Reflexic (Spinal) Erection
Reflexic erections are mediated by a spinal reflex pathway whereby sensory informa-tion from the penis and genitalia is transmitted by the dorsal nerve of the penis and contin-ues via the pudendal nerve to reach the sacral spinal cord. This constitutes the afferent limb of the sacral reflex arc. The efferent limb arises in the sacral parasympathetic center and contributes fibers to the pelvic nerve, which, in turn, enters the erectile tissue as the cavernosal nerve. These terminal parasympathetic fibers release ACh, VIP, and NO as well as additional vasorelaxant neuropeptides (substance P and calcitonin gene-related peptide).
Pudendal afferent pathways terminate in the dorsal commissure and medial dorsal horn. In addition to activating the sacral preganglionic neurons that initiate erection, interneu-rons in these regions presumably are involved in transmitting sensations to the brain and processing supraspinal inputs. The parasympathetic preganglionic neurons are located in the intermediolateral cell column and send dendritic projections to the same regions that receive penile afferent input.
Reflexic erections can be observed in men with com-plete spinal cord lesions above the sacral segments. In such men, there is obviously no sensation; therefore, erection depends on an intact sacral cord mechanism in isolation from the rest of the central nervous system (CNS). Afferent impulses traveling via the dorsal nerve of the penis and pudendal nerve activate spinal interneurons, which then activate preganglionic neurons within the sacral parasympathetic center.