Now that you are familiar with the basic mechanisms of muscle contraction at the level of the individual muscle fiber, we can begin to examine the performance of skeletal muscles--organs of the muscular system. In this section, we will consider the coordinated contractions of an entire population of skeletal muscle fibers.
The amount of tension produced by an individual muscle fiber depends solely on the number of pivoting cross-bridges. If a muscle fiber at a given resting length is stimulated to contract, it will always produce the same amount of tension. There is no mechanism to regulate the amount of tension produced in that contraction by changing the number of contracting sarcomeres. When calcium ions are released, they are released from all triads in the muscle fiber. As a result, all the myosin heads within zones of overlap will interact with thin filaments. Thus a muscle fiber is either "ON" (producing as much tension as possible at that resting length) or "OFF" (relaxed). This feature of muscle mechanics is known as the all-or-none principle. The amount of tension produced in the skeletal muscle as a whole is therefore determined by (1) the frequency of stimulation and (2) the number of muscle fibers stimulated.
The Frequency of Muscle Stimulation
A twitch is a single stimulus–contraction–relaxation sequence in a muscle fiber. Twitches vary in duration. Twitches in one eye muscle fiber can be as brief as 7.5 msec, but a twitch in a muscle fiber from the soleus, a small calf muscle, lasts about 100 msec. Figure 10-12a is a myogram, or graph of tension development in various muscles during a twitch contraction.
Figure 10-12b details the phases of a 40-msec twitch in the gastrocnemius muscle, a prominent calf muscle. A single twitch can be divided into a latent period, a contraction phase, and a relaxation phase:
1.The latent period begins at stimulation and typically lasts about 2 msec. Over this period, the action potential sweeps across the sarcolemma and the sarcoplasmic reticulum releases calcium ions. The muscle fiber does not produce tension during the latent period, because the contraction cycle has yet to begin.
2.In the contraction phase, tension rises to a peak. As tension rises, calcium ions are binding to troponin, active sites on thin filaments are being exposed, and cross-bridge interactions are occurring. The contraction phase ends roughly 15 msec after stimulation.
3.The relaxation phase then continues for about another 25 msec. During this period, calcium levels are falling, active sites are being covered by tropomyosin, and the number of active cross-bridges is declining.
A single stimulation produces a single twitch, but twitches in a skeletal muscle do not accomplish anything useful. All normal activities involve sustained muscle contractions. The mechanism involved is most easily understood by examining the responses of an isolated skeletal muscle when it is stimulated under laboratory conditions. Under these conditions, the myogram of tension production changes dramatically as the rate of stimulation increases (Figure 10-13).
Wave Summation and Incomplete Tetanus If a second stimulus arrives before the relaxation phase has ended, a second, more powerful contraction occurs. The addition of one twitch to another in this way constitutes the summation of twitches, or wave summation (Figure 10-13a). The duration of a single twitch determines the maximum time available to produce wave summation. For example, if a twitch lasts 20 msec (1/50 sec), subsequent stimuli must be separated by less than 20 msec--a stimulation rate of more than 50 stimuli per second. Rather than refer to stimulation rate, we usually use frequency, which is a number per unit time. In this instance, a stimulus frequency of greater than 50 per second produces wave summation, whereas a stimulus frequency below 50 per second will produce individual twitches.
If the stimulation continues and the muscle is never allowed to relax completely, tension will rise to a peak (Figure 10-13b). A muscle producing peak tension during rapid cycles of contraction and relaxation is in incomplete tetanus.
Complete Tetanus Complete tetanus is obtained by increasing the stimulation rate until the relaxation phase is eliminated (Figure 10-13c). During complete tetanus, action potentials arrive so rapidly that the sarcoplasmic reticulum does not have time to reclaim the calcium ions. The high Ca2+ concentration in the cytoplasm prolongs the contraction state, making it continuous. Virtually all normal muscular contractions involve complete tetanus of the participating muscle fibers.
