Male sexual circuitry
Thanks again to Kiwi for getting a full copy of the text of this article. Unfortunately, the graphs and photos are not available. This thing was setting in the Links Forum for quite awhile as a zip file download. It had only been downloaded 38 times or something like that. So, I unzipped it and posted it so it was easier to get at.
Male sexual circuitry
Scientific American; New York; Aug 2000; Irwin Goldstein;
Start Page: 70-75
Subject Terms: Sexual disordersMenMedical research
Recent research into the central nervous system may prove invaluable in the search for a cure of erectile dysfunction and impotence. By understanding what parts of the brain are stimulated and from where on the spinal cord erections are controlled, scientists hope to understand a variety of sexual phenomenon.
Copyright Scientific American, Incorporated Aug 2000
Five hundred years ago Leonardo da Vinci made an observation about the penis that rings true even today for many men and their partners. The Renaissance scientist, inventor and artist-one in a long line of investigators who have attempted to solve the riddle of penile rigidity-observed that this seemingly wayward organ has a will of its own. “The penis does not obey the order of its master, who tries to erect or shrink it at will, whereas instead the penis erects freely while its master is asleep. The penis must be said to have its own mind, by any stretch of the imagination,” he wrote.
Da Vinci, who dissected cadaverous penises from men who had been executed by hanging, was the first scientist to recognize that during an erection, the penis fills with blood. In his perception that the penis acts of its own free will, however this multitalented scholar was wrong.
Far from having a mind of its own, the penis is now known to be under the complete control of the central nervous system-the brain and spinal cord. As William D. Steers, chair of the department of urology at the University of Virginia, has noted, any disturbance in the network of nerve pathways that connects the penis and the central nervous system can lead to erection problems.
In the past few decades the study of erections has been redefined. Thanks to advances in molecular biology, we now have a better understanding of the processes within the penis that lead to erection and detumescence, the return of the penis to a flaccid state. Armed with this knowledge, we have begun to explore how the brain and spinal cord control erections and other sexual functions. The field is still young, but we are optimistic that these efforts will lead to new therapies for the millions of men who suffer from sexual dysfunction— and we expect that some of these findings will also inform treatments for women. Although research on women has lagged far behind that on men, we are beginning to elucidate the striking similarities-as well as differences-between the sexes in regard to sexual function.
An erection is a carefully orchestrated series of events, with the central nervous system in the role of conductor. Even when the penis is at rest, the nervous system is at work. When a man is not sexually aroused, parts of the sympathetic nervous system actively limit blood flow to the penis, keeping it limp. The sympathetic nervous system is one of two branches of the autonomic nervous system-the part of the central nervous system that controls largely “automatic” internal responses, such as blood pressure and heart rate.
Within the penis, and throughout the nervous system, a man’s sexual response reflects a dynamic balance between excitatory and inhibitory forces. Whereas the sympathetic nervous system tends to inhibit erections, the parasympathetic system-the other branch of the autonomic nervous system-is one of several important excitatory pathways. During arousal, excitatory signals can originate in the brain, triggered perhaps by a smell or by the sight or thought of an alluring partner, or by physical stimulation of the genitals.
Regardless of where the signals come from, the excitatory nerves in the penis respond by releasing so-called proerectile neurotransmitters, including nitric oxide and acetylcholine. These chemical messengers signal the muscles of the penile arteries to relax, causing more blood to flow into the organ. Spongy chambers inside the penis fill up with blood. As these expand, they compress the veins that normally drain blood from the penis. This pressure squeezes the veins until they are nearly closed, trapping blood within the chambers and producing an erection. (Viagra-also known as sildenafil-works by slowing the breakdown of one of the chemicals that keeps the muscles relaxed, thereby holding blood in the penis.)
During an erection, the penis not only receives nerve signals but also sends them to the spinal cord and brain. The penis has an unusually high density of specialized tactile receptors; when these receptors are stimulated, their signals course to the spinal cord and brain, where they influence nerve pathways from these higher centers. So although the penis does not “think” for itself, it keeps the brain and spinal cord well apprised of its feelings. After a man climaxes or the arousal has diminished, the erection quickly subsides. The sympathetic nervous system again limits blood flow into the penis, which returns to its soft state.
Circumstances that increase the activity of the sympathetic nervous system— such as stress or exposure to cold-can temporarily shrink the penis by making it more flaccid. Conversely, switching off the activity of the sympathetic nervous system enhances erections. Nocturnal erections are a good example of this phenomenon. These occur primarily during rapid eye movement (REM) sleep, the stage in which dreaming occurs. During REM sleep, sympathetic neurons are turned off in the locus coeruleus, a specific area of the brain stem, the part of the brain that connects to the spinal cord. According to one theory, when this sympathetic brain center is quiet, proerectile pathways predominate, allowing nocturnal erections to occur. We often refer to such erections as “battery-recharging mechanisms” for the penis, because they increase blood flow, bringing in fresh oxygen to reenergize the organ. (Episodes of nocturnal arousal also occur in women. Four or five times a night that is, during each episode of REM-women experience labial, vaginal and clitoral engorgement.)
