An Acridid Auditory Interneurone

Abstract

1. An auditory interneurone (‘alpha neurone’) of Gastrimargus africanus is described. It runs from the mesothoracic ganglion to the brain, and receives summating excitatory inputs from both tympanic nerves. There is no inhibition between the two units or their inputs. 2. The lowest effective frequency is 5-8 kcyc./sec. and response is greatest at the highest frequency tested, 40 kcyc./sec. Threshold at 30 kcyc./sec. is approximately +50 dB above 0.0002 dynes/cm², and response increases with amplitude from 60-90 dB, after which there is no further increase. Increase of amplitude also shortens response latency; the correlation of latency and response strength is low and very variable. The response is slowly adapting, and mean impulse frequency declines exponentially with increasing length of short sound pulses. 3. Directionality of unilateral sound is signalled by the alpha neurones in only 70% or less of trials, and at least 10% are discriminated ‘wrongly’. The low correlation between response strength and latency improves the signalling of directionality. 4. A second ‘beta’ neurone is described briefly. It signals directionality of a unilateral sound with 100% accuracy, due to mutual inhibition between ipsilateral and contralateral systems, as in some tettigoniid interneurones. 5. Ten or more apparently separate auditory interneurones have now been described in acridids; this parallels the diversity of receptors described in the ear.