Abstract
A large descending interneuron in the crab (fibre 5) has three input channels which have been shown to be separate by selective inactivation. These are (1) statocyst input (2) leg joint input and (3) a non specific input. Excitatory statocyst input has been reported previously (Fraser and Sandeman, 1975; Fraser, 1975), and inhibitory statocyst input has been demonstrated by examining the combined effect of statocyst input and other inputs. Leg joint input occurs when the coxopodite of any leg is retracted at the thoracic-coxopodite (T-C) joint. The effect of this input is to increase the frequency and duration of fibre 5 firing to a displacement of the body of the crab relative to the legs (Fig. 2). Non specific input occurs most readily in a well rested animal which is inverted. A single touch anywhere on the crab elicits a sustained high frequency train of spikes which is correlated with cyclical leg movements (Fig. 1). Cutting the optic tracts abolishes this pathway. Fibre 5 inScylla has an overt behavioural correlate in the stereotyped pattern of leg movements associated with the non specific input. This same behavioural sequence can be evoked during rotation of the crab and on low current electrical stimulation of fibre 5. Fibre 5 can thus be regarded as a command fibre for this reflex.
Published Version
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