The Discharge in Single Touch Receptors Elicited by Defined Mechanical Stimuli

To understand how a touch stimulus is coded in nerve impulses it is fundamental to know the significance of its wave form. This has been studied in the toad by means of a special technique for stimulation of individual touch receptors in situ with simultaneous recording of the skin displacement. Stimuli of various gradients, amplitudes and durations were applied to the skin surface over the receptor, and the impulse discharge in the afferent fibre was recorded from thin filaments of the dorsal root. The threshold amplitude was remarkably independent of the stimulus gradient. When the latter was increased gradually from the minimum effective value (critical slope) and upwards, the threshold fell sharply at first but then remained at the same level (Fig. 3 and 4). Prolongation of a threshold stimulus by adding a plateau phase to the initial moving phase did not result in a repetition of the response and did not make it possible to reduce the amplitude (Fig. 1). The repetitive response usually obtained on suprathreshold stimulation was either limited to the moving phase or had the highest impulse frequency during this phase (Fig. 2). Since the strongest excitation occurred during the moving phase of the stimuli, it was concluded that the deformation is the adequate stimulus. The deformed state is normally followed by no response at all or, in some receptors, by a rapidly adapting discharge.