In order to carry out correct behavior—that is to say, correct in relation to the survival of the individual—humans have developed innate drives, desires, and emotions and the ability to remember and learn. These fundamental features of living depend on the entire brain, yet there is one part of the brain that organizes metabolism, growth, sexual differentiation, and the desires and drives necessary to achieve these aspects of life. This is the hypothalamus and a region in front of it comprising the septal and preoptic areas. That such basic aspects of life might depend on a small region of the brain was conceived in the 1920s by the Swiss physiologist Walter Rudolf Hess and later amplified by German physiologist Erich von Holst. Hess implanted electrodes in the hypothalamus and in septal and preoptic nuclei of cats, stimulated them, and observed the animals’ behavior. Finally, he made minute lesions by means of these electrodes and again observed the effects on behavior. With this technique he showed that certain kinds of behavior were organized essentially by just a few neurons in these regions of the brain. Later, von Holst stimulated electrodes by remote control after placing the animals in various biologically meaningful conditions.
When such acts result from artificial stimulation of the neurons, the accompanying emotion also occurs, as do the movements expressing that emotion.
The hypothalamus, in company with the pituitary gland, controls the emission of hormones, body temperature, blood pressure and the rate and force of the heartbeat, and water and electrolyte levels. The maintenance of these and other changing events within normal limits is called homeostasis; this includes behavior aimed at keeping the body in a correct and thus comfortable environment.
The hypothalamus is also the center for organizing the activity of the two parts of the autonomic system, the parasympathetic and the sympathetic (see above The autonomic nervous system). Above the hypothalamus, regions of the cerebral hemispheres most closely connected to the parasympathetic regions are the orbital surface of the frontal lobes, the insula, and the anterior part of the temporal lobe. The regions most closely connected to the sympathetic regions are the anterior nucleus of the thalamus, the hippocampus, and the nuclei connected to these structures.
In general, the regions of the cerebral hemispheres that are closely related to the hypothalamus are those parts that together constitute the limbic lobe, first considered as a unit and given its name in 1878 by the French anatomist Paul Broca. Together with related nuclei, it is usually called the limbic system, consisting of the cingulate and parahippocampal gyri, the hippocampus, the amygdala, the septal and preoptic nuclei, and their various connections.
The autonomic system also involves the hypothalamus in controlling movement. Emotional expression, which depends greatly on the sympathetic nervous system, is controlled by regions of the cerebral hemispheres above the hypothalamus and by the midbrain below it.
Emotion
A great deal of human behavior involves social interaction. Although the whole brain contributes to social activities, certain parts of the cerebral hemispheres are particularly involved. The surgical procedure of leucotomy, cutting through the white matter that connects parts of the frontal lobes with the thalamus, upsets this aspect of behavior. This procedure, proposed by the Spanish neurologist Egas Moniz, used to be performed for severe depression or obsessional neuroses. After the procedure, patients lacked the usual inhibitions that were socially demanded, appearing to obey the first impulse that occurred to them. They told people what they thought of them without regard for the necessary conventions of civilization.
Which parts of the cerebral hemispheres produce emotion has been learned from patients with epilepsy and from surgical procedures under local anesthesia in which the brain is electrically stimulated. The limbic lobe, including the hippocampus, is particularly important in producing emotion. Stimulating certain regions of the temporal lobes produces an intense feeling of fear or dread; stimulating nearby regions produces a feeling of isolation and loneliness, other regions a feeling of disgust, and yet others intense sorrow, depression, anxiety, ecstasy, and, occasionally, guilt.
In addition to these regions of the cerebral cortex and the hypothalamus, regions of the thalamus also contribute to the genesis of emotion. The hypothalamus itself does not initiate behavior; that is done by the cerebral hemispheres.
The defense reaction
When certain neurons of the hypothalamus are excited, an individual either becomes aggressive or flees. These two opposite behaviors are together called the defense reaction, or the fight-or-flight response; both are in the repertoire of all vertebrates. The defense reaction is accompanied by strong sympathetic activity. Aggression is also influenced by the production of androgen hormones.
Mating
The total act of copulation is organized in the anterior part of the hypothalamus and the neighboring septal region. In the male, erection of the penis and the ejaculation of semen are organized in this area, which is adjacent to the area that controls urination. Under normal circumstances, the neurons that organize mating behavior do so only when they receive relevant hormones in their blood supply. But when the septal region is electrically stimulated in conscious patients, sexual emotions and thoughts are produced.
