Is Sexual Attraction Neurological ?
Sexuality is a highly complex, multidimensional phenomenon that ranges from the biological machinery of reproduction to diverse cultural traditions organizing human intimacy. This account enters into chosen dimensions, starting with autonomic and spinal neurons regulating genital functions, then mating strategy, and finishing with brain mechanisms essential for monogamy and parental drives.
Studies of human sexual physiology have focused predominantly on men, but we strive to provide information on both genders. Arousal causes blood engorgement in certain external genital organs—labia and clitoris in females, penis in males. These organs contain rich supplies of mechanoreceptors, particularly in the clitoris and penile glans, which react to stimulation, at times reflexively causing engorgement. Evidence for this comes from men with total spinal cord transection at thoracic or lumbar levels who continue to experience erections in response to mechanical penile stimulation.
Reproductive Organs and Their Control
In spite of dramatic anatomical differences, neural control of male and female reproductive organs is surprisingly similar. Sexual arousal is the result of psychological and sensory stimuli—visual, olfactory, somatosensory—and direct genital touch.
The sexual response cycle includes arousal, plateau, orgasm, and resolution, differing in length but uniform in physiological response. Cerebral cortex function, which is responsible for erotic thought, is integrated with spinal mechanisms that mediate sensory input and autonomic response. External and internal reproductive organs follow established somatosensory system pathways.
Mechanoreceptors in the genitalia convey signals via dorsal roots of the sacral spinal cord, extending to dorsal horns and columns along the way to the brain.
Engorgement and erection are derived from parasympathetic autonomic axons. These neurons, housed in the sacral spinal cord, are activated by mechanosensory stimulation or descending brain impulses, with the result enabling responses to mental arousal. Blood flow physiology controls engorgement of the clitoris and penis. Parasympathetic nerves release acetylcholine, vasoactive intestinal polypeptide (VIP), and nitric oxide (NO) into erectile tissues, causing relaxation of arterial smooth muscles, engorgement of spongy structures, and leading to tumescence. Viagra (sildenafil) reverses erectile dysfunction by augmenting NO effects. As the penis enlarges, inner tissues push against elastic connective coverings, providing rigidity. To provide smooth penetration, parasympathetic messages also cause lubrication from vaginal wall and bulbourethral gland.
Completion of sexual response cycle requires sympathetic autonomic involvement. Increased genital sensory activity and cerebral inputs activate spinal sympathetic neurons at thoracic and lumbar levels, causing ejaculation in men—semen, driven through muscular contractions, issues from the urethra, coinciding with orgasmic feeling. In women, orgasm triggering is less specified but probably entails sympathetic mechanisms culminating in thickening of the vaginal walls and rhythmic contractions. Probing the neural substrate of orgasm presents daunting scientific challenges. Accommodating partners in an MRI apparatus poses practical obstacles, and interpretation of subjective responses to pleasure makes analysis more difficult.
Experiments document widespread cortical and subcortical activations during orgasm, but localized neural sources of pleasure elude detection.
Epileptic seizures give clues; a few people have pre-seizure sexual auras associated with temporal lobe activity.
Surgical procedures that activate medial temporal or basal forebrain areas sometimes induce sexual arousal, and very rarely orgasmic feelings. After orgasm, men need a refractory period before they can experience another climax. Women have more variability in the frequency and intensity of orgasm. Resolution phase marks termination of sexual excitation, with blood leaving engorged tissues to return to baseline physiological conditions.
Mammals exhibit a remarkable diversity of reproductive behaviors.
Each strategy ultimately has one evolutionary purpose: securing the perpetuation of genes through maximizing offspring survival.
Mating system differences between species are usually determined by the amount of effort that females and males expend in raising their offspring, but exceptions exist. A common strategy in mammals is polygyny (from the Greek for “many women”), where one male mates with several females, yet each female mates with a single male in one or more breeding seasons. Polygynous reproduction (sustained by giraffes, orangutans, and the majority of mammalian communities) is of short-term occurrence, and with no subsequent participation of the male in his companions’ or their offspring’s affairs. There can be an ensuing harem association in certain societies, with the same top animal monopolizing courtship over the female collective to form an exclusive union, i.e., amongst gorillas, elephant seals, and certain patriarchal human civilizations. Polyandry (“many men”), where a woman mates with multiple men but each man mates with her alone, is rare among mammals and vertebrates.
A rare exception is the phalarope, a shorebird that breeds in the Arctic tundra.
Some species practice simultaneous polyandry, in which a woman lays eggs in the nests of several males and these males subsequently raise the chicks in her territory. Other animals practice sequential polyandry, in which the female abandons her mate and offspring after laying eggs. Polyandrous behavior is also seen in some marmoset and tamarin species. Polyandry in human cultures has been geographically widespread in the past but is now uncommon and occurs in only a minority of cultures. Polygyny and polyandry are both special cases of polygamy—having multiple mates. In monogamous systems (“one spouse”), a male and a female form an intimate partnership entailing exclusive (or near-exclusive) mating with each other. Monogamy occurs in merely 3% of mammalian species but in higher frequency in primates (around 12%) and most predominantly in birds (around 90%). Exclusive bonds may remain for life or continue only as long as the individual is until a new partner is chosen (serial monogamy). Human societies have displayed a variety of mating patterns throughout history and across cultures.
Generally, people prefer monogamy, if only for the short term, although polygyny is accepted in some cultures.
Even within societies that do accept polygyny, a majority of marriages are monogamous. Reproductive polyandry occurs infrequently, and in most societies in the past, women suspected of such behavior have been severely punished. Whereas evolutionary hypotheses strive to account for human mating systems, it continues to be challenging to identify the exact roles played by genetics and culture in constructing these behaviors.
