Home » Erectile Dysfunction, Health Care » The Physiology of Ejaculation
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Emission Phase

The anatomical structures involved in emission include the epididymis, vas deferens, seminal vesicles, prostate gland, prostatic portion of the urethra, and bladder neck. These structures have both sympathetic and parasympathetic innervation with nerve fibers that arise predominately from the pelvic plexus. These nerve fibers are located in the retroperitoneum, traveling alongside the rectum and also lying posterolateral to the seminal vesicles. Pelvic plexus nerve fibers come superiorly from the hypogastric and pelvic nerves, and inferiorly from the caudal paravertebral sympathetic chain.

Emission is initiated when afferent stimulatory input, primarily arising from sensory fibers within the glans penis, is integrated at the level of the spinal cord. Sympathetic nerves (T10-L2) mediate the release of several neurotransmitters, including norepinephrine, causing epithelial cell secretion and smooth muscle cell contraction throughout the excurrent ductal system. As a result, accessory gland secretions  are admixed with spermatozoa and ejected into the posterior urethra.

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Expulsion Phase

The anatomical structures involved in seminal expulsion include the bladder neck, urethra, and striated pelvic muscles. Expulsion is a spinal cord reflex triggered once inevitability, or “the point of no return” is reached during sexual activity. During expulsion, the bladder neck smooth muscle fibers, under sympathetic fiber stimulation, forcibly contact to prevent retro-grade ejaculation. Next, the striated pelvic floor muscles, in particular the ischiocavernosus and bulbocavernosus muscles, contract in an intermittent, rhythmic fashion, and the external ure-thral sphincter relaxes.

While these muscles are innervated solely by the somatic nervous system (S2–4), the expulsion phase of ejaculation Viagra Australia does not appear to have any component of volitional control. In the setting of tight bladder neck con-traction, the series of striated pelvic muscular contractions leads to antegrade propulsion of semen through the prostatic, bulbar, and penile urethra and out the urethral meatus. To date, the specific trigger for the expulsion phase has not been clearly elucidated. Early work in a rat model suggested that the presence of semen in the bulbous urethra is the predominant factor that triggers seminal expulsion. Subsequent manuscripts describe the presence of a spinal ejaculatory generator that leads to the expulsion of seminal fluid once a critical level of spinal activation has been achieved.

The spinal ejaculatory center is believed to integrate stimuli from peripheral and central sites, with efferent output through both parasympathetic and somatic pathways. In 2002, Truitt and Coolen reported that neurons having a role in generating ejaculation are located within lamina X and the medial portion of lamina VII of lumbar segments 3 and 4. These neurons receive descending input from the nucleus paragigantocellularis, the medial preoptic area, and the paraventricular nucleus of the hypothalamus, each providing supraspinal modulatory effects on the spinal ejaculatory generator. While descending cortical input may influence ejaculation, it is not essential for ejaculation to occur.

Men with complete spinal cord transection superior to the tenth thoracic segmental level (superior to the location of the spinal ejaculatory generator) exemplify this point; in these men, the ejaculatory reflex is typically still feasible. Penile vibratory stimulation is routinely used in such patients to induce the ejaculatory response for reproductive purposes, in order to collect sperm for assisted reproductive techniques, such as intrauterine insemination or in vitro fertilization. The intact function of the spinal ejaculatory generator neurons is essential for normal ejaculatory function, as their ablation leads to the complete loss of ejaculatory function.