The hypothalamus is a small but important area in the center of the brain. It plays an important role in hormone production and helps to stimulate many important processes in the body and is located in the brain, between the pituitary gland and thalamus. When the hypothalamus is not working properly, it can cause problems in the body that lead to a wide range of rare disorders.
Maintaining hypothalamic health is vital because of this. Homeostasis means a healthful, balanced bodily state. The body is always trying to achieve this balance. The hypothalamus acts as the connector between the endocrine and nervous systems to achieve this. It plays a part in many essential functions of the body such as:. As different systems and parts of the body send signals to the brain, they alert the hypothalamus to any unbalanced factors that need addressing.
The hypothalamus then responds by releasing the right hormones into the bloodstream to balance the body. One example of this is the remarkable ability of a human being to maintain an internal temperature of If the hypothalamus receives a signal that the internal temperature is too high, it will tell the body to sweat.
If it receives the signal that the temperature is too cold, the body will create its own heat by shivering. To maintain homeostasis, the hypothalamus is responsible for creating or controlling many hormones in the body. The hypothalamus works with the pituitary gland, which makes and sends other important hormones around the body.
Together, the hypothalamus and pituitary gland control many of the glands that produce hormones of the body, called the endocrine system.
This includes the adrenal cortex, gonads, and thyroid. The hypothalamus also directly influences growth hormones. It commands the pituitary gland to either increase or decrease their presence in the body, which is essential for both growing children and fully developed adults. A hypothalamic disease is any disorder that prevents the hypothalamus from functioning correctly. These diseases are very hard to pinpoint and diagnose because the hypothalamus has a wide range of roles in the endocrine system.
The hypothalamus also serves the vital purpose of signaling that the pituitary gland should release hormones to the rest of the endocrine system. For example, neither the body temperature nor the levels of salts and minerals i. Communication among various regions of the body also is essential for enabling the organism to respond appropriately to any changes in the internal and external environments.
Two systems help ensure communication: the nervous system and the hormonal i. The nervous system generally allows rapid transmission i. Conversely, hormonal communication, which relies on the production and release of hormones from various glands and on the transport of those hormones via the bloodstream, is better suited for situations that require more widespread and longer lasting regulatory actions.
Thus, the two communication systems complement each other. In addition, both systems interact: Stimuli from the nervous system can influence the release of certain hormones and vice versa. Generally speaking, hormones control the growth, development, and metabolism of the body; the electrolyte composition of bodily fluids; and reproduction.
This article provides an overview of the hormone systems involved in those regulatory processes. The article first summarizes some of the basic characteristics of hormone-mediated communication within the body, then reviews the various glands involved in those processes and the major hormones they produce.
For more in-depth information on those hormones, the reader should consult endocrinology textbooks e. Finally, the article presents various endocrine systems in which hormones produced in several organs cooperate to achieve the desired regulatory effects. The discussions focus primarily on the system responses in normal, healthy people. Hormones are molecules that are produced by endocrine glands, including the hypothalamus, pituitary gland, adrenal glands, gonads, i.
Some hormones have only a few specific target cells, whereas other hormones affect numerous cell types throughout the body. The target cells for each hormone are characterized by the presence of certain docking molecules i. Schematic representation of the location of the major hormone-producing i. For the purposes of illustration, both male and female endocrine organs are presented here.
Several classes of hormones exist, including steroids, amino acid derivatives, and polypeptides and proteins. Those hormone classes differ in their general molecular structures e. As a result of the structural differences, their mechanisms of action e.
Steroids, which are produced by the gonads and part of the adrenal gland i. The molecules can enter their target cells and interact with receptors in the fluid that fills the cell i. Amino acid derivatives are modified versions of some of the building blocks of proteins. The thyroid gland and another region of the adrenal glands i.
Like steroids, amino acid derivatives can enter the cell, where they interact with receptor proteins that are already associated with specific DNA regions.
The interaction modifies the activity of the affected genes. Polypeptide and protein hormones are chains of amino acids of various lengths from three to several hundred amino acids.
These hormones are found primarily in the hypothalamus, pituitary gland, and pancreas. In some instances, they are derived from inactive precursors, or pro-hormones, which can be cleaved into one or more active hormones. Because of their chemical structure, the polypeptide and protein hormones cannot enter cells. Instead, they interact with receptors on the cell surface. To achieve this control, many bodily functions are regulated not by a single hormone but by several hormones that regulate each other see figure 2.
