Penile erections are elicited by local sensory stimulation ofthe frenulum, glans penis, and perigenital skin (reflexic erections) as well as by central stimuli received by, or generated within, the brain (psychogenic erections). Both types of erectile mechanisms likely act in a synergistic manner and are controlled by the autonomic nervous system (see Fig. 6; Table 7).
Reflexic (Spinal) Erection
Reflexic erections are mediated by a spinal reflex pathway whereby sensory information from the penis and genitalia is transmitted by the dorsal nerve of the penis and continues via the pudendal nerve to reach the sacral spinal cord. This constitutes the afferent limb ofthe 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).
Table 7 Penile Erectile Reflexes
Tactile stimuli Dorsal nerve Friction of genitalia of the penis,
Spinal (spinal reflex pathway is also modulated by brain, e.g., tactile stimuli ♦ spinothalamic tract ♦ thalamic VLN + ILN ♦ somatic sensory ♦ postcentral gyrus
Pelvic and cavernous N.
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