Physiology of peripheral neurons innervating semicircular canals of the squirrel monkey. 3. Variations among units in their discharge properties.

Abstract

WERSXLL (17) was the first to describe the two kinds of hair cells found in the sensory epi thelium and to demonstrate that they had different nerve supplies. The type I hair cell, peculiar to birds and mammals, is a relatively large, flask-shaped cell and is innervated by a calycoid ending derived from a single, usually thick, afferent fiber. The type II hair cell, common throughout the vertebrate scale, is thin and cylindrically shaped. It receives bud-shaped endings from several afferents. Wersall’s observations were foreshadowed in the classical silver studies of Cajal (2, 3) and of Lorente de No (10) and have been amply confirmed in more recent ultrastructural (1, 4, 9, 11, 15, 18) and light microscopic studies (1, 9). These morphological complexi ties should be reflected in the physiology of mammalian vestibular afferents. On the basis of the two preceding papers (6, s>, it may be concluded that neurons innervating the semicircular canals of mammals do vary from one another in their discharge characteristics. Neurons differ in the regularity of spacing of their action potentials, in the level of their resting activity, in their sensitivity to angular accelerations, and in their dynamic properties. Were these differences a reflection of differences in the i nnerva tion patterns of the parent sensory axons, it might be ex-