For example, for many hormone systems, the hypothalamus secretes so-called releasing hormones, which are transported via the blood to the pituitary gland. There, the releasing hormones induce the production and secretion of pituitary hormones, which in turn are transported by the blood to their target glands e. In those glands, the interaction of the pituitary hormones with their respective target cells results in the release of the hormones that ultimately influence the organs targeted by the hormone cascade.
Schematic representation of negative feedback mechanisms that control endocrine system activity. For a short-loop negative feedback mechanism, pituitary hormones act directly back on the hypothalamus, inhibiting the release of hypothalamic hormones.
Constant feedback from the target glands to the hypothalamus and pituitary gland ensures that the activity of the hormone system involved remains within appropriate boundaries. In some instances, a so-called short-loop feedback occurs, in which pituitary hormones directly act back on the hypothalamus. The sensitivity with which these negative feedback systems operate i. For example, the progressive reduction in sensitivity of the hypothalamus and pituitary to negative feedback by gonadal steroid hormones plays an important role in sexual maturation.
Such a mechanism requires a specific threshold level, however, at which the positive feedback loop is turned off in order to maintain a stable system. Hormones Produced by the Major Hormone-Producing i. The hypothalamus is a small region located within the brain that controls many bodily functions, including eating and drinking, sexual functions and behaviors, blood pressure and heart rate, body temperature maintenance, the sleep-wake cycle, and emotional states e.
Hypothalamic hormones play pivotal roles in the regulation of many of those functions. Because the hypothalamus is part of the central nervous system, the hypothalamic hormones actually are produced by nerve cells i. In addition, because signals from other neurons can modulate the release of hypothalamic hormones, the hypothalamus serves as the major link between the nervous and endocrine systems.
For example, the hypothalamus receives information from higher brain centers that respond to various environmental signals. Consequently, hypothalamic function is influenced by both the external and internal environments as well as by hormone feedback.
Stimuli from the external environment that indirectly influence hypothalamic function include the light-dark cycle; temperature; signals from other members of the same species; and a wide variety of visual, auditory, olfactory, and sensory stimuli.
The communication between other brain areas and the hypothalamus, which conveys information about the internal environment, involves electrochemical signal transmission through molecules called neurotransmitters e. The complex interplay of the actions of various neurotransmitters regulates the production and release of hormones from the hypothalamus.
The hypothalamic hormones are released into blood vessels that connect the hypothalamus and the pituitary gland i. Because they generally promote or inhibit the release of hormones from the pituitary gland, hypothalamic hormones are commonly called releasing or inhibiting hormones.
The major releasing and inhibiting hormones include the following also see table , p. Corticotropin-releasing hormone CRH , which is part of the hormone system regulating carbohydrate, protein, and fat metabolism as well as sodium and water balance in the body.
Gonadotropin-releasing hormone GnRH , which helps control sexual and reproductive functions, including pregnancy and lactation i.
Thyrotropin-releasing hormone TRH , which is part of the hormone system controlling the metabolic processes of all cells and which contributes to the hormonal regulation of lactation. Somatostatin, which also affects bone and muscle growth but has the opposite effect as that of GHRH. Dopamine, a substance that functions primarily as a neurotransmitter but also has some hormonal effects, such as repressing lactation until it is needed after childbirth.
The pituitary also sometimes called the hypophysis is a gland about the size of a small marble and is located in the brain directly below the hypothalamus. The pituitary gland consists of two parts: the anterior pituitary and the posterior pituitary.
The anterior pituitary produces several important hormones that either stimulate target glands e. The pituitary hormones include adrenocorticotropic hormone ACTH ; gonadotropins; thyroid-stimulating hormone TSH , also called thyrotropin; growth hormone GH ; and prolactin. Thus, ACTH stimulates the adrenal cortex to produce corticosteroid hormones—primarily cortisol—as well as small amounts of female and male sex hormones.
The gonadotropins comprise two molecules, luteinizing hormone LH and follicle-stimulating hormone FSH. These two hormones regulate the production of female and male sex hormones in the ovaries and testes as well as the production of the germ cells—that is, the egg cells i.
