Morphology and physiology of auditory interneurons in the metathoracic ganglion of the locust

Summary

• 1. Auditory interneurons in the metathoracic ganglion of the locust were characterized by their intracellularly recorded responses to sound stimuli, and by their morphology as revealed in sectioned preparations of single neurons injected with Lucifer Yellow. Accordingly, nine interneurons were identified as either local, bisegmental, ascending or T-neurons (Figs. 2–6).

• 2. All cells possess arborizations within one or both sides of a prominent area of fine neuropil (frontal auditory neuropil), but the degree of overlap with the endings of tympanic afferents is different. For 2 ascending units (AN2, AN3) there is no overlap at all, indicating that they are interneurons of higher order.

• 3. Two neurons, SN1 and TN1, were found with only excitatory input to low frequency sound, and their tuning reflected that one of low frequency receptor fibres. In contrast, the response of all other neurons to low frequency stimuli was more complex, consisting of excitatory and inhibitory synaptic potentials. The neuronal circuitry which might underlie such physiological behaviour is discussed.

• 4. Recordings made from different parts of individual neurons enabled the sites of synaptic input and output to be localized. The postsynaptic dendrites of the interneurons appear to correspond to smooth endings in contrast to the beaded appearance of the presumptive presynaptic terminals (Fig. 8).

• 5. The results include descriptions of auditory ‘sister-neurons’ (Fig. 6) which have a similar morphology and of ‘twin-neurons’ (Fig. 7), which, using our present physiological and morphological criteria, cannot reliably be distinguished one from another. The problem that arises from such doubling of cells for the concept of identified units is discussed.