Hormonal action on the brain starts early, even in utero. At six to 12 weeks gestation, a fetus with XY (male) chromosomes will get a rush of testosterone that helps create the developing penis.
"After that, there is another flood [of hormones] that basically gives the blueprint for all circuits," said Sukel.
"For so long we have talked about gender and love as being either biology or environment," she said. "But environment has a way of shaping biology and vice versa … an intricate dance."
Nowhere are brain circuits more alight than during the human orgasm, and Sukel wanted to learn more about the neurobiology of sex. Last year, she offered to support research at Rutgers University in New Jersey, plunging in to a noisy, cold MRI machine.
At first she felt competitive, wondering if any of the other subjects had failed at climaxing inside the loud machine. She was reassured by lead researcher Barry Komisaruk. "A few. But not too many," he told her.
Understanding the physiology of climaxing is still a mystery, according to Komisaruk, a professor of psychology at Rutgers and author of the 2006 book, "The Science of Orgasm."
Stimulation of the genitals triggers a cluster of nerve activity that moves to the sensory cortex in the brain, and then on to various parts of the limbic system. Most immediately activated are the amygdala, which controls emotion and heart rate; the hippocampus, which controls memory and fantasy; and the cerebellum, which controls deep muscle tension.
Next, the frontal cortex, which controls executive function, is activated, and then the hypothalamus, releasing oxytocin, and some other areas before "everything cools down," he said.
Powerful endorphins are also activated that raise a person's pain threshold. In both men and women, orgasm works roughly the same way, said Komisaruk.
His studies focus on which parts of the brain are activated in which sequence from the start of genital stimulation to orgasm.
"It's important because in many cases, people can't experience orgasm," he said. "The next step would be to find out where the blockage occurs. We want to follow this up with a possible therapeutic procedure to bypass or get around the block."
Therapy might involve neurofeedback in "real time," allowing a person to view the parts of the brain critical to orgasm and to direct the body to respond.
Komisaruk sees real potential with those who cannot experience orgasm because of paralysis or prostate surgery.
One woman, who was paralyzed from the waist down and had no external sensation in her genitals, was able to feel the orgasm in her uterus. Brain imaging has revealed that the nerves from the genitals run along the vagus nerve, outside of the spinal column, up to the brain.
"She didn't feel it in her clitoris, but could feel it in her vagina and cervix and she had muscle cramps," he said.
In another brain imaging study, some women have shown they can have an orgasm just by thinking about it -- without any stimulation.
"We obviously have the ability to think our brains into action, but we don't know yet what the limits are," said Komisaruk.
Sukel is intrigued by the way Komisaruk emphasizes "the bigger picture" in his research.
"What we are finding is the top-down control that can modulate our feelings of pleasure and pain -- it isn't limited to our organs," she said. "Think of the placebo effect … We have a lot of influence over our perception of pleasure and even over our disease state and will learn more going forward."
Orgasm just may provide a "novel way" to understand touch and allow humans to modulate their feelings, according to Sukel.
"It might go beyond biofeedback -- just believing that something is helpful," she said. "Even telling a patient their pain is not harmful reduces the sensation of pain."