TSH stimulates the thyroid gland to produce and release thyroid hormone. The remaining two pituitary hormones, GH and prolactin, directly affect their target organs. GH is the most abundant of the pituitary hormones. For example, it stimulates the linear growth of the bones; promotes the growth of internal organs, fat i. Accordingly, the GH levels in the blood are highest during early childhood and puberty and decline thereafter. Nevertheless, even relatively low GH levels still may be important later in life, and GH deficiency may contribute to some symptoms of aging.
In addition to its growth-promoting role, GH affects carbohydrate, protein, and fat i. Thus, GH increases the levels of the sugar glucose in the blood by reducing glucose uptake by muscle cells and adipose tissue and by promoting glucose production i. GH also enhances the uptake of amino acids from the blood into cells, as well as their incorporation into proteins, and stimulates the breakdown of lipids in adipose tissue.
To elicit these various effects, GH modulates the activities of numerous target organs, including the liver, kidneys, bone, cartilage, skeletal muscle, and adipose cells. For some of these effects, GH acts directly on the target cells. In other cases, however, GH acts indirectly by stimulating the production of a molecule called insulin-like growth factor 1 IGF-1 in the liver and kidneys.
The blood then transports IGF-1 to the target organs, where it binds to specific receptors on the cells. This interaction then may lead to the increased DNA production and cell division that underlie the growth process. This regulatory mechanism also involves a short-loop feedback component, by which GH acts on the hypothalamus to stimulate somatostatin release.
In addition, GH release is enhanced by stress, such as low blood sugar levels i. Acute and chronic alcohol consumption have been shown to reduce the levels of GH and IGF-1 in the blood.
Both effects have been observed in animals as well as in humans. Those deleterious effects of alcohol may be particularly harmful to adolescents, who require GH for normal development and puberty.
Together with other hormones, prolactin plays a central role in the development of the female breast and in the initiation and maintenance of lactation after childbirth. Several factors control prolactin release from the anterior pituitary.
For example, prolactin is released in increasing amounts in response to the rise in estrogen levels in the blood that occurs during pregnancy. In nursing women, prolactin is released in response to suckling by the infant. Several releasing and inhibitory factors from the hypothalamus also control prolactin release.
The most important of those factors is dopamine, which has an inhibitory effect. Alcohol consumption by nursing women can influence lactation both through its effects on the release of prolactin and oxytocin see the following section and through its effects on the milk-producing i.
The posterior pituitary does not produce its own hormones; instead, it stores two hormones—vasopressin and oxytocin—that are produced by neurons in the hypothalamus. Both hormones collect at the ends of the neurons, which are located in the hypothalamus and extend to the posterior pituitary. Thus, AVP release promotes the reabsorption of water from the urine in the kidneys. Through this mechanism, the body reduces urine volume and conserves water.
AVP release from the pituitary is controlled by the concentration of sodium in the blood as well as by blood volume and blood pressure. For example, high blood pressure or increased blood volume results in the inhibition of AVP release. Consequently, more water is released with the urine, and both blood pressure and blood volume are reduced.
Alcohol also has been shown to inhibit AVP release. Conversely, certain other drugs e. Oxytocin, the second hormone stored in the posterior pituitary, stimulates the contractions of the uterus during childbirth. In nursing women, the hormone activates milk ejection in response to suckling by the infant i. The adrenal glands are small structures located on top of the kidneys. Structurally, they consist of an outer layer i. The adrenal cortex produces numerous hormones, primarily corticosteroids i.
The cortex is also the source of small amounts of sex hormones; those amounts, however, are insignificant compared with the amounts normally produced by the ovaries and testes. The adrenal medulla generates two substances—adrenaline and noradrenaline—that are released as part of the fight-or-flight response to various stress factors. Hormones from the hypothalamus reach the anterior pituitary via the hypophyseal portal system. The anterior pituitary produces seven hormones.
The endocrine system regulates the growth of the human body, protein synthesis, and cellular replication. A major hormone involved in this process is growth hormone GH , also called somatotropin—a protein hormone produced and secreted by the anterior pituitary gland. Its primary function is anabolic; it promotes protein synthesis and tissue building through direct and indirect mechanisms Figure 4.
Figure 4. Growth hormone GH directly accelerates the rate of protein synthesis in skeletal muscle and bones. Insulin-like growth factor 1 IGF-1 is activated by growth hormone and indirectly supports the formation of new proteins in muscle cells and bone.
