A COMPARISON OF MOTOR INTEGRATION IN THE MOUSE, RAT, RABBIT, DOG AND HORSE

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Abstract
Contractions of the diaphragm of the mouse, rat, rabbit, dog and horse show characteristic electrical patterns of multifiber twitching. This similarity of twitching indicates a common evolutionary adaptation for meeting analogous mechanical requirements (an increasing resistance to an increasing pulmonary inflation as inspiration progresses). The adaptation consists of a progressive recruitment of activity of dormant reserve muscle units and an increasing frequency of twitch of the individual activated units. This adjustment is a primitive mechanism for it is found in muscular contractions of insects, reptiles and birds as well as mammals. All evidence indicates that recruitment of reserve muscle units is a far more important mechanism of gradation of strength of contraction than is adjustment of twitch frequency. Respiratory and motor nerve cell rhythm are higher in general in the smaller and nimbler animals. As a consequence of this parallelism, the number of discharges of-those motor neurons which lead inspiratory contractions tend to be approx. equal. This equality offers comparable facilities for the control of the frequency and recruitment factors and helps to explain similarity of inspiratory contractions. Thus the events transpiring in the prolonged inspiration of a horse are accelerated and compressed, without basic change, into the shortened inspiration of a mouse.

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