Human nerve potentials evoked by tactile stimuli. II. Stimulus parameters and recruitment of components

The effect of different parameters of the tactile stimulus on the compound potential of the human sural nerve was investigated. The time-integral of the maximal response evoked by a ramp indentation (120 micrometers/ms) was 30-50% greater than that evoked by the reversal of the indentation. The time-integrals and the shortest latencies of responses recorded during local anesthesia, confined to the skin, did not vary significantly for contact areas of the tactile probe ranging from 0.2 to 9.75 mm2, compatible with the wide spacing and the large receptive fields of Pacinian corpuscles, that are the main source of the sensory potentials. The velocity and particularly the acceleration and the deceleration of the ramp indentation were more critical parameters of the tactile stimulus than the depth of the indentation. The conduction velocities of the component potentials ranged from 30 to 65 m/s. Half of the later components were conducted as fast as or faster than the initial component, indicating that they were activated after a delay. An essential cause of the delay is activation by the mechanical wave evoked by an indentation and conducted at a low velocity (10-2 m/s) along the surface of the skin. Accelerations and decelerations in the mechanical waves may activate receptors at some distance from the tactile probe.