Protection from habituation of the crayfish lateral giant fibre escape response

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Abstract
Habituation of the lateral giant fiber escape response in the crayfish [Procambarus clarkii] to repetitive tactile stimuli probably results from homosynaptic depression of the 1st synapse of the reflex, between tactile afferents and interneurons. Normall, habituation of escape responses to repeated innocuous stimuli is presumed to be adaptive. Whether habituation would occur under circumstances when it would presumably be maladaptive, in particular, when tactile receptors are stimulated by an animal''s own tail-flip movements, was investigated. Experiments were carried out on the crayfish isolated abdominal nerve cord, which contains the lateral giant reflex pathway. Compound EPSP [excitatory postsynaptic potential] elicited in the lateral giant by electrical stimulation of tactile afferents decline by from 25 to 36% over a series of 11 trials at 1/5 s (control series). To determine whether such a decline would occur when sensory afferents are stimulated during a tail-flip, stimuli were given as in the control series but each stimulus occurred 20 ms after direct electrical stimulation of a medial giant or lateral giant escape-command fiber at which time tail flexion movements of an intact animal would be in progress. Under these conditions percent EPSP decline over 11 trials at 1/5 s was only 16-45% of that occurring on the control series. This protective effect starts at .apprx. 10 ms after escape command neurone firing, is maximal at 20 ms, and thereafter declines, remaining weakly detectable at 100 ms. This time course is commensurate with that required for execution of a tail-flip movement. Sensory afferent-to-lateral giant transmission is protected from depression if stimuli occur when a tail-flip movement is or should be occurring. Giant fiber spikes do not superimpose facilitation upon a depressed reflex pathway, nor accelerate rate of recovery from depression; rather, protection is attributable to actual prevention of development of the depressed state. Protection was examined at the 1st synapse the reflex, where the depression responsible for habituation is believed to occur, by recording inracellularly in the largest of the 1st-order interneurons (interneuron A) of the pathway. In absence of protection, 10 stimuli presented at 1/4 s caused a mean decline of 32% in the EPSP in interneuron A. When such stimuli followed directly evoked escape command neuron firing by 20 ms this decline was reduced by 59-100%. Protection may serve to prevent crayfish from habituating to stimuli produced by their own tail-flip movements.