Sections include: understanding basic drives, recent discoveries about basic drives, differences and disorders
We must eat to stay alive. We must have sex to reproduce, at least enough to carry on the species. We must sleep to remain healthy, though only recently have we begun to grasp how strong that connection is. The brain regulates each of these basic drives, controlling them automatically and unconsciously. At the same time, our conscious choices and unconscious psychological desires play important roles in how we express these needs. We generally decide when we will eat, what foods we eat, and how much. We choose when to go to bed, and when (or if) we set the alarm clock in the morning. Healthy sex is an inherently social act, requiring cooperation. Thus, while eating, sleeping, and sex are basic drives, our activities based on these drives are quite complex.
Given how vital and automatic these basic drives are, it is no surprise that the brain mechanisms serving them are located deep in the core of the brain. All three functions depend on nerve cells located in the hypothalamus, just above the pituitary gland. Through complex circuitry, other parts of the brain stem core orchestrate how these nerve cells are activated. The hypothalamus in turn connects to the limbic circuits, which organize the actions, emotions, and autonomic responses appropriate to satisfying each drive.
The drive to eat is a good example of how many factors are involved in these basic, life-sustaining activities. A complex interaction of pathways regulates hunger and feeding behaviors and involves the gastrointestinal tract, hormones in the blood, and pathways in the brain. In the brain, many different neurotransmitters play a role, with some stimulating and some inhibiting the drive to eat.
It seems that we are always primed to eat except when our brains sense certain appetite-suppressing neurochemicals, notably the hormone leptin. The stomach signals the brain when it is physically full, and the gut sends hormones with messages about what nutrients it has processed. The brain also seems to read the level of insulin in a person’s cerebrospinal fluid, which reflects the amount of fat stores in his or her body. All of these signals reach a brain that is also conditioned to respond to certain psychological factors: whether the occasion is right for eating, whether the clock is showing a mealtime, whether a person wants “comfort foods,” whether thea vailable food has any appeal, and so on. Only after our brains balance all those factors do we sit down for a meal.
Most of us have experienced how difficult it is to diet in the long term. After a while, it gets harder and harder to reduce food intake, and often it seems like it gets harder to lose weight, even if we are not eating very much. While we do not understand why this is, it is interesting that many of the chemicals in the brain that we know inhibit or stimulate feeding also seem to be involved in the modulation of mood, and perhaps reward. It would be reasonable to guess that this link between feeding and mood may have developed as a way for Mother Nature to motivate animals to find food, whatever the dangers. Thus, in many people, chronic dieting may be associated with uncomfortable feelings, and feeding with some sense of satisfaction or reward.
Another important connection is that feeding and energy balance are linked to energy metabolism. That is, for many of us, the body seems to be able to make some adjustment in how vigorously it burns fuel. During periods of dieting (or starvation) the body may be able to slow down its metabolism, so that it becomes harder to lose weight. And this slowdown in metabolism makes it easier to gain weight back when people resume normal eating. It may be that genetic differences in the efficiency of their metabolisms cause a predisposition for some people to gain weight and for others to be lean.
The sex drive, in turn, is a good example of how much activity in the body the hypothalamus cues. Even though sexual behavior is regulated by the brain (parts such as the hypothalamus, the amygdala, and the nucleus accumbens), other parts of the body, such as the spinal cord and some endocrine glands, are also involved. The endocrine glands, namely testicles in men, ovaries in women, and adrenal glands in both sexes, secrete so-called sex hormones (also called sex steroids), such as testosterone, estrogen, and progesterone.The secretion of these hormones is partially influenced by the brain, but the hormones also provide information (feedback) to the brain and have an effect on some brain functions.
The regulation of sexual behavior is very complex and not fully understood. The sex drive motivates a wide range of planned behaviors, from making eye contact to (according to some theorists) buying red sports cars. An individual can be sexually aroused by a vast array of sensory experiences, and simply by his or her own imagination. Arousal and the sex act itself cause the autonomic nervous system to stimulate many parts of people’s anatomy. Women and men experience its effects not simply in their genitals, but also in increased heart rate and breathing, sweating, erect nipples, muscle spasms in various parts of the body, and the pleasure of orgasm.
