Phasic and Static Excitability of Touch Receptors in Toad Skin

The excitability of the toad's touch receptors was studied during slowly increasing and static deformation. Graded mechanical stimuli of a linearly rising and a platcau phase were applied to single receptors in situ together with bricf suprrimposrd test pulses. Action potentials were recorded from the afferent fiber in the dorsal root. In the moving stimulus phase, the excitability rose rapidly and then displayed either a peak or a smooth flattening (Fig. 4). As the plateau phase of the stimulation set in, the excitability suddenly fell but a residue of raised excitability persisted statically (Fig. 5, 6).The less rapidly adapting receptors even presented a discharge in the plateau phase. The impulse frequency of this discharge rose linearly with the deformation amplitude (Fig. 9) but the maximum frequency was low as compared with that in the dynamic discharge. While the static excitability change and the plateau discharge were related to the amplitude of deformation, the dynamic excitability change, like the dynamic discharge (L indblom 19621, depended on the rate of deformation. An hypothesis that there may be two mechanisms — one phasic and one static —in one and the same receptor is discussed. the course of the dynamic excitability explains the latency variations of the dischargr following stimulation at varying deformation rates, and may also account for the regular firing at constant deformation rate (L indelom 1962). The characteristic rapid adaption is best explained as due to lack of excitation during static stimulation.