Physiological and anatomical study of click-sensitive primary vestibular afferents in the guinea pig.

We studied the sensitivity of primary vestibular afferents in anaesthetised guinea pigs to clicks. These vestibular neurons were also tested by their response to pitch and roll tilts and yaw-axis angular acceleration. The click intensity was referred to the threshold for evoking the auditory brainstem responses. Recording sites in the vestibular nerve were confirmed histologically using iontophoretic injection of FCF green dye. To confirm the site of labyrinthine origin of the click-sensitive neurons, we used retrograde tracing with biocytin. In all, 647 out of 2354 neurons in the vestibular nerves of 51 guinea pigs were activated by clicks. Most were irregularly discharging primary neurons, but some were regularly discharging. We studied responses to vestibular stimuli in 188 click-sensitive neurons. Of these, 86% responded to pitch and/or roll tilt, but none responded to yaw angular acceleration. Conversely we also recorded vestibular neurons which did not respond to clicks. None of 300 neurons sensitive to yaw angular acceleration were responsive to 80-90 dB SL clicks (0 dB SL = threshold for auditory brainstem response to clicks). The latencies of click-evoked action potentials of neurons in the vestibular nerve were very short (mean +/- SD = 0.82 +/- 0.22 ms). Changing click polarity caused a heterogeneous pattern of latency change. Thresholds for evoking spikes in primary vestibular neurons were high (62.0 +/- 12.2 dB SL, range 30-90 dB, n = 371). Retrograde tracing of the origin of the click-sensitive afferents using extracellular biocytin showed that most neurons originated in the medial (striola area) of the saccular macula.