal root in Figure 48-2 The medial branch turns medially first and Spinal nerve Lamina marginalis Substantia gelatinosa Tract of Lissauer Spinocervical tract Dorsal spinocerebellar tract Dorsal column I II III IV V VI VII Ventral spinocerebellar tract IX VIII Anterolateral spinothalamic pathway Figure 48-2. Cross section of the spinal cord, showing the anatomy of the cord gray matter and of ascending sensory tracts in the white columns of the spinal cord 609 UNIT IX SENSORY PATHWAYS FOR TRANSMITTING SOMATIC SIGNALS INTO THE CENTRAL NERVOUS SYSTEM Dorsal Column–Medial Lemniscal System Unit IX The Nervous System: A General Principles and Sensory Physiology then upward in the dorsal column, proceeding by way of the dorsal column pathway all the way to the brain The lateral branch enters the dorsal horn of the cord gray matter, then divides many times to provide terminals that synapse with local neurons in the intermediate and anterior portions of the cord gray matter These local neurons in turn serve three functions: A major share of them give off fibers that enter the dorsal columns of the cord and then travel upward to the brain Many of the fibers are very short and terminate locally in the spinal cord gray matter to elicit local spinal cord reflexes, which are discussed in Chapter 55 Others give rise to the spinocerebellar tracts, which we discuss in Chapter 57 in relation to the function of the cerebellum Cortex Internal capsule Ventrobasal complex of thalamus Midbrain Pons Dorsal Column–Medial Lemniscal Pathway. Note in Figure 48-3 that nerve fibers entering the dorsal columns pass uninterrupted up to the dorsal medulla, where they synapse in the dorsal column nuclei (the cuneate and gracile nuclei) From there, second-order neurons decussate imme diately to the opposite side of the brain stem and continue upward through the medial lemnisci to the thalamus In this pathway through the brain stem, each medial lemnis cus is joined by additional fibers from the sensory nuclei of the trigeminal nerve; these fibers subserve the same sensory functions for the head that the dorsal column fibers sub serve for the body In the thalamus, the medial lemniscal fibers terminate in the thalamic sensory relay area, called the ventrobasal complex From the ventrobasal complex, third-order nerve fibers project, as shown in Figure 48-4, mainly to the postcentral gyrus of the cerebral cortex, which is called somatic sensory area I (as shown in Figure 48-6, these fibers also project to a smaller area in the lateral parietal cortex called somatic sensory area II) Spatial Orientation of the Nerve Fibers in the Dorsal Column–Medial Lemniscal System One of the distinguishing features of the dorsal column– medial lemniscal system is a distinct spatial orientation of nerve fibers from the individual parts of the body that is maintained throughout For instance, in the dorsal columns of the spinal cord, the fibers from the lower parts of the body lie toward the center of the cord, whereas those that enter the cord at progressively higher segmen tal levels form successive layers laterally In the thalamus, distinct spatial orientation is still maintained, with the tail end of the body represented by the most lateral portions of the ventrobasal complex and the head and face represented by the medial areas of the complex Because of the crossing of the medial lemnisci in the medulla, the left side of the body is represented in 610 Medial lemniscus Medulla oblongata Lower medulla oblongata Dorsal column nuclei Ascending branches of dorsal root fibers Dorsal root and spinal ganglion Figure 48-3. The dorsal column–medial lemniscal pathway for transmitting critical types of tactile signals Chapter 48 Somatic Sensations: I General Organization, the Tactile and Position Senses Primary motor cortex Somatosensory area I Postcentral gyrus Lower extremity Upper extremity Somatosensory area II UNIT IX Thigh Thorax Neck Shoulder Hand Leg Fingers Arm Tongue Face Intra-abdominal Trunk Face Figure 48-6. Two somatosensory cortical areas, somatosensory areas I and II Ventrobasal complex of thalamus Mesencephalon Spinothalamic tract Medial lemniscus Figure 48-4. Projection of the dorsal column–medial lemniscal system through the thalamus to the somatosensory cortex (Modified from Brodal A: Neurological Anatomy in Relation to Clinical Medicine New York: Oxford University Press, 1969.) Central fissure 10 7A 40 39 46 11 47 22 45 44 41 18 37 17 21 38 Lateral fissure 42 19 20 Figure 48-5. Structurally distinct areas, called Brodmann’s areas, of the human cerebral cortex Note specifically areas 1, 2, and 3, which constitute primary somatosensory area I, and areas and 7A, which constitute the somatosensory association area the right side of the thalamus, and the right side of the body is represented in the left side of the thalamus SOMATOSENSORY CORTEX Figure 48-5 is a map of the human cerebral cortex, showing that it is divided into about 50 distinct areas called Brodmann’s areas based on histological structural differences This map is important because virtually all neurophysiologists and neurologists use it to refer by number to many of the different functional areas of the human cortex Note in Figure 48-5 the large central fissure (also called central sulcus) that extends horizontally across the brain In general, sensory signals from all modalities of sensation terminate in the cerebral cortex immediately posterior to the central fissure Generally, the anterior half of the parietal lobe is concerned almost entirely with reception and interpretation of somatosensory signals, but the posterior half of the parietal lobe provides still higher levels of interpretation Visual signals terminate in the occipital lobe, and auditory signals terminate in the temporal lobe Conversely, the portion of the cerebral cortex anterior to the central fissure and constituting the posterior half of the frontal lobe is called the motor cortex and is devoted almost entirely to control of muscle contractions and body movements A major share of this motor control is in response to somatosensory signals received from the sensory portions of the cortex, which keep the motor cortex informed at each instant about the positions and motions of the different body parts Somatosensory Areas I and II. Figure 48-6 shows two separate sensory areas in the anterior parietal lobe called somatosensory area I and somatosensory area II The reason for this division into two areas is that a distinct and separate spatial orientation of the different parts of the body is found in each of these two areas However, somatosensory area I is so much more extensive and so much more important than somatosensory area II that in popular usage, the term “somatosensory cortex” almost always means area I Somatosensory area I has a high degree of localization of the different parts of the body, as shown by the names of virtually all parts of the body in Figure 48-6 By contrast, localization is poor in somatosensory area II, 611 Unit IX The Nervous System: A General Principles and Sensory Physiology Trunk Neck Head Shoulder Arm Elbow rm a Fore st Wri d ger n in er Ha e f ng ttl f i Li ing R M In id Th dex dle Ey umb fin fing e ge e Nos r r e Face Upper lip Hip Leg I ot Fo s Toe s l enita G II III IV Lips Lower lip Teeth, gums, and jaw V Tongue Pharynx Intra-abdominal Figure 48-7. Representation of the different areas of the body in somatosensory area I of the cortex (From Penfield W, Rasmussen T: Cerebral Cortex of Man: A Clinical Study of Localization of Function New York: Hafner, 1968.) although roughly, the face is represented anteriorly, the arms centrally, and the legs posteriorly Much less is known about the function of somatosen sory area II It is known that signals enter this area from the brain stem, transmitted upward from both sides of the body In addition, many signals come secondarily from somatosensory area I, as well as from other sensory areas of the brain, even from the visual and auditory areas Projections from somatosensory area I are required for function of somatosensory area II However, removal of parts of somatosensory area II has no apparent effect on the response of neurons in somatosensory area I Thus, much of what we know about somatic sensation appears to be explained by the functions of somatosensory area I Spatial Orientation of Signals from Different Parts of the Body in Somatosensory Area I. Somatosen sory area I lies immediately behind the central fissure, located in the postcentral gyrus of the human cerebral cortex (in Brodmann’s areas 3, 1, and 2) Figure 48-7 shows a cross section through the brain at the level of the postcentral gyrus, demonstrating repre sentations of the different parts of the body in separate regions of somatosensory area I Note, however, that each lateral side of the cortex receives sensory information almost exclusively from the opposite side of the body Some areas of the body are represented by large areas in the somatic cortex—the lips the greatest of all, followed by the face and thumb—whereas the trunk and lower part of the body are represented by relatively small areas The sizes of these areas are directly proportional 612 VIa VIb Figure 48-8. Structure of the cerebral cortex I, molecular layer; II, external granular layer; III, layer of small pyramidal cells; IV, internal granular layer; V, large pyramidal cell layer; and VI, layer of fusiform or polymorphic cells (From Ranson SW, Clark SL: Anatomy of the Nervous System Philadelphia: WB Saunders, 1959.) to the number of specialized sensory receptors in each respective peripheral area of the body For instance, a great number of specialized nerve endings are found in the lips and thumb, whereas only a few are present in the skin of the body trunk Note also that the head is represented in the most lateral portion of somatosensory area I, and the lower part of the body is represented medially Layers of the Somatosensory Cortex and Their Function The cerebral cortex contains six layers of neurons, begin ning with layer I next to the brain surface and extending progressively deeper to layer VI, shown in Figure 48-8 As would be expected, the neurons in each layer perform functions different from those in other layers Some of these functions are: The incoming sensory signal excites neuronal layer IV first; the signal then spreads toward the surface of the cortex and also toward deeper layers Layers I and II receive diffuse, nonspecific input signals from lower brain centers that facilitate specific regions of the cortex; this system is described in Chapter 58 This input mainly controls the overall level of excitability of the respective regions stimulated Chapter 48 Somatic Sensations: I General Organization, the Tactile and Position Senses The Sensory Cortex Is Organized in Vertical Columns of Neurons; Each Column Detects a Different Sensory Spot on the Body with a Specific Sensory Modality Functionally, the neurons of the somatosensory cortex are arranged in vertical columns extending all the way through the six layers of the cortex, with each column having a diameter of 0.3 to 0.5 millimeter and containing perhaps 10,000 neuronal cell bodies Each of these columns serves a single specific sensory modality; some columns respond to stretch receptors around joints, some to stimulation of tactile hairs, others to discrete localized pressure points on the skin, and so forth At layer IV, where the input sensory signals first enter the cortex, the columns of neurons function almost entirely separately from one another At other levels of the columns, inter actions occur that initiate analysis of the meanings of the sensory signals In the most anterior to 10 millimeters of the post central gyrus, located deep in the central fissure in Brodmann’s area 3A, an especially large share of the vertical columns respond to muscle, tendon, and joint stretch receptors Many of the signals from these sensory columns then spread anteriorly, directly to the motor cortex located immediately forward of the central fissure These signals play a major role in controlling the effluent motor signals that activate sequences of muscle contraction As one moves posteriorly in somatosensory area I, more and more of the vertical