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SYMPATHETIC, MOTOR AND SENSORY FIBERS IN PUDENDAL NERVE

of their axons down to the erection-generating neurons in the lower spinal cord. There the PGN nerve endings release the neurotransmitter serotonin—a chemical messenger that inhibits erections by opposing the effects of pro-erectile neurotransmitters.

This discovery may have important implications for the millions of men and women who take serotonin-enhancing drugs to treat depression and other mental health problems. Drugs such as Prozac and Paxil, which belong to the widely used class of drugs called selective serotonin reuptake inhibitors (SSRIs), work in part by increasing brain levels of serotonin. These drugs often cause sexual dysfunction as an unwanted side effect, most commonly delayed or blocked ejaculation in men and, in women, reduced sexual desire and difficulty reaching orgasm.

The work of McKenna and his colleagues provides an explanation for how this side effect may occur. By increasing serotonin in the central nervous system, SSRIs may tighten the brain's built-in brakes on erection, ejaculation and other sexual functions in some people.

As often happens in medicine, however, one person's side effect can be another's therapy. The inhibitory properties of SSRIs have been shown to be helpful for men with premature ejaculation, a condition in which a man climaxes too quickly, typically before vaginal penetration or a few seconds thereafter. SSRIs are effective in delaying orgasm in these men, most likely because they increase central inhibition. Although more research is needed, SSRIs may also hold promise in treating sexual disorders that are associated with excessive or inappropriate sexual urges, such as paraphilias— for example, pedophilia, a sexual interest in children.

Considering that sex makes the world go 'round, or at least keeps us on the planet, it is not clear why these elaborate inhibitory controls have evolved. Although no one knows for sure, some intriguing theories have been advanced. John Bancroft of Indiana University believes that for most men this central inhibition is adaptive, keeping them out of trouble that might arise from excessive or risky pursuit of sexual enjoyment. These internal brakes also may help prevent a man from having repeated ejaculations during sexual encounters, which could lower his sperm store and reduce fertility.

Also, as with many pleasures in life, an erection can become too much of a good thing if it lasts too long. An erection that persists longer than four hours—a phenomenon that may occur in men with sickle cell anemia and in those who use certain drugs—is considered a medical emergency. Called priapism, this condition traps blood within the erect penis, leading to permanent damage if not treated promptly: if freshly oxygenated blood is not brought in, tissue starvation can occur.

Despite the benefits of central inhibition for most men, Bancroft believes it can cause problems for others if it is too strong or too weak. If a man has too much central inhibitory control—if, say, his brain serotonin levels are too high— he may develop sexual dysfunction. Conversely, if his central inhibition is too low, he may be more inclined to engage in high-risk sexual behaviors, such as recklessly ignoring the threat of sexually transmitted diseases in the pursuit of sexual gratification.

Inside the Brain

Many regions throughout the brain contribute to the male sexual response, ranging from centers in the hindbrain, which regulates basic body functions, to areas of the cerebral cortex, the organ of higher thought and intellect. The brain sites we have identified so far appear to be extensively interconnected. We now think the brain's control of sexual function works as a unified network, rather than as a chain of relay sites. In other words, the control of erection does not appear to be organized in a tightly linked chain of command centers but rather is distributed throughout multiple areas in the brain and spinal cord. Therefore, should injury or disease destroy one or more of these regions, the capacity for erections often remains intact.

One of the important brain regions regulating sexual behavior is the hypothalamus. This small area plays a vital role in linking the nervous and endocrine, or hormonal, systems and is involved in the control of certain basic behaviors, such as eating and aggression. A cluster of neurons in the hypothalamus, called the medial preoptic area, or MPOA, seems to have a crucial role in sexual function and, accordingly, is being intensively studied at the moment.

