The different hypothalamic mechanisms for controlling multiple functions of the body are so important that they are discussed in multiple chapters throughout this text. For instance, the role of the hypothalamus to help regulate arterial pressure is discussed in Chapter 18, thirst and water conservation in Chapter 30, appetite and energy expenditure in Chapter 72, temperature regulation in Chapter 74, and endocrine control in Chapter 76. To illustrate the organization of the hypothalamus as a functional unit, we summarize the more important of its vegetative and endocrine functions here as well.
Figures 1 and 2 show enlarged sagittal and coronal views of the hypothalamus, which represents only a small area in Figure 3. Take a few minutes to study these diagrams; especially note in Figure 1 the multiple activities that are excited or inhibited when respective hypothalamic nuclei are stimulated. In addition to the centers shown in Figure 1, a large lateral hypothalamic area (shown in Figure 2) is present on each side of the hypothalamus. The lateral areas are especially important in controlling thirst, hunger, and many of the emotional drives.
Fig1. Control centers of the hypothalamus (sagittal view).
Fig2. Coronal view of the hypothalamus, showing the mediolateral positions of the respective hypothalamic nuclei.
Fig3. Anatomy of the limbic system, shown in the dark pink area. (Modified from Warwick R, Williams PL: Gray’s Anatomy, 35th ed. London: Longman Group Ltd, 1973.)
A word of caution must be issued when studying these diagrams because the areas that cause specific activities are not nearly as accurately localized as suggested in the figures. Also, it is not known whether the effects noted in the figures result from stimulation of specific control nuclei or merely from activation of fiber tracts leading from or to control nuclei located elsewhere. With this caution in mind, we can give the following general description of the vegetative and control functions of the hypothalamus.
Cardiovascular Regulation. Stimulation of different areas throughout the hypothalamus can cause many neurogenic effects on the cardiovascular system, including changes in arterial pressure and heart rate. In general, stimulation in the posterior and lateral hypothalamus increases the arterial pressure and heart rate, whereas stimulation in the preoptic area often has opposite effects, causing a decrease in both heart rate and arterial pressure. These effects are transmitted mainly through specific cardiovascular control centers in the reticular regions of the pons and medulla.
Body Temperature Regulation. The anterior portion of the hypothalamus, especially the preoptic area, is concerned with regulation of body temperature. An increase in temperature of the blood flowing through this area increases activity of temperature-sensitive neurons, where as a decrease in temperature decreases their activity. In turn, these neurons control mechanisms for increasing or decreasing body temperature, as discussed in Chapter 74.
Body Water Regulation. The hypothalamus regulates body water in two ways: (1) by creating the sensation of thirst, which drives the animal or person to drink water, and (2) by controlling the excretion of water into the urine. An area called the thirst center is located in the lateral hypothalamus. When the fluid electrolytes in either this center or closely allied areas become too concentrated, the animal develops an intense desire to drink water; it will search out the nearest source of water and drink enough to return the electrolyte concentration of the thirst center to normal.
Control of renal excretion of water is vested mainly in the supraoptic nuclei. When the body fluids become too concentrated, the neurons of these areas become stimulated. Nerve fibers from these neurons project downward through the infundibulum of the hypothalamus into the posterior pituitary gland, where the nerve endings secrete the hormone antidiuretic hormone (also called vasopressin). This hormone is then absorbed into the blood and transported to the kidneys, where it acts on the collecting ducts of the kidneys to cause increased reabsorption of water. This action decreases loss of water into the urine but allows continuing excretion of electrolytes, thus decreasing the concentration of the body fluids back toward normal. These functions are presented in Chapter 29.
Regulation of Uterine Contractility and Milk Ejection from the Breasts. Stimulation of the paraventricular nuclei causes their neuronal cells to secrete the hormone oxytocin. Secretion of this hormone in turn causes increased contractility of the uterus, as well as contraction of the myoepithelial cells surrounding the alveoli of the breasts, which then causes the alveoli to empty their milk through the nipples.
At the end of pregnancy, especially large quantities of oxytocin are secreted, and this secretion helps promote labor contractions that expel the baby. Then, whenever the baby suckles the mother’s breast, a reflex signal from the nipple to the posterior hypothalamus also causes oxytocin release, and the oxytocin now performs the necessary function of contracting the ductules of the breast, thereby expelling milk through the nipples so that the baby can nourish itself. These functions are discussed in Chapter 83.
Gastrointestinal and Feeding Regulation. Stimulation of several areas of the hypothalamus causes an animal to experience extreme hunger, a voracious appetite, and an intense desire to search for food. One area associated with hunger is the lateral hypothalamic area. Conversely, damage to this area on both sides of the hypothalamus causes the animal to lose desire for food, sometimes causing lethal starvation, as discussed in Chapter 72.
A center that opposes the desire for food, called the satiety center, is located in the ventromedial nuclei. When this center is stimulated electrically, an animal that is eating food suddenly stops eating and shows complete indifference to food. However, if this area is destroyed bilaterally, the animal cannot be satiated; instead, its hypothalamic hunger centers become overactive, so it has a voracious appetite, resulting eventually in tremendous obesity. The arcuate nucleus of the hypo thalamus contains at least two different types neurons that, when stimulated, lead either to increased or de creased appetite. Another area of the hypothalamus that enters into overall control of gastrointestinal activity is the mammillary bodies, which control at least partially the patterns of many feeding reflexes, such as licking the lips and swallowing.
Hypothalamic Control of Endocrine Hormone Secretion by the Anterior Pituitary Gland. Stimulation of certain areas of the hypothalamus also causes the anterior pituitary gland to secrete its endocrine hormones. This subject is discussed in detail in Chapter 75 in relation to neural control of the endocrine glands. Briefly, the basic mechanisms are as follows: The anterior pituitary gland receives its blood supply mainly from blood that flows first through the lower part of the hypothalamus and then through the anterior pituitary vascular sinuses. As the blood courses through the hypothalamus before reaching the anterior pituitary, specific releasing and inhibitory hormones are secreted into the blood by various hypothalamic nuclei. These hormones are then transported via the blood to the anterior pituitary gland, where they act on the glandular cells to control release of specific anterior pituitary hormones.
Summary. Several areas of the hypothalamus control specific vegetative and endocrine functions. The functions of these areas are not fully understood, so the specification given earlier of different areas for different hypothalamic functions is still partially tentative.