columns respond to slowly adapting cutaneous receptors, and then still farther pos teriorly, greater numbers of the columns are sensitive to deep pressure In the most posterior portion of somatosensory area I, about percent of the vertical columns respond only when a stimulus moves across the skin in a particular direction Thus, this is a still higher order of interpretation of sensory signals; the process becomes even more complex as the signals spread farther backward from somatosensory area I into the parietal cortex, an area called the somatosensory association area, as we discuss subsequently Functions of Somatosensory Area I Widespread bilateral excision of somatosensory area I causes loss of the following types of sensory judgment: The person is unable to localize discretely the dif ferent sensations in the different parts of the body However, he or she can localize these sensations crudely, such as to a particular hand, to a major level of the body trunk, or to one of the legs Thus, it is clear that the brain stem, thalamus, or parts of the cerebral cortex not normally considered to be con cerned with somatic sensations can perform some degree of localization The person is unable to judge critical degrees of pressure against the body The person is unable to judge the weights of objects The person is unable to judge shapes or forms of objects This condition is called astereognosis The person is unable to judge texture of materials because this type of judgment depends on highly critical sensations caused by movement of the fingers over the surface to be judged Note that in the list nothing has been said about loss of pain and temperature sense In the specific absence of only somatosensory area I, appreciation of these sensory modalities is still preserved both in quality and intensity However, the sensations are poorly localized, indicating that pain and temperature localization depend greatly on the topographical map of the body in somato sensory area I to localize the source SOMATOSENSORY ASSOCIATION AREAS Brodmann’s areas and of the cerebral cortex, located in the parietal cortex behind somatosensory area I (see Figure 48-5), play important roles in deciphering deeper meanings of the sensory information in the somatosen sory areas Therefore, these areas are called somatosensory association areas Electrical stimulation in a somatosensory association area can occasionally cause an awake person to experi ence a complex body sensation, sometimes even the “feeling” of an object such as a knife or a ball Therefore, it seems clear that the somatosensory association area combines information arriving from multiple points in the primary somatosensory area to decipher its meaning This occurrence also fits with the anatomical arrange ment of the neuronal tracts that enter the somatosen sory association area because it receives signals from (1) somatosensory area I, (2) the ventrobasal nuclei of the thalamus, (3) other areas of the thalamus, (4) the visual cortex, and (5) the auditory cortex Effect of Removing the Somatosensory Association Area—Amorphosynthesis. When the somatosensory association area is removed on one side of the brain, the person loses the ability to recognize complex objects and 613 UNIT IX The neurons in layers II and III send axons to related portions of the cerebral cortex on the opposite side of the brain through the corpus callosum The neurons in layers V and VI send axons to the deeper parts of the nervous system Those in layer V are generally larger and project to more distant areas, such as to the basal ganglia, brain stem, and spinal cord, where they control signal transmission From layer VI, especially large numbers of axons extend to the thalamus, providing signals from the cerebral cortex that interact with and help to control the excitatory levels of incoming sensory signals entering the thalamus Unit IX The Nervous System: A General Principles and Sensory Physiology OVERALL CHARACTERISTICS OF SIGNAL TRANSMISSION AND ANALYSIS IN THE DORSAL COLUMN–MEDIAL LEMNISCAL SYSTEM Basic Neuronal Circuit in the Dorsal Column–Medial Lemniscal System. The lower part of Figure 48-9 shows the basic organization of the neuronal circuit of the spinal cord dorsal column pathway, demonstrating that at each synaptic stage, divergence occurs The upper curves of the figure show that the cortical neurons that discharge to the greatest extent are those in a central part of the cortical “field” for each respective receptor Thus, a weak stimulus causes only the most central neurons to fire A stronger Discharges per second Strong stimulus Moderate stimulus Weak stimulus Cortex stimulus causes still more neurons to fire, but those in the center discharge at a considerably more rapid rate than those farther away from the center Two-Point Discrimination. A method frequently used to test tactile discrimination is to determine a person’s so-called “two-point” discriminatory ability In this test, two needles are pressed lightly against the skin at the same time, and the person determines whether one point or two points of stimulus is/are felt On the tips of the fingers, a person can normally distinguish two separate points even when the needles are as close together as to millimeters However, on the person’s back, the needles usually must be as far apart as 30 to 70 millime ters before two separate points can be detected The reason for this difference is the different numbers of spe cialized tactile receptors in the two areas Figure 48-10 shows the mechanism by which the dorsal column pathway (as well as all other sensory path ways) transmits two-point discriminatory information This figure shows two adjacent points on the skin that are strongly stimulated, as well as the areas of the somato sensory cortex (greatly enlarged) that are excited by signals from the two stimulated points The blue curve shows the spatial pattern of cortical excitation when both skin points are stimulated simultaneously Note that the resultant zone of excitation has two separate peaks These two peaks, separated by a valley, allow the sensory cortex to detect the presence of two stimulatory points, rather than a single point The capability of the sensorium to distinguish this presence of two points of stimulation is strongly influenced by another mechanism, lateral inhibition, as explained in the next section Discharges per second complex forms felt on the opposite side of the body In addition, he or she loses most of the sense of form of his or her own body or body parts on the opposite side In fact, the person is mainly oblivious to the opposite side of the body—that is, forgets that it is there Therefore, the person also often forgets to use the other side for motor functions as well Likewise, when feeling objects, the person tends to recognize only one side of the object and forgets that the other side even exists This complex sensory deficit is called amorphosynthesis Thalamus Cortex Dorsal column nuclei Two adjacent points strongly stimulated Single-point stimulus on skin Figure 48-9. Transmission of a pinpoint stimulus signal to the cerebral cortex 614 Figure 48-10. Transmission of signals to the cortex from two adjacent pinpoint stimuli The blue curve represents the pattern of cortical stimulation without “surround” inhibition, and the two red curves represent the pattern when “surround” inhibition does occur Chapter 48 Somatic Sensations: I General Organization, the Tactile and Position Senses Effect of Lateral Inhibition (Also Called Surround Inhibition) to Increase the Degree of Contrast in the Perceived Spatial Pattern. As pointed out in Chapter Transmission of Rapidly Changing and Repetitive Sensations. The dorsal column system is also of particu lar importance in apprising the sensorium of rapidly changing peripheral conditions Based on recorded action potentials, this system can recognize changing stimuli that occur in as little as 1/400 of a second Vibratory Sensation. Vibratory signals are rapidly repetitive and can be detected as vibration up to 700 cycles per second The higher-frequency vibratory signals originate from the Pacinian corpuscles in the skin and deeper tissues, but lower-frequency signals (below about 200 per second) can originate from Meissner’s corpuscles as well These signals are transmitted only in the dorsal column pathway For this reason, application of vibration (e.g., from a “tuning fork”) to different peripheral parts of the body is an important tool used by neurologists for testing functional integrity of the dorsal columns Interpretation of Sensory Stimulus Intensity The ultimate goal of most sensory stimulation is to apprise the psyche of the state of the body and its surroundings Therefore, it is important that we discuss briefly some of the principles related to transmission of sensory stimulus intensity to the higher levels of the nervous system How is it possible for the sensory system to transmit sensory experiences of tremendously varying intensities? For instance, the auditory system can detect the weakest Importance of the Tremendous Intensity Range of Sensory Reception. Were it not for the tremendous inten sity range of sensory reception that we can experience, the various sensory systems would more often than not be operating in the wrong range This principle is demon strated by the attempts of most people, when taking photographs with a camera, to adjust the light exposure without using a light meter Left to intuitive judgment of light intensity, a person almost always overexposes the film on bright days and greatly underexposes the film at twi light Yet that person’s own eyes are capable of discriminat ing with great detail visual objects in bright sunlight or at twilight; the camera cannot perform this discrimination without very special manipulation because of the narrow critical range of light intensity required for proper exposure of film Judgment of Stimulus Intensity Weber-Fechner Principle—Detection of “Ratio” of Stimulus Strength. In the mid-1800s, Weber first and Fechner later proposed the principle that gradations of stimulus strength are discriminated approximately in proportion to the logarithm of stimulus strength That is, a person already holding 30 grams weight in his or her hand can barely detect an additional 1-gram increase in weight, and, when already 615 UNIT IX 47, virtually every sensory pathway, when excited, gives rise simultaneously to lateral inhibitory signals; these inhibitory signals spread to the sides of the excitatory signal and inhibit adjacent neurons For instance, con sider an excited neuron in a dorsal column nucleus Aside from the central excitatory signal, short lateral pathways transmit inhibitory signals to the surrounding neurons— that is, these signals pass through additional interneurons that secrete an inhibitory transmitter The importance of lateral inhibition is that it blocks lateral spread of the excitatory signals and, therefore, increases the degree of contrast in the sensory pattern perceived in the cerebral cortex In the case of the dorsal column system, lateral inhibi tory signals occur at each synaptic level—for instance, in (1) the dorsal column nuclei of the medulla, (2) the ven trobasal nuclei of the thalamus, and (3) the cortex itself At each of these levels, the lateral inhibition helps to block lateral spread of the excitatory signal As a result, the peaks of excitation stand out, and much of the surround ing diffuse stimulation is blocked This effect is demon strated by the two red curves in Figure 48-10, showing complete separation of the peaks when the intensity of lateral inhibition is great possible whisper but can also discern the meanings of an explosive sound, even though the sound intensities of these two experiences can vary more than 10 billion times; the eyes can see visual images with light intensities that vary as much as a half million times; and the skin can detect pressure differences of 10,000 to 100,000 times As a partial explanation of these effects, Figure 47-4 in the previous chapter shows the relation of the receptor potential produced by the Pacinian corpuscle to the inten sity of the sensory stimulus At low stimulus intensity, slight changes in intensity increase the potential