Erections

In both males and females, erections occur as a result of parasympathetic nervous system action causing an increased blood flow to erectile tissue. Let me describe this in a little more detail for you...

In males, the PNS causes vasodilation of the arteries into the penis. As these arteries dilate, since they lie adjacent to veins, they end up compressing the veins in the penis. The vein compression further increases blood pressure within the penis, causing the tissues of the penis to accumulate blood and swell further. As blood accumulates, the pressure within the penis increases and it elongates and swells. This is the male erection.

In females, the PNS causes vasodilation of the arteries into the female erectile tissues. These tissues are found in the clitoris and the region around the vaginal entrance. Again, because of vasodilation, the blood supply to these tissues increase and they swell. This is the female erection.

The one confusing point of all of this may be the first one: how can the PNS cause vasodilation? Remember, when you learned about the ANS, you learned that blood vessels vasoconstrict due to sympathetic stimulation, and vasodilate due to sympathetic relaxation. The parasympathetic nervous system had nothing to do with it. But now, we see that it does! What's up with that?

Well, the postganglionic neurons of the PNS do not directly synapse onto the arteries to the erectile tissues with acetylcholine. Instead, in the penis, these neurons release NO, which stands for nitric oxide... NO is a different neurotransmitter and it causes relaxation of the arterial smooth muscles. In the female, the postganglionic neurons actually synapse onto sympathetic neurons! The effect of this synaptic connection is to inhibit the sympathetic neurons to the arteries of the erectile tissues. And, as you learned already, when the sympathetic neurons are less active, vasodilation occurs.

Finally, as well as causing the female erection, the PNS stimulation causes the glands in the vestibule to secrete mucus. This lubricates the female reproductive organs for intercourse.

The sensory stimulation that accompanies intercourse is what causes orgasm. The orgasm is a sensation experienced through our nervous systems.

In males, the orgasm is accompanied by emission and ejaculation. Emission is simply the formation of semen-- it occurs as all the spermatozoa move into the urethra, mixing with the newly secreted solutions of the seminal vesicles and prostate gland. The semen components mix together as rhythmic and ordered peristaltic contractions of smooth muscle of the male reproductive tract occur. Ejaculation is the release of the semen from the penis. In females, orgasm is accompanied by rhythmic peristaltic contractions of the smooth muscles of the female reproductive tract... these facilitate the movement of spermatozoa toward the ovaries.