Crayfish antennal neuropil. I. Reciprocal synaptic interactions and input-output characteristics of first-order interneurons.

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Abstract
Primary afferents and sensory interneurons which subserve the antennae of the crayfish [Procambarus clarkii] were studied with intracellular recordings in the supraesophageal ganglion and extracellular recordings from the antennal nerve and the ipsilateral circumesophageal connective. Primary afferents exhibit synchronous responses to mechanical stimuli. The synchronization is attributable to strong but independent correlations to the antennal stimulus. First-order interneurons reveal phase-locked compound EPSP [excitatory postsynaptic potential] to primary afferent volleys with minimum latencies of 2-12 ms. Longer latency (20-70 ms) labile inputs are suggestive of input from other interneurons. About a third of the 1st-order interneurons subserving the antenna exhibit subthreshold synaptic potentials on stimulation of remote body parts, such as the tail. The 1st-order interneurons are postsynaptic and presynaptic to other interneurons. Reciprocal connections are quite common and function in the synchronization of the discharge among selected interneurons. Many of the synapses are labile and exhibit frequency-dependent antidefacilitation. Some reciprocal synapses exhibit transmission delays of 0.1 ms or less and are capable of 1:1 following at rates of at least 80 pulses/s. These junctions transmit both depolarization and hyperpolarization, and are probably electrotonic. On depolarization with extrinsic current, many 1st-order interneurons exhibit repetitive bursting with temporal features similar to the pulse trains elicited by sensory stimuli. For a narrow range of currents, the burst repetition rate increases with current intensity. Interneurons coupled to the current-driven interneuron tend to burst in synchrony with the latter.

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