mark edly, whereas at high levels of stimulus intensity, further increases in receptor potential are slight Thus, the Pacinian corpuscle is capable of accurately measuring extremely minute changes in stimulus at low-intensity levels, but at high-intensity levels, the change in stimulus must be much greater to cause the same amount of change in receptor potential The transduction mechanism for detecting sound by the cochlea of the ear demonstrates still another method for separating gradations of stimulus intensity When sound stimulates a specific point on the basilar membrane, weak sound stimulates only those hair cells at the point of maximum sound vibration However, as the sound inten sity increases, many more hair cells in each direction farther away from the maximum vibratory point also become stimulated Thus, signals are transmitted over pro gressively increasing numbers of nerve fibers, which is another mechanism by which stimulus intensity is trans mitted to the central nervous system This mechanism, plus the direct effect of stimulus intensity on impulse rate in each nerve fiber, as well as several other mechanisms, makes it possible for some sensory systems to operate rea sonably faithfully at stimulus intensity levels changing as much as millions of times Unit IX The Nervous System: A General Principles and Sensory Physiology holding 300 grams, he or she can barely detect a 10-gram increase in weight Thus, in this instance, the ratio of the change in stimulus strength required for detection remains essentially constant, about to 30, which is what the logarithmic principle means To express this principle mathematically, Interpreted signal strength = Log (Stimulus ) + Constant More recently, it has become evident that the WeberFechner principle is quantitatively accurate only for higher intensities of visual, auditory, and cutaneous sensory expe rience and applies only poorly to most other types of sensory experience Yet, the Weber-Fechner principle is still a good one to remember because it emphasizes that the greater the background sensory intensity, the greater an additional change must be for the psyche to detect the change Power Law. Another attempt by physiopsychologists to find a good mathematical relation is the following formula, known as the power law: Interpreted signal strength = K × (Stimulus − k )y In this formula, the exponent y and the constants K and k are different for each type of sensation When this power law relation is plotted on a graph using double logarithmic coordinates, as shown in Figure 48-11, and when appropriate quantitative values for y, K, and k are found, a linear relation can be attained bet ween interpreted stimulus strength and actual stimulus strength over a large range for almost any type of sensory perception POSITION SENSES The position senses are frequently also called proprioceptive senses They can be divided into two subtypes: (1) static position sense, which means conscious percep tion of the orientation of the different parts of the body Interpreted stimulus strength (arbitrary units) Position Sensory Receptors. Knowledge of position, both static and dynamic, depends on knowing the degrees of angulation of all joints in all planes and their rates of change Therefore, multiple different types of receptors help to determine joint angulation and are used together for position sense Both skin tactile receptors and deep receptors near the joints are used In the case of the fingers, where skin receptors are in great abundance, as much as half of position recognition is believed to be detected through the skin receptors Conversely, for most of the larger joints of the body, deep receptors are more important For determining joint angulation in midranges of motion, the muscle spindles are among the most impor tant receptors They are also exceedingly important in helping to control muscle movement, as we shall see in Chapter 55 When the angle of a joint is changing, some muscles are being stretched while others are loosened, and the net stretch information from the spindles is trans mitted into the computational system of the spinal cord and higher regions of the dorsal column system for deci phering joint angulations At the extremes of joint angulation, stretch of the ligaments and deep tissues around the joints is an addi tional important factor in determining position Types of sensory endings used for this are the Pacinian cor puscles, Ruffini’s endings, and receptors similar to the Golgi tendon receptors found in muscle tendons The Pacinian corpuscles and muscle spindles are espe cially adapted for detecting rapid rates of change It is likely that these are the receptors most responsible for detecting rate of movement Processing of Position Sense Information in the Dorsal Column–Medial Lemniscal Pathway. Referring to Figure 48-12, one sees that thalamic neurons respond ing to joint rotation are of two categories: (1) those maxi mally stimulated when the joint is at full rotation and (2) those maximally stimulated when the joint is at minimal rotation Thus, the signals from the individual joint receptors are used to tell the psyche how much each joint is rotated 500 200 100 50 TRANSMISSION OF LESS CRITICAL SENSORY SIGNALS IN THE ANTEROLATERAL PATHWAY 20 10 0 10 100 1000 10,000 Stimulus strength (arbitrary units) Figure 48-11. Graphical demonstration of the “power law” relation between actual stimulus strength and strength that the psyche interprets it to be Note that the power law does not hold at either very weak or very strong stimulus strengths 616 with respect to one another, and (2) rate of movement sense, also called kinesthesia or dynamic proprioception The anterolateral pathway for transmitting sensory signals up the spinal cord and into the brain, in contrast to the dorsal column pathway, transmits sensory signals that not require highly discrete localization of the signal source and not require discrimination of fine grada tions of intensity These types of signals include pain, heat, cold, crude tactile, tickle, itch, and sexual sensations In Unit XIV Endocrinology and Reproduction 12 Boys Girls Stands alone 10 Walks with support 50 Age (months) Height (inches) 60 40 Pulls up Grasps 20 Age (months) 12 16 20 Age (years) be achieved only at normal oxygen concentration in the air breathed Growth and Development of the Child The major physiological problems of the child beyond the neonatal period are related to special metabolic needs for growth, which have been fully covered in the sections of this book on metabolism and endocrinology Figure 84-8 shows the changes in heights of boys and girls from the time of birth until the age of 20 years Note especially that these heights parallel each other almost exactly until the end of the first decade of life Between the ages of 11 and 13 years, the female estrogens begin to be formed and cause rapid growth in height but early uniting of the epiphyses of the long bones at about the 14th to 16th year of life, so growth in height then ceases In contrast, the effect of testosterone in the male causes extra growth at a slightly later age—mainly between ages 13 and 17 years The male, however, undergoes more prolonged growth because of delayed uniting of the epiphyses, so his final height is considerably greater than that of the female .t c t t h s p Behavioral Growth a k / :/ Behavioral growth is principally related to maturity of the nervous system It is difficult to dissociate maturity of the anatomical structures of the nervous system from maturity caused by training Anatomical studies show that certain major tracts in the central nervous system are not completely myelinated until the end of the first year of life For this reason, it is frequently stated that the nervous system is not fully functional at birth The brain cortex and its associated functions, such as vision, seem to require several months after birth for final functional development to occur At birth, the infant brain mass is only 26 percent of the adult brain mass and 55 percent at year, but it reaches almost adult proportions by the end of the second year Crawls Sits briefly Birth / 9 Rolls over Hand control r i h Head control Vocalizes Smiles a t r/ Figure 84-8. Average height of boys and girls from infancy to 20 years of age 1080 30 12 16 20 24 Walks alone 11 Suckles Figure 84-9. Behavioral development of the infant during the first year of life e s This process is also associated with closure of the fontanels and sutures of the skull, which allows only 20 percent additional growth of the brain beyond the first years of life Figure 84-9 shows a normal progress chart for the infant during the first year of life Comparison of this chart with the baby’s actual development is used for clinical assessment of mental and behavioral growth /r u Bibliography Brew N, Walker D, Wong FY: Cerebral vascular regulation and brain injury in preterm infants Am J Physiol Regul Integr Comp Physiol 306:R773, 2014 Coceani F, Baragatti B: Mechanisms for ductus arteriosus closure Semin Perinatol 36:92, 2012 Forhead AJ, Fowden AL: Thyroid hormones in fetal growth and prepartum maturation J Endocrinol 221:R87, 2014 Fowden AL, Giussani DA, Forhead AJ: Intrauterine programming of physiological systems: causes and consequences Physiology (Bethesda) 21:29, 2006 Gao Y, Raj JU: Regulation of the pulmonary circulation in the fetus and newborn Physiol Rev 90:1291, 2010 Gluckman PD, Hanson MA, Cooper C, Thornburg KL: Effect of in utero and early-life conditions on adult health and disease N Engl J Med 359:61, 2008 Grijalva J, Vakili K: Neonatal liver physiology Semin Pediatr Surg 22:185, 2013 Hilaire G, Duron B: Maturation of the mammalian respiratory system Physiol Rev 79:325, 1999 Hines MH: Neonatal cardiovascular physiology Semin Pediatr Surg 22:174, 2013 Johnson MH: Functional brain development in humans Nat Rev Neurosci 2:475, 2001 Kugelman A, Colin AA: Late preterm infants: near term but still in a critical developmental time period Pediatrics 132:741, 2013 Luyckx VA, Bertram JF, Brenner BM, et al: Effect of fetal and child health on kidney development and long-term risk of hypertension and kidney disease Lancet 382:273, 2013 70 Chapter 84 Fetal and Neonatal Physiology t c a k / :/ Salmaso N, Jablonska B, Scafidi J, et al: Neurobiology of premature brain injury Nat Neurosci 17:341, 2014 Sferruzzi-Perri AN, Vaughan OR, Forhead AJ, Fowden AL: Hormonal and nutritional drivers of intrauterine growth Curr Opin Clin Nutr Metab Care 16:298, 2013 Sulemanji M, Vakili K: Neonatal renal physiology Semin Pediatr Surg 22:195, 2013 a t r/ / 9 r i h /r u e s t t h s p 1081 UNIT XIV Muglia LJ, Katz M: The enigma of spontaneous preterm birth N Engl J Med 362:529, 2010 Osol G, Mandala M: Maternal uterine vascular remodeling during pregnancy Physiology (Bethesda) 24:58, 2009 Palinski W: Effect of maternal cardiovascular conditions and risk factors on offspring cardiovascular disease Circulation 129:2066, 2014 Raju TN: Developmental physiology of late and moderate prematurity Semin Fetal Neonatal Med 17:126, 2012 85 CHAPTER There are few stresses to which the body is exposed that approach the extreme stresses of heavy exercise In fact, if some of the extremes of exercise were continued for even moderately prolonged periods, they might be lethal Therefore, sports physiology is mainly a discussion of the ultimate limits to which several of the bodily mechanisms can be stressed To give one simple example: In a person who has extremely high fever approaching the level of lethality, the body metabolism increases to about 100 percent above normal By comparison, the metabolism of the body during a marathon race may increase to 2000 percent above normal .t c s p a k / :/ t t h e s /r u Most of the quantitative data that are given in this chapter are for the young male athlete, not because it is desirable to know only these values, but because it is only in male athletes that relatively complete measurements have been made However, for measurements that have been made in the female athlete, similar basic physiological principles apply, except for quantitative differences caused by differences in body size, body composition, and the presence or absence of the male sex hormone