Excitation and habituation of the crayfish escape reflex: the depolarizing response in lateral giant fibres of the isolated abdomen.

Crayfish commonly evade capture by darting backwards when one tries to pick them up or otherwise disturbs them. They do this by sharply flexing the abdomen, the tail fan acting as a paddle; at the same time they streamline themselves by thrusting their appendages forward. Like many other escape and startle reflexes throughout the animal kingdom, this one is exceedingly labile. A crayfish held by its carapace will usually perform an escape response the first time its tail is briefly squeezed in a mock attempt at capture. But it is uncommon to find an individual which will respond to more than the first few such stimuli when these are presented at 5 min. intervals, and recovery from such waning requires a number of hours of rest (Krasne & Woodsmall, in preparation). It is the purpose of the work whose beginnings are described here to analyse the physiological mechanisms responsible for this phenomenon in the hope that an understanding of this relatively simple sort of behavioural plasticity will ultimately contribute in some measure to our understanding of how nervous systems allow for learning and memory. The present behaviour pattern was chosen for analysis because it is commonly (Wiersma, 1947), though apparently not necessarily (D. Kennedy, personal communication), mediated by giant fibres whose position as interneurones in the reflex arc provide a convenient vantage point for looking for sites of lability. Direct stimulation of either the medial or lateral giant fibres of the nerve cord can call out the escape manoeuvre reliably, though not necessarily with full vigour (Kennedy & Takeda, 1965), at least as often as once every 10 sec. for many trials (F. B. Krasne, unpublished observations). Therefore, it is not possible that the total failure of reflex responses described above can be due to changes occurring efferent to giant fibres. Consequently, it seemed appropriate to start analysis of escape reflex lability by examining reflex transmission between afferent nerves and giant axons. Wiersma (1947) had shown that it is the lateral giant fibres which can be fired in response to abdominal stimulation, and Kao & Grundfest (1956) and Kao (i960) had demonstrated that subthreshold depolarizing responses could be recorded from these fibres in their course through the abdomen. These observations were the point of departure for the work reported here.