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Figure

Liver cells can convert lactic acid, generated by muscles anaerobically, into glucose.

Muscle Fatigue

A muscle exercised persistently for a prolonged period may lose its ability to contract, a condition called fatigue. This condition may result from a number of causes, including decreased blood flow, ion imbalances across the sarcolemma resulting from repeated stimulation, and psychological loss of the desire to continue the exercise. However, muscle fatigue is most likely to arise from accumulation of lactic acid in the muscle as a result of anaerobic ATP production. The lowered pH from the lactic acid prevents muscle fibers from responding to stimulation.

Occasionally a muscle fatigues and cramps at the same time. A cramp is a painful condition in which a muscle undergoes a sustained, involuntary contraction. Cramps are thought to occur when changes in the extracellular fluid, particularly a decreased electrolyte concentration, surrounding the muscle fibers and their motor neurons somehow trigger uncontrolled stimulation of the muscle.

As muscle metabolism shifts from aerobic ATP production to anaerobic ATP production, lactic acid begins to accumulate in muscles and to appear in the bloodstream (lactic acid threshold). This leads to muscle fatigue. How quickly this happens varies from individual to individual, although people who regularly exercise aerobically produce less lactic acid than those who do not. Physically fit people make less lactic acid, because the strenuous exercise of aerobic training stimulates new capillaries to grow within the muscles, supplying more oxygen and nutrients to the muscle fibers. Such physical training also causes muscle fibers to produce additional mitochondria, increasing their ability to carry on aerobic respiration. Some muscle fibers may be more likely to accumulate lactic acid than others, as described in the section titled "Fast and Slow Muscle Fibers."

Heat Production

Heat is a by-product of cellular respiration; all active cells generate heat. Since muscle tissue represents such a large proportion of the total body mass, it is a major source of heat.

Less than half of the energy released in cellular respiration is available for use in metabolic processes; the rest becomes heat. Active muscles release a great deal of heat. Blood transports this heat throughout the body, which helps to maintain body temperature. Homeostatic mechanisms promote heat loss when the temperature of the internal environment begins to rise (see chapters 1 and 6, pp. 6 and 182, respectively).

H What are the sources of energy used to regenerate ATP?

^9 What are the sources of oxygen required for aerobic respiration?

^9 How do lactic acid and oxygen debt relate to muscle fatigue?

□ What is the relationship between cellular respiration and heat production?

Muscular Responses

One way to observe muscle contraction is to remove a single muscle fiber from a skeletal muscle and connect it to a device that senses and records changes in the fiber's length. An electrical stimulator is usually used to promote muscle contraction.

Threshold Stimulus

When an isolated muscle fiber is exposed to a series of stimuli of increasing strength, the fiber remains unresponsive until a certain strength of stimulation is applied. This minimal strength required to cause contraction is called the threshold stimulus (thresh'old stim'u-lus). An impulse in a motor neuron normally releases enough ACh to bring the muscle fibers in its motor unit to threshold.

Recording a Muscle Contraction

To record how a whole muscle responds to stimulation, a skeletal muscle can be removed from a frog or other small animal and mounted in a special laboratory apparatus that stretches it to an optimal length. The muscle is then stimulated electrically, and when it contracts, it pulls on a lever. The lever's movement is recorded, and the resulting pattern is called a myogram (mi'o-gram).

If a muscle is exposed to a single stimulus of sufficient strength to activate some of its motor units, the muscle will contract and then relax. This action—a single contraction that lasts only a fraction of a second—is called a twitch. A twitch produces a myogram like that in figure 9.16. Note there was a delay between the time the stimulus was applied and the time the muscle responded. This is the latent period. In a frog muscle, the latent period lasts for about 0.01 second; in a human muscle, it is even shorter. The latent period is followed by a period of contraction when the muscle pulls at its attachments, and a period of relaxation when the apparatus stretches it to its former length.