testosterone In general, most quantitative values for women—such as muscle strength, pulmonary ventilation, and cardiac output, all of which are related mainly to the muscle mass—vary between two thirds and three quarters of the values recorded in men, although there are many exceptions to this generalization When measured in terms of strength per square centimeter of cross-sectional area, the female muscle can achieve almost exactly the same maximal force of contraction as that of the male muscle—between and kg/cm2 Therefore, most of the difference in total muscle performance lies in the extra percentage of the male body that is muscle, which is caused partly by endocrine differences that we will discuss later The performance capabilities of the female versus male athlete are illustrated by the relative running speeds for a marathon race In a comparison, the top female performer had a running speed that was 11 percent less than that of the top male performer For other events, however, women have at times held records faster than men—for instance, for the two-way swim across the English Channel, for which the availability of extra fat seems to be an advantage for heat insulation, buoyancy, and extra long-term energy a t r/ r i h Muscles in Exercise Strength, Power, and Endurance of Muscles The final common determinant of success in athletic events is what the muscles can for you—that is, what strength they can give when it is needed, what power they can achieve in the performance of work, and how long they can continue their activity The strength of a muscle is determined mainly by its size, with a maximal contractile force between and kg/cm2 of muscle cross-sectional area Thus, a man who is well supplied with testosterone or who has enlarged his muscles through an exercise training program will have correspondingly increased muscle strength To give an example of muscle strength, a world-class weight lifter might have a quadriceps muscle with a crosssectional area as great as 150 square centimeters This measurement would translate into a maximal contractile strength of 525 kilograms (or 1155 pounds), with all this force applied to the patellar tendon Therefore, one can readily understand how it is possible for this tendon at times to be ruptured or actually to be avulsed from its insertion into the tibia below the knee Also, when such forces occur in tendons that span a joint, similar forces are applied to the surfaces of the joint or sometimes to ligaments spanning the joints, thus accounting for such Female and Male Athletes / 9 Testosterone secreted by the male testes has a powerful anabolic effect in causing greatly increased deposition of protein everywhere in the body, but especially in the muscles In fact, even a male who participates in very little sports activity but who nevertheless has a normal level of testosterone will have muscles that grow about 40 percent larger than those of a comparable female without the testosterone The female sex hormone estrogen probably also accounts for some of the difference between female and male performance, although not nearly so much as testosterone Estrogen increases the deposition of fat in the female, especially in the breasts, hips, and subcutaneous tissue At least partly for this reason, the average nonathletic female has about 27 percent body fat composition, in contrast to the nonathletic male, who has about 15 percent This increased body fat composition is a detriment to the highest levels of athletic performance in events in which performance depends on speed or on the ratio of total body muscle strength to body weight 1085 UNIT XV Sports Physiology Unit XV Sports Physiology t c a k / :/ kg-m/min First to 10 seconds 7000 Next minute 4000 Next 30 minutes 1700 s p Thus, it is clear that a person has the capability of extreme power surges for short periods, such as during a 100-meter dash that is completed entirely within 10 seconds, whereas for long-term endurance events, the power output of the muscles is only one fourth as great as during the initial power surge This does not mean that one’s athletic performance is four times as great during the initial power surge as it is for the next 30 minutes, because the efficiency for translation of muscle power output into athletic performance is often much less during rapid activity than during less rapid but sustained activity Thus, the velocity of the 100-meter dash is only 1.75 times as great as the velocity of a 30-minute race, despite the fourfold difference in short-term versus long-term muscle power capability Another measure of muscle performance is endurance Endurance, to a great extent, depends on the nutritive support for the muscle—more than anything else, it depends on the amount of glycogen that has been stored in the muscle before the period of exercise A person who consumes a high-carbohydrate diet stores far more t t h 1086 glycogen in muscles than does a person who consumes either a mixed diet or a high-fat diet Therefore, endurance is enhanced by a high-carbohydrate diet When athletes run at speeds typical for the marathon race, their endurance (as measured by the time that they can sustain the race until complete exhaustion) is approximately the following: Minutes High-carbohydrate diet 240 Mixed diet 120 / 9 High-fat diet 85 The corresponding amounts of glycogen stored in the muscle before the race started explain these differences The amounts stored are approximately the following: ir g/kg Muscle h a t r/ High-carbohydrate diet Mixed diet High-fat diet 40 20 Muscle Metabolic Systems in Exercise e s The same basic metabolic systems are present in muscle as in other parts of the body; these systems are discussed in detail in Chapters 68 through 74 However, special quantitative measures of the activities of three metabolic systems are exceedingly important in understanding the limits of physical activity These systems are (1) the phosphocreatinecreatine system, (2) the glycogen–lactic acid system, and (3) the aerobic system Adenosine Triphosphate The source of energy actually used to cause muscle contraction is adenosine triphosphate (ATP), which has the following basic formula: /r u happenings as displaced cartilages, compression fractures about the joint, and torn ligaments The holding strength of muscles is about 40 percent greater than the contractile strength That is, if a muscle is already contracted and a force then attempts to stretch out the muscle, as occurs when landing after a jump, this action requires about 40 percent more force than can be achieved by a shortening contraction Therefore, the force of 525 kilograms previously calculated for the patellar tendon during muscle contraction becomes 735 kilograms (1617 pounds) during holding contractions, which further compounds the problems of the tendons, joints, and ligaments It can also lead to internal tearing in the muscle In fact, forceful stretching of a maximally contracted muscle is one of the surest ways to create the highest degree of muscle soreness Mechanical work performed by a muscle is the amount of force applied by the muscle multiplied by the distance over which the force is applied The power of muscle contraction is different from muscle strength because power is a measure of the total amount of work that the muscle performs in a unit period of time Power is therefore determined not only by the strength of muscle contraction but also by its distance of contraction and the number of times that it contracts each minute Muscle power is generally measured in kilogram meters (kg-m) per minute That is, a muscle that can lift kilogram weight to a height of meter or that can move some object laterally against a force of kilogram for a distance of meter in minute is said to have a power of kg-m/min The maximal power achievable by all the muscles in the body of a highly trained athlete with all the muscles working together is approximately the following: Adenosine-PO3 ~ PO3 ~ PO3− The bonds attaching the last two phosphate radicals to the molecule, designated by the symbol ~, are high-energy phosphate bonds Each of these bonds stores 7300 calories of energy per mole of ATP under standard conditions (and even slightly more than this under the physical conditions in the body, which is discussed in detail in Chapter 68) Therefore, when one phosphate radical is removed, more than 7300 calories of energy are released to energize the muscle contractile process Then, when the second phosphate radical is removed, still another 7300 calories become available Removal of the first phosphate converts the ATP into adenosine diphosphate (ADP), and removal of the second converts this ADP into adenosine monophosphate (AMP) The amount of ATP present in the muscles, even in a well-trained athlete, is sufficient to sustain maximal muscle power for only about seconds, which might be enough for one half of a 50-meter dash Therefore, except for a few seconds at a time, it is essential that new ATP be formed continuously, even during the performance of short athletic events Figure 85-1 shows the overall metabolic system, demonstrating the breakdown of ATP first to ADP and then to AMP, with the release of energy to the muscles for contraction The left-hand side of the figure shows the Chapter 85 Sports Physiology Creatine + PO3− II Glycogen Lactic acid three metabolic systems that provide a continuous supply of ATP in the muscle fibers Phosphocreatine-Creatine System Phosphocreatine (also called creatine phosphate) is another chemical compound that has a high-energy phosphate bond, with the following formula: Creatine ~ PO3− Phosphocreatine can decompose to creatine and phosphate ion, as shown in Figure 85-1, and in doing so releases large amounts of energy In fact, the high-energy phosphate bond of phosphocreatine has more energy than the bond of ATP: 10,300 calories per mole compared with 7300 for the ATP bond Therefore, phosphocreatine can easily provide enough energy to reconstitute the high-energy bond of ATP Furthermore, most muscle cells have two to four times as much phosphocreatine as ATP A special characteristic of energy transfer from phosphocreatine to ATP is that it occurs within a small fraction of a second Therefore, all the energy stored in muscle phosphocreatine is almost instantaneously available for muscle contraction, just as is the energy stored in ATP The combined amounts of cell ATP and cell phosphocreatine are called the phosphagen energy system These substances together can provide maximal muscle power for to 10 seconds, almost enough for the 100-meter run Thus, the energy from the phosphagen system is used for maximal short bursts of muscle power Glycogen–Lactic Acid System The stored glycogen in muscle can be split into glucose, and the glucose can then be used for energy The initial stage of this process, called glycolysis, occurs without use of oxygen and, therefore, is said to be anaerobic metabolism (see Chapter 68) During glycolysis, each glucose molecule is split into two pyruvic acid molecules, and energy is released to form four ATP molecules for each original glucose molecule, as explained in Chapter 68 Ordinarily, the pyruvic acid then enters the mitochondria of muscle cells and reacts with oxygen to form still many more ATP molecules However, when there is insufficient oxygen for this second stage (the oxidative stage) of glucose metabolism to occur, most of the pyruvic acid then is converted into lactic acid, which diffuses out of the muscle cells into the interstitial fluid and blood Therefore, much of the muscle glycogen is transformed to lactic acid, but in doing so, considerable amounts of ATP are formed entirely without consumption of oxygen .t c AMP CO2 + H2O + Urea / 9 Another characteristic of the glycogen–lactic acid system is that it can form ATP molecules about 2.5 times as rapidly as can the oxidative mechanism of mitochondria Therefore, when large amounts of ATP are required for short to moderate periods of muscle contraction, this anaerobic glycolysis mechanism can be used as a rapid source of energy However, it is only about one half as rapid as the phosphagen system Under optimal conditions, the glycogen–lactic acid system can provide 1.3 to 1.6 minutes of maximal muscle