Treppe If a skeletal muscle is stimulated a second time immediately after the relaxation phase has ended, the contraction that occurs will develop a slightly higher maximum tension than did the contraction after the first stimulation. The increase in peak tension indicated in Figure 10-13d will continue over the first 30–50 stimulations. Thereafter, the amount of tension produced will remain constant at roughly 25 percent of the maximal tension that would be produced in complete tetanus. Because the tension rises in stages, like the steps in a staircase, this phenomenon is called treppe, a German word meaning "stairs." The rise is thought to result from a gradual increase in the concentration of calcium ions in the sarcoplasm, in part because the ion pumps in the sarcoplasmic reticulum are unable to recapture them in the time between stimulations.
Internal Tension and External Tension
We have already discussed the relationship between resting sarcomere length and tension production in individual muscle fibers. When we consider an entire skeletal muscle, the situation is complicated by the fact that the muscle fibers are not directly connected to the structures they pull against. The extracellular fibers of the endomysium, perimysium, epimysium, and tendons are flexible and somewhat elastic. When a skeletal muscle contracts, the myofibrils in the muscle fibers generate internal tension. This internal tension is applied to the extracellular fibers. The tension in the extracellular fibers is called external tension.
As the external tension rises, the extracellular fibers stretch; for this reason, they are called series elastic elements. The series elastic elements behave like fat rubber bands. They stretch easily at first, but as they elongate they become stiffer and more effective at transferring tension to the resistance. As a result, external tension does not climb as quickly as internal tension when a contraction occurs.
To understand this relationship, attach a rubber band to one of the rings in a three-ring notebook. Put a finger through the rubber band and use it to pull the notebook across a table. Your finger represents the muscle fibers; the rubber band, the attached tendon; and the notebook, a bone of the skeleton. When you first apply tension, the rubber band stretches and becomes stiffer. Over this period, external tension rises. The notebook starts to move when the rubber band becomes sufficiently taut--that is, when the tension in the rubber band (the external tension) overcomes friction and the weight of the notebook (the resistance).
If you now relax your hand, the rubber band will pull your finger toward the notebook. The same thing happens in a muscle: When the contraction ends, the series elastic elements recoil and pull on the muscle. This recoil helps return the muscle to its original resting length.
A myogram performed in the laboratory generally measures the tension in a tendon and so is reporting external tension rather than internal tension. Figure 10-14a compares the internal and external tensions during a twitch contraction. A single twitch is so short in duration that there isn't enough time for the external tension to rise as high as the internal tension. Twitches are therefore ineffective in terms of performing useful work. Notice that the external tension remains elevated until the relaxation phase has ended. If a second twitch occurs before the external tension returns to zero, the external tension will peak at a higher level, because more of the internal tension will be conveyed to the series elastic elements. Think of pushing a child on a swing: You push once to start the swing moving; if you push a second time, the child swings higher because the energy of the second push is added to the energy remaining from the first. This mechanism of elevated external tension is now thought to be the primary basis of wave summation. During a tetanic contraction, there is sufficient time for internal and external tensions to equalize (Figure 10-14b).
You have a remarkable ability to control the amount of tension exerted by your skeletal muscles. During a normal contraction, tension rises smoothly, not jerkily, because activated muscle fibers are stimulated to complete tetanus. The total force exerted by the skeletal muscle depends on how many muscle fibers are activated.
A typical skeletal muscle contains thousands of muscle fibers. Although some motor neurons control a few muscle fibers, most control hundreds of them. All the muscle fibers controlled by a single motor neuron constitute a motor unit. The size of a motor unit is an indication of how fine the control of movement can be. In the muscles of the eye, where precise control is extremely important, a motor neuron may control 4–6 muscle fibers. We have much less precise control over our leg muscles, where a single motor neuron may control 1000–2000 muscle fibers (depending on the specific muscle and the reference consulted). The muscle fibers of each motor unit are intermingled with those of other motor units (Figure 10-15a). Because of this intermingling, the direction of pull exerted on the tendon does not change despite variations in the numbers of activated motor units.
When you decide to perform a specific arm movement, specific groups of motor neurons in the spinal cord are stimulated. The contraction begins with the activation of the smallest motor units in the stimulated muscle. These motor units generally contain muscle fibers that contract relatively slowly. Over time, larger motor units containing faster and more powerful muscle fibers are activated, and tension production rises steeply. The smooth but steady increase in muscular tension produced by increasing the number of active motor units is called recruitment, or multiple motor unit summation.
