Presynaptic contributions to response shape in an auditory neuron of the grasshopper

Neuron 714 is morphologically one of the most prominent neurons in the central auditory pathway of the grasshopper with arborizations extending from the abdominal neuromeres of the metathoracic ganglion to the brain. The aim of this study is to explore auditory information flow involving neuron 714 at the level of the ventral nerve cord. Paired intracellular recordings were made from neuron 714 in the mesothorax on the one hand, and from candidate presynaptic auditory neurons of the metathorax on the other. Electrical stimulation of the tympanal nerves provides an estimate of the synaptic distance between these interneurons and auditory afferents. Four, including neuron 714, are monosynaptically connected to afferents, the remainder disynaptically. Current-injection and spike-triggered averaging reveal that of nine neurons examined, seven make either monosynaptic, disynaptic or polysynaptic connections onto neuron 714. All connections are excitatory. Paired recordings show that response duration and response amplitude in synaptically linked cells vary according to the frequency of the stimulus. Measurements of the latency of the first excitatory post-synaptic potential evoked in neuron 714 by afferents and by metathoracic interneurons show how the synaptic drive from these sources shapes the auditory response of neuron 714. Accepted: 14 October 1998