Synaptic Excitation of the Second Order Auditory Neurons in Avian Brain Stem

A~traet---Synaptic potentials were examined in the secondand third-order auditory neurons of nucfeus magnocellularis and nucleus iaminaris in the chick. Brain stems of mature chick embryos were explanted and maintained in vitro for 4 to 8 h. Field potentials, extracellular spike potentials and intracellular potentials evoked by 8th-nerve stimulation were examined. Eighth-nerve stimulation reliably elicited four identifiable field potentials which could be attributed to; (i) the afferent volley of the &h-nerve axons, (ii) postsynaptic responses of n. magnocelhdaris neurons, and (iii) ips~laterally and, (iv) ContraIateraily-evoked n. laminaris postsy~aptic responses. intracellularrecorded postsynaptic potentials from n. magnoce~~uia~s neurons were of two classes. ‘Fast’ excitatory postsynaptic potentials were characterized by a rapid rise time and short duration. They were apparently monosynaptic with a synaptic delay of 0.4 ms. In each n. magnocellularis neuron the ‘fast’ excitatory postsynaptic potentials were composed of 1 to 3 all-or-none components. ‘Slow’ excitatory postsynaptic potentials were characterized by a longer latency, a longer duration and graded amplitude variation in proportion to the intensity of Ith-nerve stimulation. Both ‘fast’ and ‘slow’ excitatory postsynaptic potentials had similar reversal potentials. Since the 8th nerve makes monosynaptic connection with n. magnocellularis neurons, it is likety that at this synapse the ‘fast’ excitatory postsynaptic potentials were produced, while the ‘slow’ potential may be attributable to the convergence of many boutonal synapses of unknown origin. Intracellular injections of horseradish peroxidase into n. magnocellularis revealed that its efferents bifurcate below the nucleus and send one axon to the contralateral n. laminaris while the other axon forms a highiy divergent projection to the ipsilateral laminar nucleus. The intracellular records obtained from n. laminaris are consistent with this anatomical finding in that graded excitatory postsynaptjc potentials were elicited by &h-nerve stimulation.