The anterior intermediate sensory neuropile (aISN) is a prominent neuropile in the ventral nerve cord of locusts and bushcrickets. Previous studies have shown that it receives its main sensory input from auditory receptors. In this paper we examine the structural and physiological relationship between tympanal receptor terminations and the dendrites of sound-sensitive interneurones in the homologous neuropile of locusts and bushcrickets. Each individual receptor fibre of the bushcricket terminates in a somewhat different target area of the neuropile. The ordering is with respect to the characteristic frequency of the fibres (tonotopic) in the anterior-posterior and dorsoventral axis. In the locust, representatives of the four tympanal receptor groups branch in different areas of the aISN. Most of the dorsal neuropilar region, and the anterior ventral region, do not receive input from tympanal receptors. The dendrites of identified sound-sensitive interneurones were examined in the context of this afferent projection. Local interneurones as well as intersegmental interneurones in bushcrickets have dendritic branches in the whole aISN or part of it and thus overlap with at least some receptors. By recording intracellularly from their main neurites, short-latency synaptic potentials were found in response to receptor spikes indicating monosynaptic input. The tuning of these neurones could be predicted by their dendritic morphology. In contrast, in the locust only local and bisegmental neurones are monosynaptically connected with tympanal receptors, but not the studied intersegmental neurones. This is consistent with the finding that most or all branches of intersegmental neurones lie in the dorsal area of neuropile where no receptors terminate. Anatomical and physiological evidence is presented for identified local neurones providing the excitatory and inhibitory synaptic input for such intersegmental neurones. The difference in the basic wiring diagram in the homologous neuropile of the two orthopteran groups is discussed with respect to the possible different roles that sound plays in their behaviour.
In the prothoracic ganglia of the cricket Gryllus bimaculatus two local auditory interneurones, ON1 and ON2, were labelled for electron microscopy by intracellular injection of horseradish peroxidase following physiological characterisation. The neurones branch in the median ventral association centre and the root of nerve 5 on both sides of the ganglion. As they are very similar in shape and position they may share a common embryological origin. Differences are found in the details of the fine branching pattern and in their physiology as ON1 is tuned particularly to low sound frequencies of 4-5 kHz whereas ON2 is more sensitive to frequencies above 8 kHz. Although the ON1 neurones inhibit each other and are involved in the inhibition of other auditory neurones they were not labelled by antibodies against the inhibitory transmitter GABA and their vesicles differ significantly from those in neurones that are. The same is true of the ON2 neurones whose vesicles also differ significantly from those in ON1 supporting light-microscope evidence that they may use different transmitters. The distribution of input and output synapses on the ipsilateral and contralateral branches of ON1 and ON2, and the proportion of the synapses made from and onto neuropilar processes immunoreactive for GABA was determined. In ON1 94% of the input synapses were received on the ipsilateral branches and 62% of the outputs made from the contralateral branches. This confirms previous physiological evidence that input is received ipsilaterally and output made contralaterally but the presence of some contralateral input and a significant ipsilateral output was unsuspected. Thirty percent of the input synapses on the ipsilateral side and 75% on the contralateral side were made from GABA-immunoreactive processes but processes postsynaptic to ON1 were rarely immunoreactive. The distribution of input synapses on ON2 was similar with 90% received on ipsilateral branches but a higher proportion of outputs (83%) was made from the contralateral side than in ON1. Thirty one percent of ipsilateral inputs were GABA-immunoreactive but only 14% on the contralateral side.
Two identified cricket auditory interneurones, AN1 and AN2, were intracellularly labelled with horseradish peroxidase following physiological characterisation. The neurones, which have some structural similarities, have their somata in the prothoracic ganglion and axons that project to the brain. Although both carry auditory information, they have different response properties and participate in different types of phonotactic behaviour. Ultrathin sections from selected regions of their prothoracic arborisations were examined in the electron microscope after postembedding immunostaining for the inhibitory transmitter GABA. In the prothoracic ganglion AN1 branches only in the medial ventral association centre (mVAC) contralateral to the soma, and receives only iput synapses. Twenty-seven percent of these were made by processes immunoreactive for GABA. AN2 branches not only in mVAC on both sides of the ganglion but also in several other areas. It makes output synapses from large diameter neurites in mVAC on both sides of the ganglion as well as from neurites in more posterior regions of the neuropile. Most input synapses are received onto branches in the contralateral mVAC where about 19% were made from GABA-immunoreactive processes.
The stereotyped response of the female Leptophyes punctutissimu Bosc. to a representation of the male call has been taken as a behavioural measure to determine the directional sensitivity of monaurally deafened animals. By using the responses of an auditory interneuron (T-fibre), the directional sensitivity could also be measured neurophysiologically on the same preparation. Thus it was possible to compare directly both a behavioural and a neurophysiological polar directional plot for an individual. Both methods provided very similar results, showing an overall directionality of about 15 dB, with the maximum sensitivity of the ear occurring on the ipsilateral side.
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