Components of the tract and their innervation

A diagram of the human male genital tract is given in Fig. 10.19.

Testes and epididymides

These are the organs of spermatogenesis. They are located outside the abdominal cavity in the scrotum, a thin-skinned pouch with a layer of smooth muscle (the dartos muscle), which on contraction in the cold or during exercise allows the testes to be pulled nearer to the body for protection or increased warmth, and when relaxed allows elongation and thus lowering of the testes away from the abdomen for cooling. Correct tempera- chapter 10 ture is necessary for normal development of the sperm. Renal, Urinary

The testes are ovoid structures covered by a thick white capsule, the and Reproductive tunica albuginea. Inside, each testis is divided into about 250 compartments Tract Systems or lobes, each containing one to three highly convoluted seminiferous tubules. These tubules join a series of some 30 short collecting tubules, the straight seminiferous tubules, which converge into the rete testis, a network of spaces on the medial aspect of the testis under the epididymis. From the rete, efferent ductules enter the head of the epididymis, which is a tortuous canal in which the coils form a compact body extending from the cranial to the caudal pole of the testis.

There is smooth muscle both in the capsule of the testis and in the walls of the epididymis. In both these organs, the smooth muscles display auto-maticity, i.e. they appear spontaneously active. This activity in the testis may be important in transporting spermatozoa into the epididymis. Both organs are innervated by sympathetic and parasympathetic nerves, entering mainly with the testicular arteries. The main targets of these adrenergic nerves seem to be the blood vessels, although in the more distal part of the ducts some control of the smooth muscle contractile activity may be exerted by the innervation. Both cholinergic and adrenergic nerves are excitatory.

Vasa deferentia and ejaculation

Each vas deferens is a continuation of the duct of the epididymis. Initially, the vas deferens is tortuous, but it becomes straighter as it passes back along the testis and up through the spermatic cord and the inguinal canal into the pelvic cavity. Here it runs between the peritoneum and the lateral wall of the pelvis, from whence it runs backwards, downwards and medially, crossing the ureter to run behind the bladder, approaching its pair between the seminal vesicles at the base of the bladder. Here the tube swells into the wider ampulla of the vas deferens. On each side, the vas deferens then begins to penetrate into the substance of the prostate, and joins the duct from the seminal vesicle to form an ejaculatory duct. These ducts run through the prostate and open upwards into the prostatic urethra on the top of the urethral crest, close to the openings of the prostatic utricle.

The vas deferens is a thick-walled tube about 4-5 cm long. The wall contains three layers of smooth muscle cells in the human, a small inner and variable outer longitudinal layer and a middle circular layer. The lumen of the tube is very small and is lined by a mucous membrane made up of a lamina propria of connective tissue with elastic fibres and a lining epithelium.

More detail is known about the innervation and excitation of the smooth muscle of vas deferens than for any other smooth muscle-containing organ. This is probably because it is easily removed from the body with its main innervation intact, and since it lacks spontaneous electrical activity, it is chapter 10

Renal, Urinary and Reproductive Tract Systems

Seminal vesicles

These paired organs do not in fact store sperm (they are stored in the epididymis and vas deferens), but with the secretions of the prostate they produce the bulk of the seminal fluid. The seminal vesicles secrete a slightly alkaline fluid containing fructose, which is an energy source for the sperm, and a coagulating enzyme. The vesicles are in fact sacculated tubes, lined with secretory epithelium containing goblet cells. Their walls contain a longitudinal and a circular layer of smooth muscle, innervated predominantly by the sympathetic nervous system, which when activated propels the contents of the glands into the ejaculatory ducts.