activity in addition to the to 10 seconds provided by the phosphagen system, although at somewhat reduced muscle power Aerobic System The aerobic system is the oxidation of foodstuffs in the mitochondria to provide energy That is, as shown to the left in Figure 85-1, glucose, fatty acids, and amino acids from the foodstuffs—after some intermediate processing—combine with oxygen to release tremendous amounts of energy that are used to convert AMP and ADP into ATP, as discussed in Chapter 68 In comparing this aerobic mechanism of energy supply with the glycogen–lactic acid system and the phosphagen system, the relative maximal rates of power generation in terms of moles of ATP generation per minute are the following: a t r/ e s /r u r i h Moles of ATP/min Phosphagen system Glycogen–lactic acid system 2.5 Aerobic system When comparing the same systems for endurance, the relative values are the following: Time Phosphagen system 8-10 seconds Glycogen–lactic acid system 1.3-1.6 minutes Aerobic system Unlimited time (as long as nutrients last) Thus, one can readily see that the phosphagen system is used by the muscle for power surges of a few seconds and the aerobic system is required for prolonged athletic activity In between is the glycogen–lactic acid system, which is especially important for providing extra power during such intermediate races as 200- to 800-meter runs What Types of Sports Use Which Energy Systems? By considering the vigor of a sports activity and its duration, t t h s p a k / :/ + O2 Energy for muscle contraction ADP Figure 85-1. Important metabolic systems that supply energy for muscle contraction III Glucose Fatty acids Amino acids ATP 1087 UNIT XV I Phosphocreatine Phosphagen System, Almost Entirely 100-meter dash Jumping Weight lifting Diving Football dashes Baseball triple Phosphagen and Glycogen–Lactic Acid Systems 200-meter dash Basketball Ice hockey dashes Glycogen–lactic acid system, mainly 400-meter dash 100-meter swim Tennis Soccer Alactacid oxygen debt = 3.5 liters Exercise Table 85-1 Energy Systems Used in Various Sports Rate of oxygen uptake (L/min) Unit XV Sports Physiology Lactic acid oxygen debt = liters Figure 85-2. Rate of oxygen uptake by the lungs during maximal exercise for minutes and then for about 40 minutes after the exercise is over This figure demonstrates the principle of oxygen debt Glycogen–Lactic Acid and Aerobic Systems 800-meter dash 200-meter swim 1500-meter skating Boxing 2000-meter rowing 1500-meter run 1-mile run 400-meter swim a t r/ r i h Oxygen Debt The body normally contains about 10,000-meter skating Cross-country skiing Marathon run (26.2 miles, 42.2 kilometers) Jogging t c one can estimate closely which of the energy systems is used for each activity Various approximations are presented in Table 85-1 a k / :/ Recovery of the Muscle Metabolic Systems After Exercise In the same way that the energy from phospho- creatine can be used to reconstitute ATP, energy from the glycogen–lactic acid system can be used to reconstitute both phosphocreatine and ATP Energy from the oxidative metabolism of the aerobic system can then be used to reconstitute all the other systems—the ATP, phosphocreatine, and glycogen–lactic acid systems Reconstitution of the lactic acid system means mainly the removal of the excess lactic acid that has accumulated in the body fluids Removal of the excess lactic acid is especially important because lactic acid causes extreme fatigue When adequate amounts of energy are available from oxidative metabolism, removal of lactic acid is achieved in two ways: (1) A small portion of it is converted back into pyruvic acid and then metabolized oxidatively by the body tissues, and (2) the remaining lactic acid is reconverted into glucose mainly in the liver, and the glucose in turn is used to replenish the glycogen stores of the muscles Recovery of the Aerobic System After Exercise Even during the early stages of heavy exercise, a portion of one’s aerobic energy capability is depleted This depletion results from two effects: (1) the so-called oxygen debt and (2) depletion of the glycogen stores of the muscles s p t t h liters of stored oxygen that can be used for aerobic metabolism even without breathing any new oxygen This stored oxygen consists of the following: (1) 0.5 liter in the air of the lungs, (2) 0.25 liter dissolved in the body fluids, (3) liter combined with the hemoglobin of the blood, and (4) 0.3 liter stored in the muscle fibers, combined mainly with myoglobin, an oxygen-binding chemical similar to hemoglobin In heavy exercise, almost all this stored oxygen is used within a minute or so for aerobic metabolism Then, after the exercise is over, this stored oxygen must be replenished by breathing extra amounts of oxygen over and above the normal requirements In addition, about liters more oxygen must be consumed to reconstitute both the phosphagen system and the lactic acid system All this extra oxygen that must be “repaid,” about 11.5 liters, is called the oxygen debt Figure 85-2 shows this principle of oxygen debt During the first minutes, as depicted in the figure, the person exercises heavily, and the rate of oxygen uptake increases more than 15-fold Then, even after the exercise is over, the oxygen uptake still remains above normal; at first it is very high while the body is reconstituting the phosphagen system and repaying the stored oxygen portion of the oxygen debt, and then it is still above normal although at a lower level for another 40 minutes while the lactic acid is removed The early portion of the oxygen debt is called the alactacid oxygen debt and amounts to about 3.5 liters The latter portion is called the lactic acid oxygen debt and amounts to about liters Recovery of Muscle Glycogen Recovery from exhaustive muscle glycogen depletion is not a simple matter This process often requires days, rather than the seconds, minutes, or hours required for recovery of the phosphagen and lactic acid metabolic systems Figure 85-3 shows this recovery process under three conditions: first, in people who consume a high-carbohydrate diet; second, in people who consume a high-fat, high-protein diet; and third, in people who consume no food Note that for persons who consume a high-carbohydrate diet, full recovery occurs in about days Conversely, people who consume a high-fat, /r u e s Aerobic System 1088 / 9 12 16 20 24 28 32 36 40 44 Minutes Percent increase in strength hours of exercise 24 20 High-carbohydrate diet 16 12 No food Fat and protein diet Resistive training 30 25 20 15 10 No-load training 0 10 20 30 40 50 Hours of recovery 25 75 50 25 50 Mixed diet 10 20 40 75 Exhaustion High-fat diet Seconds Minutes Hours Duration of exercise Percent fat usage Percent carbohydrate usage High-carbohydrate diet 100 t c a k / :/ high-protein diet or no food at all show very little recovery even after as long as days The messages of this com parison are (1) it is important for athletes to consume a high-carbohydrate diet before a grueling athletic event and (2) athletes should not participate in exhaustive exercise during the 48 hours preceding the event t t h 10 Nutrients Used During Muscle Activity In addition to the use of a large amount of carbohydrates by the muscles during exercise, especially during the early stages of exercise, muscles use large amounts of fat for energy in the form of fatty acids and acetoacetic acid (see Chapter 69), as well as (to a much less extent) proteins in the form of amino acids In fact, even under the best conditions, in endurance athletic events that last longer than to hours, the glycogen stores of the muscle become almost totally depleted and are of little further use for energizing muscle contraction Instead, the muscle now depends on energy from other sources, mainly from fats Figure 85-4 shows the approximate relative usage of carbohydrates and fat for energy during prolonged r i h exhaustive exercise under three dietary conditions: a highcarbohydrate diet, a mixed diet, and a high-fat diet Note that most of the energy is derived from carbohydrates during the first few seconds or minutes of the exercise, but at the time of exhaustion, as much as 60 to 85 percent of the energy is being derived from fats rather than carbohydrates Not all the energy from carbohydrates comes from the stored muscle glycogen In fact, almost as much glycogen is stored in the liver as in the muscles, and this glycogen can be released into the blood in the form of glucose and then taken up by the muscles as an energy source In addition, glucose solutions given to an athlete to drink during the course of an athletic event can provide as much as 30 to 40 percent of the energy required during prolonged events such as marathon races Therefore, if muscle glycogen and blood glucose are available, they are the energy nutrients of choice for intense muscle activity Even so, for a long-term endurance event, one can expect fat to supply more than 50 percent of the required energy after about the first to hours a t r/ e s /r u Figure 85-4. The effect of duration of exercise, as well as type of diet, on relative percentages of carbohydrate or fat used for energy by muscles (Data from Fox EL: Sports Physiology Philadelphia: Saunders College Publishing, 1979.) s p Figure 85-5. Approximate effect of optimal resistive exercise training on increase in muscle strength over a training period of 10 weeks days Figure 85-3. The effect of diet on the rate of muscle glycogen replenishment after prolonged exercise (Modified from Fox EL: Sports Physiology Philadelphia: Saunders College Publishing, 1979.) 100 / 9 Weeks of training Effect of Athletic Training on Muscles and Muscle Performance Importance of Maximal Resistance Training One of the UNIT XV Muscle glycogen content (g/kg muscle) Chapter 85 Sports Physiology cardinal principles of muscle development during athletic training is the following: Muscles that function under no load, even if they are exercised for hours on end, increase little in strength At the other extreme, muscles that contract at more than 50 percent maximal force of contraction will develop strength rapidly even if the contractions are performed only a few times each day Using this principle, experiments on muscle building have shown that six nearly maximal muscle contractions performed in three sets days a week give approximately optimal increase in muscle strength without producing chronic muscle fatigue The upper curve in Figure 85-5 shows the approximate percentage increase in strength that can be achieved in a previously untrained young person by this resistive training program, demonstrating that the muscle strength increases about 30 percent during the first to weeks but almost plateaus after that time Along with this increase in strength is an approximately equal percentage increase in muscle mass, which is called muscle hypertrophy 1089 Unit XV Sports Physiology 1090 a t r/ e s /r u r i h Fast-Twitch Slow-Twitch Marathoners 18 82 Swimmers 26 74 Average male 55 45 Weight lifters 55 45 Sprinters 63 37 Jumpers 63 37 Respiration in Exercise Although one’s respiratory ability is of relatively little concern in the performance of sprint types of athletics, it is critical for maximal performance in endurance athletics Oxygen Consumption and Pulmonary Ventilation in Exercise Normal oxygen consumption for a young man at rest is about 250 ml/min However, under maximal conditions, this consumption can be increased to approximately the following average levels: ml/min Untrained average male 3600 Athletically trained average male 4000 Male marathon runner 5100 Figure 85-6 shows the relation between oxygen consumption and total pulmonary ventilation at different levels of exercise As would be expected, there is a linear relation Both oxygen consumption and total pulmonary ventila tion increase about 20-fold between the resting state and maximal intensity of exercise in the well-trained athlete Limits of Pulmonary Ventilation How severely we stress our respiratory systems during exercise? This t t h / 9 have considerably more fast-twitch than slow-twitch fibers, and others have more slow-twitch fibers; this factor could determine to some extent the athletic capabilities of different individuals Athletic training