Muscle Twitch

Time of stimulation

Time

Figure 9.16

A myogram of a single muscle twitch.

Time of stimulation

Time

Figure 9.16

A myogram of a single muscle twitch.

If a muscle is exposed to two stimuli (of threshold strength or above) too quickly, it may respond with a twitch to the first stimulus but not to the second. This is because it takes an instant following a contraction for muscle fibers to become responsive to further stimulation. Thus, for a very brief moment following stimulation, a muscle remains unresponsive. This time is called the refractory period.

All-or-None Response

A muscle fiber that is not brought to threshold will not contract. One that is exposed to a stimulus of threshold strength or above responds with a complete twitch. Increasing the strength of the stimulus does not affect the strength of the contraction. This phenomenon is called the all-or-none response.

Staircase Effect

The force a muscle fiber exerts in a twitch may depend on whether it has recently been stimulated to contract. A muscle fiber that has been inactive can be subjected to a series of stimuli, such that it undergoes a series of twitches with complete relaxation in between (fig. 9.17a). However, the strength of each successive contraction increases slightly, soon reaching a maximum. This phenomenon, called the staircase effect (treppe), is small and brief. Muscle fiber contraction is otherwise an all-or-none response.

The staircase effect seems to involve a net increase in the concentration of calcium ions available in the sar-coplasm of the muscle fibers. This increase might occur if each stimulus in the series caused the release of calcium ions and if the sarcoplasmic reticulum failed to recapture those ions immediately.

Time

Figure 9.17

Time

Figure 9.17

Myograms of (a) a series of twitches showing the staircase effect, (b) summation, and (c) a tetanic contraction. Note that stimulation frequency increases from one myogram to the next.

Summation

The force that a muscle fiber can generate is not limited to the maximum force of a single twitch. A muscle fiber exposed to a series of stimuli of increasing frequency reaches a point when it is unable to completely relax before the next stimulus in the series arrives. When this happens, the individual twitches begin to combine and the muscle contraction becomes sustained. In such a sustained contraction, the force of individual twitches combines by the process of summation (fig. 9.17fo). When the resulting forceful, sustained contraction lacks even partial relaxation, it is called a tetanic (te-tan-ik) contraction (tetanus) (fig. 9.17c).

Recruitment of Motor Units

The number of muscle fibers in a motor unit varies considerably. The fewer muscle fibers in the motor units, however, the more precise the movements that can be produced in a particular muscle. For example, the motor units of the muscles that move the eyes may contain fewer than ten muscle fibers per motor unit and can produce very slight movements. Conversely, the motor units of the large muscles in the back may include a hundred or more muscle fibers. When these motor units are stimulated, the movements that result are less gradual compared to those of the eye.

Since the muscle fibers within a muscle are organized into motor units and each motor unit is controlled by a single motor neuron, all the muscle fibers in a motor unit are stimulated at the same time. Therefore, a motor unit also responds in an all-or-none manner. A whole muscle, however, does not behave like this, because it is composed of many motor units controlled by different motor neurons, some of which are more easily stimulated than others. Thus, if only the more easily stimulated motor neurons are involved, few motor units contract. At higher intensities of stimulation, other motor neurons respond, and more motor units are activated. Such an increase in the number of activated motor units is called multiple motor unit summation, or recruitment (re-kroot'ment). As the intensity of stimulation increases, recruitment of motor units continues until finally all possible motor units are activated in that muscle.

Sustained Contractions

During sustained contractions, smaller motor units, which have smaller diameter axons, tend to be recruited earlier. The larger motor units, which contain larger diameter axons, respond later and more forcefully. The product is a sustained contraction of increasing strength.

Typically, many action potentials are triggered in a motor neuron when it is called into action, thus individual twitches do not normally occur. Tetanic contractions of muscle fibers are common. On the whole-muscle level, contractions are smooth rather than irregular or jerky because a mechanism within the spinal cord stimulates contractions in different sets of motor units at different moments.