A glucose-sparing effect occurs when GH stimulates lipolysis, or the breakdown of adipose tissue, releasing fatty acids into the blood. As a result, many tissues switch from glucose to fatty acids as their main energy source, which means that less glucose is taken up from the bloodstream.
GH also initiates the diabetogenic effect in which GH stimulates the liver to break down glycogen to glucose, which is then deposited into the blood. Blood glucose levels rise as the result of a combination of glucose-sparing and diabetogenic effects.
GH indirectly mediates growth and protein synthesis by triggering the liver and other tissues to produce a group of proteins called insulin-like growth factors IGFs. These proteins enhance cellular proliferation and inhibit apoptosis, or programmed cell death. IGFs stimulate cells to increase their uptake of amino acids from the blood for protein synthesis. Skeletal muscle and cartilage cells are particularly sensitive to stimulation from IGFs. For example, gigantism is a disorder in children that is caused by the secretion of abnormally large amounts of GH, resulting in excessive growth.
A similar condition in adults is acromegaly , a disorder that results in the growth of bones in the face, hands, and feet in response to excessive levels of GH in individuals who have stopped growing. Abnormally low levels of GH in children can cause growth impairment—a disorder called pituitary dwarfism also known as growth hormone deficiency. The activity of the thyroid gland is regulated by thyroid-stimulating hormone TSH , also called thyrotropin.
TSH is released from the anterior pituitary in response to thyrotropin-releasing hormone TRH from the hypothalamus. As discussed shortly, it triggers the secretion of thyroid hormones by the thyroid gland. In a classic negative feedback loop, elevated levels of thyroid hormones in the bloodstream then trigger a drop in production of TRH and subsequently TSH. ACTH come from a precursor molecule known as pro-opiomelanotropin POMC which produces several biologically active molecules when cleaved, including ACTH, melanocyte-stimulating hormone, and the brain opioid peptides known as endorphins.
The release of ACTH is regulated by the corticotropin-releasing hormone CRH from the hypothalamus in response to normal physiologic rhythms. A variety of stressors can also influence its release, and the role of ACTH in the stress response is discussed later in this chapter.
The endocrine glands secrete a variety of hormones that control the development and regulation of the reproductive system these glands include the anterior pituitary, the adrenal cortex, and the gonads—the testes in males and the ovaries in females. Much of the development of the reproductive system occurs during puberty and is marked by the development of sex-specific characteristics in both male and female adolescents.
Puberty is initiated by gonadotropin-releasing hormone GnRH , a hormone produced and secreted by the hypothalamus. GnRH stimulates the anterior pituitary to secrete gonadotropins —hormones that regulate the function of the gonads.
The levels of GnRH are regulated through a negative feedback loop; high levels of reproductive hormones inhibit the release of GnRH. Throughout life, gonadotropins regulate reproductive function and, in the case of women, the onset and cessation of reproductive capacity.
The gonadotropins include two glycoprotein hormones: follicle-stimulating hormone FSH stimulates the production and maturation of sex cells, or gametes, including ova in women and sperm in men. FSH also promotes follicular growth; these follicles then release estrogens in the female ovaries. Luteinizing hormone LH triggers ovulation in women, as well as the production of estrogens and progesterone by the ovaries.
LH stimulates production of testosterone by the male testes. As its name implies, prolactin PRL promotes lactation milk production in women. During pregnancy, it contributes to development of the mammary glands, and after birth, it stimulates the mammary glands to produce breast milk.
However, the effects of prolactin depend heavily upon the permissive effects of estrogens, progesterone, and other hormones.
And as noted earlier, the let-down of milk occurs in response to stimulation from oxytocin. In a non-pregnant woman, prolactin secretion is inhibited by prolactin-inhibiting hormone PIH , which is actually the neurotransmitter dopamine, and is released from neurons in the hypothalamus. Only during pregnancy do prolactin levels rise in response to prolactin-releasing hormone PRH from the hypothalamus. The cells in the zone between the pituitary lobes secrete a hormone known as melanocyte-stimulating hormone MSH that is formed by cleavage of the pro-opiomelanocortin POMC precursor protein.
Local production of MSH in the skin is responsible for melanin production in response to UV light exposure. The role of MSH made by the pituitary is more complicated.
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