There are a lot of things scientists don't understand about sex. But now two researchers have found an intriguing piece of the puzzle: a long-sought bit of circuitry called the ejaculation generator. And no, they didn't find it at a sex shop or on some sleazy Web site; they found it in the spinal cord.
Ejaculation is a reflex, but it's no simple knee jerk. What triggers it is poorly understood--sometimes it takes a lot of sexual activity, other times just a little--and it kicks off a complicated pattern of muscle contractions. From animal experiments and clinical studies of men with spinal cord injuries, researchers only knew this activity was coordinated somewhere in the lower third of the spinal cord.
Things got a little hotter last year, when neuroscientist Lique Coolen and postdoc William Truitt of the University of Cincinnati College of Medicine in Ohio identified a group of neurons in male rats that light up after ejaculation. These so-called lumbar spinothalamic (LSt) neurons are scattered through two segments of the lumbar region of the spinal cord. To determine whether LSt neurons were actually running the show, the pair injected male rats with a toxin that selectively destroys these cells. After the rats recovered, the researchers put them, one at a time, into a cage with a ready and willing female. Truitt kept a play-by-play of the romance up to ejaculation.
Shortly after this last hurrah, Coolen and Truitt killed the rats and tallied the LSt cells remaining in their spinal cords. Not one of the rats with less than one-third the normal number of LSt cells had been able to ejaculate, despite otherwise showing off the same moves as untreated males, the team reports in the 30 August issue of Science. The findings indicate that LSt neurons are a critical component of the ejaculation generator--if not the whole kit and caboodle, Coolen says.
The study has titillated other experts. "I think it's fabulous," says Kevin McKenna, a neuroscientist at Northwestern University in Evanston, Illinois. Knowing where ejaculation is coordinated, he says, is an important step toward understanding male sexual function. At the same time, new questions have popped up. For instance: What do LSt cells do in females? Coolen is planning to find out.