Learning Objective

Understand the physiological mechanisms of male sexual function, including erection, emission, and ejaculation, and the neural and molecular pathways involved.

Erection

Erection is initiated by parasympathetic stimulation, causing vasodilation of the erectile tissue of the penis (corpora cavernosa and ischiocavernosus sinuses). Increased blood inflow compresses the penile veins against the Buck’s and Dartos fasciae, maintaining rigidity.

This vasodilation is mediated by nitric oxide (NO) acting through cGMP.

Emission

Emission refers to the movement of semen from the epididymis, vas deferens, seminal vesicles, and prostate into the ejaculatory ducts. This process is controlled by sympathetic (thoracolumbar) adrenergic nerves.

• Simultaneously, there is contraction of the internal sphincter of the bladder to prevent retrograde ejaculation. Surgical damage (e.g., prostatectomy) can disrupt this, causing retrograde flow.
• Emission usually precedes ejaculation but continues during it.

Ejaculation

Ejaculation occurs via rhythmic contractions of the bulbospongiosus and ischiocavernosus muscles, which are striated muscles innervated by somatic motor nerves.

These contractions expel semen through the urethra along the path of least resistance.

Activity