Excitability and functional organization of cutaneous tactile units of the bullfrog (R. catesbeiana).

Frog skin touch receptors are discrete structures which appear as dome-shaped translucent elevations of the epidermis. These domes are particularly responsive to direct touch. Tactile stimulation of adjacent skin never caused impulse activity, except when the skin distortion disturbed the domes. On stimulation at threshold intensity, the response of all units studied always consisted of a single impulse whether the stimulus energy was electrical or mechanical. While most tactile units discharged one impulse upon application of mechanical stimulation, a few also discharged one impulse upon stimulus release (on-off response). No after discharge was seen at stimulus strengths of three or four times threshold or after repetitive stimulation at 1,000 pps. The range of distribution of active domes/receptive field was 1--14. Generally the most sensitive domes appeared to be in or near center field. Twice as many of the tactile units responded more readily to cold than to warmth. The impulse frequency of units responding to thermal stimulation ranged from 3-11/sec. The conduction velocity of tactile units measured was within a range of 9--25 m/sec, with a mean of 12 m/sec. The results suggest that the following: 1) the existence of discrete structures which respond to tactile stimuli; a phylogenetic relationship might be postulated; 2) cold/touch receptors which appear to be present in greater quantity in the frog's skin than are warm/touch receptors, 3) touch units which should be considered primarily as mechanoreceptors with a possible secondary function as thermoreceptors.