Intracellular studies on auditory processing in the metathoracic ganglion of the locust

Summary

• 1. Postsynaptic potentials of various auditory interneurons were recorded intracellularly in the metathoracic ganglion of the locust. Although the sound stimulus consisted of bursts of pure tones (20 or 100 ms duration), only a minority of intracellular responses (8 out of 43) exhibited simply configurated depolarizing potentials (Fig. 2a, b). Most responses were composed of excitatory and inhibitory postsynaptic potentials (Fig. 2c-e) with different temporal interactions between EPSP and IPSP. Thus the multifold spike patterns, known in principle from previous extracellular recordings, can be explained.

• 2. In the neuron of Fig. 3 a the threshold curves of EPSP and IPSP differ significantly in the frequencyintensity field. Based on a comparison with previous findings on receptors and one thoracic interneuron (Rehbein et al. 1974), a model for synaptic connectivity is proposed, although detailed morphological data are not available.

• 3. The degree of EPSP- and IPSP-interaction depends strongly on sound direction. The ipsilaterally generated excitation can be totally eliminated when the contralateral ear is more strongly stimulated (Figs. 4, 5). Two different modes of temporal interaction between EPSP and IPSP can be detected: on the one hand — if both potentials appear — the inhibition follows the excitation. Thus the resulting spiking activity is modified without changing the latency of the corresponding response (Figs. 4, 6). On the other hand the inhibition precedes excitation (Fig. 5), which in addition influences the initial phase (latency) of the response. These data indicate the existence of an underlying synaptic mechanism for previous extracellular findings (see Mörchen 1980).