Peak tension production occurs when all motor units in the muscle contract in a state of complete tetanus. Such powerful contractions do not last long, however, because the individual muscle fibers soon use up their available energy reserves. During a sustained tetanic contraction, motor units are activated on a rotating basis, so some of them are resting and recovering while others are actively contracting. This "relay team" approach, called asynchronous motor unit summation, lets each motor unit recover somewhat before it is stimulated again (Figure 10-15b). As a result, when your muscles contract for sustained periods, they produce slightly less than maximal tension.
Muscle Tone In any skeletal muscle, some motor units are always active, even when the entire muscle is not contracting. Their contractions do not produce enough tension to cause movement, but they do tense and firm the muscle. This resting tension in a skeletal muscle is called muscle tone. A muscle with little muscle tone appears limp and flaccid, whereas one with moderate muscle tone is firm and solid. The identity of the stimulated motor units changes constantly, so a constant tension in the attached tendon is maintained, but individual muscle fibers can relax.
Resting muscle tone stabilizes the position of bones and joints. For example, in muscles involved with balance and posture, enough motor units are stimulated to produce the tension needed to maintain body position. Muscle tone also helps prevent sudden, uncontrolled changes in the position of bones and joints. In addition to bracing the skeleton, the elastic nature of muscles and tendons lets skeletal muscles act as shock absorbers that cushion the impact of a sudden bump or shock. Heightened muscle tone accelerates the recruitment process during a voluntary contraction, because some of the motor units are already stimulated. Strong muscle tone also makes skeletal muscles appear firm and well defined, even at rest.
Isotonic and Isometric Contractions
We can classify muscle contractions as isotonic or isometric on the basis of the pattern of tension production.
Isotonic Contractions In an isotonic contraction, tension rises and the skeletal muscle's length changes. Lifting an object off a desk, walking, and running involve isotonic contractions.
There are two types of isotonic contractions: (1) concentric and (2) eccentric. In a concentric contraction, the muscle tension exceeds the resistance and the muscle shortens. Consider the experiment summarized in Figure 10-16. A skeletal muscle 1 cm2 in cross-sectional area can produce roughly 4 kg of tension in complete tetanus. If we hang a 2-kg weight from that muscle and stimulate it, the muscle will shorten (Figure 10-16a). Before the muscle can shorten, the cross-bridges must produce enough tension to overcome the resistance--in this case, the 2-kg weight. Over this period, internal tension in the muscle fibers rises until the external tension in the tendon exceeds the amount of resistance. As the muscle shortens, the internal and external tensions in the skeletal muscle remain constant at a value that just exceeds the resistance (Figure 10-16b). The term isotonic originated from this type of experiment.
In the body, however, the situation is more complicated. For example, muscles are not always positioned directly above the resistance, and they are attached to bones rather than to static weights. Changes in the relative positions of the muscle and the articulating bones, the effects of gravity, and other mechanical and physical factors interact to increase or decrease the amount of resistance the muscle must overcome as a movement proceeds. Nevertheless, at any time during a concentric contraction, the tension produced exceeds that resistance.
The speed of shortening varies with the difference between the amount of tension produced and the amount of resistance. If all the muscle units are stimulated and the resistance is relatively small, the muscle will shorten very quickly. In contrast, if the muscle barely produces enough tension to overcome the resistance, it will shorten very slowly.
In an eccentric contraction, the peak tension developed is less than the resistance, and the muscle elongates owing to the contraction of another muscle or the pull of gravity. Think of a tug-of-war team trying to stop a moving car. Although everyone pulls as hard as they can, the rope slips through their fingers. The speed of elongation depends on the difference between the amount of tension developed by the active muscle fibers and the amount of resistance. In our analogy, the team might slow down a small car but would have little effect on a large truck.
