Some Factors in the Activation of Crab Movement Receptors

The activation of movement receptors in Pachygrapsus crassipes is examined. Lengthening and shortening of the elastic strand of the propodite-dactylopodite (PD) organ are respectively the adequate stimuli for movement receptors responding uniquely either to closing or opening. These stimuli are also effective on the isolated PD organ. Twisting of the strand is without effect. Intracellular records from the receptor cells show apparent intermittent generator potentials during effective stimulation. No electrical activity is observed at sub-threshold speeds or during movement in the inappropriate direction. The action potentials are initiated in the distal process at a considerable distance from the soma. Nicotine applied to the PD organ in high concentration elicits spike discharges of high frequency and long duration; it has no effect when applied to the axons. Applied to the PD organ in low concentration it potentiates the effect of mechanical stimulation without itself eliciting spikes. These findings are discussed in relation to the structure of the receptor endings and a mechanism is tentatively suggested to account for the unidirectional sensitivity