Erection, accompanied by increases in the length and width of the penis, is achieved as a result of blood flow into the "erectile tissues" of the penis. These erectile tissues include two paired structures—the corpora cavernosa—located on the dorsal side of the penis, and one unpaired corpus spongiosum on the ventral
V v T * Symphysis pubis-
Ductus (vas) deferens
Ampulla of ductus deferens
Seminal vesicle Ejaculatory duct Prostate
Bulbourethral gland Anus
Ductus (vas) deferens
■ Figure 20.21 The organs of the male reproductive system. The male organs are seen here in a sagittal view.
side (fig. 20.22). The urethra runs through the center of the corpus spongiosum. The erectile tissue forms columns that extend the length of the penis, although the corpora cavernosa do not extend all the way to the tip.
Erection is achieved by parasympathetic nerve-induced vasodilation of arterioles that allows blood to flow into the corpora cavernosa of the penis. The neurotransmitter that mediates this increased blood flow is now believed to be nitric oxide (fig. 20.23). Nitric oxide released by parasympathetic axons and produced by the endothelial cells of penile blood vessels activates guanylate cyclase in the vascular smooth muscle cells. Guanylate cyclase catalyzes the production of cyclic GMP (cGMP), which closes Ca2+ channels in the plasma membrane (fig. 20.23). This decreases the cytoplasmic Ca2+ concentration, causing smooth muscle relaxation (chapter 12). The penile blood vessels thereby dilate to increase the blood flow into the erectile tissue, producing an erection.
As the erectile tissues become engorged with blood and the penis becomes turgid, venous outflow of blood is partially occluded, thus aiding erection. The term emission refers to the movement of semen into the urethra, and ejaculation refers to the forcible expulsion of semen from the urethra out of the penis. Emission and ejaculation are stimulated by sympathetic nerves, which cause peristaltic contractions of the tubular system, contractions of the seminal vesicles and prostate, and contractions of muscles at the base of the penis. Sexual function in the male thus requires the synergistic action (rather than antagonistic action) of the parasympathetic and sympathetic systems.
Erection is controlled by two portions of the central nervous system—the hypothalamus in the brain and the sacral portion of the spinal cord. Conscious sexual thoughts originating in the cerebral cortex act via the hypothalamus to control the sacral region, which in turn increases parasympathetic nerve activity to promote vasodilation and erection of the penis. Conscious thought is not required for erection, however, because sensory stimulation of the penis can more directly activate the sacral region of the spinal cord and cause an erection.
Nitric oxide, released in the penis in response to parasympathetic nerve stimulation, diffuses into the smooth muscle cells of blood vessels and stimulates the production of cyclic guanosine monophosphate (cGMP). The cGMP, in turn, causes the vascular smooth muscle to relax, so that blood can flow into the corpora cavernosa (fig. 20.23). This physiology is exploited by sildenafil (trade named Viagra), which can be taken as a pill to treat erectile dysfunction. Sildenafil blocks cGMP phosphodiesterase, an enzyme that functions to break down cGMP. This increases the concentration of cGMP and thus promotes vasodilation, leading to increased engorgement of the erectile spongy tissue with blood and consequently promoting erection (fig. 20.23).
Cremasteric fascia Epididymis Scrotum
Suspensory ligament of penis
Body of penis
Dorsal vein of penis Dorsal artery of penis
Corona glandis Glans penis Prepuce
Skin Areolar tissue
Dorsal vein of penis Deep dorsal vein of penis Dorsal nerve of penis Dorsal artery of penis Deep artery of penis Corpora cavernosa penis
Corpus spongiosum penis
■ Figure 20.22 The structure of the penis. The attachment, blood and nerve supply, and arrangement of the erectile tissue are shown in both longitudinal and cross section.
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