Modern science has broadened our understanding of human sexual functioning. Normal men and women experience a sequence of physiological responses to sexual stimulation. The sexual response cycle is usually divided into four phases: desire, excitement, orgasm, and resolution. Men experience one orgasm per cycle, women may experience more than one. Men and women may respond to different external erotic stimuli—men are more visual, women may respond more to romantic stories or tactile stimulation. The frequency and intensity of sexual intercourse vary from individual to individual. A person’s satisfaction with his or her sexual life is important, and popular generalizations about specific frequency of sexual intercourse do not necessarily apply to everyone.
Sexual behavior is diverse and determined by various factors. It is affected by relationships, life circumstances, stage of development, and culture. Some sexual behavior may be considered acceptable in one culture and not acceptable in another (extramarital sex, masturbation, oral sex). Sexual behavior develops throughout the life cycle. Early sexual experience may involve genital play in infants, which is considered part of normal development. Gender identity (“I am male/female”) is established by the age of 2 or 3. Puberty is usually marked by a rapid development of secondary sexual characteristics and the ability to engage in sexual intercourse and reproduction. Sexuality usually peaks in early adulthood and gradually declines thereafter. However, contrary to popular beliefs, a satisfactory sexual functioning is possible even in advanced age (if the person is healthy and physically fit).
We know from daily experience how much thought can go into eating and mating, while sleep may appear to be a much simpler activity. In fact, it has complexities that we are only beginning to understand. There are really two states of sleep: rapid eye movement (REM) and non-REM (NREM). In REM sleep, a person’s eyes move quickly back and forth behind his or her eyelids. The sleeper’s muscles are still, but his or her brain is active: REM sleep is when most dreaming occurs, and the electroencephalograph (EEG) readings of a brain in that state can resemble those of an alert person. NREM sleep comes in stages, from light dozing to deep, sound slumber. A regular night’s sleep for an adult includes several hours of both NREM and REM states, coming in cycles. And each of these states, studies have shown, serves a vital purpose for the brain.
Forming Concepts of the Basic Drives
The concept of a “drive” has undergone many changes over the last century. Sigmund Freud shaped the modern awareness of basic drives such as sexuality, aggression, and sleep, with a particular emphasis on dreaming. He championed the view that all of our mental life derives from deep instinctual forces of which we are unaware. Freud felt a keen need to anchor his psychoanalytic theory in brain science but, having little knowledge to build on, created it using only ingenuity, imagination, and clinical acumen. Since 1900, psychoanalysis has exerted a strong cultural and literary influence—without, however, advancing scientific status.
In the first half of the twentieth century experimental psychology focused strongly on the connections between human physiology and behavior. We learned a great deal about hunger, thirst, and sexual behavior, translating the concept
of drives into the language of motivation and learning. Neuroscientists then began to focus on the brain circuits that underlie these motivated behaviors. Studying people with brain lesions and stimulating particular neural areas showed the importance of subcortical regions, especially the hypothalamus and the limbic system, but these techniques could not specify the precise regulatory mechanisms involved.
Since 1950, we have made considerable progress thanks to the ability to test the effect of particular chemicals on individual brain cells. Studying the flow of blood within the human brain through positron-emission tomography (PET) and magnetic resonance imaging (MRI) scans also helps us understand the importance of particular neural regions. Today we have a detailed picture of how, for example, chemically coded neurons govern the circadian rhythm of rest and activity, and the sleep-wake cycle. The next frontier to explore is the genetic basis of these systems. It is already clear that the activation of the brain cells that serve basic drives affects how genes turn on or off and what proteins they produce.
