Most animals use sex steroids in differentiation of male and female
reproductive structures and oxytocin for the majority of the acute,
sudden events that occur at these structures. Oxytocin and vasopressin
family members are known from 4 invertebrate phyla and all groups of
vertebrates (Youson, 1999). Oxytocin and vasopressin are short peptides
composed of nine amino acids (nanopeptides) which are made in the paraventricular
and supraoptic nuclei of the hypothalamus. Vasopressin is also made
in several other regions of the brain (Debiec, 2007).Oxytocin functions
in a paracrine hormone in mammals, being produced in the hypothalamus,
pituitutary, mammary gland, ovary, uterus, testis, and prostate. The
processes regulated by oxytocin tend to utilize positive feedback mechanisms
(such as in birth, milk ejection mediated by neurons followed by recovery,
and in the deterioration of the corpus luteum (Ivell, 1999; Caldwell,
Vasopressin increases sex drive (Brizendine, 2006).
Different researchers have classified different stages of sexual pleasure and orgasm. Alternate enumeration of the stages of sexual experience include a three step model of desire, arousal, and orgasm and a 4 step model of excitement, plateau, orgasm, resolution. Many now join the plateau phase with the excitation phase and add two stages of desire, one spontaneous and a second resulting from arousal (Levin, 2007). Orgasm effects a variety of body regions such as the heart (tachycardia--up to 180 beats per minute), blood vessels (initial increase then decrease in blood pressure and the "sex flush"), lungs (hyperventilation), sweat glands, muscles (spasms, pelvic motions, shuddering), and vocalizations (Mah, 2001). During a woman's orgasm, the circumvaginal musculature contracts involuntarily but rhythmically 3-15 times (at about 0.8 second intervals) accompanied by contractions of uterine and anal muscles. Orgasms may last 3 seconds to about 2 minutes. Other physiological changes such as also occur. Consciousness may fade somewhat (Bianchi-Demicheli, 2007).
In men, but not women, there is a post-ejaculatory refractory time (PERT) during which time a second erection and ejaculation are impossible. Multiple orgasms are possible in young males but the incidence decreases rapidly with age (Mah, 2001; Levin, 2007).
Twin studies suggest there is a heritable basis to orgasm although
other factors such as her partner, education level, sexual experience,
body-image, mental state, and culture can also affect orgasm. For example,
a woman might easily reach orgasm with one partner but not achieve orgasm
with a second (Bianchi-Demicheli, 2007).In women, sexual function can
be affected by sociocultural and economic factors (such as sex education,
access to health care, stress associated with cultural norms, perceived
body image in light of cultural expectations, and fatigue due to work
obligations), issues relating to her sexual partner (including poor
communication, reaction to betrayal, imbalance of power in the relationship,
fear of abuse, differences in sexual drive, health problems in her partner),
psychological factors (aversion or mistrust of sexual pleasure, inhibition
because of fear of consequences), and medical factors (health disorders,
side effects of medication, pregnancy, STDs) (Ramage, 2007).