Page 479 - Anatomy and Physiology of Farm Animals, 8th Edition

464 / Anatomy and Physiology of Farm Animals

stimulation results in the parasympathetic the sigmoid flexure and functional length-
ening of the penis.
nerves releasing acetylcholine, which in
VetBooks.ir turn stimulates endothelial cells lining the moved from storage sites in the epididy-
Prior to ejaculation, spermatozoa are
penile blood vessels to produce nitric oxide
(NO) from oxygen and L‐arginine. Nitric mides through the ductus deferentia to the
oxide penetrates the cytoplasm of smooth pelvic urethra. This movement is termed
muscle cells and binds to guanylyl cyclase, emission and is assumed to result from
resulting in the production of 3′‐5′‐cyclic contractions of smooth muscle in the wall
guanosine monophosphate (cGMP). Cyclic of these tubular structures. Within the
GMP activates a protein kinase, reduces pelvic urethra, secretions of the accessory
intracellular calcium levels, and causes sex glands are mixed with the spermato-
relaxation of smooth muscle. In the mus- zoa. Ejaculation of the mixture (semen)
culocavernous penis, vascular smooth through the penile urethra is associated
muscle relaxes, dilating penile arteries, with more contractions of the epididy-
resulting in venoconstriction, filling of mides and ductus deferentia and addi-
sinusoids with blood, and the increase in tional contractions by muscles of the penis
diameter and rigidity expected with penile surrounding the penile urethra. Emission
erection. In the fibroelastic penis, the and ejaculation are autonomic reflexes
smooth muscle that relaxes is the retractor involving both the sympathetic and para-
penis muscle, resulting in the relaxation of sympathetic divisions.