ASA 127th Meeting M.I.T. 1994 June 6-10

1pPP3. Response properties of medial olivocochlear neurons: A sound-evoked reflex.

M. C. Brown

M. C. Lieberman

Dept. of Cellular and Molecular Physiol. and Dept. of Otol. and Laryngol., Harvard Med. School, Harvard---MIT Div. of Health Sci. Technol. and Eaton--Peabody Lab., Mass. Eye & Ear Infirmary, 243 Charles St., Boston, MA 02114

The responses of single medial olivocochlear (MOC) neurons have been studied in anesthetized animals. MOC neurons have low rates of spontaneous discharge (<20 spikes/s). Responses to tone bursts show sharp tuning with well-defined characteristic frequency and large dynamic range with maximum discharge rates of up to 150 spikes/s. In response to monaural stimulation, about 2/3 of MOC neurons response best to sound in the ear they innervate, the other 1/3 respond to sound presented contralaterally, and a small number respond to sound in either ear. However, virtually all response most vigorously to binaural stimulation. Single-neuron labeling shows projections from a single MOC efferent fiber span up to an octave of best frequency on the basilar membrane, with tonotopy similar to that of auditory-nerve fibers. The strength of the OC reflex in laboratory animals and humans has been assayed by measuring the suppression of response (compound action potentials or ear-canal emissions) produced by addition of contralateral sound, although care must be taken to anesthetized cat and awake human. [Supported by NIH Grants DC 01089, 00119, and 00188.]