How does hypothalamus regulate hormone action

Insulin causes blood glucose levels to drop, which signals the pancreas to stop producing insulin in a negative feedback loop. Hormonal stimuli refers to the release of a hormone in response to another hormone. A number of endocrine glands release hormones when stimulated by hormones released by other endocrine glands.

For example, the hypothalamus produces hormones that stimulate the anterior portion of the pituitary gland. The anterior pituitary in turn releases hormones that regulate hormone production by other endocrine glands.

The anterior pituitary releases the thyroid-stimulating hormone, which then stimulates the thyroid gland to produce the hormones T 3 and T 4. As blood concentrations of T 3 and T 4 rise, they inhibit both the pituitary and the hypothalamus in a negative feedback loop.

In some cases, the nervous system directly stimulates endocrine glands to release hormones, which is referred to as neural stimuli. Recall that in a short-term stress response, the hormones epinephrine and norepinephrine are important for providing the bursts of energy required for the body to respond.

Here, neuronal signaling from the sympathetic nervous system directly stimulates the adrenal medulla to release the hormones epinephrine and norepinephrine in response to stress.

Hormone levels are primarily controlled through negative feedback, in which rising levels of a hormone inhibit its further release. The three mechanisms of hormonal release are humoral stimuli, hormonal stimuli, and neural stimuli. Humoral stimuli refers to the control of hormonal release in response to changes in extracellular fluid levels or ion levels. Hormonal stimuli refers to the release of hormones in response to hormones released by other endocrine glands.

Neural stimuli refers to the release of hormones in response to neural stimulation. Skip to content Chapter The Endocrine System. Learning Objectives By the end of this section, you will be able to: Explain how hormone production is regulated Discuss the different stimuli that control hormone levels in the body. Figure The branches of the superior hypophyseal artery form the hypophyseal portal system see Figure 3. Hypothalamic releasing and inhibiting hormones travel through a primary capillary plexus to the portal veins, which carry them into the anterior pituitary.

Hormones produced by the anterior pituitary in response to releasing hormones enter a secondary capillary plexus, and from there drain into the circulation. Figure 3. The anterior pituitary manufactures seven hormones. The hypothalamus produces separate hormones that stimulate or inhibit hormone production in the anterior pituitary.

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. This hormonal cascade is referred to as the hypothalamic-pituitary-thyroid HPT axis and will be explored in detail in part 3 of this series.

ACTH primarily regulates the production and secretion of cortisol from the adrenal cortex outer portion of the adrenal gland. Cortisol is a long-term stress hormone and a steroidal hormone synthesised from cholesterol.

It is referred to as a glucocorticoid because it is produced by the adrenal cortex and influences the concentration of glucose in the blood VanPutte et al, Following periods of chronic stress including classic biological stressors such as starvation or physical injury , the hypothalamus releases corticotropin-releasing hormone. This initiates the release of ACTH from the anterior pituitary and, subsequently, stimulates the release of cortisol from the adrenal cortex Table 1. Cortisol plays a key role in regulating metabolism and, during periods of food deprivation, stimulates the breakdown of protein and fat to generate glucose for use as fuel in glucose-dependent tissues, such as the brain.

This process is called gluconeogenesis literally, the creation of new glucose. ACTH also helps to regulate the release of other steroid hormones produced by the adrenal cortex, including aldosterone which regulates the concentration of sodium and potassium in the blood and the group of testosterone-like hormones known as androgens Gallo-Payet, The complex interplay between the hypothalamus, anterior pituitary and the adrenal cortex is referred to as the HPT axis and will be examined in detail in part 4 of this series.

ACTH is also part of the melanocortin group of hormones, which influence skin pigmentation see below. MSH is synthesised by the pars intermedia region of the pituitary gland. Although this region marks the boundary where the anterior and posterior portions of the pituitary gland fuse, it is generally considered part of the anterior pituitary. The pars intermedia atrophies shrinks with age and, in adults, may only be present as a vestigial remnant or, in some cases, is not recognisable at all.

MSH exists in a range of structurally similar forms known as melanocortins, which are all small peptides. As implied by its name, MSH stimulates the pigment-producing cells melanocytes in the epidermis to release the dark pigment known as melanin, which is largely responsible for skin colour.

All races are thought to have similar numbers of melanocytes in their epidermis; it is the relative activity of these cells and the amount of melanin they synthesise and release that ultimately determines skin colour. This is essential to protect the actively dividing cells of the epidermis from the harmful effects of UV, known to cause DNA damage that can lead to mutations and, potentially, skin cancers.

