The synaptic origins of receptive field properties in the cricket cercal sensory system

1. The synaptic connections made by identified sensory neurons were studied electrophysiologically in the cercal sensory system of the cricket (Acheta domestica).2. The results confirmed that the synaptic connections made by a particular sensory neuron were correlated with the precise topographic projection of the sensory neurons and the location of the postsynaptic dendrites within this afferent projection. Where an afferent axon overlapped the dendrites of an interneuron a monosynaptic connection was often found, where there was no overlap there was no connection.3. However, the synaptic connections expected, based on anatomical overlap, were not always detected and the present study has revealed two factors that account for this difference between expectation and reality.4. First, the synaptic connections made by a particular sensory neuron were not invariant, rather they were probabilistic. For a given sensory neuron-interneuron pair, there was a certain probability that a synapse between the two neurons would be detected in any given animal and this probability was seldom 100%.5. Second, the size of the receptor hair was directly correlated with the interneurons with which the sensory neuron formed synapses. For example, one interneuron, MGI, received input only from sensory neurons associated with small receptor hairs. In contrast, interneuron 10-3 received input from sensory neurons associated with large hairs. These results confirm the hypothesis of Shimozawa and Kanou (1984).6. Our conclusion was that two properties of the receptive fields of cricket giant interneurons, directional sensitivity and acceleration/velocity sensitivity, are the direct result of the monosynaptic inputs from sensory neurons