Sperm Transport in the Male Tract: Erection of the Penis
The sperm are immotile when released into the lumen of the seminiferous tubules, where they float in fluid secreted by the sustentacular (Sertoli) cells of the epithelial lining.
Their passage through the rete testis into the head of the epididymis is effected by the current generated by the combination of the testicular secretory pressure and the resorption of fluid by the lining of the efferent ductules. Onward progress through the epididymis appears to depend on several factors, among which spontaneous peristalsis of the muscular epididymal duct is probably most important. Hydrostatic pressure may continue to play a part, and in many species the sperm have themselves acquired the capacity for coordinated movement by the time they reach the tail of the epididymis. The physiologic maturation of the sperm—which take some days to complete their passage through the epididymis — may be a result of aging and specific molecules in their environment. Fertilization with epididymal sperm, especially the one from the tail, has been achieved under experimental conditions. Secretory activity of the lining of the epididymal duct is maintained by androgens, and it is possible that these substances also have a direct influence on sperm. The deferent duct also exhibits peristalsis, which gradually moves the sperm toward the ampullary region. In sexually inactive animals, sperm are lost from here by seepage into the urethra and flushed away by urine. A few may be resorbed by the lining of the duct system.This regular but slow emission of sperm contrasts with the vigorous ejaculation that occurs during coitus through a penis erected by the engorgement of the cavernous and spongy spaces. The penis thus stiffened and enlarged protrudes its free extremity to enter the vagina. The details of the process, which differs significantly among species, largely depend on the structure of the penis.
The fibroelastic penis needs little additional blood to fully distend the cavernous spaces, does not increase greatly in size, and uses effacement of the preexisting sigmoid flexure for its protrusion. Moreover, because relatively little additional blood is required, full erection of the fibroelastic penis may be achieved rapidly. The cavernous spaces are much larger and more dilatable in the musculocavernous penis possessed by horses and dogs. In these species a much greater increase in both length and girth occurs. The process requires more time for its completion.Erection of penis in two distinct phases. In the first stages of sexual excitement, blood flow into the penis increases as the walls of the supplying arteries relax, with simultaneous obstruction of the venous outflow. The pressure within the cavernous spaces rises rapidly and soon equals that within the arteries that deliver blood to the corpus cavernosum via the crura and to the corpus spongiosum via the bulb. The venous outflow is restricted at the proximal extremity of the organ, where the veins are compressed against the ischial arch; this has more effect on the drainage of the crura and corpus cavernosum than on that of the corpus spongiosum, whose more distal outlet is as yet unaffected (see Fig. 15.20).
The process continues and intensifies after intromission. Rhythmic contractions of the ischiocavernosus and bulbospongiosus muscles now begin, impelling blood forward through the corpus cavernosum and corpus spongiosum. The internal pressures fluctuate in time with this activity. The additional blood pumped distally within the corpus cavernosum cannot escape because the emissary veins are compressed; the pressure therefore rises further. In contrast, the contractions of the bulbospongiosus produce only intermittent rises in pressure because some blood continues to escape at the free extremity of the penis; the effect of this flow is to massage the urethra, which supplies a further impulse to the forward movement of semen when ejaculation takes place.
In most species the pressures drop rapidly after ejaculation, first reaching that within the arteries and then dropping to the resting pressure (a mere 15 to 20 mm Hg). As the blood escapes, the penis shrinks, becomes more flaccid, and is returned to the prepuce. The return is brought about by the active involvement of the retractor penis muscles (see Fig. 29.34).
The volume and composition of the ejaculate vary with the species and also with recent sexual activity. Only a small part of the semen is provided by the sperm-rich fraction emanating from the testes and epididymides, the bulk coming from the accessory reproductive glands. Because semen volume depends on the bulk of these glands, it could be anticipated that the ejaculate would be greatest in the boar. The various contributions to the semen are very imperfectly mixed when expelled into the urethra. The semen is moved through the urethra by the activity of striated muscles (urethralis, bulbospongiosus), and its ejaculation into the vagina or cervix (according to the species) is therefore forceful.
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