The tail-flip escape reflex of the crayfish shows marked habituation and is very amenable to detailed electrophysiological study. It thus provides a model system for comprehending the neural basis for one sort of learning phenomenon. This paper describes the electrical events which can be recorded by micropipettes placed in lateral giant axons of isolated crayfish abdomens.

1. There is little spontaneous activity. Depolarizing responses can be evoked in a given segment by natural or electrical stimulation ipsilateral or contralateral to the recording electrode and at any of a number of segments.

2. Electrical shocks to the ipsilateral second root of the impaled ganglion evoke a depolarization with an early stable phase and a later labile phase which shows largely reversible decrements when stimulated as seldom as once a minute. Only the labile portion is ever big enough to trigger a propagated response.

3. The initial, potentially spike initiating, and largest portion of the labile phase has a sufficiently short latency to suggest that it arrives along a disynaptic or at most trisynaptic pathway.

4. The labile phase appears to be built up of a number of measurably large, unitary, subcomponents. Decrements during repetitive stimulation seem to result from the all-or-none dropping out of these subcomponents following increases in their latency. This suggests that blockage of transmission occurs at synapses on to neurones intercalated between afferents and giant fibres or at parts of the axonal tree which are electrotonically remote and support branch spikes.

5. The lability described here is probably not sufficient to account for habituation in the intact animal.

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