Recent Discoveries About Basic Drives
For many years Western culture seemed to consider obesity to be a moral “weakness” that happened because people did not control their food intake. However, in the past decade, a number of strains of rodents were discovered that were very obese. This led to new discoveries of brain chemicals that played an important role in regulating feeding behavior. One finding that seemed to make an important change in the understanding of obesity was the discovery that some obese rodents had a disturbance of leptin, which resulted in their overeating and becoming obese. Leptin, a hormone secreted by fat cells, circulates in blood in proportion to body fat content. There are leptin receptors in the brain, and when leptin is administered directly into the brains of experimental animals, it reduces food intake. By contrast, a deficiency of leptin increases food intake. The reason these rodents had become obese is that a mutation had occurred in the gene that codes for leptin. Since then, disturbances of other brain neurochemicals have been found in other rodents that develop obesity.
However, the story of obesity is not so simple in humans. First, very few people have been found whose obesity was caused by a solitary defect in leptin, or, for that matter, any other brain chemical. Second, while it is known that the genes you inherit contribute to causing obesity,the population of the United States has become more and more obese over the past several decades. Clearly, genes don’t change in such a short period of time, so cultural factors also appear to contribute to the problem. The cultural factors are not well understood and may be related to lack of exercise or eating too much junk food, but this remains uncertain. At this point, obesity appears most commonly to be a combination
of genes (which ones remains unknown) and cultural and psychological factors. It is most likely that there are many different causes of obesity and that the proportion of the influence of genes and environment is different for each of us.
Recent research on sleep and dreaming reveals previously unsuspected links between this behavior and the other basic drives—a direct link to feeding and an indirect link to sexuality.
Animals that are deprived of sleep develop incapacitating and ultimately fatal disorders of temperature control, metabolic balance, and immune function. Sleep itself obviously saves calories. Being still is integral to deep sleep, and a still body uses less energy. Furthermore, a sleeping body in a bedroom (or a nest) and under cover (or feathers), especially snuggled beside a mate or offspring, does not radiate as much heat as it does out-of-doors. So in the short term we save in two ways: body heat and metabolic energy.
But that is not all: sleep somehow helps us restore our capacity to regulate both body heat and metabolism. After sleep we are better able to adjust to changes in the temperature around us so as to make efficient use of food. These facts emerged from long-term sleep-deprivation studies of rats. After about two weeks rats deprived of sleep begin to overeat, in spite of which they still lose weight. They seek sources of heat, but their body temperature falls anyway. These rats are like a wood stove with the draft open: all the heat (and nutritional benefit) goes up the chimney.
These changes lead to death within four weeks if a rat is totally deprived of sleep, or within six weeks if only the REM sleep is interrupted. Since REM sleep occupies only 15 percent to 25 percent of sleep, it would seem to constitute a supersleep state in terms of its efficiency in the maintenance of these thermoregulatory functions. If sleep is allowed, recovery is rapid and dramatic, but if deprivation is continued, the ensuing death may be associated with massive infection caused by bacterial invasion of the bloodstream from the animal’s own digestive tract. This finding shows that immune functions also depend on sleep. While no such studies have een done on humans, unscrupulous interrogators
have long known that sleep deprivation is one of the most powerful ways to break down a prisoner’s will.
Learning is also highly sleep dependent, as recent experiments have clearly shown. Subjects who learn a visual discrimination task in which they are asked to detect a stimulus with different properties from the array in which it is embedded actually improve their skill on the next day’s retest if they have ample NREM sleep in the first quarter and REM sleep in the last quarter of the night. If they are systematically deprived of sleep, they have to relearn the skill from scratch. This finding suggests that sleep not only consolidates new learning but may even improve it.
At a behavioral level, it is obvious that sleeping with another person provides regular mating opportunities. It is probably no accident that the vernacular phrase for sexual intercourse is “sleeping together.” But there are deeper links as well. Major hormones essential to sexual development and reproductive capability are released on a schedule related to sleep. Body development, including the ratio of muscle to body mass, is regulated by growth hormone, about 95 percent of which is released in sleep. In some animals, sexual intercourse is immediately followed by sleep; lying still at this time may favor fertility. So sexual capacity, performance, and efficiency are all enhanced by sleep.
Differences and Disorders
All three basic drives are quite variable in their expression, both among individuals and between two people. Just as there are Jack Sprats who eat no fat and Mrs. Sprats who eat no lean, there are sexually uninterested and sexually insatiable individuals. There are adults who sleep as long as ten hours a day, or as short as four hours a day.
