The sexual response depends on a complicated series of reflexes that involve the neuromuscular transmissions stimulated by a wide variety of visual, tactile (touch), olfactory (smell), and emotional sensations. Sexual excitement and response begin in the brain. Electrical signals are transmitted from the brain areas involved via the spinal cord to the sexual organs or genitals, through nerves that exit near the base of the spinal cord. The pathways between the brain and the genitals are long and complex, and demyelination may "short-circuit" them.
Impulses leave the CNS from the sacral spinal cord via the auto-nomic nervous system, which controls bodily functions that are considered "automatic." For example, this system controls the arousal that men and women experience without external stimulation, such as that which occurs during sleep. There are two divisions to the autonomic nervous system, the parasympathetic and the sympathetic. The parasympathetic section controls the erectile response. Erections in men may be stimulated by visual stimuli. obviously, for a visual stimulus to cause an erection, there must be an intact pathway from the brain down the spinal cord to the sexual organs. Demyelination may interfere with the connections from the "brain erection center" to the target organ, the penis.
There is clear evidence of a spinal center for erection as well as the brain center. As a result, reflex erections still may occur, but even when desired, willed erections may become impossible. Stimulation of the penis by masturbation or as part of sexual fore-play may allow an erection to occur if the pathway from the penis to the spinal cord and within the spinal cord back to the penis remains intact. This stimulation may require greater intensity if there is numbness or if sensation to the stimulus is decreased. Finally, erections may occur during sleep that may or may not have to do with these centers.
The normal male sexual response has three phases: desire, lubrication-swelling (excitement, plateau phases), and orgasm. The first response to sexual stimulation is erection, which is accompanied by increases in muscle tension, heart rate, blood pressure, and respiration. This then "plateaus" with advanced lubrication and swelling and is followed by a series of contractions by which the sympathetic nervous system allows for ejaculation (emission). Finally, the body returns to its resting state during the resolution stage.
The penis has soft, spongy tissue that easily expands when it is filled with blood. The tip of the penis, the bulb, is very sensitive to stimulation and sends messages to the various centers if it is appropriately stimulated. These centers allow the parasympathetic system to be stimulated, causing blood to be trapped within the spongy tissue of the penis to produce an erection. Ejaculation, the expulsion of liquid (semen) from the penis, is handled by the sympathetic division. When the stimulus ends or ejaculation occurs, the blood flows out of the penis and the erection disappears.
The external female genitalia, or vulva, consists of the labia majora (large outer lips of the vagina), the labia minora (smaller inner lips), the clitoris, and the vestibule. Like the male penis, the clitoris contains spongy tissue and a significant number of blood vessels. Bartholin glands, which produce a lubrication fluid, lie adjacent to the vagina.
As in men, the phases of normal female sexual response include desire, lubrication-swelling (excitement and plateau phase), and orgasm. The factors involved in the desire phase are not well understood but clearly may be affected by MS. Increased sexual excitement is accompanied by muscle tension, increased blood flow to the clitoris, and the beginning of vaginal lubrication, which then plateaus with increased lubrication and swelling. Orgasm consists of rhythmic contractions of the muscles around the vagina and uterus.
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