Signal processing in brainstem auditory neurons which receive giant endings (calyces of Held) in the medial nucleus of the trapezoid body of the cat

Abstract

In the medial nucleus of the trapezoid body (MNTB), each principal neuron receives one large axonal ending (a calyx of Held) and many small endings. In this same region, microelectrode recordings show unusual ‘unit’ waveforms which have two components separated by about 0.5 ms. We show that the first component (C 1) of such a waveform corresponds to a spike from the calyx of Held and that the second component (C 2) corresponds to a spike from the MNTB principal neuron. There are two kinds of evidence for these correspondences. First, electrical stimulation of calyciferous axons in the contralateral trapezoid body evokes C 1 spikes with latencies of 0.1–0.3 ms. These latencies are too short for there to be an intervening synapse and are consistent with C 1 being a presynaptic spike. Second, shocks in the lateral superior olive (which receives projections from MNTB principal-neurons) evoke ‘A’ spikes in the MNTB which can be shown by their waveshapes and mutual refractoriness with C 2 spikes to result from antidromic activation of the neurons producing C 2 spikes. Spontaneous and sound-evoked responses in dozens of cats anesthetized by barbiturates or Ketamine always had a C 2 spike following each C 1 spike. This implies that there is normally one-to-one spike transmission from the calyx of Held input to the MNTB principal neuron output. The small endings on MNTB principal neurons are also capable of evoking spikes. Electric shocks (and in one case, sound), evoked long latency (1–3 ms) ‘LC 2’ spikes, which (by mutual refractoriness and waveshape) are from the same neural elements as C 2 and ‘A’ spikes. Since LC 2 spikes are not preceded by C 1 spikes, LC 2 spikes must be mediated by small axonal endings on MNTB principal neurons. We found some evidence of inhibition, possibly recurrent inhibition, in MNTB principal neurons. In a few neurons, sound or shocks inhibited ‘A’ spikes or LC 2 spikes. In some cases, after each C 2 spike, LC 2 spikes were blocked or reduced in amplitude for several milliseconds. Our data firmly establish that there is fast, secure spike transmission from calyces of Held to MNTB principal neurons and suggest that under some circumstances there is additional signal processing in MNTB principal neurons.