On- versus off-responses of raccoon glabrous skin rapidly adapting cutaneous mechanoreceptors.

1. The on- and off-responses of 50 raccoon median nerve fibers associated with rapidly adapting cutaneous mechanoreceptors in glabrous skin were examined under experimental conditions designed to allow comparable opportunities for on- and off-responses to occur. Trapezoidal stimuli were utilized, providing for equal stimulus indentation and retraction velocities and equal static displacement times and intertrial intervals. Principal findings were as follows: 2. At stimulus levels well above displacement and velocity thresholds for on-responses, 80% of units yielded a more vigorous on-response than off-response (as measured by the total number of ramp impulses); in 6%, the reverse was true; while in the remaining 14%, the off-discharge was absent. 3. On and off displacement thresholds were approximately equal (on median, 43 micron; off median, 42 micron). However, on velocity thresholds were significantly lower than off velocity thresholds (on median, 1.0 micron/ms; off median, 3.8 micron/ms). 4. Exponents (b) of power functions relating discharge rate to ramp velocity (frequency = a x velocityb) were consistently greater for on-responses than for off-responses, but intercept constants (a) were consistently greater for off-responses than for on-responses. 5. Previous findings that mammalian rapidly adapting (RA) mechanoreceptors possess a "linear directionality" generally favoring on-responses were confirmed. 6. Results are discussed in relation to the role of viscoelastic properties of RA mechanoreceptors and neighboring skin. It is suggested that, when considerations is also given to the mechanical properties of surrounding tissues, the Loewenstein and Skalak (18) analysis of the mode of operation of Pacinian corpuscles might also apply, at least qualitatively, to the simple dermal (rapidly adapting) corpuscle of raccoon glabrous skin.