Morphology and physiology of auditory and vibratory ascending interneurones in bushcrickets

Auditory/vibratory interneurones of the bushcricket species Decticus albifrons and Decticus verrucivorus were studied with intracellular dye injection and electrophysiology. The morphologies of five physiologically characterised auditory/vibratory interneurones are shown in the brain, subesophageal and prothoracic ganglia. Based on their physiology, these five interneurones fall into three groups, the purely auditory or sound neurones: S-neurones, the purely vibratory V-neurones, and the bimodal vibrosensitive VS-neurones. The S1-neurones respond phasically to airborne sound whereas the S4-neurones exhibit a tonic spike pattern. Their somata are located in the prothoracic ganglion and they show an ascending axon with dendrites located in the prothoracic, subesophageal ganglia, and the brain. The VS3-neurone, responding to both auditory and vibratory stimuli in a tonic manner, has its axon traversing the brain, the suboesophageal ganglion and the prothoracic ganglion although with dendrites only in the brain. The V1- and V2-neurones respond to vibratory stimulation of the fore- and midlegs with a tonic discharge pattern, and our data show that they receive inhibitory input suppressing their spontaneous activity. Their axon transverses the prothoracic ganglion, subesophageal ganglion and terminate in the brain with dendritic branching. Thus the auditory S-neurones have dendritic arborizations in all three ganglia (prothoracic, subesophageal, and brain) compared to the vibratory (V) and vibrosensitive (VS) neurones, which have dendrites almost only in the brain. The dendrites of the S-neurones are also more extensive than those of the V-, VS-neurones. V- and VS-neurones terminate more laterally in the brain. Due to an interspecific comparison of the identified auditory interneurones the S1-neurone is found to be homologous to the TN1 of crickets and other bushcrickets, and the S4-neurone also can be called AN2. J. Exp. Zool. 286:219-230, 2000.