Electrophysiological responses of cochlear root neurons

Cochlear root neurons (CRNs) are second-order neurons interspersed among th e fibers of the cochlear nerve in certain rodents. They project. among othe r nuclei, mainly to the pontine reticular nucleus. and participate in the a coustic startle response (ASR), a short-latency motor reflex initiated by s udden 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 leve l and sharpness of tuning similar to that of cochlear nerve fibers. The per istimulus time histograms were primary-like with notch. The observed respon se properties were consistent with the suggestion that CRNs provide the sho rt-latency acoustic input to the reticular formation that leads to an ASR. (C) 2001 Elsevier Science B.V. All rights reserved.