Responses in neck and facial muscles to sudden free fall and a startling auditory stimulus

Abstract

EMG responses elicited by sudden onset of free fall and a startling auditory stimulus were investigated in healthy subjects while lying on a couch with their eyes closed. Muscle responses were recorded from masseter (V cranial nerve), orbicularis oculi and mentalis (VII nerve) and sternomastoid and trapezoid (XI nerve). A similar sequence of muscle activation and absolute latencies occurred in response to both stimulus modalities, consisting of a blink (30 msec) followed simultaneously by mentalis, sternomastoid and trapezoid (55 msec). Masseter could either be simultaneously activated with the latter muscles or follow after a delay of 10-20 msec. A patient with bilateral cochleo-vestibular nerve section had responses at comparable latencies in the free fall experiment. The similarities between the reaction to free fall and a startling auditory stimulus indicate that the early response to free fall constitutes a startle and that various stimuli converge onto a common response generator. The latency pattern of neck and facial muscles does not follow a sequence of innervation with increasing segmental distance from a single centre. Therefore, our data do not support the concept that the startle response is produced by a caudally and rostrally spreading volley from a putative pontomedullary centre. It is suggested that the startle response is a polysynaptically generated patterned muscle activation.