DISCHARGE PATTERN AND INHIBITION OF PRIMARY AUDITORY NERVE FIBERS IN THE MONKEY

Abstract

The electrical signs of action potentials in the auditory nerve fibers of 32 monkeys were recorded with fine microelectrodes placed in extra-axonal positions. Histological studies of the segment of the cochlear nerve from which recordings were made revealed it to be composed only of primary auditory neurons. These first-order auditory fibers were clearly divided into 2 groups, one which displayed "spontaneous" activity, and one which did not. The fibers were also separated into 2 groups in terms of the slopes of their sound intensity - impulse frequency relations. The group of fibers with graded slopes for this relation is thought to consist of the spiral fibers, while the group showing more abrupt slopes is composed of the radial fibers. The impulses in first-order fibers elicited by a continuous tone could be inhibited temporarily by presenting a 2nd tone burst, if the frequencies and the intensities of the 2 tones were chosen properly. Although the nature of the inhibition observed is not clear, it can not be due to action by an efferent fiber group, for its latency is very short. The simultaneous presentation of 2 pure tones, each outside the previously determined response area of a given fiber, could evoke impulses in that fiber.