Mechanoreceptor response to mechanical and thermal stimuli in the glans penis of the dog.

In the dog, we isolated 126 mechanoreceptive afferent fibers of the A-delta myelinated fiber class from the dorsal nerve of the penis using microdissection and extracellular electrophysiological techniques. Receptive fields on the glans penis were stimulated with a computer-controlled mechanostimulator and Peltier effect thermostimulator. The great majority of units were categorized as either rapidly adapting (RA) or slowly adapting (SA) and were located primarily proximally and distally, respectively, on the glans. In comparison to values from other glabrous skin regions in other species, mean displacement and force thresholds of penile mechanoreceptors were high, whereas the mean velocity thresholds were low. SA units, generally poor encoders of static displacement, were distinguishable into two types based on static response firing pattern but were not homologous to either the SA I or SA II mechanoreceptors found in other skin regions. Fifty-five units were given simultaneous mechanical and thermal stimulation. Very few units responded to pure thermal stimulation or increased their discharge frequency to skin cooling. Warm receptive-field temperatures between 35 and 43 degrees C increased mechanical sensitivity, measured by displacement and velocity coding functions, in almost all units tested. We conclude that canine penile mechanoreceptors, capable of encoding a variety of skin movements when the penis is warm, provide the spinal cord with the sensory input necessary to drive the spinal sexual reflexes. However, many appear to be at least physiologically different from mechanoreceptors native to other skin areas.