Auditory interneurons in the prothoracic ganglion of the cricket, gryllus bimaculatus deGeer

Summary

• 1. The large segmental auditory neuron (LSAN) was identified both morphologically and physiologically in experiments with extracellular recording of spike potentials by electrodes filled with 2–3 M CoCl₂.

• 2. The LSAN connects left and right auditory neuropiles of the prothoracic ganglion and has no branches outside them (Fig. 3). Soma-ipsilateral arborizations are physiologically interpreted as dendritic, receiving information from different primary tympanal organ fibres. The axon crosses the midline of the ganglion via an antero-ventral commissural bridge and gives off 2 main branches, medial and lateral, projecting into all 3 parts of the contralateral auditory neuropile. There they form terminal arborizations with characteristic large (1–1.5 μm) synaptic boutons (Fig. 1B, C).

• 3. The LSAN is a tonic unit working in the frequency range of 2–3 kHz to 20–45 kHz with a narrow sensitivity maximum at 5 kHz, and moderate sensitivity at high frequencies above 8 kHz. The exact form of the tuning curves depends on the direction of sound incidence and absolute intensity (Fig. 6).

• 4. The LSAN has sharp directional sensitivity only within a limited frequency range very close to the 5 kHz optimum (Fig. 5), corresponding to the peak frequency of the conspecific calling song. At 5 kHz it is most sensitive to frontal stimulation, at high frequencies above 18 kHz — to caudal stimulation.

• 5. The LSAN precisely reproduces the time-pattern of the calling song and accurately codes the absolute intensity of such signals (Fig. 7, 8).

• 6. It is suggested that the LSAN plays an important role in the location of singing conspecifics in the field.