Two pioneering researchers, William Masters and Virginia Johnson, identified four stages of sexual response. These stages—excitement, plateau, orgasm, and resolution—are known as the human sexual response cycle. Two basic processes occur during sexual response: vasocongestion and myotonia. Vasocongestion refers to the concentration of blood in the blood vessels and in the tissues of the genitals and breasts. In men, vasocongestion occurs when arterial blood flow to the penis increases and venous outflow decreases. Erection is a result of increased blood flow to the spongy tissue (corpora cavernosa) of the penis, causing these tissues to expand. In women, this inflow of blood causes the clitoris to enlarge, the labia to swell, and the vagina to lubricate. Myotonia, or neuromuscular tension, refers to the increase of energy in nerves and muscles. During sexual activity, myotonia takes place throughout the body, affecting both involuntary and skeletal muscles.
Complications of MS
MS can interfere with the excitement and plateau phase of the sexual response cycle.
MS lesions in the brain can interfere with the perception of sexual stimulation so it no longer has an arousing effect. Lesions of the spinal cord can interfere with the transmission of nerve signals to the genitals and prevent vasocongestion, resulting in diminished or absent erections, clitoral swelling, or vaginal lubrication.
In men, orgasm takes place in two phases, emission and ejaculation. In the emission phase, increased tension leads to a series of rhythmic contractions that drives semen into the bulb of the urethra. The second phase, ejaculation, is marked by contractions that force semen out of the urethra.
Some men with MS experience orgasmic contractions without ejaculation (nonejacu-latoy orgasms) or ejaculation without orgasm. Some men and women with MS report loss of orgasm, although their excitement and plateau phases are unaffected. More commonly reported are less frequent orgasms, which can stem from direct physical, indirect, or psychosocial causes.
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