Eccentric contractions are very common, and they are an important part of a variety of movements. In these movements, you exert precise control over the amount of tension produced. By varying the tension in an eccentric contraction, you can control the rate of elongation, just as you can vary the tension in a concentric contraction. For example, precisely controlled eccentric contractions occur each time you walk down stairs or settle into a chair. During physical training, people commonly perform cycles of concentric and eccentric contractions, as when you hold a weight in your hand and flex and extend your elbow.
Isometric Contractions In an isometric contraction, the muscle as a whole does not change length, and the tension produced never exceeds the resistance. Figure 10-16c shows what happens if we attach a weight heavier than 4 kg to the experimental muscle and then stimulate the muscle. Although cross-bridges form and tension rises to peak values, the muscle cannot overcome the resistance of the weight and so cannot shorten (Figure 10-16d). Examples of isometric contractions include holding a heavy weight above the ground, pushing against a locked door, or trying to pick up a car. These are rather unusual movements. However, many of the reflexive muscle contractions that keep your body upright when you stand or sit involve the isometric contractions of muscles that oppose the force of gravity.
You may have noticed that when you perform an isometric contraction, the contracting muscle bulges (but not as much as it does during an isotonic contraction). In an isometric contraction, although the muscle as a whole does not shorten, the individual muscle fibers shorten until the tendons are taut and the external tension equals the internal tension generated by the muscle fibers. The muscle fibers cannot shorten further, because the external tension does not exceed the resistance.
Normal daily activities therefore involve a combination of isotonic and isometric muscular contractions. As you sit and read this text, isometric contractions of postural muscles stabilize your vertebrae and maintain your upright position. When you turn a page, the movements of your arm, forearm, hand, and fingers are produced by a combination of concentric and eccentric isotonic contractions.
You can lift a light object more rapidly than you can lift a heavy one because the resistance and the speed of contraction are inversely related. If the resistance is less than the tension produced, an isotonic, concentric contraction will occur; the muscle will shorten. The heavier the resistance, the longer it takes for the movement to begin, because muscle tension, which increases gradually, must exceed the resistance before shortening can occur (Figure 10-17). The contraction itself proceeds more slowly. At the cellular level, the speed of cross-bridge pivoting is reduced as the load increases.
For each muscle, an optimal combination of tension and speed exists for any given resistance. If you have ever ridden a 10-speed bicycle, you are probably already aware of this fact. When you are cruising along comfortably, your thigh and leg muscles are working at an optimal combination of speed and tension. When you come to a hill, the resistance increases. Your muscles must now develop more tension, and they move more slowly; they are no longer working at optimal efficiency. You then shift to a lower gear. The load on your muscles decreases, the speed increases, and the muscles are once again working efficiently.
Muscle Relaxation and the Return to Resting Length
As we noted earlier, there is no active mechanism for muscle fiber elongation. The sarcomeres in a muscle fiber can shorten and develop tension, but the power stroke cannot be reversed to push the Z lines farther apart. After a contraction, a muscle fiber returns to its original length through a combination of elastic forces, opposing muscle contractions, and gravity.
Elastic Forces When the contraction ends, some of the energy initially "spent" in stretching the series elastic elements is recovered as they recoil. The recoil of the series elastic elements gradually helps return the muscle fiber to its original resting length.
Opposing Muscle Contractions The contraction of opposing muscles can return a muscle to its resting length more quickly than elastic forces can. Consider the muscles of the arm that flex or extend the elbow. Contraction of the biceps brachii muscle on the anterior part of the arm flexes the elbow; contraction of the triceps brachii muscle on the posterior part of the arm extends the elbow. When the biceps brachii contracts, the triceps brachii is stretched. When the biceps brachii relaxes, contraction of the triceps brachii extends the elbow and stretches the muscle fibers of the biceps brachii to their original length.
Gravity may assist opposing muscle groups in quickly returning a muscle to
its resting length after a contraction. For example, imagine the biceps
brachii muscle fully contracted with the elbow pointed at the ground. When
the muscle relaxes, gravity will pull the forearm down and stretch the
muscle. Although gravity can provide assistance in stretching muscles,
some active muscle tension is needed to control the rate of movement and
to prevent damage to the joint. In the previous example, eccentric
contraction of the biceps brachii muscle can control the movement.