New information now suggests that alterations in brain chemistry may contribute to such eating disorders as anorexia nervosa. This disorder most commonly occurs in young women who see themselves as too fat, go on extreme diets, and become very thin. There is evidence that people with anorexia nervosa have too much activity of the neurotransmitter serotonin, which may increase anxiety and satiety. It may be that people with anorexia starve themselves because food restriction causes a reduction in tryptophan, an essential amino acid that we get only from food and that is the chemical which makes serotonin. This may be a mechanism whereby people with anorexia can reduce serotonin activity and make themselves less anxious.
Information is widely available about the beneficial influences of a healthy diet and exercise. Such advice is often not followed, which may contribute to the high incidence of obesity and adult-onset diabetes in our culture. Whether this is related to some genetic influence in people that drives hunger or the difficulties people have in balancing short-term rewards (for example, too many appealing desserts) with long-term benefits (not getting diabetes 30 years in the future) is not clear. The current treatments for obesity, whether medication or some form of talk therapy, have limited success, and most people eventually relapse. A better understanding of these problems would thus be likely to result in more effective treatment. (See also our section on eating disorders.)
Sexual functioning could be impaired in two ways: as a disturbance related to a particular phase of the sexual cycle, or as a disturbance involving unusual objects or activity. The first group, sexual dysfunctions, include sexual desire disorders (characterized either by a lack of sexual fantasies and desire for sexual activity or by aversion to or avoidance of genital contact), sexual arousal disorders (characterized by the failure to attain or maintain erection in men, and failure to attain or maintain lubrication in women), and orgasm disorders (recurrent delay or inability to achieve orgasm or, in men, inability to achieve ejaculation or ejaculating before they wish—so-called premature ejaculation). Other dysfunction may include painful sex. The second group, so-called paraphilias, involve unusual fantasies, urges, or practices (exposing oneself, sex with minors, sadistic sex, and so on). Homosexuality is not considered a dysfunction or abnormal behavior.
People may experience various sexual dysfunctions that may not have a detectable or obvious cause or that may result from various medical conditions (for example, diabetes mellitus, prostate surgery, trauma), substances (chronic abuse of alcohol and other drugs), medications (for hypertension, depression, heart conditions, and others), or psychological problems (conflict with partner, depression, anxiety).
Good sexual functioning requires good physical health and a good relationship with the partner. Obesity, lack of physical stamina, illnesses, chronic exhaustion, smoking, substance abuse, chronic conflict may be some of the causes of sexual dysfunction. Healthy diet and exercise, and a healthy lifestyle in general, help maintain a good sexual life. However, various misconceptions about sex could hamper sexual functioning further. The best approach to impairment of sexual functioning is to discuss it with a physician, as complete physical examination and laboratory tests may help reveal the underlying cause. It is not advisable to treat oneself with various over-the-counter substances, as these are mostly of unproven efficacy and could even be dangerous.
As old age arrives, changes in sleep patterns often develop. A person’s capacity for deep, refreshing, and prolonged sleep may deteriorate, with more frequent interruptions at night and a lingering sense of fatigue in the daytime. Old age has been justly characterized as more time in which to do less. Individuals can best respond to these biological changes by making deliberate and determined changes in their lifestyles.
Insomnia (or naturally diminished sleep capacity) is the leading complaint people bring to their physicians for remedy. Too often, doctors prescribe sedatives indiscriminately. While it may initially be effective, sleep medication is not physiological and may induce tolerance and dependency. Users need to be cautioned and should exercise both flexibility and restraint. The same counsel should be offered to those suffering from minor excesses and deficiencies of appetite, whether sexual or culinary. As with more problematic eating and sexual behaviors, disorders of sleep like narcolepsy, sleep apnea syndrome, and periodic leg movement, require a specialist’s evaluation and treatment.
In all of these domains, brief psychotherapy can help inculcate a healthy, versatile, and philosophical attitude that walks the fine line between unrealistic optimism (the fountain of youth) and nihilistic despair (being old before one’s time).
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