ERECTION EJACULATION

erection and ejaculation
Innervation
Autonomic: sympathetic nerves originating from T11-L2, and parasympathetic nerves originating from S2-4, join to form the pelvic plexus. The cavernosal nerves are branches of pelvic plexus (i.e. parasympathetic) that innervate the penis. Parasympathetic stimulation causes erection; sympathetic activity causes ejaculation and detumescence (loss of erection).
Somatic: somatosensory (afferent) information travels via the dorsal penile and pudendal nerves, and enters the spinal cord at S2-4. Onuf s nucleus (segments S2-4) is the somatic centre for efferent (i.e. somatomotor) innervation of the ischiocavernosus and bulbocavernosus muscles of the penis.
Central: medial preoptic area (MPOA) and paraventricular nucleus (PVN) in the hypothalamus are important centres for sexual function and penile erection.

Mechanism of erection
Neuroendocrine signals from the brain, created by audiovisual or tactile stimuli, activate the autonomic nuclei of the spinal erection centre (T11-L2 and S2-4). Signals are relayed via the cavernosal nerve to the erectile tissue of the copora cavernosa, activating the veno-occlusive mechanism. This triggers increased arterial blood flow into sinusoidal spaces (secondary to arterial and arteriolar dilatation), relaxation of cavernosal smooth muscle, and opening of the vascular space. The result is expansion of the sinusoidal spaces against the tunica albuginea, which compresses the subtunical venous plexuses, decreasing venous outflow. Maximal stretching of the tunica albuginea acts to compress the emissary veins which lie within its inner circular and outer longitudinal layers, reducing venous flow even further. Rising intracavernosal pressure and contraction of the ischiocavernosus muscles produces a rigid erection. Following orgasm and ejaculation, vasoconstriction (due to increased sympathetic activity, endothelin, PGF2, and breakdown of cGMP) produces detumescence.

Ejaculation
Tactile stimulation of the glans penis causes sensory information to travel (via the pudendal nerve) to the lumbar spinal sympathetic nuclei. Sympathetic efferent signals (travelling in the hypogastric nerve) cause contraction of smooth muscle of the epididymis, vas deferens, and secretory glands, propelling spermatozoa and glandular secretions into the prostatic urethra. There is simultaneous closure of the internal urethral sphincter and relaxation of the extrinsic sphincter, directing sperm into the bulbourethra (emission), but preventing sperm entering the bladder. Rhythmic contraction of the bulbocavernosus muscle (somatomotor innervation) leads to the pulsatile emission of the ejaculate from the urethra. During ejaculation, the alkaline prostatic secretion is discharged first, followed by spermatozoa and, finally, seminal vesicle secretions (ejaculate volume 2-5ml).