Responses of vestibular-nerve afferents in the squirrel monkey to externally applied galvanic currents

Vestibular-nerve afferents classified as regularly or irregularly discharging also differ in their response dynamicsa, 4,13,14, in their sensitivites to natural stimulationa,4,14 and to electrical stimulation of efferent pathways 6, and in their conduction velocitiesS, 14. The cellular mechanisms determining the differences in discharge properties of the afferents are only poorly understood. In the present study, galvanic currents were used to define possible stages of the transduction process at which differences in response dynamics and discharge regularity may arise. Evidence will be presented that externally applied currents bypass earlier, mechanical stages and that tb_ey may actually influence discharge by acting near the postsynaptic trigger site the place in the afferent axon where impulses normally arise. Data were obtained from 16 barbiturate-anesthe tized adult male squirrel monkeys (Saimiri seiureus). Surgical and recording procedures have been described previously, as have the methods for natural stimulation and for identifying the end organ innervated by any afferent 2. Regularity of discharge was quantified by a normalized coefficient of variation (cv*), appropriate to a mean interval of 15 m s ! Units were classified as regular (cv* < 0.10), intermediate (0.10 _< cv* < 0.30) or irregular (cv* > 0.30). Electric currents were delivered between two chiorided silver wires (0.25 mm diameter), which were insulated to within 1 mm of their tips. One electrode was snugly fit into the perilymphatic space of the vestibule through a small hole made in the bony promontory; the other electrode was lodged in the hypotympanic space of the middle ear. Galvanic stimuli are called cathodal (-) or anodal (+) , in conformity with the polarity of the perilymphatic electrode. Cathodal currents were invariably found to increase afferent discharge, whereas anodal currents always decreased it. Sinusoidal head rotations and sinusoidal galvanic currents both led to responses with low harmonic distortion (near 5 ~o). Gains and phases extracted by Fourier analysis and expressed relative to peak head velocities or to peak currents were not systematically affected by stimulus magnitude. Results for 9 semicircular-canal units are summarized in Bode plots (Fig. 1). Responses to natural stimulation (Fig. 1A, C) are similar to those described previously1 and include two effects not seen in