There are times when you're just not interested in sex. You know it would be fun once you got into it, but on a bad day the passion can seem much too far away. Sex starts in the brain before it moves into the body, and there comes a point where a healthy sex life isn't possible without a healthy life. Sex ceases to be enjoyable, and instead becomes work.

Most people think of the nervous system as an electrical highway of sorts that conducts information about physical experience from different parts of the body to the brain. In this model, the central nervous system, or CNS, is the central processing unit of the body. Consisting of the brain and spine, the CNS connects to the rest of the body via the peripheral nervous system (PNS), and information and instructions flow back and forth in both directions.

The role of the nervous system in sexual arousal is both conscious and unconscious. Although much of arousal begins in the brain and is communicated to the genital area, there is also an element of reflex involved. Research on individuals with spinal cord injuries has demonstrated that even when there is loss of arousal controlled by the CNS, arousal by local stimulation can still remain intact. This is true even if the person cannot feel the sensation of being touched, as long as the relevant nerve pathways are still present.

Although sexuality has a reflexive component, we can override it with the activity of our brain. Thinking too much can stop the arousal process in its tracks just as efficiently as biological problems can. There is so much in a sexual encounter that can go wrong-thinking the wrong thoughts, not having enough blood flow, a failure of communication in the nervous system, a failure of communication with your partner...If you think about it, it's pretty amazing that anyone ever manages to have sex at all.

Stress has a profound effect on sexual health. Not just a mental issue, stress has a strong biological component that can be measured by its effects on body levels of cortisol, a steroid that has various metabolic and anti-inflammatory effects in the body, and other hormones. By reducing circulating levels of sex hormones, stress can reduce sexual desire and function in men and women. Stress also has negative effects on the cardiovascular system. It increases blood pressure and causes vessels to contract, both of which further impair sexual function—remember that vasodilation, or dilation of vessels, is crucial to sexual arousal. Fortunately, reducing stress relieves its effects and allows people to resume their normal sexual lives.

Depression and other mental disorders are intimately linked with sexual function. First, the very parts of the brain that are involved in sexual arousal and function are those that are impaired in many individuals with mental illness. Second, many of the drugs used to treat the common mental illnesses of depression and schizophrenia also disable the pathways involved in sexual response. Depending on their precise chemical action, antidepressants have been shown to affect sexual interest, the ability to become aroused, the ability to reach orgasm, and the experience of orgasm. Fortunately, this problem can often be solved by switching to a different antidepressant. Anti-psychotic treatments for schizophrenia actually work to limit dopamine secretion by the limbic system. Dopamine is a neurotransmitter associated with mood enhancement, including sexual pleasure. Limiting dopamine can have the effect of damping down sexual feeling as well as other emotions.

Emotions themselves are, in other circumstances, the source of sexual difficulties. Sexual desire and sexual pain disorders in both men and women can have both psychological and organic origins. The lack of sexual interest that one woman has because of a low testosterone level may, in another woman, be the result of a traumatic sexual experience. Similarly, in men, premature ejaculation and related disorders are equally likely to be caused by anxiety or biology. Thought and worry can trap people in an unending cycle of sexual disorder: being afraid of, or uncomfortable about, sex makes it difficult to become aroused, which makes sex less enjoyable, which causes even more fear-and the more concerned you are about having trouble getting aroused, the more you think about it and analyze it, the harder it is to relax enough to let your body turn itself on. It's the centipede's dilemma: if he had to think about how to coordinate all 100 feet, he'd never manage to actually walk anywhere.

The mind and body are fundamentally linked in both the joys of sex and in its disorders, and it takes patience and sometimes professional help to break the cycle once problems become entrenched.

As men and women age, their bodies change, and so do the ways they respond sexually. They may feel less attractive, or that they're not supposed to be as interested in sex, but sexual activity is as healthy a part of life in a person's 80s as it is in their 20s. Sex is as normal and natural for older people as it is for younger people. Having a healthy sex life can add richness and vigor to people's later years.

