Comparison of sinusoidal responses to externally applied currents and angular head

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- 100uA. 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 vestibular nerve afferent. 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.