Fiber counts and architecture of the human dorsal penile nerve

The human penis transmits behaviorally important sensory information via the dorsal penile nerve, which is required for initiation and maintenance of erection. The human penis differs from the penes of other hominids. The lack of a baculum makes the human penis dependent on erectile tissue, which is under control of neural signals activated by tactile stimulation. Accordingly, the penile sensory innervation is crucial for human sexual behavior. To clarify penile innervation, we analyzed the architecture of the dorsal penile nerve of five male subjects who donated their body. We stained the sensory fibers in the penile dorsal nerve with anti-neurofilament H antibody, and identified myelinated axons with Luxol fast blue staining. Furthermore, we visualized nerve bundles as they travel along the shaft of the penis by performing microfocus computed tomography scans after counterstaining penes with iodine. Our results show that the dorsal penile nerve is organized in 25-45 loosely packed nerve bundles, running mediodorsally in the shaft of the penis. This organization corresponds to that in penes of other mammalian species, but differs from the organization of the other peripheral sensory nerves. Around half of the dorsal penile nerve fibers were myelinated and a human hemipenis contained a total of 8290 ± 2553 (mean ± SD) axons. Thus, the number of sensory axons in the human dorsal penile nerve is higher than in other species described so far. The large fraction of unmyelinated nerve fibers suggests that the conduction speed is not a crucial aspect of penile sensory transmission.