Variation in response dynamics of regular and irregular vestibular-nerve afferents during sinusoidal head rotations and currents in the chinchilla

摘要

In mammals, primary vestibular afferents that innervate only type I hair cells (calyx-only afferents) respond nearly in phase with head acceleration for high-frequency motion, whereas afferents that innervate both type I and type II (dimorphic) or only type II (bouton-only) hair cells respond more in phase with head velocity. Afferents that exhibit irregular background firing rates have a larger phase lead re head velocity than those that fire more regularly. We wanted to examine what is the cause of the variation in phase lead between regular and irregular afferents at high frequency head rotations. Under the assumption that externally applied galvanic currents act directly on the nerve, we derived a transfer function describing the dynamics of a semicircular canal and its hair cells through comparison of responses to sinusoidally modulated head velocity and currents. All afferent responses were well fit with a transfer function with one zero (lead term). Best-fit lead terms describing responses to current for each group of afferents were similar to the lead term describing responses to head velocity for regular afferents (0.006s + 1). This shows that the pre synaptic/synaptic inputs to regular afferents are pure velocity transducers. However, the variation in phase lead between regular and irregular afferents cannot be explained solely by the ratio of type I to II hair cells () suggesting that the variation is caused by a combination of pre- (type of hair cell) and post- synaptic properties.