Comparison of sinusoidal responses to externally applied currents and angular head movements in the vestibular nerve of the chinchilla

초록

Extracellular recordings were made from 65 afferents innervating the semicircular canals in 8 adult, barbiturateanesthetized chinchillas. Responses to sinusoidal stimuli (rotations and externally applied galvanic currents to the middle ear) were measured. Afferents were divided into three groups: bouton and dimorphic regular (R), dimorphic irregular (D) and calyx-only irregular (C) afferents based upon their discharge regularity and sensitivity to head movements at 2 Hz. The frequency of applied current ranged from 0.1 to 20 Hz with peak amplitude from 10-100 &#9127;A. The rotational frequency varied from 2 to 16 Hz. Because of variability in the amount of current stimulation from animal to animal, we calculated a normalized measure of galvanic sensitivity at 2 Hz based upon the discharge regularity. As reported in Baird et al. (1988), this current sensitivity increased logarithmically with increasing CV*. Transfer functions were fit to the sensitivity and phase across the frequency spectrum for each group of afferents. Phase lead at 20 Hz to externally applied currents measured 29 ± 13° (R), 32 ± 12° (D), 26 ± 9° (C) (p > 0.1). In contrast, phase at 16 Hz in response to head velocity measured 31 ± 25° (R), 57 ± 6° (D), 83 ± 22° (C) (p < 0.01). Externally applied galvanic currents act directly on vestibularnerve afferents. Comparison of rotational and galvanic responses allowed us to compute the relative contributions of synaptic and post-synaptic mechanisms. One conclusion emerging from this analysis is that the dynamics of the type I hair cell synapse are associated with an acceleration-sensitive component at higher frequencies.