The sexual response changes with age, due in part to lower levels of hormones. In women, the excitement phase takes far more time. Lubrication is delayed, but although the amount of vasocongestion is lower than in younger women, it is usually sufficient for normal sexual function. Because of the lowered hormone levels, however, vaginal tissues are thinner, and the vagina is also less able to expand, which can lead to pain during penetration. The plateau phase is similar to that of younger women, although the uterus does not become as elevated or the orgasmic platform as pronounced. Fortunately, most postmenopausal women don't experience a change in their frequency of orgasm, although the number of contractions may be reduced, and resolution occurs more quickly afterwards due to the overall decrease in vasocongestion. Many of the negative effects of aging on female sexuality can be minimized by hormone therapy. However for many women hormone therapy isn't appropriate, or even necessary.

In men, aging primarily affects the duration and intensity of their sexual response. Erection takes longer, and may require more direct stimulation than when a man was younger. During the plateau phase, men experience less muscle tension, and the testicles do not elevate as close to the perineum (in men, the region between the scrotum and the anus). One benefit of aging is that men can, in general, sustain an erection for a longer period of time. They also have greater ejaculatory control, which may increase their and their partner's pleasure. Orgasm remains pleasurable, but men may have reduced numbers of contractions and less ejaculatory force and volume. Finally, the resolution phase is much faster in older men, and the refractory period may lengthen from a couple of minutes to several hours or even days.

For both sexes, however, the most important factor in maintaining a healthy sex life into old age is continuing to have sex. A regular sex life, on top of good general health, helps to keep everything working the way that it should.

The nervous system is not simply composed of the brain and the spinal cord. Although these are the central core of the system, the peripheral nervous system is where much of the information processed there comes from, relaying signals from the rest of the body. Three specific groups of peripheral nerves are particularly important in the sexual response: the pudendal (external genital), the hypogastric, and the pelvic. The pelvic nerves in women are the main vaginal and cervical sensory nerves, and in addition they control engorgement of the clitoris. In men, the pelvic nerves control erection of the penis. The hypogastric nerves are sympathetic nerves. Because they innervate the testicles, cervix, and uterine area, they are in charge of ejaculation and vaginal lubrication. Finally, the pudendal nerves control conscious contraction of the vaginal muscles and the muscles at the base of the penis, as well as the urethral and anal sphincters. The pudendal nerves also receive sensation from the penile skin, scrotum, labia, and clitoris.

Receiving and relaying sensation is not the only function of the peripheral nervous system. A very important component of the PNS, the autonomic nervous system, has another role entirely. It manages the systems of the body that need to function without conscious control.

The autonomic nervous system consists of two parts-the sympathetic and parasympathetic systems. Although both systems affect organs throughout the body, they tend to work in opposite ways. The sympathetic nervous system is responsible for what's known as the "fight or flight” reflex. Among other effects, arousal of the sympathetic system increases heart rate, breathing, and blood pressure as well as releasing the hormones commonly called adrenaline. In contrast, the parasympathetic system tends to inhibit the reflexive alarm responses of the sympathetic system.

Most research suggests that it is the parasympathetic, or calming, part of the autonomic system that is most responsible for the subconscious aspects of sexual arousal. However, some controversial research suggests that activation of the sympathetic nervous system by exercise or other stimuli may leave an individual more biologically susceptible to sexual arousal, even if the person doesn't consciously notice a difference. Translation: being scared may stimulate your interest in having sex, but you need to be relaxed to become physically aroused. It makes sense: if you were in a "fight or flight" situation, it wouldn't be very useful to your survival to become distracted by thoughts of sex!

People believe that alcohol increases sexual function. Getting slightly drunk does, in fact, lower inhibitions and increase sexual risk-taking, but at higher levels alcohol actually decreases sexual ability. Alcohol is a systemic depressant that reduces both the capacity for arousal and the enjoyment of orgasm. It can also make it difficult for a man to achieve an erection. While alcohol may make it psychologically easier for a person to have sex, it makes it physiologically far more difficult-and it blunts good judgment, besides.

In the short term, drinking too much alcohol can make you do some pretty stupid things. But although most people think regrets and hangovers are the only lasting effects of alcohol use, in the long term excessive drinking can damage not only your body but your brain. People are frequently warned about liver damage from alcohol abuse, but very few individuals are ever told about the frightening fact that chronic drinking can actually cause loss of tissue throughout the brain.