Tetanic contractions occur frequently in skeletal muscles during everyday activities. In many cases, the condition occurs in only a portion of a muscle. For example, when a person lifts a weight or walks, sustained contractions are maintained in the upper limb or lower limb muscles for varying lengths of time. These contractions are responses to a rapid series of stimuli transmitted from the brain and spinal cord on motor neurons.

Even when a muscle appears to be at rest, a certain amount of sustained contraction is occurring in its fibers. This is called muscle tone (tonus), and it is a response to nerve impulses originating repeatedly in the spinal cord and traveling to a few muscle fibers. The result is a continuous state of partial contraction.

Muscle tone is particularly important in maintaining posture. Tautness in the muscles of the neck, trunk, and lower limbs enables a person to hold the head upright, stand, or sit. If tone is suddenly lost, such as when a person loses consciousness, the body will collapse. Muscle tone is maintained in health but is lost if motor nerve axons are cut or if diseases interfere with conduction of nerve impulses.

Figure

(a and b) Isotonic contractions include concentric and eccentric contractions. (c) Isometric contractions occur when a muscle contracts but does not shorten.

When skeletal muscles are contracted very forcefully, they may generate up to 50 pounds of pull for each square inch of muscle cross section. Consequently, large muscles such as those in the thigh can pull with several hundred pounds of force. Occasionally, this force is so great that the tendons of muscles tear away from their attachments to the bones.

Types of Contractions

Sometimes muscles shorten when they contract. For example, if a person lifts an object, the muscles remain taut, their attached ends pull closer together, and the object is moved. This type of contraction is termed isotonic (equal force—change in length), and because shortening occurs, it is called concentric.

Another type of isotonic contraction, called a lengthening or an eccentric contraction, occurs when the force a muscle generates is less than that required to move or lift an object, as in laying a book down on a table. Even in such a contraction, cross-bridges are working but not generating enough force to shorten the muscle.

At other times, a skeletal muscle contracts, but the parts to which it is attached do not move. This happens, for instance, when a person pushes against the wall of a building. Tension within the muscles increases, but the wall does not move, and the muscles remain the same length. Contractions of this type are called isometric (equal length—change in force). Isometric contractions occur continuously in postural muscles that stabilize skeletal parts and hold the body upright. Figure 9.18 illustrates isotonic and isometric contractions.

Most body actions involve both isotonic and isometric contraction. In walking, for instance, certain leg and thigh muscles contract isometrically and keep the limb stiff as it touches the ground, while other muscles contract isotonically, bending the limb and lifting it. Similarly, walking down stairs involves eccentric contraction of certain thigh muscles.

Fast and Slow Muscle Fibers

Muscle fibers vary in contraction speed (slow twitch or fast twitch) and in whether they produce ATP oxidatively or glycolytically. Three combinations of these characteristics are found in humans. Slow-twitch fibers (type I) are always oxidative and are therefore resistant to fatigue. Fast-twitch fibers (type II) may be primarily glycolytic (fa-tigueable) or primarily oxidative (fatigue resistant).

Slow-twitch (type I) fibers, such as those found in the long muscles of the back, are often called red /ibers because they contain the red, oxygen-storing pigment myoglobin. These fibers are well supplied with oxygen-carrying blood. In addition, red fibers contain many mitochondria, an adaptation for aerobic respiration. These fibers have a high respiratory capacity and can generate ATP fast enough to keep up with the ATP breakdown that occurs when they contract. For this reason, these fibers can contract for long periods without fatiguing.

Fast-twitch glycolytic fibers (type IIa) are often called white /ibers because they contain less myoglobin and have a poorer blood supply than red fibers. They include fibers found in certain hand muscles as well as in muscles that move the eye. These fibers have fewer mitochondria and thus have a reduced respiratory capacity. However, they have a more extensive sarcoplasmic

Use and Disuse of Skeletal Muscles

Skeletal muscles are very responsive to use and disuse. Those that are forcefully exercised tend to enlarge. This phenomenon is called muscular hypertrophy. Conversely, a muscle that is not used atrophies — it decreases in size and strength.

