31.10 : Male Sexual Response: Erection & Ejaculation

Sexual stimulation can take various forms, such as physical touch and visual or auditory cues. When this happens, the parasympathetic reflex in the sacral portion of the spinal cord is activated. This reflex stimulates the release of nitric oxide (NO), which then dilates the arterioles in the penis, increasing blood flow to the erectile tissues - the corpora cavernosa and corpus spongiosum.

The blood filling the erectile tissues compresses the veins, which helps to prevent blood from leaving the penis and leads to an erection. When sexual stimulation culminates at a critical threshold, a sympathetic reflex is initiated by the sympathetic fibers located in the lumbar part (typically T12-L2) of the spine.

This reflex triggers the contraction of the smooth muscle in the walls of the vas deferens, seminal vesicles, and prostate gland. These contractions propel seminal fluid and sperm into the prostatic urethra in a process known as emission. After the semen enters the penis, muscles like the bulbospongiosus and ischiocavernosus in the pelvic floor contract, forcefully expelling the semen from the urethra through a process called ejaculation.

All these physiological processes work together to facilitate the male sexual response, resulting in an erection and ejaculation.