Auditory neurones in the brain of the cricketGryllus bimaculatus (De Geer)

Abstract

1. Auditory neurones in the brain (supraoesophageal ganglion) of the cricketGryllus bimaculatus (De Geer) were intracellularly recorded and stained with Cobalt or Lucifer. 2. The cell bodies and arborisations of all recorded neurones are contained entirely within the supraoesophageal ganglion. Further, the cells all have arborisations which overlap extensively in the ventral-anterior protocerebrum, but there is no direct contact with the mushroom bodies. 3. The auditory neurones described fall into three morphological types. A. The first morphological type — theUnisegmentalauditorybrainneurone (UABN) — is restricted entirely to the protocerebrum and has no identifiable axon or terminal arborisations. Its Best Frequency (BF) is 16 kHz, near the conspecific courtship song frequency, and it is inhibited by low frequency sounds at high intensities. The intensity-response curve at 16 kHz has a greater dynamic range than those at higher or lower frequencies. B. The second morphological type — thePlurisegmentalauditorybrainneurone 1 (PABN1) — runs ventro-dorsally from proto- to deutocerebrum. The PABN1 has a bimodal tuning curve with a BF at 16 kHz and a secondary peak at 10 kHz. The intensity-response curve showing the greatest dynamic range occurs at 16 kHz. C. The third morphological type — thePlurisegmentalauditorybrainneurone 2 (PABN2) — sends branches from proto- to deutocerebrum, collaterals to its protocerebral and deutocerebral arborisations, and branches into the contralateral half of the brain. The PABN2 has a broader tuning curve than the other cells but is still most sensitive around 16 kHz. 4. All the cells exhibit spiking responses to sound, with tonic or phasic/tonic bursts which may exceed the duration of shorter stimulus tones. Sensitivities of the brain cells at their BF's are similar to those of receptors and ascending interneurones in this species. 5. All the brain cells recorded here can follow the pulse pattern of a synthetic calling song, adapting throughout a chirp, but only when chirps are presented at the BF. The initial response pattern is maintained over successive chirp presentations. This frequency-dependent encoding of the temporal properties of a stimulus is independent of sound intensity and, therefore, of auditory threshold. The underlying mechanism is probably different for each cell type. 6. The brain neurones have response properties and areas of arborisation which are similar to those of a previously identified ascending auditory interneurone. This ascending interneurone is thought to play a role in encoding the conspecific courtship song, with which these brain neurones may also be involved. The ventral-anterior protocerebrum may therefore represent a localised high-frequency auditory neuropile.