The way a muscle responds to use also depends on the type of exercise. For instance, when a muscle contracts weakly, as during swimming and running, its slow, fatigue-resistant red fibers are most likely to be activated. As a result, these fibers develop more mitochondria and more extensive capillary networks. Such changes increase the fibers' abilities to resist fatigue during prolonged exercise, although their sizes and strengths may remain unchanged.

Forceful exercise, such as weightlifting, in which a muscle ex erts more than 75% of its maximum tension, uses the muscle's fast, fati-gable white fibers. In response, existing muscle fibers develop new filaments of actin and myosin, and as their diameters increase, the entire muscle enlarges. However, no new muscle fibers are produced during hypertrophy.

Since the strength of a contraction is directly proportional to the diameter of the muscle fibers, an enlarged muscle can contract more strongly than before. However, such a change does not increase the muscle's ability to re sist fatigue during activities such as running or swimming.

If regular exercise stops, capillary networks shrink, and the number of mitochondria within the muscle fibers fall. Actin and myosin filaments diminish, and the entire muscle atrophies. Injured limbs immobilized in casts, or accidents or diseases that interfere with motor nerve impulses, commonly cause muscle atrophy. A muscle that cannot be exercised may shrink to less than one-half its usual size within a few months.

Muscle fibers whose motor neurons are severed not only shrink but also may fragment and, in time, be replaced by fat or fibrous tissue. However, reinnervation of such a muscle within the first few months following an injury can restore function.

reticulum to store and reabsorb calcium ions, and their ATPase is faster than that of red fibers. Because of these factors, white muscle fibers can contract rapidly, although they tend to fatigue as lactic acid accumulates and as the ATP and the biochemicals to regenerate ATP are depleted.

A third kind of fiber, the fast-twitch fatigue-resistant fibers (type IIb), are sometimes called intermediate fibers. These fibers have the fast-twitch speed associated with white fibers combined with a substantial oxidative capacity more characteristic of red fibers.

While some muscles may have mostly one fiber type or another, all muscles contain a combination of fiber types. The speed of contraction and aerobic capacities of the fibers present reflect the specialized functions of the muscle. For example, muscles that move the eyes contract about ten times faster than those that maintain posture, and the muscles that move the limbs contract at intermediate rates. Clinical Application 9.2 discusses very noticeable effects of muscle use and disuse.

Birds that migrate long distances have abundant dark, slow-twitch muscles — this is why their meat is dark. In contrast, chickens that can only flap around the barnyard have abundant fast-twitch muscles, and mostly white meat.

World-class distance runners are the human equivalent of the migrating bird. Their muscles may contain over 90% slow-twitch fibers! In some European nations, athletic coaches measure slow-twitch to fast-twitch muscle fiber ratios to predict who will excel at long-distance events and who will fare better in sprints.

Define threshold stimulus.

What is an all-or-none response?

Distinguish between a twitch and a sustained contraction.

Define muscle tone.

Explain the differences between isometric and isotonic contractions.

Distinguish between fast-contracting and slow-contracting muscles fibers.

Smooth Muscles

The contractile mechanisms of smooth and cardiac muscles are essentially the same as those of skeletal muscles. However, the cells of these tissues have important structural and functional differences.

Smooth Muscle Fibers

As discussed in chapter 5 (page 160), smooth muscle cells are shorter than the fibers of skeletal muscle, and they have single, centrally located nuclei. Smooth muscle cells are elongated with tapering ends and contain filaments of actin and myosin in myofibrils that extend throughout their lengths. However, the filaments are very thin and more randomly organized than those in skeletal muscle fibers. As a result, smooth muscle cells lack stria-tions. They also lack transverse tubules, and their sar-coplasmic reticula are not well developed.