has not been shown to change the relative proportions of fast-twitch and slowtwitch fibers, however much an athlete might want to develop one type of athletic prowess over another Instead, the relative proportions of fast-twitch and slow-twitch fibers seem to be determined almost entirely by genetic inheritance, which in turn helps determine which area of athletics is most suited to each person: some people appear to be born to be marathoners, whereas others are born to be sprinters and jumpers For example, the following values are recorded percentages of fast-twitch versus slow-twitch fiber in the quadriceps muscles of different types of athletes: s p a k / :/ Hereditary Differences Among Athletes for FastTwitch Versus Slow-Twitch Muscle Fibers Some people t c The number of capillaries is greater in the vicinity of slow-twitch fibers than in the vicinity of fast-twitch fibers In summary, fast-twitch fibers can deliver extreme amounts of power for a few seconds to a minute or so Conversely, slow-twitch fibers provide endurance, delivering prolonged strength of contraction over many minutes to hours In old age, many people become so sedentary that their muscles atrophy tremendously In these instances, however, muscle training may increase muscle strength more than 100 percent Muscle Hypertrophy The average size of a person’s muscles is determined to a great extent by heredity plus the level of testosterone secretion, which, in men, causes considerably larger muscles than in women With training, however, the muscles can become hypertrophied perhaps an additional 30 to 60 percent Most of this hypertrophy results from increased diameter of the muscle fibers rather than increased numbers of fibers However, a very few greatly enlarged muscle fibers are believed to split down the middle along their entire length to form entirely new fibers, thus increasing the number of fibers slightly The changes that occur inside the hypertrophied muscle fibers include (1) increased numbers of myofibrils, proportionate to the degree of hypertrophy; (2) up to 120 percent increase in mitochondrial enzymes; (3) as much as 60 to 80 percent increase in the components of the phosphagen metabolic system, including both ATP and phosphocreatine; (4) as much as 50 percent increase in stored glycogen; and (5) as much as 75 to 100 percent increase in stored triglyceride (fat) Because of all these changes, the capabilities of both the anaerobic and the aerobic metabolic systems are increased, especially increasing the maximum oxidation rate and efficiency of the oxidative metabolic system as much as 45 percent Fast-Twitch and Slow-Twitch Muscle Fibers In the human being, all muscles have varying percentages of fast-twitch and slow-twitch muscle fibers For instance, the gastrocnemius muscle has a higher preponderance of fast-twitch fibers, which gives it the capability of forceful and rapid contraction of the type used in jumping In contrast, the soleus muscle has a higher preponderance of slow-twitch muscle fibers and therefore is used to a greater extent for prolonged lower leg muscle activity The basic differences between the fast-twitch and the slow-twitch fibers are the following: Fast-twitch fibers are about twice as large in diameter compared with slow-twitch fibers The enzymes that promote rapid release of energy from the phosphagen and glycogen–lactic acid energy systems are two to three times as active in fast-twitch fibers as in slow-twitch fibers, thus making the maximal power that can be achieved for very short periods by fast-twitch fibers about twice as great as that of slow-twitch fibers Slow-twitch fibers are mainly organized for endurance, especially for generation of aerobic energy They have far more mitochondria than the fasttwitch fibers In addition, they contain considerably more myoglobin, a hemoglobin-like protein that combines with oxygen within the muscle fiber; the extra myoglobin increases the rate of diffusion of oxygen throughout the fiber by shuttling oxygen from one molecule of myoglobin to the next In addition, the enzymes of the aerobic metabolic system are considerably more active in slow-twitch fibers than in fast-twitch fibers Chapter 85 Sports Physiology 0 Severe exercise 1.0 2.0 3.0 4.0 O2 consumption (L/min) Figure 85-6. Effect of exercise on oxygen consumption and ventilatory rate (Modified from Gray JS: Pulmonary Ventilation and Its Physiological Regulation Springfield, Ill: Charles C Thomas, 1950.) • Vo2max (L/min) 3.8 3.6 3.4 Training frequency = days/wk = days/wk = days/wk 3.2 3.0 2.8 10 Weeks of training 12 14 t c a k / :/ question can be answered by the following comparison for a normal young man: L/min Pulmonary ventilation at maximal exercise 100-110 Maximal breathing capacity 150-170 s p t t h Thus, the maximal breathing capacity is about 50 percent greater than the actual pulmonary ventilation during maximal exercise This difference provides an element of safety for athletes, giving them extra ventilation that can be called on in such conditions as (1) exercise at high altitudes, (2) exercise under very hot conditions, and (3) abnormalities in the respiratory system The important point is that the respiratory system is not normally the most limiting factor in the delivery of oxygen to the muscles during maximal muscle aerobic metabolism We shall see shortly that the ability of the heart to pump blood to the muscles is usually a greater limiting factor Effect of Training on VO2max The abbreviation for the rate of oxygen usage under maximal aerobic metabolism is VO2 max Figure 85-7 shows the progressive effect of athletic training on VO2max recorded in a group of subjects beginning at the level of no training and then while pursuing the training program for to 13 weeks In this study, it is surprising that the VO2max increased only about 10 a t r/ r i h e s /r u Figure 85-7. Increase in VO2 max over a period of to 13 weeks of athletic training (Modified from Fox EL: Sports Physiology Philadelphia: Saunders College Publishing, 1979.) / 9 Moderate exercise 20 40 60 80 100 percent Furthermore, the frequency of training, whether two times or five times per week, had little effect on the increase in VO2max Yet, as pointed out earlier, the VO2 max of a marathoner is about 45 percent greater than that of an untrained person Part of this greater VO2max of the marathoner probably is genetically determined; that is, people who have greater chest sizes in relation to body size and stronger respiratory muscles select themselves to become marathoners However, it is also likely that many years of training increase the marathoner’s VO2max by values considerably greater than the 10 percent that has been recorded in short-term experiments such as that in Figure 85-7 Oxygen-Diffusing Capacity of Athletes The oxygendiffusing capacity is a measure of the rate at which oxygen can diffuse from the pulmonary alveoli into the blood This capacity is expressed in terms of milliliters of oxygen that will diffuse each minute for each millimeter of mercury difference between alveolar partial pressure of oxygen and pulmonary blood oxygen pressure That is, if the partial pressure of oxygen in the alveoli is 91 mm Hg and the oxygen pressure in the blood is 90 mm Hg, the amount of oxygen that diffuses through the respiratory membrane each minute is equal to the diffusing capacity The follow ing values are measured values for different diffusing capacities: ml/min Nonathlete at rest 23 Nonathlete during maximal exercise 48 Speed skater during maximal exercise 64 Swimmer during maximal exercise 71 Oarsman during maximal exercise 80 The most startling fact about these results is the severalfold increase in diffusing capacity between the resting state and the state of maximal exercise This finding results mainly from the fact that blood flow through many of the pulmonary capillaries is sluggish or even dormant in the resting state, whereas in maximal exercise, increased blood flow through the lungs causes all the pulmonary capillaries to be perfused at their maximal rates, thus providing a far greater surface area through which oxygen can diffuse into the pulmonary capillary blood It is also clear from these values that athletes who require greater amounts of oxygen per minute have higher diffusing capacities Is this the case because people with naturally greater diffusing capacities choose these types of sports, or is it because something about the training procedures increases the diffusing capacity? The answer is not known, but it is very likely that training, particularly endurance training, does play an important role Blood Gases During Exercise Because of the great usage of oxygen by the muscles in exercise, one would expect the oxygen pressure of the arterial blood to decrease markedly during strenuous athletics and the carbon dioxide pressure of the venous blood to increase far above normal However, this normally is not the case Both of these values remain nearly normal, demonstrating the extreme ability 110 1091 UNIT XV Total ventilation (L/min) 120 t t h s p a k / :/ Cardiovascular System in Exercise Muscle Blood Flow A key requirement of cardiovascular function in exercise is to deliver the required oxygen and other nutrients to the exercising muscles For this purpose, the muscle blood flow increases drastically during exercise Figure 85-8 shows a recording of muscle blood flow in the calf of a person for a period of minutes during moderately strong intermittent contractions Note not only the great increase in flow—about 13-fold—but also the flow decrease during each muscle contraction Two points can be made from this study: The actual contractile process itself temporarily decreases muscle blood flow because the contracting skeletal muscle compresses the intramuscular blood vessels; therefore, strong tonic muscle contractions 1092 / 9 10 16 Minutes 18 r i h Figure 85-8. Effects of muscle exercise on blood flow in the calf of a leg during strong rhythmical contraction The blood flow was much less during contraction than between contractions (Modified from Barcroft J, Dornhorst AC: The blood flow through the human calf during rhythmic exercise, J Physiol 109:402, 1949.) a t r/ can cause rapid muscle fatigue because of lack of delivery of enough oxygen and other nutrients during the continuous contraction The blood flow to muscles during exercise increases markedly The following comparison shows the maximal increase in blood flow that can occur in a well-trained athlete e s /r u ml/100 g Muscle/min Resting blood flow 3.6 Blood flow during maximal exercise 90 Thus, muscle blood flow can increase a maximum of about 25-fold during the most strenuous exercise Almost one half this increase in flow results from intramuscular vasodilation caused by the direct effects of increased muscle metabolism, as explained in Chapter 21 The remaining increase results from multiple factors, the most important of which is probably the moderate increase in arterial blood pressure that occurs in exercise, which is usually about a 30 percent increase The increase in pressure not only forces more blood through the blood vessels but also stretches the walls of the arterioles and further reduces the vascular resistance Therefore, a 30 percent increase in blood pressure can often more than double the blood flow, which multiplies the great increase in flow already caused by the metabolic vasodilation at least another twofold Work Output, Oxygen Consumption, and Cardiac Output During Exercise Figure 85-9 shows the inter t c 20 It is widely known that smoking can decrease an athlete’s “wind.” This is true for many reasons First, one effect of nicotine is constriction of the terminal bronchioles of the lungs, which increases the resistance of airflow into and out of the lungs Second, the irritating effects of the smoke cause increased fluid secretion into the bronchial tree, as well as some swelling of the epithelial linings Third, nicotine paralyzes the cilia on the surfaces of the respiratory epithelial cells that normally beat continuously to remove excess fluids and foreign particles from the respiratory passageways As a result, much debris accumulates in the passageways and adds further to the difficulty of breathing After