Melanin is excellent at absorbing UV wavelengths of light and, as it accumulates in the epidermis the skin, darkens and develops a protective suntan. During pregnancy, levels of MSH tend to increase, which, together with changes to the sex hormones oestrogen and progesterone, often leads to hyper-pigmentation around the eye sockets, cheekbones, lips and forehead.

ACTH described above is another hormone that can influence skin pigmentation through the direct stimulation of melanocytes. These act on the gonads testes and ovaries to stimulate the production of sex hormones and sperm or ova in males and females respectively see below.

The main gonadrotrophins are FSH and LH; the release of both is regulated by gonadotropin-releasing hormone, which is produced by the hypothalamus Table 1. In females, each month FSH initiates the development of immature follicles in the ovaries. As each follicle enlarges, it secretes the female sex hormone oestrogen, before maturing into a Graafian follicle, a fluid-filled, pressurised sac containing a mature ovum egg , primed and ready to rupture.

Ovulation is triggered by LH, which initiates rupturing of the follicle and ovarian wall; this explosive event propels the ovum into its adjacent fallopian tube. Following ovulation, the remnants of the Graafian follicle collapse to form a structure known as the corpus luteum yellow body.

This produces the second major female sex hormone, progesterone, which maintains the integrity of the endometrial lining of the uterus to allow for the implantation of a fertilised ovum VanPutte et al, Despite their names being reflective of the role played in the female ovarian cycle, FSH and LH also play crucial roles in male reproductive physiology.

FSH is essential in stimulating spermatogenesis, where diploid cells containing 46 chromosomes undergo meiotic division to produce vast numbers of haploid spermatozoa each containing 23 chromosomes. LH stimulates the interstitial cells Leydig cells of the testes to synthesise and release the male sex hormone testosterone Babu et al, This powerful anabolic steroid stimulates skeletal muscle development, growth of facial and body hair, expansion of the larynx causing the deepening of the voice and spermatogenesis, and is largely responsible for the male sex drive.

The role of the gonadotropins and male and female sex hormones will be discussed further in part 7 of this series. The hypothalamus contains discrete, organised clusters of neurons called the hypothalamic nuclei, which synthesise the hypothalamic releasing and inhibiting hormones that regulate the activity of the anterior pituitary.

Both the hypothalamus and pituitary gland are highly vascularised and have a dedicated network of blood vessels called the hypothalamic-pituitary portal system, which ensures rapid and efficient delivery of the releasing and inhibiting hormones from the hypothalamus to the anterior pituitary below Bear et al, Secretion of the hypothalamic releasing and inhibiting hormones is determined by multiple sensory inputs, which continually monitor the changing physiological status of the body.

Multiple parameters monitored continuously and in real time include temperature, pH, solute concentrations and current levels of circulating hormones.

The hypothalamus functions as the key bridge between the nervous and endocrine systems, but many of the interactions between the two remain poorly understood.

Table 1 summarises the key hormones of the hypothalamus and pituitary, and their relationships. Some of the better-studied interactions between the hypothalamus, pituitary and peripheral endocrine glands such as the HPT axis and hypothalamic-pituitary-adrenal axis will be explored later in this series.

Part 3 focuses on the thyroid and parathyroid glands. You are here: Long-term conditions. Endocrine system 2: hypothalamus and pituitary gland. Abstract The endocrine system consists of glands and tissues that produce and secrete hormones to regulate and coordinate vital bodily functions.

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Key points The hypothalamus and pituitary gland both lie in the cranial cavity of the skull Two major hormones released by the posterior pituitary gland are antidiuretic hormone and oxytocin The anterior pituitary gland produces several stimulating hormones that regulate the activity of other endocrine glands Although the pituitary gland is often referred to as the master gland, many of its actions are directed by the hypothalamus Clusters of neurons in the hypothalamus synthesise releasing and inhibiting hormones that regulate the activity of the anterior pituitary.

Also in this series Endocrine system 1: overview of the endocrine system and hormones Endocrine system 3: thyroid and parathyroid glands Endocrine system 4: adrenal gland Endocrine system 5: the functions of the pineal and thymus glands Endocrine System 6: pancreas, stomach, small intestine and liver Endocrine system 7: ovaries and testes, placenta pregnancy.

Asian Journal of Neurosurgery ; 8: 4, Indian Journal of Clinical Biochemistry ; 1, Bear MH et al Neuroanatomy, Hypothalamus. StatPearls Publishing. De Herder WW Acromegaly and gigantism in the medical literature.