Other legal intoxicants also affect a person's sexual function. A groundbreaking study on the causes of sexual dysfunction found that for any given age of men, the most important risk factor for erectile difficulty was cigarette smoking. Contrary to the image of the cigarette as an accessory to the sexual act-a signal for flirtation, or a way to ignite a literal afterglow, smoking is actually harmful to your sexual health. Smoking is a major risk factor for impotence in men, affecting the health of the vasculature, overall blood flow, and male hormone levels. Smoking also has negative effects on female sexual function. By reducing estrogen levels, it increases the risk of vaginal dryness, painful periods, and hormonal contraceptive failure. What's more, smoking decreases fertility in both men and women. Although it may seem erotic to light a woman's cigarette with a match struck to flame, in the long run the sexy effect is overshadowed by the very real loss of your health.

In the 1950s and 1960s, methamphetamines (popularly known today as crystal meth) were used as a treatment for obesity and depression, but today the drug is causing more than its fair share of physical and mental health problems. In the beginning of 2006, the United Nations declared that use of crystal meth was approaching pandemic proportions-with frightening health consequences, both direct and indirect. Methamphetamine use is dangerous, and users risk numerous health issues, including stroke, cardiac arrhythmia, paranoia, hallucinations, and structural changes to the brain. It's also the leading cause of drug related emergency room visits in several states. Why do people use methamphetamines? Methamphetamines are easy to make from commonly available ingredients - labs can avoid the very real possibility of explosions. Methamphetamines make you feel good, keep you awake for days, and turn you on-which leads to their indirect consequences. Users want sex, and they tend not to be too careful about how they get it. Crystal meth use is associated with a significantly increased risk of STD transmission, because people taking it are far too intent on having sex to think about using condoms and practicing safer sex.

Although there is definitely a conscious component to arousal that is based in the CNS, there is also a component that is pure reflex. Some men and women with spinal cord damage can still experience physical arousal and orgasm even if they have no genital sensation. Sexual response can also be conditioned: the body can be retrained to respond to stimuli that it normally wouldn't perceive as sexual. These functions work through very different pathways, although their effects are both on the subconscious level. In the case of individuals with spinal cord injury, local nerve responses seem to be responsible for the ability to orgasm. The conditioned response, on the other hand, involves a change in brain activation where the mind itself is trained to respond in a sexual way to nonsexual stimuli.

Even though a serious sensation-destroying, or paralytic, spinal cord injury can leave a person still capable of sexual function, some much less severe injuries can demolish sexual capacity while leaving the rest of the body intact. Bicycle riding on an inappropriate saddle can, for example, lead to erectile dysfunction that can last from months to years. Other traumatic injuries to the pelvic region, like those that affect genital blood flow, vasculature, or nerves, can also cause erectile difficulty. So can penile fracture. In women, injury and scarring can lead to difficulty with penetration, as well as loss of sensation and interest in sex.

Peyronie's disease is a condition in which the connective tissue of the penis becomes scarred, possibly leading to penile curvature, shortening, disfigurement, or loss of capacity for erection. It's thought to occur in approximately three percent of older men, although numbers are probably much higher since it is an erectile dysfunction condition that is difficult for many men to discuss with their physicians. Although the exact cause of the disease is not known, it's thought to begin with some sort of trauma to the penis, possibly from damage occurring during vigorous sex. Then, if the damage doesn't heal successfully (perhaps due to an autoimmune response or excess inflammation), scar tissue can form and permanently injure the structure of the penis. Because of the nature of the disease, erectile dysfunction drugs and other medical treatments are not normally effective for resolving complications of Peyronie's disease. When erectile function is impaired or curvature becomes too extreme to allow for vaginal penetration, surgery is frequently the only option.

Just as every part of your body can be involved in sexual response, so can every part of your brain. Sexual thoughts and fantasies arise in the outer part of the brain-the cerebral cortex-and hormones are controlled by the hypothalamus. But our emotional and instinctual responses originate elsewhere: in the limbic system.

The limbic system is a network of brain structures involved in the experience and expression of emotion. Depression and other emotional problems can be the result of damage to the limbic system, which is also known as the "pleasure center" of the brain. Sexual emotions, and our responses to drugs, gambling, and the other pleasures of life are all processed in the limbic system, and direct stimulation of these areas of the brain can lead to physical arousal.

