Excitation and Habituation of the Crayfish Escape Reflex: The Depolarizing Response in Lateral Giant Fibres of the Isolated Abdomen

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.

Escape from self-produced rates of brain stimulation

Rats were allowed to self-stimulate while their responses were being recorded on tape. Subsequently, prerecorded patterns of their brain stimulation were "played back" to them. All subjects learned to escape brain stimulation delivered in exactly the same manner as they had previously elected to receive it.

Some Features of the Central Co-Ordination of a Fast Movement in the Crayfish

(i) The nervous and muscular activity pattern produced in an abdominal segment by a single giant-fibre impulse when all peripheral reflexes are abolished is described.

(ii) This description depends on criteria for the identification of efferent neurones to the tail muscles. These are discussed and the identification of the inhibitor to the fast flexor muscles is established by anatomical homology.

(iii) The pattern of response to a giant-fibre impulse is summarized in Fig. 4.

(iv) Inhibition of the fast flexors, the lack of an extension command, the slow flexor contraction, the accessory neurone discharge and central nervous inhibition during the escape movement are among the features discussed.

Morphine: single-dose tolerance

Rats show a significant degree of tolerance to a second dose of morphine, with the degree of tolerance increasing the longer the delay between the two doses of morphine. To measure the morphine effect a foot-shock attenuation procedure that allowed the animal to adjust the shock intensity was used in studying delays of up to 180 days.