Prostate

This is a partly glandular and partly fibromuscular organ situated at the base of the bladder, and surrounding the prostatic urethra. It secretes an important component of the seminal fluid. The secretion is slightly acidic and contains many important constituents, including acid phosphatase, fib-rinolysin and prostaglandins. The stroma of the gland is composed of a meshwork of smooth muscle, in which the glandular tissue is embedded.

relatively easy to study with electrophysiological techniques. Some further information about it is given in Chapter 4. The main excitatory innervation is via the sympathetic nervous system, although there is a sparse cholinergic innervation, more clearly seen in the circular smooth muscle layer. The smooth muscle is very densely innervated throughout its thickness by classical varicose postganglionic sympathetic nerve terminals. Many of the ganglion cell bodies lie in the hypogastric nerve close to the vas deferens. The walls of the tube are normally relaxed, and the tube is usually filled with sperm. Contraction of the smooth muscle is brought about through activation of the nerves in the ejaculatory reflex, in which a nearly simultaneous activation of the smooth muscle constricts the lumen and supposedly sweeps the spermatozoa into the urethra. In animals, the sympathetic innervation provides a dual excitatory input to the smooth muscles, the transmitters being ATP and noradrenaline. ATP, acting through P2x purinoceptors, is responsible for excitatory junction potentials, which can summate to initiate action potentials that are transmitted from cell to cell in the longitudinal direction through gap junctions between the smooth muscle cells. Noradrenaline, on the other hand, results in elevation of intracellular free calcium concentration by activating a-adrenoceptors, with the probable involvement of the phosphatidylinositol pathway, but with little effect on the membrane potential. The relative importance of noradrenaline and ATP as excitatory transmitters is very species-dependent, and the purinergic component is almost absent in man. It is thought that the cholinergic nerves may also play a role in modulating the activity of the sympathetic nerves.

The glandular tissue is composed of follicles that open into elongated chapter 10 canals, which join to form the thirty or so collecting ducts on each side. The Renal, Urinary collecting ducts drain the prostatic secretions into the urethra via the prosta- and Reproductive tic sinuses, which lie on either side of the urethral crest. The prostate is Tract Systems innervated by both the sympathetic and parasympathetic nervous systems, but the main activation of the smooth muscle in the ejaculatory reflex is brought about through the sympathetic innervation and a-adrenoceptor activation.

Bulbo-urethral glands

These glands lie, one on each side, between the lower part of the prostate and the bulb of the penis. Their ducts enter the bulb of the penis and run down either side of the urethra, opening in the posterior floor of the spongy urethra. They are mucus-secreting glands and contribute a small amount to the seminal fluid. The arrangement of all of these accessory glands is shown in Fig. 10.19.

Penis and erection

The penis contains three longitudinally arranged erectile bodies that run from the base to the tip of the organ (Fig. 10.20). They are the paired corpora cavernosa on the dorsal aspect of the penis, and the ventral corpus spongiosum. The corpora cavernosa begin as the crus penis, attached to the bony pelvis where the pubis and ischium join, and extend the length of the shaft of the penis. The corpus spongiosum starts as the bulb of the penis, and contains the urethra. Distally, it expands into the glans penis, which carries the exit of the urethra (external urethral meatus). These three erectile bodies are covered by a tough fibrous envelope, the tunica albuginea. Outside this is the superficial fascia and the thin, loosely fitting skin. The erectile tissue itself consists of numerous trabeculae arising from the inner surface of the tunica, and dividing the bodies into cavernous spaces, giving them a sponge-like form. The trabeculae contain fibrous tissue, elastin and smooth muscle cells, and in them run nerves and blood vessels. The deep arteries of the penis and the urethral arteries supply the erectile tissue either directly or through the convoluted helicine arteries, and end in a capillary network that opens into the cavernous spaces. These spaces are lined throughout by endothelium, and drain into a series of emissary veins that leave the erectile tissue through the tunica and drain either into the circumflex veins or directly into the deep dorsal vein of the penis.

