Even in women without any sexual dysfunction, there is highly variable correlation between the degree of subjective sexual excitement and the increase in congestion around the vagina (23,26). This poor correlation was shown repeatedly over the past 30 years based on psychophysiologic studies using the vaginal photoplethysmograph. Congestion in response to a sexual video is reduced in women with disruption of the autonomic nerve supplying the vulva and vagina (e.g., from nonnerve-sparing radical hysterectomy). Otherwise healthy women experiencing chronic lack of arousal (including lack of subjective excitement and lack of any awareness of genital congestion) show increases in vaginal congestion from erotic stimuli that are similar those in control women (23,26). With the so-called cervico-motor reﬂex, cervical touch (in the laboratory with a balloon-tipped catheter to replicate penile pressure) leads to a reduction in pressure in the upper portion of the vagina and an increase in pressure in the middle and lower portions. Simultaneously, an increase in electromyographic activity in the levator ani and puborectalis muscles was recorded. It is thought that during intercourse penile thrusting on the cervix might cause contraction of the pelvic muscles to facilitate “ballooning” of the upper vagina, perhaps to facilitate pooling of semen. The same muscle contraction constricts the lower vagina, which may afford increased stimulation of the partner’s penis, thereby maintaining rigidity (27).
A further reﬂex demonstrated in laboratory studies shows reduced uterine tone in response to mechanical or electrical stimulation of the glans of the clitoris. Background activity of the uterine muscle was abolished by clitoral stimulation if either the glans clitoris or the uterus was anesthetized. Uterine pressure declined on clitoral stimulation. This reﬂex may underlie the known increase in size and the elevation of the uterus with sexual arousal (28).
Orgasm Orgasms is a brain event, typically triggered by genital stimulation that can occur during sleep or from stimulation of other body parts including the breast and nipple or by fantasy, occasionally by medication, and in spinal cord injured women by vibrostimulation of the cervix. In able-body women, it involves a myotonic response of smooth and striated muscle associated with feelings of sudden release of the sexual tension built up during arousal. It is described as the most intensely pleasurable of the sexual sensations. Reﬂex rhythmic contractions (3–20 0.8/sec) of the muscles surrounding the vagina and anus occur. Some women may subjectively perceive uterine contractions during orgasm and some may report a difference in their perception of orgasm after hysterectomy, but this is not objectively documented. An objective quantitative measure was established that shows strong correspondence with the subjective experience of orgasm. Analysis of rectal pressure data while volunteers imitated orgasm, tried to achieve orgasm and failed, or experienced orgasm showed a significant and important difference in this analysis between orgasm and both control tasks (29).
Brain imaging studies of women during orgasm showed brain activations and deactivations similar but not identical to those found in men (30). There is profound deactivation in the anterior part of the orbitofrontal cortex (OFC). This area is thought to be involved in urge suppression and behavioral release. This area is activated when experiences are particularly hedonic, with further activation increasing satiation and deactivated with feelings of satiety. The medial OFC is part of the neuronal network underlying self-monitoring and is connected to the amygdala. The latter is deactivated during the genital stimulation and arousal and remains deactivated during orgasm. Deactivation of this network is associated with a more carefree state of mind. The subjective description of orgasm is very much in keeping with this depiction (31).
The majority of women most easily experience orgasm from direct clitoral stimulation.