putting all these factors together, even a light smoker often feels respiratory strain during maximal exercise, and the level of performance may be reduced Much more severe are the effects of chronic smoking There are few chronic smokers in whom some degree of emphysema does not develop In this disease, the following mechanisms occur: (1) chronic bronchitis, (2) obstruction of many of the terminal bronchioles, and (3) destruction of many alveolar walls In persons with severe emphysema, as much as four fifths of the respiratory membrane can be destroyed; then even the slightest exercise can cause respiratory distress In fact, many such patients cannot even perform the simple feat of walking across the floor of a single room without gasping for breath Rhythmic exercise 40 Effect of Smoking on Pulmonary Ventilation in Exercise of the respiratory system to provide adequate aeration of the blood even during heavy exercise This demonstrates another important point: The blood gases not always have to become abnormal for respiration to be stimulated in exercise Instead, respiration is stimulated mainly by neurogenic mechanisms during exercise, as discussed in Chapter 42 Part of this stimulation results from direct stimulation of the respiratory center by the same nervous signals that are transmitted from the brain to the muscles to cause the exercise An additional part is believed to result from sensory signals transmitted into the respiratory center from the contracting muscles and moving joints All this extra nervous stimulation of respiration is normally sufficient to provide almost exactly the necessary increase in pulmonary ventilation required to keep the blood respiratory gases—the oxygen and the carbon dioxide—very near to normal Calf blood flow (100 mL/min) Unit XV Sports Physiology relations among work output, oxygen consumption, and cardiac output during exercise It is not surprising that all these factors are directly related to one another, as shown by the linear functions, because the muscle work output increases oxygen consumption, and increased oxygen consumption in turn dilates the muscle blood vessels, thus increasing venous return and cardiac output Typical cardiac outputs at several levels of exercise are as follows: n yge n ptio m nsu co Ox 0 200 400 600 800 1000120014001600 Work output during exercise (kg-m/min) Figure 85-9. Relation between cardiac output and work output (solid line) and between oxygen consumption and work output (dashed line) during different levels of exercise The different colored dots and squares show data derived from different studies in humans (Modified from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation Philadelphia: WB Saunders, 1973.) L/min Cardiac output in a young man at rest 5.5 Maximal cardiac output during exercise in a young untrained man 23 Maximal cardiac output during exercise in an average male marathoner 30 s p Role of Stroke Volume and Heart Rate in Increasing the Cardiac Output Figure 85-10 shows the approximate changes in stroke volume and heart rate as the cardiac output increases from its resting level of about 5.5 L/min to 30 L/min in the marathon runner The stroke volume 75 105 75 50 Maximum Nonathlete Marathoner 110 162 195 185 150 a t r/ 135 120 / 9 r i h Stroke volume 165 e s /r u marathoners can achieve maximal cardiac outputs that are about 40 percent greater than those achieved by untrained persons This results mainly from the fact that the heart chambers of marathoners enlarge about 40 percent; along with this enlargement of the chambers, the heart mass also increases 40 percent or more Therefore, not only the skeletal muscles hypertrophy during athletic training, but so does the heart However, heart enlargement and increased pumping capacity occur almost entirely in the endurance types, not in the sprint types, of athletic training Even though the heart of the marathoner is considerably larger than that of the normal person, resting cardiac output is almost exactly the same as that in a normal person However, this normal cardiac output is achieved by a large stroke volume at a reduced heart rate Table 85-2 compares stroke volume and heart rate in the untrained person and the marathoner Thus, the heart-pumping effectiveness of each heartbeat is 40 to 50 percent greater in the highly trained athlete than in the untrained person, but there is a corresponding decrease in the heart rate at rest t t h Resting Nonathlete Marathoner Effect of Training on Heart Hypertrophy and on Cardiac Output From the foregoing data, it is clear that a k / :/ Heart Rate (beats/min) 105 Thus, the normal untrained person can increase cardiac output a little over fourfold, and the well-trained athlete can increase output about sixfold (Individual marathoners have been clocked at cardiac outputs as great as 35 to 40 L/ min, or seven to eight times normal resting output.) t c Stroke Volume (ml) 170 150 130 110 90 Heart rate 10 15 20 25 Cardiac output (L/min) 190 70 Heart rate (beats/min) C 10 ca utp co ia ard 15 ex ind 50 30 Figure 85-10. Approximate stroke volume output and heart rate at different levels of cardiac output in a marathon athlete increases from 105 to 162 milliliters, an increase of about 50 percent, whereas the heart rate increases from 50 to 185 beats/min, an increase of 270 percent Therefore, the heart rate increase by far accounts for a greater proportion of the increase in cardiac output than does the increase in stroke volume during sustained strenuous exercise The stroke volume normally reaches its maximum by the time the cardiac output has increased only halfway to its maximum Any further increase in cardiac output must occur by increasing the heart rate Relation of Cardiovascular Performance to VO2max 20 nd ut a c rdia Stroke volume (ml/beat) 25 Table 85-2 Comparison of Cardiac Function Between Marathoner and Nonathlete 10 Cardiac output (L/min) 15 30 UNIT XV Cardiac index (L/min/m2) 35 Oxygen consumption (L/min) Chapter 85 Sports Physiology During maximal exercise, both the heart rate and stroke volume are increased to about 95 percent of their maximal levels Because the cardiac output is equal to stroke volume times heart rate, one finds that the cardiac output is about 90 percent of the maximum that the person can achieve, which is in contrast to about 65 percent of maximum for pulmonary ventilation Therefore, one can readily see that the cardiovascular system is normally much more limiting on VO2max than is the respiratory system, because oxygen utilization by the body can never be more than the rate at which the cardiovascular system can transport oxygen to the tissues For this reason, it is frequently stated that the level of athletic performance that can be achieved by the marathoner mainly depends on the performance capability of his or her heart, because this is the most limiting link in the delivery of adequate oxygen to the exercising muscles Therefore, the 40 percent greater cardiac output that the marathoner can achieve over the average untrained male is 1093 Unit XV Sports Physiology cardiovascular system places on maximal performance in endurance athletics, one can readily understand that any type of heart disease that reduces maximal cardiac output will cause an almost corresponding decrease in achievable total body muscle power Therefore, a person with congestive heart failure frequently has difficulty achieving even the muscle power required to climb out of bed, much less to walk across the floor The maximal cardiac output of older people also decreases considerably; there is as much as a 50 percent decrease between ages 18 and 80 years Also, there is even more of a decrease in maximal breathing capacity For these reasons, as well as because of reduced skeletal muscle mass, the maximal achievable muscle power is greatly reduced in old age Body Heat in Exercise Almost all the energy released by the body’s metabolism of nutrients is eventually converted into body heat This applies even to the energy that causes muscle contraction for the following reasons: First, the maximal efficiency for conversion of nutrient energy into muscle work, even under the best of conditions, is only 20 to 25 percent; the remainder of the nutrient energy is converted into heat during the course of the intracellular chemical reactions Second, almost all the energy that does go into creating muscle work still becomes body heat because all but a small portion of this energy is used for (1) overcoming viscous resistance to the movement of the muscles and joints, (2) overcoming the friction of the blood flowing through the blood vessels, and (3) other, similar effects, all of which convert the muscle contractile energy into heat Now, recognizing that the oxygen consumption by the body can increase as much as 20-fold in the well-trained athlete and that the amount of heat liberated in the body is almost exactly proportional to the oxygen consumption (as discussed in Chapter 73), one quickly realizes that tremendous amounts of heat are injected into the internal body tissues when performing endurance athletic events Next, with a vast rate of heat flow into the body, on a very hot and humid day that prevents the sweating mechanism from eliminating the heat, an intolerable and even lethal condition called heatstroke can easily develop in the athlete Heatstroke During endurance athletics, even under normal environmental conditions, the body temperature often rises from its normal level of 98.6°F to 102°F or 103°F (37°C to 40°C) With very hot and humid conditions or excess clothing, the body temperature can easily rise to 106°F to 108°F (41°C to 42°C) At this level, the elevated temperature becomes destructive to tissue cells, especially the brain cells When this phenomenon occurs, multiple symptoms begin to appear, including extreme weakness, exhaustion, headache, dizziness, nausea, profuse sweating, confusion, staggering gait, collapse, and unconsciousness This entire complex is called heatstroke, and failure to treat it immediately can lead to death In fact, even though t c s p a k / :/ t t h 1094 r i h Body Fluids and Salt in Exercise a t r/ / 9 As much as a 5- to 10-pound weight loss has been recorded in athletes in a period of hour during endurance athletic events under hot and humid conditions Essentially all this weight loss results from loss of sweat Loss of enough sweat to decrease body weight only percent can significantly diminish a person’s performance, and a to 10 percent rapid decrease in weight can often be serious, leading to muscle cramps, nausea, and other effects Therefore, it is essential to replace fluid as it is lost /r u e s Replacement of Sodium Chloride and Potassium Effect of Heart Disease and Old Age on Athletic Performance Because of the critical limitation that the the person has stopped exercising, the temperature does not easily decrease by itself, partly because at these high temperatures, the temperature-regulating mechanism often fails (see Chapter 74) A second reason is that in heatstroke, the very high body temperature approximately doubles the rates of all intracellular chemical reactions, thus liberating still more heat The treatment of heatstroke is to reduce the body temperature as rapidly as possible The most practical way to reduce the body temperature is to remove all clothing, maintain a spray of cool water on all surfaces of the body or continually sponge the body, and blow air over the body with a fan Experiments have shown that this treatment can reduce the temperature either as rapidly or almost as rapidly as any other procedure, although some physicians prefer total immersion of the body in water containing a mush of crushed ice if available Sweat contains a large amount of sodium chloride, for which reason it has long been stated that all athletes should take salt (sodium chloride) tablets when performing exercise on hot and humid days However, overuse of salt tablets has often done as much harm as good Furthermore, if an athlete becomes acclimatized to the heat by progressive increase in athletic exposure over a period of to weeks rather than performing maximal athletic feats on the first day, the sweat glands also become acclimatized, so the amount of salt lost in the sweat becomes