Other parts of the brain are engaged in sexual processing as well. When, for example, men are exposed to erotic imagery, several parts of their brains become active. The thinking parts of their brain pay attention to the stimulus, while the limbic system and hypothalamus provide the motivation and activity needed for erection. Another part of the brain, the amygdala, seems to play an important role in coordinating the sexual response, and some people believe it actually determines whether any given stimulus is perceived as sexual. The amygdala is a component of the temporal lobe and, has been shown to play a role in the sexual responsiveness of many animal species. It is intricately intertwined with the many different levels of the brain, from the cortex to the hypothalamus.

Although penile erection is basically a local reflex, it is usually stimulated by sensations that are processed in the brain. Smells, for example, can initiate sexual arousal, as can sights, sounds, or fantasies. One important area of the brain responsible for controlling erection is a specialized area of the hypothalamus known as the paraventricular nuclei (PVN). Nerve cells that start in the PVN make a hormone called oxytocin, and send it directly into not only other areas of the brain, but also the spinal cord and bloodstream. Some of these nerves send oxytocin directly into the autonomic pathways responsible for erection, taking much less time to stimulate the penis then it would if the oxytocin had to circulate through the blood.

If there is one chemical in charge of mediating sexual functioning, it is nitric oxide (NO). Although many chemicals are involved in the processes that control sexual arousal in men and women, nitric oxide plays an essential role in regulating blood flow. Blood flow is central to most of the obvious physical manifestations of the sexual response. Nitric oxide doesn't affect blood flow directly, but instead initiates a series of chemical reactions that allow the blood vessels to dilate. The most important milestones along this chemical pathway are two proteins known as cyclic GMP (CGMP) and PDE 5. CGMP is the protein that signals the muscles in the blood vessel walls to relax and fill with blood. It's critical that the amount of blood that flows into the affected vessels be strictly controlled, filling them to capacity-but not to bursting. That is the role of PDE 5. PDE 5 breaks down cGMP to prevent overstimulation of the blood vessels and excessive relaxation. When the system works, it provides an extremely effective mechanism to create and control arousal. However, problems start to occur if not enough cGMP is made or if cGMP is broken down too quickly. In either case, the blood vessels don't remain relaxed long enough to fill with sufficient blood to allow the penis to become erect, the clitoris to engorge, or vaginal lubrication to occur.

Current erectile dysfunction drugs work by blocking the effect of PDE 5 and allowing cGMP to remain active longer. These drugs can have beneficial sexual effects for men-and, some studies have found, for women. They make it easier for men to both achieve and maintain an erection, and for women to experience the vaginal lubrication and the enhanced clitoral sensitivity associated with normal sexual arousal.

Diabetes can have serious sexual side effects in men, including decreased libido, difficulty achieving orgasm, and retrograde ejaculation. The most well-known sexual complication of diabetes, however, is erectile dysfunction, which occurs in between 20 percent to 70 percent of diabetic men. Studies that have compared diabetic to non-diabetic men find that men with diabetes are almost twice as likely to have problems with erection.

Diabetic men are at increased risk for erectile dysfunction because of, among other reasons, the damage to nerves that diabetes causes throughout the body. When this damage occurs to the parasympathetic nerves of the penis, it directly affects local sensation and function. The less well controlled a patient's blood sugar levels, and the longer they've had diabetes, the more likely they are to experience nerve damage and erectile dysfunction.

Diabetic men can also have problems with their NO cascade. Not only may it be difficult for their brains to stimulate release of NO because of nerve damage, but diabetics also have increased production of arginase, an enzyme found in the liver.

This can make it difficult for them to make enough NO, because arginine is a necessary ingredient in NO production and too much arginase in the body can use it up.

The blood vessel damage that is so well known in diabetics can also cause erectile dysfunction. In uncontrolled diabetes, poorly managed blood flow can cause the smooth muscle of the penis to be replaced by less effective fibrous muscle. If enough fibrous muscle is present, it can make it impossible for a man to have an erection without a penile implant. Blood vessel changes in diabetic men can also reduce the volume of blood that enters the penis, and without sufficient blood flow it may be impossible to get or maintain an erection. One study found that more than 95 percent of diabetic men have impaired blood flow in their penile arteries.

