Electromyographic Study of Click Evoked Middle Ear Muscle Activity in Cats

Abstract

ELECTROMYOGRAPHIC STUDY OF CLICK EVOKED MIDDLE EAR MUSCLE ACTIVITY IN CATS * G. Salomon and A. Starr In waking cats middle ear muscles responded to clicks with activity, which on emg. consisted of an initial burst of activity (latency 4.5 - 10 msec.), a pause, and a later sustained period of activity (latency 34 - 56 msec., duration 1 sec.). The latency of the first response decreased with increasing sound in- tensity, whereas the latency of the second response varied independently of intensity. These two responses behaved differently during repetitive stimulat- ion with clicks; the first response remained constant while the second de- creased in amplitude and disappeared. The second response would reappear promptly, however, by adding extra sounds, by decreasing or increasing the click intensity, by cutaneous stimulation, or by the animal's own movements. Sound transmission through the middle ear did not change during the first response (measuring cochlea output elicited by tone of constant intensity) but an attenuation of sound transmission occurred during the second res- ponse. The lack of attenuation during the first response was due to the brief duration of this response. Sudden strong tones evoked activity in middle ear muscles after a latency of 5 - 10 msec. This activity consisted of a 20 - 30 msec. discharge, a silent period, and a sustained activity. The silent period was most pronounced in muscle tensor tympani. After termination of a tone activity persisted in both muscles and returned only gradually to spontaneous activity levels. lntercollicular decerebration modified middle ear muscle responses to click and continuous sounds: the second long latency response to clicks was abo- lished and the after discharge to tone bursts no longer occurred. These results demonstrate that descending influences from dienchephalic centers modify brainstem mechanisms regulating reflex middle ear muscle activity. These higher controls most likely account for the variability of middle ear muscle responses described in humans and in waking animals. UNE ETUDE ELECTRO-MYOGRAPHIQUE DE L'ACTIVITE MUSCULAIRE DE L'OREILLE MOYENNE ENGENDREE PAR VOIE DE CLICK CHEZ LES CHATS Des clicks ont engendre, chez des chats eveilles, une activite des deux muscles de l'oreille moyenne qui, d'apres l'electro-myogramme, consistait en une premiere reponse de courte duree (periode latente de 4, 5 a 10 msec), * This work was supported in part by the Secretariat of Science of the Copenhagen Municipal Hospitals.