Responses of cats to sudden falls: an otolith-originating reflex assisting landing.

A short-latency electromyographic response has been demonstrated in the human gastrocnemius during unexpected falls (11). The hypothesis that it is a form of otolith-spinal reflex has been tested in the cat. Electromyographic activity was measured in four muscles while suddenly and unexpectedly dropping a series of six cats from 50 cm above the ground. In normal animals, electrical activation of all muscles tested occurred in two distinct phases. An early burst commenced as early as 15 ms from the instant of release, was usually spindle shaped, and often but not always disappeared by 100 ms. This burst was totally and permanently abolished by bilateral labyrinthectomy, but not by selective inactivation of the relevant semicircular canals. A later phase of muscle activity commenced 75-140 ms after release and gradually built up and then subsided, rougly centered about the movement of landing on the ground. This activity was not greatly altered by labyrinthectomy. The present results appear to confirm that the early electromyographic response to sudden falls is due to an otolith-spinal reflex. A later component is nonlabyrinthine in origin, however. Smooth landings from short falls are not possible in the absence of either phase of the response.