Discovering Human Sexuality 4e Appendix B Summary

Sexual functions are regulated by the nervous and endocrine systems. In the absence of sexual stimulation, erection of the penis or clitoris is prevented by ongoing activity in the adrenergic sympathetic axons that innervate the genitals. This activity causes constriction of the arterioles supplying the erectile tissue. Erection involves a spinal reflex that begins with stimulation of nerve endings in the genital skin, followed by processing in the spinal cord and outgoing activity in the cholinergic sympathetic axons that innervate the genitals. This activity dilates the arterioles, so more blood enters the sinusoids within the erectile tissue.
In males, seminal emission (loading of the urethra with semen) involves the coordinated action of sympathetic inputs to the prostate gland, seminal vesicles, and vasa deferentia. Ejaculation is brought about by activity in spinal motor neurons that inner-vate the muscles of the pelvic floor, causing them to contract in a pulsatile fashion and eject semen from the urethral opening.
The brain influences these processes in various ways. The hypothalamus influences genital reflexes, so they are modulated by circumstances and erotic images and thoughts. Activity in the locus coeruleus increases the activity of sympathetic neurons in the spinal cord: A decrease in activity in the locus coeruleus during REM sleep leads to a decrease in adrenergic sympathetic input to the genitals and thus to a nocturnal erection. Regions called the pelvic organ stimulating center (POSC) and the pelvic floor stimulating center (PFSC) trigger the pulsatile muscle contractions associated with male ejaculation and female orgasm.