Auditory interneurones in the metathoracic ganglion of the grasshopper Chorthippus biguttulus. I, Morphological and physiological characterization

1.Auditory intemeurones originating in the metathoracic ganglion of females of the grasshopper Chorthippus biguttulus can be classified as local (SN), bisegmental (BSN), T-shaped (TN) and ascending neurones (AN). A comparison of branching patterns and physiological properties indicates that auditory interneurones of C. biguttulus are homologous with those described for the locust. 2.Eighteen types of auditory neurones are morphologically characterized on the basis of Lucifer Yellow staining. All of them branch bilaterally in the metathoracic ganglion. Smooth dendrites, from which postsynaptic potentials (PSPs) can be recorded, predominate on the side ipsilateral to the soma. If ‘beaded’ branches exist, they predominate contralaterally. The ascending axon runs contralaterally to the soma, except in T-fibres. 3.Auditory receptors respond tonically. The dynamic range of their intensity-response curve covers 20–25 dB. Local, bisegmental and T-shaped neurones are most sensitive to stimulation ipsilateral to the soma. The responses of SN1 and TNI to white-noise stimuli are similar to those of receptors, while phasic-tonic responses are found in SN4, SN5, SN7 and BSN1. The bisegmental neurones receive side-dependent inhibition that corresponds to a 20–30dB attenuation. One local element (SN6) is predominantly inhibited by acoustic stimuli. 4.Ascending neurones are more sensitive to contralateral stimulation (i.e. on their axon side). Only one of them (AN6) responds tonically to white-noise stimuli at all intensities; others exhibit a tonic discharge only at low or at high intensities.One neurone (AN12) responds with a phasic burst over a wide intensity range. The most directional neurones (AN1, AN2) are excited by contralateral stimuli and (predominantly) inhibited by ipsilateral stimuli. Three ascending neurones (AN13-AN15) are spontaneously active and are inhibited by acoustic stimuli. 5.All auditory intemeurones, except SN5, are more sensitive to pure tones below 10 kHz than to ultrasound.