Some erections, called reflexive erections, are generated entirely in the spinal cord. Much like touching a finger to a hot burner triggers a rapid withdrawal of the hand, physical stimulation of the penis can set off a spinal erection reflex in some situations. So crucial is reproduction to our perpetuation as a species that it appears that the capacity to create an erection has been wired into nerve circuits near the base of a man’s spine.
In humans, most of the evidence for this finding has come from observations of soldiers with spinal cord injuries, particularly veterans wounded in World War II. Before then, the general belief was that men with spinal cord injuries were permanently and completely impotent and sterile. Although we now know that this view is mistaken, it is understandable. The spinal cord is the information superhighway for the nervous system, shuttling nerve stimuli to and from the brain and the peripheral nerves of the rest of the body. If the spinal cord is damaged, this flow of nerve impulses can be interrupted in myriad ways, depending on where the injury occurs and how extensive it is.
Yet, as physician Herbert Talbot reported in a classic study in 1949, men with severe or complete spinal cord injuries often continue to have erections. In his examination of 200 men with paraplegia, two thirds were able to achieve erections, and some were able to engage in vaginal intercourse and have an orgasm. Even though devastating war injuries left these men paralyzed and unable to control many basic bodily functions, the ability to have erections was often preserved.
These observations-and information from studies in laboratory animals as far back as the 1890s-led to the discovery that an “erection-generating center” is located in the sacral segments of the spinal cord (that is, just above the tail end of the spine, between the S3 and T12 vertebrae). Physical stimulation of the penis sends sensory signals via the pudendal nerve to this erection center. The incoming signals activate connector nerve cells called interneurons, which then stimulate nearby parasympathetic neurons. These neurons send erection— inducing signals from the sacral spine to the penile blood vessels. As long as this reflex arc remains intact, an erection is possible.
The Brain's Brakes
Observations of men and laboratory animals with spinal cord damage have led to another intriguing finding: when the brain is disconnected from the erection-generating center in the spinal cord, erections typically occur more frequently and with less tactile stimulation than they did before the injury. For instance, Benjamin D. Sachs, an experimental psychologist at the University of Connecticut, found in 1979 that spinal transection in rats caused an increase of more than 1,000 percent in the number of erections and a 94 percent reduction in the time it took for the animals to become erect.
It seemed as if, in the disconnection of the brain from the body, some inhibitory control over erections was removed. This proved to be the case. In 1990 physiologists Kevin E. McKenna and Lesley Marson, then at Northwestern University, identified the brain center that keeps the brakes on spinal-mediated erections. They found that a specific cluster of neurons in the hindbrain (an evolutionarily ancient part of the brain that controls such basic functions as blood pressure and heart rate) is in charge of this central inhibition. When McKenna and Marson destroyed this group of neurons-called the paragigantocellular nucleus, or PGN-in a male rat’s brain, the inhibition disappeared, causing more frequent and intense erections.
These researchers then made another significant discovery about the brain’s role in suppressing erections. They found that the PGN neurons send most 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 proerectile neurotransmitters.
Erections are continuously inhibited by the sympathetic nervous system (blue). During REM sleep, however, when the sympathetic neurons in the locus coeruleus are turned off, erections occur spontaneously The other brain structure that inhibits erections is the paragigantocellular nucleus (PGN). Conversely, the parasympathetic nervous system (red) is excitatory. Tactile stimuli or stimuli processed in the cortex may be integrated in the paraventricular nucleus and the medial preoptic area (MPOA), triggering an erection. Some erections (called reflexive) occur entirely in the erection-generating center of the spinal cord, which runs from vertebra S3 to vertebra T12.
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 paraphiliasfor 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 highhe 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 aggeession. 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 Francois Giuliano of the Faculte de Medecine of the Universite Paris-Sud and his colleagues have recently 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 paraventricular 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 conduction 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 autonomic 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, NJ., 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 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
My 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 the next 25 years-ultimately affecting more than 330 million men worldwide.
If nerve stimuli cannot reach the penis for any reason, an erection problem is inevitable. Such dysfunction can also be an unfortunate complication of surgery to remove the prostate gland to treat prostate cancer, because this procedure can damage penile nerves. Diabetes can lead to nerve and blood vessel damage in the penis as well. Many neurological conditions-including spinal cord injury, Parkinson’s disease, multiple sclerosis and stroke-can cause problems. And because a man’s moods and mental wellbeing affect the flow of nerve messages to the penis, it is not surprising that stress, depression, anxiety or anger often underlies erection difficulties.
Using their growing knowledge of central nervous system control, researchers have begun to develop medications that target the central nervous system. A drug called apomorphine will most likely be the first in a new generation of therapies that acts directly on the brain as opposed to the penis, as Viagra does. Apomorphine-brand name Uprimamimics the neurotransmitter dopamine, enhancing erections by binding to specific receptors on nerve cells in the paraventricular nucleus and the MPOA, thereby turning on proerectile pathways.
