Muscle Response to Nerve Stimulation

Measurement of the effects of stimulation frequency is possible under similar conditions of isometric contraction. Given that there is all-or-nothing activation and contraction of fibers within a motor unit after stimulation of a single motor neuron, single stimuli produce identical twitch responses with identical tension created. If a second stimulus is given after full relaxation of the muscle, this second twitch intensity will be the same as the first. If, however, the second stimulus is given before full relaxation (i.e., while the muscle is still at least partially contracted) the tension created by the second stimulus will surpass that created by the first. As the frequency of stimulation is increased, this phenomenon, called summation, is enhanced. At high enough stimulation frequencies, muscle fibers are unable to relax between stimuli, and they produce a prolonged single-peaked contraction, known as tetanus. This is one mechanism by which muscles generate varying degrees of force.

Isometric contractions also allow for the measurement of the effect of sarcomere length on the development of force generation, that is, the muscle length-tension relationship. Maximum tensile force results when the greatest number of cross-bridges forms. This occurs when the muscle is at its resting length and all or most of the cross-bridges are available (i.e., there is maximum overlap of myosin heads with actin), and stimulation will produce maximum tension. If the sarcomere is shortened, actin filaments begin to overlap or repulse each other and interfere with cross-bridge formation. As a result, there fewer cross-bridges form, resulting in less tension. Conversely, when the sarcomere is stretched, actin filaments are pulled laterally and myosin heads at the middle of the sarcomere no longer overlap with actin. Therefore fewer cross-bridges can form, also resulting in less tension. At the extreme, there is no actin-myosin overlap, and no tension develops after stimulation.

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