Children are often told to be careful of rusty nails. Parents should worry most not about the rust or the nail but about infection with the very common bacterium Clostridium tetani, which can cause tetanus. Although they have the same name, the disease tetanus has no relation to the normal muscle response to neural stimulation. The Clostridium bacterium occurs virtually everywhere in the environment, but it can thrive only in tissues that contain low amounts of oxygen. For this reason, a deep puncture wound, such as that from a nail, carries a much greater risk of producing tetanus than does a shallow, open cut that bleeds freely.
When active in body tissues, these bacteria release a powerful toxin that affects the central nervous system. Motor neurons, which control skeletal muscles throughout the body, are particularly sensitive to it. The toxin suppresses the mechanism that regulates motor neuron activity. The result is a sustained, powerful contraction of skeletal muscles throughout the body.
The incubation period (the time from exposure to the development of symptoms) is generally less than 2 weeks. The most common complaints are headache, muscle stiffness, and difficulty in swallowing. Because it soon becomes difficult to open the mouth, this disease is also called lockjaw. Widespread muscle spasms typically develop within 2 or 3 days of the initial symptoms and continue for a week before subsiding. After 2–4 weeks, patients who survive recover with no after-effects.
Severe tetanus has a 40–60 percent mortality rate; that is, for every 100 people who develop severe tetanus, 40 to 60 die. Fortunately, immunization is effective in preventing the disease. Approximately 500,000 cases of tetanus occur worldwide each year, but only about 100 of them occur in the United States, thanks to an effective immunization program. (“Tetanus shots” are recommended, with booster shots every 10 years.) Severe symptoms in unimmunized patients can be prevented by early administration of an antitoxin, in most cases human tetanus immune globulin. Such treatment does not reduce symptoms that have already appeared, however.
|FIGURE 10-12 The Twitch and Development of Tension. (a) A myogram showing differences in tension over time for a twitch contraction in different skeletal muscles. (b) The details of tension over time for a single twitch contraction in the gastrocnemius muscle. Notice the presence of a latent period, which corresponds to the time needed for the conduction of an action potential and the subsequent release of calcium ions by the sarcoplasmic reticulum.|
|FIGURE 10-13 Effects of Repeated Stimulations. (a) Wave summation occurs when successive stimuli arrive before the relaxation phase (the downturn of the curve) has been completed. (b) Incomplete tetanus occurs if the rate of stimulation increases further. Tension production will rise to a peak, and the periods of relaxation will be very brief. (c) During complete tetanus, the frequency of stimulation is so high that the relaxation phase is eliminated; tension plateaus at maximal levels. (d) Treppe is an increase in peak tension with each successive stimulus delivered shortly after the completion of the relaxation phase of the preceding twitch.|
|FIGURE 10-14 Internal and External Tensions. Internal tension rises as the muscle fiber contracts. External tension rises more slowly as the series elastic elements are stretched. (a) During a single-twitch contraction, external tension cannot rise as high as internal tension before the relaxation phase begins. (b) During a tetanic contraction, external tension soon plateaus at a level roughly equivalent to internal tension. External tension remains elevated for the duration of the contraction.|
|FIGURE 10-15 The Arrangement of Motor Units in a Skeletal Muscle. (a) Muscle fibers of different motor units are intermingled, so the forces applied to the tendon remain roughly balanced regardless of which muscle groups are stimulated. (b) The tension applied to the tendon remains relatively constant even though individual motor units cycle between contraction and relaxation.|
|FIGURE 10-16 Isotonic and Isometric Contractions. (a,b) This muscle is attached to a weight less than its peak tension capabilities. On stimulation, it develops enough tension to lift the weight. Tension remains constant for the duration of the contraction, although the length of the muscle changes. This is an example of isotonic contraction. (c,d) The same muscle is attached to a weight that exceeds its peak tension capabilities. On stimulation, tension will rise to a peak, but the muscle as a whole cannot shorten. This is an isometric contraction.|
|FIGURE 10-17 The Resistance and Speed of Contraction. The heavier the resistance on a muscle, the longer it will take for the muscle to begin to shorten and the less the muscle will shorten.|