Response properties of mechanosensitive nerve fibers innervating cephalic skin of the Texas rat snake

The response properties of 146 single maxillary nerve fibers innervating the cephalic integument of 11 Texas rat snakes were studied. Twenty-one of these fibers (14%) were exclusively activated by mechanical stimulation of specialized cutaneous corpuscles. The remaining 125 fibers (86%) were excited by mechanical deformation of skin surrounding, but not including, the specialized corpuscles; these “noncorpuscle units” were placed into three classes on the basis of adaptation rate. (i) Sixty-eight percent of the fibers were rapidly adapting units, discharging only during the moving phase of skin deformation. All rapidly adapting units gave “on” responses, and 46% also gave “off” responses; 72% of these units were able to faithfully discharge one impulse during each cycle of a 256-Hz vibratory stimulus. The rapidly adapting units had the highest stimulus thresholds ( X = 590.6 mg ) and the smallest receptive fields ( X = 12.0 mm 2 ). (ii) Twenty-six percent of the fibers were slowly adapting units, discharging during both the dynamic and static phases of skin deflection. All slowly adapting units gave “on” responses, but only 22% gave “off” responses; 31% of the units were able to respond faithfully during each cycle of vibratory stimulation at a frequency of 256 Hz. The slowly adapting units had the lowest stimulus thresholds ( X = 453.0 mg ) and the largest receptive fields ( X = 18.3 mm 2 ). (iii) Six percent of the fibers had adaptation rates and other response properties intermediate between those of the other two fiber types, and were classified as intermediately adapting units. The receptive fields of most noncorpuscle fibers within each class were relatively small, and were confined to one or two supralabial scales.