Erection is brought about by relaxation of the smooth muscle in the trabeculae, allowing filling of the cavernous spaces with blood, combined with relaxation of the smooth muscle of the incoming arteries, allowing the intra-cavernosal pressure to rise to values close to the mean arterial pressure, conferring considerable rigidity to the erect penis. This high pressure requires

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Renal, Urinary and Reproductive Tract Systems

Structure Corpora Cavernosa
Fig. 10.21 Wedge segments of the penis to show relationship between the cavernosal arteries, sinuses and circumflex veins in the relaxed (a) and erect (b) penis. The distance from the centre to the outside of the erect penis in (b) would be much greater than actually shown.

both relaxation of the arteries and an increase in resistance of the venous vessels draining the spaces. The latter is brought about by compression of the emissary veins due to the expanding cavernosal spaces pressing on them as they pass through the fibrous tunica albuginea (Fig. 10.21). Detumescence requires constriction of the smooth muscle in the trabeculae, and their continued constriction is necessary to maintain a flaccid penis. The changes in tone in the various smooth muscles leading to erection

Arteries (constricted)

Dorsal

Veins

Veins

(compressed)

Arteries (dilated)

Arteries (constricted)

Veins

Veins

(compressed)

Arteries (dilated)

Cavernosal Smooth Muscle The Penis
Fig. 10.20 Diagrammatic cross-section of the penis. (Left) flaccid, (right) erect.

Fig. 10.22 Diagram of the innervation of the penis. The sympathetic outflow arises from segments T9 to L2 of the spinal cord (SC) and runs to the prevertebral ganglia (coeliac plexus CP and superior hypogastric plexus SHP) via the splanchnic nerves, and from thence via the hypogastric nerves to the pelvic plexus (PP). Parasympathetic input comes from segments S2 to S4 of the sacral cord, and travels to the pelvic plexus via the pelvic nerves (PEL). The autonomic nerves then travel to the penis in the cavernous nerves (CN). The sensory output from the penis travels in the perineal nerves (PN) and the dorsal nerve of the penis (DNP) to the pudendal nerve (PUD), which enters the cord through the S2-S4 roots. (SN) sciatic nerve.

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Renal, Urinary and Reproductive Tract Systems chapter 10

Renal, Urinary and Reproductive Tract Systems and detumescence are entirely neural in origin. The penis is innervated by parasympathetic, sympathetic and somatic nerves. Both sympathetic (from T9 to L2) and parasympathetic (S2-S4) inputs converge at the level of the pelvic plexus (Fig. 10.22), and the innervation to the erectile tissues runs into the penis in the cavernous nerves, which contain preganglionic and postganglionic autonomic axons, sensory nerves and also the cell bodies of many postganglionic neurones. Several different potential transmitters and modulatory peptides have been shown in nerves in the penis, and clearly the processes of erection and detumescence are complex. Erection probably involves reduction in the effectiveness of the constrictor innervation to the arteries and trabecular smooth muscle, as well as activation of inhibitory nerves. Experiments on isolated segments of the corpus caver-nosum demonstrate the presence of an excitatory adrenergic innervation and an NANC inhibitory innervation. Tone in the arteries, which controls the amount of blood flowing into the cavernous bodies in the flaccid penis, is clearly due to ongoing sympathetic activity, working through noradrena-

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Renal, Urinary and Reproductive Tract Systems line release and a-adrenoceptors. Vasodilation and an increase in tumescence can be brought about by a-receptor blockers, although they will not reliably produce full erection. Erections can, however, be abruptly terminated by injection of a-agonists into the cavernosal bodies. Stimulation of the parasympathetic sacral outflow can induce full erections, suggesting that it is this system that is responsible for relaxation of the trabecular smooth muscles, as well as relaxation of the arterial muscles. Currently, the evidence strongly suggests that the NANC inhibitory transmitter to the trabecular smooth muscle is NO, although VIP is also present with inhibitory effects, and may play some role. The nerve-mediated relaxations are inhibited by drugs that block nitric oxide synthase and by substances that can scavenge NO from the extracellular fluid. Postganglionic cholinergic nerves are present in the penis, both innervating the arteries and the trabeculae, but they are relatively sparse, and far less common than adrenergic fibres. Although acetylcholine can cause some dilation of the blood vessels and relaxation of the trabecular smooth muscle, it appears to be acting by the release of endothelium-derived relaxing factor (EDRF, nitric oxide) from the endothelium. Acetylcholine released from nerves probably plays a role in modulating transmitter release - possibly facilitating the NANC transmitter release and inhibiting noradrenaline release from adrenergic nerve fibres.

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