Twitch Summation and Tetanus

When the muscle is stimulated with a single electric shock of sufficient voltage, it quickly contracts and relaxes. This response is called a twitch. Increasing the stimulus voltage increases the strength of the twitch, up to a maximum. The strength of a muscle contraction can thus be graded, or varied—an obvious requirement for the proper control of skeletal movements. If a second electric shock is delivered immediately after the first, it will produce a second twitch that may partially "ride piggyback" on the first. This response is called summation.

Stimulation of fibers within a muscle in vitro with an electric stimulator, or in vivo by motor axons, usually results in the full contraction of the individual fibers. Stronger muscle contractions are produced by the stimulation of greater numbers of muscle fibers. Skeletal muscles can thus produce graded contractions, the strength of which depends on the number of fibers stimulated rather than on the strength of the contractions of individual muscle fibers.

If the stimulator is set to deliver an increasing frequency of electric shocks automatically, the relaxation time between successive twitches will get shorter and shorter as the strength of contraction increases in amplitude. This effect is known as incomplete tetanus (fig. 12.18). Finally, at a particular "fusion frequency" of stimulation, there is no visible relaxation between successive twitches. Contraction is smooth and sustained, as it is during normal muscle contraction in vivo. This smooth, sustained contraction is called complete tetanus. (The term tetanus should not be confused with the disease of the same name, which is accompanied by a painful state of muscle contracture, or tetany.) The tetanus produced in vitro by the asynchronous twitches of muscle fibers simulates the normal, smooth contraction produced in vivo by the asynchronous activation of motors units.

Chapter Twelve

Twitch Summation

Twitch Summation

■ Figure 12.17 Recording muscle contractions. Recorder tracings demonstrating twitch and summation of an isolated frog gastrocnemius muscle.

Incomplete tetanus

Complete tetanus

Incomplete tetanus

Complete tetanus

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Tetanus

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10 Shocks per second

60 Shocks per second

5 Shocks per second

10 Shocks per second

60 Shocks per second

■ Figure 12.18 Incomplete and complete tetanus. When an isolated muscle is shocked repeatedly, the separate twitiches summate to produce a sustained contracton. At a relatively slow rate of simulation (5 or 10 per second), the separate muscle twitches can still be observed. This is incomplete tetanus. When the frequency of stimulation increases to 60 shocks per second, however, complete tetanus—a smooth, sustained contraction—is observed. If the stimulation is continued, the muscle will demonstrate fatigue.

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