. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 596 Comparative Animal Physiology smaller (5 /x) nerve fibers elicits local graded potentials, called small-fiber junctional potentials, which summate on repetitive stimulation and may be accompanied by local low-tension contractions, 10-15 per cent of twitch tension in magnitude (Fig. 229). The slow contractions may relax very slowly, but interposition of a twitch response, when repetitive stimulation of the slow nerve fiber has stopped, results in rapid relaxation. The slow contraction develops tension faster
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 596 Comparative Animal Physiology smaller (5 /x) nerve fibers elicits local graded potentials, called small-fiber junctional potentials, which summate on repetitive stimulation and may be accompanied by local low-tension contractions, 10-15 per cent of twitch tension in magnitude (Fig. 229). The slow contractions may relax very slowly, but interposition of a twitch response, when repetitive stimulation of the slow nerve fiber has stopped, results in rapid relaxation. The slow contraction develops tension faster with high frequency than with low fre- quency stimulation. Whether one muscle fiber receives both types of in- nervation is not yet certain, but the net effect of the two types of fiber is peripheral gradation of response, the slow response being "tonic" in character. In mammalian striated muscle, small motor nerve fibers activate intrafusal muscle fibers and so regulate the afferent discharge originating in sensory spindles (Ch. 14). The most studied examples of multiple motor innervation are in arthro- pods. Early histological studies^"'"' ^-^ showed that whole muscles in many crustaceans and insects are innervated by only two (or a few) axons and that each muscle fiber has multiple innervation; , each muscle fiber re-. Fig. 229. Response of frog muscle to stimulation of large and small fibers. Action potential from gastrocnemius, progressive pressure block to sciatic: 1, partial block, threshold nerve stimulation, only spike response; 2, increased strength, spike followed by small junction potential; 3, pressure increased, stimulus as in 2, only small junction potential remains. From Kuffler and Gerard.^^" ceives branches from two or more nerve fibers, often from each of the motor fibers to the muscle. Triple innervation of claw muscles is common among crustaceans,''' and even quintuple innervation has been seen in a muscle of the leg of Panulini
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