Unfortunately, because of the types of physical damage associated with diabetes, standard medical treatments for erectile dysfunction are frequently less effective in diabetics than they are in other men. Researchers continue to investigate new treatments for this high-risk population.

Women with diabetes may also experience sexual health difficulties, including reduced desire, decreased capacity for arousal, and painful intercourse. Decreased vaginal lubrication, however, is the most significant component of sexual dysfunction in diabetic women. It is probably caused by vascular damage similar to that seen in men. Without sufficient blood flow into the vagina, there is insufficient transudation of fluid, and vaginal lubrication is reduced.

There has been relatively little quantitative research on sexual dysfunction in women with diabetes, although there has been intense study of diabetic men. The research that has been done suggests that between 20 percent to 70 percent of women with type 1 diabetes and approximately 40 percent of women with type 2 diabetes have some degree of sexual dysfunction. Figures vary widely, but studies have generally found that the prevalence of dysfunction in diabetic women is about twice that of non-diabetic women from the same population. Some scientists have suggested that this data may be complicated by body-image issues caused by cofactors of diabetes, like age and obesity. In other words, because type 1 diabetes is found in older, heavier women, it's hypothesized that biology may be less of the problem than loss of attractiveness. Can you imagine anyone suggesting that being bald or fat is an important cause of sexual dysfunction in men? This is just additional proof that further research is needed to clarify the nature of sexual dysfunction in diabetic women, so that appropriate treatment regimens can be developed. One thing, however, is clear: the more complications of diabetes a woman has, the more likely she is to have difficulties with her sex life.

Obesity and the health problems associated with it—vascular disease, hypertension, high cholesterol, and diabetes—are all linked to sexual dysfunction in both men and women. These diseases are associated with a process called endothelial dysfunction, a disruption of the interior layer of the blood vessels, which can affect the nitric oxide cycle in the body. In fact, because of the association between endothelial dysfunction and serious cardiovascular illness, some scientists have suggested that erectile dysfunction may be a useful warning sign for the early stages of heart disease in men.

Besides their effects on the nitric oxide cycle, these conditions and their treatments all have negative effects on systemic circulation. If blood vessels are too hardened to dilate, physiological arousal is impossible. Anti-hypertensive drugs can control high blood pressure, but they may make it difficult to attain the local increases in blood pressure necessary for erection and lubrication. Anything that strains the cardiovascular system also puts strain on the mechanics of sexual function.

Chronic diseases can also have debilitating effects on sexual function. They can affect the brain, the vasculature, and the nervous system, and can have direct effects on sexual function. Nerve damage from multiple sclerosis can affect sexual function by causing two opposite effects. It can create a reduction in genital sensation and a loss of interest in sexual activity; conversely, it can cause a hypersensitivity to genital stimulation, making sex uncomfortable to the point of pain. Kidney disease, in contrast, affects a person's sex life by altering both gonadal function and hormone production in the brain. When the kidneys are no longer able to filter the blood properly, it becomes contaminated with components that should be excreted in the urine. This can cause mixed signals affecting the hypothalamic and pituitary control of hormone synthesis, and it can also directly impair function of the ovaries and testicles. Furthermore, since high blood pressure is a common correlate of kidney disease, many of the sexual issues found in individuals with hypertension are also found in those with kidney disease.

Cancer and its treatments also have multiple deleterious effects on sexual function. The pain associated with the disease, and the massive discomfort, nausea, and fatigue common to its treatment, can reduce sexual interest and interfere with arousal. The physical changes that cancer patients experience, from hair loss to amputation, can also drastically affect self-image and diminish their interest in sex. Cancer patients may be unwilling to let a partner witness the effects of the disease on their body. Reproductive cancers can be particularly devastating to sexual function, due to changes in sensation or function and even loss of reproductive organs.

All in all, any illness that affects a person's ability to function in daily life is also likely to have negative effects on his or her sexual life. Expanding the definition of sex beyond the standard repertoire of genital sensation may allow people with chronic diseases to find greater happiness in their sexual activities and improve their overall quality of life as well.