Researcher François Giuliano of the Faculté de Médecine of the Université Paris-Sud and his colleagues have re cently shown that electrical or chemical stimulation of the MPOA causes erections in rats. The MPOA appears to integrate stimuli from many areas of the brain, helping to organize and direct the complex patterns of sexual behavior. Some scientists speculate that the MPOA may also be involved in the recognition of a sexual partner.

The hypothalamus also contains the paraventricular nucleus, another group of neurons with an important role in male sexual function. Like the MPOA, this nucleus is a processing center that sends and receives messages from different parts of the brain and spinal cord. During sexual arousal, the paraventric-ular nucleus releases oxytocin. This hormone has long been known to stimulate the release of milk in breast-feeding women and uterine contractions during delivery of a baby; in many species, oxytocin is a chemical "love" messenger that promotes bonding and social attachments. But it also proves to be a brain neurotransmitter that has a powerful proerectile effect in men. Like other neurotransmitters, oxytocin binds to target neurons and regulates the con-

SINUSOID FILLED SMOOTH MUSCLE WITH BLOOD CELL RELAXED

ENGORGEMENT begins when nerves release transmitters that diffuse into the smooth muscle cells around the arteries in the penis, causing the normally contracted cells to relax and blood to flow in (top). As they relax, the muscles elongate, pushing against the veins that drain blood from the penis. The blood becomes trapped in sinusoids—the chambers between muscle cells—and the penis becomes erect (bottom).

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duction of nerve impulses. In this case, oxytocin activates excitatory nerve pathways running from the spinal erection-generating center to the penis.

Higher brain centers are involved in male sexual response as well, but we know much less about them. Nevertheless, the few studies to date have provided some intriguing results. Researcher Serge Stoleru of Inserm in Paris recently used positron emission tomography (PET) to reveal which parts of the cerebral cortex are activated when men are sexually aroused. He compared PET scans in a group of men who were presented with three kinds of films: sexually explicit, humorous and emotionally neutral (such as a documentary on the Amazon). Stoleru found that when men were sexually aroused, specific parts of the cerebral cortex were activated, including regions associated with emotional experiences and control of the au-tonomic nervous system.

In addition, scientists are exploring how higher brain functions, such as memory and learning, help to control erections. Psychologist Raymond Rosen of Robert Wood Johnson Medical School in New Brunswick, N.J., showed that healthy men can be taught to have erections on demand, in response to mental imagery or nonsexual cues. In one study, men were instructed to use their minds to arouse themselves in exchange for a financial reward. When they were given feedback on their performance via a light display, they rapidly learned to increase their erections—in the absence of direct physical stimulation—through the use of imagery and fantasy techniques. To keep their motivation high, the men earned financial

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SIDE VIEW

FRONT VIEW

AROUSAL has been mapped in these PET scans of men watching sexually explicit films. Although research on the sexual organization of the higher brain is just beginning, these scans show that several areas of the cerebral cortex are clearly involved.

TOP VIEW

TOP VIEW

AROUSAL has been mapped in these PET scans of men watching sexually explicit films. Although research on the sexual organization of the higher brain is just beginning, these scans show that several areas of the cerebral cortex are clearly involved.

bonuses that depended on the number and degree of erections they achieved.

This experiment was one of many that have shown that learning and memory strongly influence erections. Indeed, the ability of the brain to associate sexual arousal and orgasm with cues helps to explain why an astounding number of fetish objects—such as high-heeled shoes, leather whips and lingerie—can often enhance sexual arousal.

When Things Go Wrong

By understanding the role of the central nervous system in controlling erection and other sexual functions, we hope to set the stage for new therapies. Erectile dysfunction, which is defined as a consistent inability to get or keep an erection that is satisfactory for sexual performance, is an increasingly common health problem. A study we conducted a few years ago in the Boston area estimated that some degree of erectile dysfunction affects about 40 percent of men over age 40 and up to 70 percent of men 70 years old. As baby boomers grow old and the global population ages, we estimate that the number of men who have this condition will more than double in

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