only a small fraction of that lost before acclimatization This sweat gland acclimatization results mainly from increased aldosterone secretion by the adrenal cortex The aldosterone in turn has a direct effect on the sweat glands, increasing reabsorption of sodium chloride from the sweat before the sweat issues forth from the sweat gland tubules onto the surface of the skin Once the athlete is acclimatized, only rarely salt supplements need to be considered during athletic events Exercise-associated hyponatremia (low plasma sodium concentration) can sometimes occur after sustained physical exertion In fact, severe hyponatremia can be an important cause of fatalities in endurance athletes As noted in Chapter 25, severe hyponatremia can cause tissue edema, especially in the brain, which can be lethal In persons who experience life-threatening hyponatremia after heavy exercise, the main cause is not simply the loss of sodium due to sweating; instead, the hyponatremia is often due to ingestion of hypotonic fluid (water or sports drinks that usually have a sodium concentration of less than 18 mmol/L) in excess of sweat, urine, and insensible (mainly respiratory) fluid losses This excess fluid consumption can be probably the single most important physiological benefit of the marathoner’s training program Chapter 85 Sports Physiology Drugs and Athletes Without belaboring this issue, let us list some of the effects of drugs in athletics First, some persons believe that caffeine increases athletic performance In one experiment performed by a marathon runner, running time for the marathon was improved by percent through judicious use of caffeine in amounts similar to those found in one to three cups of coffee Yet experiments by other investigators have failed to confirm any advantage, thus leaving this issue in doubt Second, use of male sex hormones (androgens) or other anabolic steroids to increase muscle strength undoubtedly can increase athletic performance under some conditions, especially in women and even in men However, anabolic steroids also greatly increase the risk of cardiovascular disease because they often cause hypertension, decreased high-density blood lipoproteins, and increased low-density lipoproteins, all of which promote heart attacks and strokes In men, any type of male sex hormone preparation also leads to decreased testicular function, including both decreased formation of sperm and decreased secretion of the person’s own natural testosterone, with residual effects sometimes lasting at least for many months and perhaps indefinitely In a woman, even more significant effects such as facial hair, a bass voice, ruddy skin, and cessation of menses can occur because she is not normally adapted to the male sex hormone Other drugs, such as amphetamines and cocaine, have been reputed to increase athletic performance It is equally true that overuse of these drugs can lead to deterioration of performance Furthermore, experiments have failed to prove the value of these drugs except as a psychic stimulant Some athletes have been known to die during athletic events because of interaction between such drugs and the norepinephrine and epinephrine released by the sympathetic nervous system during exercise One of the possible causes of death under these conditions is overexcitability of the heart, leading to ventricular fibrillation, which is lethal within seconds .t c a k / :/ Body Fitness Prolongs Life Multiple studies have shown that people who maintain appropriate body fitness, using judicious regimens of exercise and weight control, have the additional benefit of prolonged life Especially between the ages of 50 and 70 years, a t r/ / 9 r i h e s /r u s p t t h studies have shown mortality to be three times less in the most fit people than in the least fit people Why does body fitness prolong life? The following reasons are some of the most important Body fitness and weight control greatly reduce cardiovascular disease This results from (1) maintenance of moderately lower blood pressure and (2) reduced blood cholesterol and low-density lipoprotein along with increased high-density lipoprotein As pointed out earlier, these changes all work together to reduce the number of heart attacks, brain strokes, and kidney disease The athletically fit person has more bodily reserves to call on when he or she does become sick For instance, an 80-year-old nonfit person may have a respiratory system that limits oxygen delivery to the tissues to no more than L/min; this means a respiratory reserve of no more than threefold to fourfold However, an athletically fit old person may have twice as much reserve This extra reserve is especially important in preserving life when the older person experiences conditions such as pneumonia that can rapidly require all available respiratory reserve In addition, the ability to increase cardiac output in times of need (the “cardiac reserve”) is often 50 percent greater in the athletically fit old person than in the nonfit person Exercise and overall body fitness also reduce the risk for several chronic metabolic disorders associated with obesity such as insulin resistance and type diabetes Moderate exercise, even in the absence of significant weight loss, has been shown to improve insulin sensitivity and reduce, or in some cases eliminate, the need for insulin treatment in patients with type diabetes Improved body fitness also reduces the risk for several types of cancers, including breast, prostate, and colon cancer Much of the beneficial effects of exercise may be related to a reduction in obesity However, studies in animals used in experiments and in humans have also shown that regular exercise reduces the risk for many chronic diseases through mechanisms that are incompletely understood but are, at least to some extent, independent of weight loss or decreased adiposity Bibliography Allen DG, Lamb GD, Westerblad H: Skeletal muscle fatigue: cellular mechanisms Physiol Rev 88:287, 2008 Booth FW, Laye MJ, Roberts MD: Lifetime sedentary living accelerates some aspects of secondary aging J Appl Physiol 111:1497, 2011 Casey DP, Joyner MJ: Compensatory vasodilatation during hypoxic exercise: mechanisms responsible for matching oxygen supply to demand J Physiol 590:6321, 2012 González-Alonso J: Human thermoregulation and the cardiovascular system Exp Physiol 97:340, 2012 Joyner MJ, Green DJ: Exercise protects the cardiovascular system: effects beyond traditional risk factors J Physiol 587:5551, 2009 Kent-Braun JA, Fitts RH, Christie A: Skeletal muscle fatigue Compr Physiol 2:997, 2012 Lavie CJ, McAuley PA, Church TS, et al: Obesity and cardiovascular diseases: implications regarding fitness, fatness, and severity in the obesity paradox J Am Coll Cardiol 63:1345, 2014 Powers SK, Jackson MJ: Exercise-induced oxidative stress: cellular mechanisms and impact on muscle force production Physiol Rev 88:1243, 2008 1095 UNIT XV driven by thirst but also may be due to conditioned behavior that is based on recommendations to drink fluid during exercise to avoid dehydration Copious supplies of water are also generally available in marathons, triathlons, and other endurance athletic events Experience by military units exposed to heavy exercise in the desert has demonstrated still another electrolyte problem—the loss of potassium Potassium loss results partly from the increased secretion of aldosterone during heat acclimatization, which increases the loss of potassium in the urine, as well as in the sweat As a consequence of these findings, some of the supplemental fluids for athletics contain properly proportioned amounts of potassium along with sodium, usually in the form of fruit juices Unit XV Sports Physiology Powers SK, Smuder AJ, Kavazis AN, Quindry JC: Mechanisms of exercise-induced cardioprotection Physiology (Bethesda) 29:27, 2014 Rosner MH: Exercise-associated hyponatremia Semin Nephrol 29: 271, 2009 Sandri M: Signaling in muscle atrophy and hypertrophy Physiology (Bethesda) 23:160, 2008 Schiaffino S, Dyar KA, Ciciliot S, et al: Mechanisms regulating skeletal muscle growth and atrophy FEBS J 280:4294, 2013 .t c s p a k / :/ t t h 1096 Seals DR, Edward F: Adolph Distinguished Lecture: the remarkable anti-aging effects of aerobic exercise on systemic arteries J Appl Physiol 117:425, 2014 Thompson D, Karpe F, Lafontan M, Frayn K: Physical activity and exercise in the regulation of human adipose tissue physiology Physiol Rev 92:157, 2012 /r u e s a t r/ r i h / 9 Normal Values for Selected Common Laboratory Measurements Average (“Normal” Value) Range Comment/Unit of Measure Sodium (Na+) 142 mmol/L 135-145 mmol/L mmol/L = Millimoles per liter Potassium (K+) 4.2 mmol/L 3.5-5.3 mmol/L Chloride (Cl−) 106 mmol/L 98-108 mmol/L Anion gap 12 mEq/L 7-16 mEq/L Bicarbonate (HCO3−) 24 mmol/L 22-29 mmol/L Hydrogen ion (H+) 40 nmol/L 30-50 nmol/L pH, arterial 7.4 7.25-7.45 pH, venous 7.37 7.32-7.42 Calcium ion (Ca++) 5.0 mg/dL 4.65-5.28 mg/dL Calcium, total 10.0 mg/dL 8.5-10.5 mg/dL Magnesium ion (Mg++) 0.8 mEq/L 0.6-1.1 mEq/L Magnesium, total 1.8 mEq/L 1.3-2.4 mEq/L Phosphate, total 3.5 mg/dL 2.5-4.5 mg/dL In plasma, HPO4= is ~1.05 mmol/L and H2PO4− is 0.26 mmol/L 4.5 g/dL 3.5-5.5 g/dL g/dL = grams per deciliter Alkaline phosphatase M: 38-126 U/L F: 70-230 U/L U/L = units per liter Bilirubin, total 0.2-1.0 mg/dL Substance mEq/L = milliequivalents per liter Anion gap = Na+ − Cl− − HCO3− nmmol/L = nanomoles per liter a t r/ 0-0.2 mg/dL 10-26 mg/dL Creatinine 1.0 mg/dL 0.6-1.3 mg/dL Glucose 90 mg/dL 70-115 mg/dL Osmolarity 282 mOsm/L 275-300 mOsm/L Protein, total 7.0 g/dL 6.0-8.0 g/dL Prothrombin time (PT) White blood cells, total Neutrophils Lymphocytes Monocytes Eosinophils Basophils 35-45 mm Hg PCO2 = partial pressure of carbon dioxide in millimeters of mercury 41-51 mm Hg PO2 = partial pressure of oxygen in millimeters of mercury 25-40 mm Hg M: 13.5-17.5 g/dL F: 12-16 g/dL M: 5.5 ì 108/àL F: 4.7 ì 108/àL 4.3-5.7 ì 108/àL 4.3-5.7 ì 108/àL Number of cells per microliter of blood 90 fl 80-100 fl fl = femtoliters 10-14 seconds Time required for the plasma to clot during a special test M: 15 g/dL F: 14 g/dL t t h Mean corpuscular (RBC) volume (MCV) Percentage of hemoglobin molecules saturated with oxygen 80-100 mm Hg M: 39%-49% F: 35%-45% s p Hemoglobin (Hgb) mOsm/L = milliosmoles per liter Osmolality is expresses as mOsm/kg of water M: 42% F: 38% Hematocrit (Hct) Hematology 45 mm Hg PCO2, venous 40 mm Hg PCO2, arterial 40 mm Hg t c a k / :/ PO2, venous Varies depending on muscle mass, age, and sex 95%-99% 90 mm Hg 98% PO2, arterial O2 sat, arterial M: 3.0-7.4 mg/dL F: 2.1-6.3 mg/dL Blood Gases Platelets /r u Uric acid 14 mg/dL Blood urea nitrogen (BUN) Red blood cells (RBCs) e s Bilirubin, conjugated r i h Nonelectrolyte Blood Chemistries Albumin / 9 mg/dL = milligrams/deciliter Average normal value can also be expressed as approximately 1.2 mmol/L or 2.4 mEq/L Electrolytes 150-450 ì 103/àL 4.5-11.0 × 103/µL 57%-67% 23%-33% 3%-7% 1%-3% 0%-1% Lipids 35 mg/dL Triglycerides M: 40-160 mg/dL F: 35-135 mg/dL Total cholesterol This table is not an exhaustive list of common laboratory values Most of these values are approximate reference values used by the University of Mississippi Medical Center Clinical Laboratories; normal ranges may vary among different clinical laboratories Average “normal” values and units of measure may also differ slightly from those cited in the Guyton and Hall Textbook of Medical Physiology, 13th edition For example, electrolytes are often reported in milliequivalents per liter (mEq/L), a measure of electrical charge of an electrolyte, or in millimoles per liter F, female; M, male ... Principles and Sensory Physiology pro-opiomelanocortin, proenkephalin, and prodynorphin Among the more important of these opiate-like substances are β-endorphin, met-enkephalin, leu-enkephalin, and. .. shape, body position and movement, and muscle force Physiol Rev 92: 1651, 20 12 Suga N: Tuning shifts of the auditory system by corticocortical and corticofugal projections and conditioning Neurosci... Slow-chronic pain fibers Figure 4 9 -2 . Transmission of both “fast-sharp” and “slow-chronic” pain signals into and through the spinal cord on their way to the brain Aδ fibers transmit fast-sharp