As long as the voltage of an artificial stimulus delivered directly to a muscle is at threshold or higher, a muscle gives a complete twitch. Increasing the voltage still more does not cause the twitches to become any stronger. There are other factors, however, that can produce stronger twitches. Indeed, an individual twitch is not strong enough to do any useful work. Muscles must be able to contract with variable strength—differently in lifting a glass of champagne than in lifting a heavy barbell, for example.
If we stimulate the nerve rather than the muscle, higher voltages produce stronger muscle contractions because they excite more nerve fibers and therefore more motor units. The more motor units that contract, the more strongly the muscle as a whole contracts (fig. 11.14). The process of bringing more motor units into play is called recruitment, or multiple motor unit (MMU) summation. It is seen not just in artificial stimulation but is part of the way the nervous system behaves normally to produce variable muscle contractions.
Another way to produce a stronger muscle contraction is to stimulate the muscle at a higher frequency. Even when voltage remains the same, high-frequency stimulation causes stronger contractions than low-frequency stimulation. In figure 11.15a, we see that when a muscle is stimulated at a low frequency (up to 10 stimuli/sec in this example), it produces an identical twitch for each stimulus and fully recovers between twitches.
Between 10 and 20 stimuli per second, the muscle still recovers fully between twitches, but each twitch develops more tension than the one before. This pattern of increasing tension with repetitive stimulation is called treppe8 (TREP-eh), or the staircase phenomenon, after the appearance of the myogram (fig. 11.15b). One cause of treppe is that when stimuli arrive so rapidly, the sarcoplasmic reticulum does not have time between stimuli to completely reabsorb all the calcium that it released. Thus, the calcium concentration in the cytosol rises higher and higher with each stimulus and causes subsequent twitches to be stronger. Another factor is that the heat released by each twitch causes muscle enzymes such as myosin ATPase to work more efficiently and produce stronger twitches as the muscle warms up. One purpose of warm-up exercises before athletic competition is to induce treppe, so that the muscle contracts more effectively when the competition begins.
At a still higher stimulus frequency (20-40 stimuli/ sec in fig. 11.15c), each new stimulus arrives before the previous twitch is over. Each new twitch "rides piggyback" on the previous one and generates higher tension.
8 treppe = staircase
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Figure 11.15 The Relationship Between Stimulus Frequency and Muscle Tension. (a) Twitch: At low frequency, the muscle relaxes completely between stimuli and shows twitches of uniform strength. (b) Treppe: At a moderate frequency of stimulation, the muscle relaxes fully between contractions, but successive twitches are stronger. (c) Wave summation and incomplete tetanus: At still higher stimulus frequency, the muscle does not have time to relax completely between twitches, and the force of each twitch builds on the previous one. (d) Complete tetanus: At high stimulus frequency, the muscle does not have time to relax at all between stimuli and exhibits a state of continual contraction with about four times as much tension as a single twitch. Tension declines as the muscle fatigues.
This phenomenon goes by two names: temporal9 summation, because it results from two stimuli arriving close together, or wave summation, because it results from one wave of contraction added to another. Wave is added upon wave, so each twitch reaches a higher level of tension than the one before, and the muscle relaxes only partially between stimuli. This effect produces a state of sustained fluttering contraction called incomplete tetanus.
At a still higher frequency, such as 40 to 50 stimuli per second, the muscle has no time to relax at all between stimuli, and the twitches fuse into a smooth, prolonged contraction called complete tetanus. A muscle in complete tetanus produces about four times as much tension as a single twitch (fig. 11.15d). This type of tetanus should not be confused with the disease of the same name caused by the tetanus toxin, explained in insight 11.1.
9tempor = time
Complete tetanus is a phenomenon seen in artificial stimulation of a muscle, however, and rarely if ever occurs in the body. Even during the most intense muscle contractions, the frequency of stimulation by a motor neuron rarely exceeds 25/sec, which is far from sufficient to produce complete tetanus. The reason for the smoothness of muscle contractions is that motor units function asynchronously; when one motor unit relaxes, another contracts and "takes over" so that the muscle does not lose tension.
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