Electrophysiology of the afferent innervation of the penis of the domestic ram.

1. The discharge of impulses in afferent fibres dissected from the dorsal nerve of the penis of chloralose-anaesthetized rams was recorded electrophysiologically during controlled natural stimulation of the surgically exposed penis maintained at body temperature and mechanically stabilized in a plaster of Paris mould. 2. Fifty-eight slowly adapting mechanorecptor units were examined and their pressure, velocity and displacement thresholds were determined. Units often responded best to integumental stretch. Few had resting discharges. During a sustained perpendicularly applied displacement most units adapted to silence within 1.5 min. The units were classified into types from an analysis of their adapted impulse trains in response to a sustained mechanical stimulus. 3. Twenty-five mechanoreceptive units had rapidly adapting responses. Most units had typical rapid adapting characteristics and discharged impulses only during the dynamic phase of the application of the displacement. A subgroup had intermediate adapting characteristics, and discharged intermittently during steady displacement of the integument. 4. The mechanical sensitivity of most receptors altered when the temperature of the receptive field was changed with a positive correlation in eleven units, a negative correlation in six. Six slowly adapting units were thermally insensitive. Twelve rapidly adapting units were tested. Six had a positive thermal correlation and four a negative correlation. 5. The conduction velocities of axons of mechanoreceptor units in the dorsal nerve of the penis were in the Aalpha range (12--77 msec-1). 6. Two specific warm and five specific cold units were found. The conduction velocities of the axons supplying warm receptors were 45.4 msec-1 (one unit) and those for cold receptors were 7.5, 7.8, 30, 45.5, 48.7 msec-1. 7. No correlation could be found between the receptor submodality and the profuse receptor end bulb population demonstrated histologically.