There are visible differences between the male and female sexes in nuclei of the central nervous system related to reproduction. These differences are a form of sexual dimorphism.
Urination and defecation
Electrical stimulation in cats of regions in and related to the anterior part of the hypothalamus can induce the behavior of expelling or retaining urine and feces. When electrodes planted in these regions are stimulated by radio waves, the cat stops whatever it is doing and behaves as though it is going to urinate or defecate. It goes through its usual behavior of digging a hole, squatting, and assuming the correct posture, and then it passes urine or feces. At the end, it even goes through its customary ritual of hiding its excreta.
Eating and drinking
The eating and drinking centers are in the lateral and ventromedial regions of the hypothalamus, although such basic aspects of living concern most of the brain. If the lateral region is experimentally destroyed, the animal consumes less food or stops eating altogether; if the ventromedial region is destroyed, it eats enormously. When neurons of the lateral region are electrically stimulated, a monkey eats, and when those of the ventromedial area are stimulated, the monkey stops eating. There is an increase in the activity of these neurons when the monkey looks at food, but only when it is hungry. Receptors in the lateral region monitor blood glucose and are stimulated only when blood glucose is low; satiety stops their response.
Hunger does not depend only on these glucose receptors. Severe hunger is associated with contractions of the stomach, which are felt almost as a sensation of pain. Yet neither is this an essential mechanism for feeling hungry, as patients who have had total removal of the stomach still feel hunger. In experiments in rats, it is found that stress may make the animal either increase or reduce the amount it eats. This is probably the same in humans.
When certain neurons in the same regions of the hypothalamus are experimentally destroyed, animals lose the urge to drink, although they continue to eat normally. Stimulation of these neurons causes them to drink excessively. Control of drinking depends on osmoreceptors located throughout the hypothalamus. When receptors detect a minimal increase in the concentration of dissolved substances in the extracellular fluid, which indicates cellular dehydration, the sensation of thirst occurs. A less-important contributor to the sensation of thirst is a reduction in blood volume. Dryness of the mouth can also be a component of thirst, noted by receptors in the mucous membrane. The feeling of having drunk enough depends not only on the hypothalamic neurons but also on receptors in the wall of the stomach, which report when the stomach is full.
Both glucose receptors and osmoreceptors are sensitive to the temperature of the passing blood. When the temperature starts to rise, one feels thirsty but not hungry; cooling the blood makes one feel hungry.
Temperature regulation
To maintain homeostasis, heat production and heat loss must be balanced. This is achieved by both the somatomotor and sympathetic systems. The obvious behavioral way of keeping warm or cool is by moving into a correct environment. The posture of the body is also used to balance heat production and heat loss. When one is hot, the body stretches out—in physiological terms, extends—thus presenting a large surface to the ambient air and losing heat. When one is cold, the body curls itself up—in physiological terms, flexes—thus presenting the smallest area to the ambient temperature.
The sympathetic system is the most important part of the nervous system for controlling body temperature. On a long-term basis, when the climate is cold, the sympathetic system produces heat by its control of certain fat cells called brown adipose tissue. From these cells, fatty acids are released, and heat is produced by their chemical breakdown.
Body temperature fluctuates regularly within 24 hours; this is a type of circadian rhythm (see below). It also fluctuates in rhythm according to the menstrual cycle. During fever, the body temperature is set at a higher point than normal.
Reward and punishment
In a fundamental discovery made in 1954, Canadian researchers James Olds and Peter Milner found that stimulation of certain regions of the brain of the rat acted as a reward in teaching the animals to run mazes and solve problems. The conclusion from such experiments is that stimulation gives the animals pleasure. The discovery has also been confirmed in humans. These regions are called pleasure, or reward, centers. One important center is in the septal region, and there are reward centers in the hypothalamus and in the temporal lobes of the cerebral hemispheres as well. When the septal region is stimulated in conscious patients undergoing neurosurgery, they experience feelings of pleasure, optimism, euphoria, and happiness.
Regions of the brain also clearly cause rats distress when electrically stimulated; these are called aversive centers. However, the existence of an aversive center is less certain than that of a reward center. Electrodes stimulating neurons or neural pathways may cause an animal to have pain, anxiety, fear, or any unpleasant feeling or emotion. These pathways are not necessarily centers that provide punishment in the sense that a reward center provides pleasure. Therefore, it is not definitely known that connections to aversive centers punish the animal for biologically wrong behavior, but it is thought that correct behavior is rewarded by pleasure provided by neurons of the brain.
