Electrophysiological responses of cochlear root neurons.

Cochlear root neurons (CRNs) are second-order neurons interspersed among the fibers of the cochlear nerve in certain rodents. They project, among other nuclei, mainly to the pontine reticular nucleus, and participate in the acoustic startle response (ASR), a short-latency motor reflex initiated by sudden intense sounds. The sound-evoked activity of CRNs has not previously been described. Here we describe extracellular responses of CRNs located in the infranuclear portion of the cochlear nerve root. CRNs exhibited secure responses to tone bursts, with first-spike latencies of approximately 2.2 ms. The characteristic frequencies of the recorded CRNs were about 30 kHz, and the best-characterized CRN had a threshold of 10 dB sound pressure level and sharpness of tuning similar to that of cochlear nerve fibers. The peristimulus time histograms were primary-like with notch. The observed response properties were consistent with the suggestion that CRNs provide the short-latency acoustic input to the reticular formation that leads to an ASR.