Muscle Responses

Muscles can be studied in the laboratory in an effort to understand how they respond when in the body.

In the Laboratory
When a muscle fiber is isolated, placed on a microscope slide, and provided with ATP plus the various electrolytes it requires, it contracts completely along its entire length. This observation has resulted in the all-or-none law: A muscle fiber contracts completely or not at all. In contrast, a whole muscle shows degrees of contraction. To study whole muscle contraction in the laboratory, an isolated muscle is stimulated electrically, and the mechanical force of contraction is recorded as a visual pattern called a myogram. When the strength of the stimulus is above a threshold level, the muscle contracts and then relaxes. This action-a single contraction that lasts only a fraction of a second-is called a muscle twitch. Figure 7.7 is a myogram of a muscle twitch, which is customarily divided into three stages: the latent period, or the period of time between stimulation and initiation of contraction; the contraction period, when the muscle shortens; and the relaxation period, when the muscle returns to its former length. It’s interesting to use our knowledge of muscle fiber contraction to understand these events. From our study thus far, we know that a muscle fiber in an intact muscle contracts when calcium leaves storage sacs and relaxes when calcium returns to storage sacs. But unlike the contraction of a muscle fiber, a muscle has degrees of contraction, and a twitch can vary in height (strength) depending on the degree of stimulation. Why should that be? Obviously, a stronger stimulation causes more individual fibers to contract than before.

Figure 7.7 A myogram showing a single muscle twitch.

If a whole muscle is given a rapid series of stimuli, it can respond to the next stimulus without relaxing completely. Summation is increased muscle contraction until maximal sustained contraction, called a tetanic contraction, is achieved (Fig. 7.8). The myogram no longer shows individual twitches; rather, the twitches are fused and blended completely into a straight line. Tetanus continues until the muscle fatigues due to depletion of energy reserves. Fatigue is apparent when a muscle relaxes even though stimulation continues.

In the Body
In the body, muscles are innervated to contract by nerves. As mentioned, each axon within a nerve stimulates a number of muscle fibers. A nerve fiber together with all of the muscle fibers it innervates is called a motor unit. A motor unit obeys the all-or-none law. Why? Because all the muscle fibers in a motor unit are stimulated at once, and they all either contract or do not contract. A variable of interest is the number of muscle fibers within a motor unit. For example, in the ocular muscles that move the eyes, the innervation ratio is one motor axon per 23 muscle fibers, while in the gastrocnemius muscle of the lower leg, the ratio is about one motor axon per 1,000 muscle fibers. No doubt, moving the eyes requires finer control than moving the legs.

Figure 7.8 Myograms showing (a) a series of twitches, (b) summation, and (c) a tetanic contraction. Note that an increased frequency of stimulations has resulted in these different responses.

Tetanic contractions ordinarily occur in the body because, as the intensity of nervous stimulation increases, more and more motor units are activated. This phenomenon, known as recruitment, results in stronger and stronger muscle contractions. But while some muscle fibers are contracting, others are relaxing. Because of this, intact muscles rarely fatigue completely. Even when muscles appear to be at rest, they exhibit tone, in which some of their fibers are always contracting. Muscle tone is particularly important in maintaining posture. If all the fibers within the muscles of the neck, trunk, and legs were to suddenly relax, the body would collapse.

Athletics and Muscle Contraction
Athletes who excel in a particular sport, and much of the general public as well, are interested in staying fit by exercising.
Exercise and Size of Muscles Muscles that are not used or that are used for only very weak contractions decrease in size, or atrophy.

     Atrophy can occur when a limb is placed in a cast or when the nerve serving a muscle is damaged. If nerve stimulation is not restored, muscle fibers are gradually replaced by at and fibrous tissue. Unfortunately, atrophy can cause muscle ibers to shorten progressively, leaving body parts contracted n contorted positions. orceful muscular activity over a prolonged period causes uscle to increase in size as the number of myofibrils within he muscle fibers increases. Increase in muscle size, called hypertrophy, occurs only if the muscle contracts to at least 75% f its maximum tension. Some athletes take anabolic steroids, either testosterone or related chemicals, to promote muscle growth.

     Slow-Twitch and Fast-Twitch Muscle Fibers. We have seen that all muscle fibers metabolize both aerobically and anaerobically. Some muscle fibers, however, utilize one method more than the other to provide myofibrils with ATP. Slowtwitch fibers tend to be aerobic, and fast-twitch fibers tend to be anaerobic (Fig. 7.9).
     Slow-twitch fibers have a steadier tug and more endurance, despite having motor units with a smaller number of fibers. These muscle fibers are most helpful in sports such as longdistance running, biking, jogging, and swimming. Because they produce most of their energy aerobically, they tire only when their fuel supply is gone.

Slow-twitch fibers have many mitochondria and are dark in color because they contain myoglobin, the respiratory pigment found in muscles. They are also surrounded by dense capillary beds and draw more blood and oxygen than fast-twitch fibers. Slow-twitch fibers have a low maximum tension, which develops slowly, but these muscle fibers are highly resistant to fatigue. Because slow-twitch fibers have a substantial reserve of glycogen and fat, their abundant mitochondria can maintain a steady, prolonged production of ATP when oxygen is available.
Fast-twitch fibers tend to be anaerobic and seem to be designed for strength because their motor units contain many fibers. They provide explosions of energy and are most helpful in sports activities such as sprinting, weight lifting, swinging a golf club, or throwing a shot. Fast-twitch fibers are light in color because they have fewer mitochondria, little or no myoglobin, and fewer blood vessels than slow-twitch fibers do. Fast-twitch fibers can develop maximum tension more rapidly than slow-twitch fibers can, and their maximum tension is greater. However, their dependence on anaerobic energy leaves them vulnerable to an accumulation of lactic acid that causes them to fatigue quickly.

Figure 7.9 Slow- and fast-twitch fibers. If your muscles contain many slow-twitch fibers (dark color), you would probably do better at a sport like cross-country running. But if your muscles contain many fast-twitch fibers (light color), you would probably do better at a sport like weight lifting.