Apomorphine is under review by the U.S. Food and Drug Administration for approval, and a final decision is expected soon. Although the compound has been used in medicine for more than a century-for the treatment of Parkinson’s disease, among other disorders-it was not until the mid-1980s that investigators, including R. Taylor Segraves, a psychiatrist at Case Western Reserve University, and Jeremy P W Heaton, a urologist at Queen’s University in Ontario, began investigating it for the treatment of erectile dysfunction. Since then, clinical studies have evaluated apomorphine in more than 3,000 men and found that it can successfully treat those with many different types of erectile dysfunction.
Sex and the Sexes
Until recently, most research on sexual function focused in large part on men and the control of penile erection. Fortunately, this is changing, as we increasingly recognize that sexual dysfunction is extremely common-and treatable-in both sexes. In fact, a recent survey of more than 3,000 Americans reported that the number of women with sexual complaints was greater than the number of men: 43 percent as opposed to 31 percent.
Many researchers are studying the mechanisms that control sexual function in women and are testing therapies to treat female sexual disorders. Our laboratory is conducting a clinical trial to determine whether apomorphine can enhance sexual arousal in women with such problems. We also are testing a new FDA-approved device called the EROSClitoral Therapy Device, which is used to provide gentle suction to the clitoris, causing engorgement. In women with sexual dysfunction, it has been shown to safely improve sexual sensation, lubrication, orgasm and sexual satisfaction.
This research has made us aware of some similarities between the sexes in the central nervous system’s control of arousal, orgasm and various sexual functions. Preliminary evidence suggests that the central control of sexual function in men and women is remarkably similar. For instance, as noted earlier, both sexes experience nocturnal arousal responses, and both are vulnerable to SSRI-induced sexual dysfunction.
Of course, there are also dramatic differences-as in the postorgasmic refractory period, the normal delay after an orgasm before arousal can occur again. Women can have multiple orgasms and therefore have virtually no refractory period, but most men have a refractory period that lasts from several minutes to many hours.
We have come a long way since da Vinci’s discovery that the penis fills with blood-not air or spiritual essencesduring an erection. The past decade has revolutionized not only the field of erection research but also our societal attitudes about sexual health. Only a few years ago erectile dysfunction went generally untreated.
Today this condition and other sexual problems are more openly recognized and discussed. Millions of men are receiving care for erection troubles, thanks to a burgeoning appreciation of the importance of sexual health and the availability of more effective and convenient
treatments. In the near future we anticipate that there will be an even wider array of therapies for men and women. With our increasing insight into the brain’s role in controlling our sexuality, we are also moving toward a more holistic view of sexual well-being-one that integrates mind and body and responds to the unique needs of both sexes.
IMPOTENCE AND ITS MEDICAL AND PSYCHOLOGICAL CORRELATES: RESULTS OF THE MASSACHUSETTS MALE AGING STUDY. H. A. Feldman et al. in Journal of Urology Vol. 151, No. 1, pages 54-61; January 1994.
Caption: IMPOTENCE increases with age, according to several surveys. In 25 years, given the aging of the world’s population, it is estimated that the condition may affect more than 330 million men.
NEURAL CONTROL OF PENILE ERECTION. E Giuliano, O. Rampin, G. Benoit and A. Jardin in Urology Clinics of North America, Vol. 22, No. 4, pages 747-766; November 1995. THE BRAIN IS THE MASTER ORGAN IN SEXUAL FUNCTION: CENTRAL NERVOUS SYSTEM CONTROL OF MALE AND FEMALE SEXUAL FUNCTION. K. McKenna in International Journal of Impotence Research, Vol. 11, Supplement 1, pages 548-SSS; 1999. SEXUAL DYSFUNCTION IN THE UNITED STATES: PREVALENCE AND PREDICTORS. E. O. Laumann, A. Paik and R. C. Rosen in Journal of the American Medical Association, Vol. 281, No. 6, pages 537-544; February 10, 1999.
and the Working Group for the Study of Central Mechanisms in Erectile Dysfunction
IRWIN GOLDSTEIN is a urologist at Boston University. He is a member of the Working Group for the Study of Central Mechanisms in Erectile Dysfunction, which was formed in 1998. The other members are John Bancroft of Indiana University; Franqois Giuliano of the Faculte de Medecine, Universite Paris-Sud; Jeremy P W Heaton of Queen’s University, Ontario; Ronald W Lewis of the Medical College of Georgia; Tom E Lue of the University of California, San Francisco; Kevin E. McKenna of Northwestern University; Harin Padma-Nathan of the University of Southern California; Raymond Rosen of the Robert Wood Johnson Medical School; Benjamin D. Sachs of the University of Connecticut; R. Taylor Segraves of Case Western Reserve University; and William D. Steers of the University of Virginia. All the authors consult or investigate (or have done so in the past) for one or more pharmaceutical companies-among them Abbott, Eli Lilly Merck, Pfizer and TAP; Sachs owns stock in Abbott; Heaton shares several patents on apomorphine.