Erection and Ejaculation
Penile erection is a neural reflex initiated by appropriate tactile stimulation of the penis, visual or environmental stimuli (such as a female in estrus), or as a result of learned behavior.
For example, an erection may begin when a breeding stallion is being led to a breeding area but before a mare is present. This is believed to be a learned response. similarly, it is believe that learned responses may be responsible for lack of libido in certain males that have undergone trauma (e.g., a stallion kicked by a mare) or that associate mating with some other painful or unpleasant event.Penile erection requires vasodilation within the penis, which results in an increased blood flow into the penis. Parasympathetic nerves innervating blood vessels within the penis initiate the vasodilation. studies in experimental animals suggest that the parasympathetic nerves release acetylcholine, which in turn stimulates endothelial cells lining the blood vessels to release nitric oxide. Nitric oxide acts directly on vascular smooth muscle to bring about vasodilation.
Prior to ejaculation, spermatozoa are moved from storage sites in the epididymides through the ductus deferentia to the pelvic urethra. This movement is termed emission and is assumed to result from contractions of smooth muscle in the wall of these tubular structures. Within the pelvic urethra, secretions of the accessory sex glands are mixed with the spermatozoa. Ejaculation of the mixture (semen) through the penile urethra is associated with more contractions of the epididymides and ductus deferentia and additional contractions by muscles of the penis surrounding the penile urethra. Emission and ejaculation are autonomic reflexes involving both the sympathetic and parasympathetic divisions.