Ultrastructure of synaptic contacts between identified neurons of the auditory pathway in Gryllus bimaculatus DeGeer.

The synaptic contacts between the auditory sensory cells and identified auditory interneurons ON1 and AN2 have been examined at the ultrastructural level by selective electron-dense labeling of two interneurons or of one interneuron and the sensory fibers in the same preparation. The experiments have provided the following information. The auditory afferent fibers have a monosynaptic connection with the lateral inhibitors ON1 and the ascending interneuron AN2, allowing direct activation of these interneurons. Furthermore, our work proves that the paired, lateral, inhibitor ON1 neurons have direct output synapses onto each other. The results also show that the auditory afferent axons, themselves, receive synaptic inputs just before entering the central auditory neuropil. The effects of current injection into the ON1 neuron during auditory processing indicate that these synaptic inputs onto the afferents originate, in part, from the lateral branches of the ipsilateral ON1 neuron and that they have inhibitory function. The significance of these results for auditory processing and a future perspective for electron microscopic analysis of neuropil are discussed.