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Twitch and tetanus responses

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Twitch and tetanus responses

Skeletal muscles respond to a single electric shock of sufficient magnitude by rapid, intense contractions called phasic contractions. If the ends of a frog sartorius muscle (at 0° C) are fixedto prevent shortening, the tension increases for about 200 milliseconds and then begins to decrease, at first rather rapidly and then more slowly. More happens during this mechanical response to a single stimulation, called a twitch, than the tension record suggests.

The mechanical response to repeated stimulation depends on the rate of the stimulation. Muscle, like other excitable tissues, has a period following its action potential during which themembrane will not respond to stimulation regardless of the strength. This absolute refractory period in the frog sartorius at 0° C lasts about 10 milliseconds after stimulation. Therefore, a second pulse coming within that time span will not elicit any response. If, however, the pulses are 300 milliseconds apart, the muscle will be relaxing when the second pulse is given, and the tension will appear in waves in phase with the stimulation, causing an unfused tetanus. It is possible to stimulate the muscle at a frequency between these extremes so that the tension developed by the muscle remains constant. This latter type of contraction is called a fused tetanus, and the rate of stimulation that produces it is called the fusion frequency. The exact rate depends upon the particular muscle and the temperature.

Usually, the maximum tetanus tension is from 1.2 to 1.8 times greater than the maximum tension during a twitch. Within the muscle many elastic structures, connected in series with thecontractile elements, are stretched during contraction. The attachment of the muscle fibres to the tendons at the end of the muscle and the attachment of the thin filaments to the Z line contribute to this elastic component. In single fibres, however, most of the elasticity of the series elastic elements is contributed by the actin-myosin cross bridges themselves. Full maximum tension is not apparent at the end of the muscle until the contractile elements have shortened enough to stretch the elastic elements—somewhat like taking up the slack in rope before a pull on one end can be felt at the other end. In a twitch, the activity of the muscle is so brief that the contractile elements cannot extend the elastic elements completely before relaxation begins; as a result, the tension at the ends of the muscle does not reach the maximum possible level. During a tetanus, on the other hand, the activity of the contractile elements is maintained, and they can eventually shorten enough to extend fully the series elastic elements. When this has been accomplished, the maximum tension is apparent at the ends of the muscle.


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