The two major types of smooth muscles are multi-unit and visceral. In multiunit smooth muscle, the muscle fibers are less well organized and function as separate units, independent of neighboring cells. Smooth muscle of this type is found in the irises of the eyes and in the walls of blood vessels. Typically, multiunit smooth muscle contracts only after stimulation by motor nerve impulses or certain hormones.

Visceral smooth muscle (single-unit smooth muscle) is composed of sheets of spindle-shaped cells held in close contact by gap junctions. The thick portion of each cell lies next to the thin parts of adjacent cells. Fibers of visceral smooth muscle respond as a single unit. When one fiber is stimulated, the impulse moving over its surface may excite adjacent fibers that, in turn, stimulate others. Some visceral smooth muscle cells also display rhythmicity—a pattern of spontaneous repeated contractions.

These two features of visceral smooth muscle— transmission of impulses from cell to cell and rhythmicity— are largely responsible for the wavelike motion called peristalsis that occurs in certain tubular organs (see chapter 17, p. 688). Peristalsis consists of alternate contractions and relaxations of the longitudinal and circular muscles. These movements help force the contents of a tube along its length. In the intestines, for example, peristaltic waves move masses of partially digested food and help to mix them with digestive fluids. Peristalsis in the ureters moves urine from the kidneys to the urinary bladder.

Visceral smooth muscle is the more common type of smooth muscle and is found in the walls of hollow organs, such as the stomach, intestines, urinary bladder, and uterus. Usually there are two thickness of smooth muscle in the walls of these organs. The fibers of the outer coats are directed longitudinally, whereas those of the inner coats are arranged circularly. These muscular layers change the sizes and shapes of these organs as they function.

Smooth Muscle Contraction

Smooth muscle contraction resembles skeletal muscle contraction in a number of ways. Both mechanisms reflect reactions of actin and myosin; both are triggered by membrane impulses and release of calcium ions; and both use energy from ATP molecules. There are, however, significant differences between smooth and skeletal muscle action. For example, smooth muscle fibers lack troponin, the protein that binds to calcium ions in skeletal muscle. Instead, smooth muscle uses a protein called calmodulin, which binds to calcium ions released when its fibers are stimulated, thus activating the actin-myosin contraction mechanism. In addition, much of the calcium necessary for smooth muscle contraction diffuses into the cell from the extracellular fluid.

Acetylcholine, the neurotransmitter in skeletal muscle, as well as norepinephrine, affect smooth muscle. Each of these neurotransmitters stimulates contractions in some smooth muscles and inhibits contractions in others. The discussion of the autonomic nervous system in chapter 11 (p. 437) describes these actions in greater detail.

Hormones affect smooth muscles by stimulating or inhibiting contraction in some cases and altering the degree of response to neurotransmitters in others. For example, during the later stages of childbirth, the hormone oxytocin stimulates smooth muscles in the wall of the uterus to contract (see chapter 22, p. 918).

Stretching of smooth muscle fibers can also trigger contractions. This response is particularly important to the function of visceral smooth muscle in the walls of certain hollow organs, such as the urinary bladder and the intestines. For example, when partially digested food stretches the wall of the intestine, automatic contractions move the contents away.

Smooth muscle is slower to contract and slower to relax than skeletal muscle. On the other hand, smooth muscle can forcefully contract longer with the same amount of ATP. Unlike skeletal muscle, smooth muscle fibers can change length without changing tautness; because of this, smooth muscles in the stomach and intestinal walls can stretch as these organs fill, holding the pressure inside the organs constant.

Describe the two major types of smooth muscle.

What special characteristics of visceral smooth muscle make peristalsis possible?

How is smooth muscle contraction similar to skeletal muscle contraction?

How do the contraction mechanisms of smooth and skeletal muscles differ?

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