The pituitary and hypothalamus (2023)

The hypothalamus-pituitary complex can be considered the "command center" of the endocrine system. This complex secretes several hormones that cause reactions directly in target tissues, as well as hormones that regulate the synthesis and secretion of hormones from other glands. In addition, the hypothalamic-pituitary complex coordinates messages from the endocrine and nervous systems. In many cases, a stimulus received by the nervous system must pass through the hypothalamic-pituitary complex to be translated into hormones capable of eliciting a response.

Hehypothalamusit is a structure in the diencephalon of the brain, located in front of and below the thalamus (Figure 1). It has neural and endocrine functions and produces and secretes many hormones. Furthermore, the hypothalamus is anatomically and functionally related to thehypophysis(or pituitary gland), a bean-sized organ that hangs from a stalk called theinfuser(or pituitary stalk). The pituitary gland is inserted into the sella turcica of the sphenoid bone of the skull. It consists of two lobes that arise from different parts of the embryonic tissue: the posterior pituitary (neurohypophysis) is the neural tissue, while the anterior pituitary (also known as the adenohypophysis) is the glandular tissue that develops from the digestive tract. primitive. The hormones secreted by the posterior and anterior pituitary and the interlobular zone are summarized in Table 1.

posterior pituitary

The posterior pituitary is actually an extension of neurons from the paraventricular and supraoptic nuclei of the hypothalamus. The cell bodies of these regions rest in the hypothalamus, but their axons descend as the hypothalamic-pituitary tract within the infundibulum and terminate in axon terminals that cross the posterior pituitary (Figure 2).

The posterior pituitary does not produce hormones, but stores and secretes hormones produced by the hypothalamus. The paraventricular nuclei produce the hormone oxytocin, while the supraoptic nuclei produce ADH. These hormones travel down the axons to storage sites in the posterior pituitary axon terminals. In response to signals from the same hypothalamic neurons, hormones are released from the axon terminals into the bloodstream.

oxytocin

When fetal development is complete, the peptide-derived hormoneoxytocin(tocia- = "birth") stimulates uterine contractions and dilation of the cervix. During most of pregnancy, receptors for the hormone oxytocin are not expressed at high levels in the uterus. Towards the end of pregnancy, synthesis of oxytocin receptors in the uterus increases and smooth muscle cells in the uterus become more sensitive to its effects. Oxytocin is continuously released during labor through a positive feedback mechanism. As mentioned above, oxytocin triggers uterine contractions that push the fetal head towards the cervix. In response, cervical distention stimulates additional oxytocin, which is synthesized by the hypothalamus and released by the pituitary. This increases the intensity and effectiveness of uterine contractions, leading to further dilation of the cervix. The feedback loop continues until birth.

Although the high levels of oxytocin in the mother's blood begin to decline immediately after birth, oxytocin continues to play an important role in the health of the mother and newborn. First, oxytocin is required for the let-down reflex (commonly known as "let-down") in lactating women. When the newborn begins to breastfeed, sensory receptors in the nipples transmit signals to the hypothalamus. In response, oxytocin is released and released into the bloodstream. Within seconds, cells in the mother's milk ducts contract and release milk into the baby's mouth. Second, oxytocin is believed to contribute to the bond between parents and newborns, known as attachment, in both males and females. Oxytocin is also believed to be involved in feelings of love and closeness, as well as sexual response.

Antidiuretic hormone (ADH)

Blood solute levels, or blood osmolarity, can change in response to the consumption of certain foods and fluids, as well as in response to illness, injury, medications, or other factors. Blood osmolarity is constantly monitored byosmorezeptoren– specialized cells in the hypothalamus that are particularly sensitive to the concentration of sodium ions and other solutes.

In response to high blood osmolarity, which can occur during dehydration or after a very salty meal, osmoreceptors in the posterior pituitary send signals to releaseAntidiuretic hormone (ADH). ADH target cells are found in the tubular cells of the kidney. Its action is to increase epithelial water permeability, which allows for greater water reabsorption. The more water absorbed from the filtrate, the more water is returned to the blood and less is excreted in the urine. A higher concentration of water leads to a lower concentration of solutes. ADH is also known as vasopressin because, at very high levels, it causes blood vessels to constrict, which increases blood pressure by increasing peripheral resistance. ADH release is controlled by a negative feedback loop. When blood osmolarity decreases, hypothalamic osmoreceptors detect the change and trigger a corresponding decrease in ADH secretion. As a result, less water is absorbed from the urine filtrate.

Interestingly, drugs can affect ADH secretion. For example, drinking alcohol inhibits the release of ADH, leading to increased urine output that can lead to dehydration and hangovers. A disease called diabetes insipidus is characterized by chronic underproduction of ADH, leading to chronic dehydration. Because little ADH is produced and excreted, the kidneys do not reabsorb enough water. Although patients are thirsty and increase fluid intake, this does not effectively decrease the concentration of solutes in the blood because ADH levels are not high enough to trigger the kidneys to absorb water. Electrolyte imbalances can occur in severe cases of diabetes insipidus.

anterior pituitary

The anterior pituitary gland arises from the digestive tract in the embryo and migrates to the brain during fetal development. There are three regions: the pars distalis is the most anterior, the pars intermedia borders the posterior pituitary, and the pars tuberalis is a thin "tube" that lines the infundibulum.

Remember that the posterior pituitary gland does not produce hormones, it only stores them. Instead, the anterior pituitary gland produces hormones. However, anterior pituitary hormone secretion is regulated by two classes of hormones. These hormones secreted by the hypothalamus are releasing hormones, which stimulate secretion of anterior pituitary hormones, and inhibitory hormones, which inhibit secretion.

Hypothalamic hormones are secreted by neurons but travel through blood vessels to the anterior pituitary (Figure 3). Within the infundibulum is a capillary bridge connecting the hypothalamus to the anterior pituitary gland. This network, calledpituitary portal system, allows the transport of hypothalamic hormones to the anterior pituitary without first entering the systemic circulation. The system originates from the superior pituitary gland, which branches off the carotid arteries and supplies blood to the hypothalamus. Branches of the superior pituitary artery form the pituitary portal system (see Figure 3). Hormones that are released and inhibited by the hypothalamus travel through a primary capillary plexus to the portal veins, which transport them to the anterior pituitary. Hormones produced by the anterior pituitary (in response to hormone release) enter a secondary capillary plexus and from there drain into the circulatory system.

The anterior pituitary produces seven hormones. They are: growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), beta-endorphin and prolactin. Of the anterior pituitary hormones, TSH, ACTH, FSH, and LH are collectively known as tropic hormones (tropo- = "rotating") because they turn on or off the function of other endocrine glands.

growing hormone

The endocrine system regulates the human body's growth, protein synthesis and cell replication. An important hormone involved in this process isgrowth hormone (GH), also called somatotropin, a protein hormone produced and secreted by the anterior pituitary gland. Its main function is anabolic; it promotes protein synthesis and tissue formation through direct and indirect mechanisms (Figure 4). GH levels are controlled by the release of GHRH and GHIH (also known as somatostatin) from the hypothalamus.

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 source of energy, which means less glucose is absorbed from the bloodstream.

GH also initiates the diabetogenic effect, where GH stimulates the liver to break down glycogen into glucose, which is then deposited in the blood. The name "diabetogenic" derives from the similarity in elevated blood sugar levels seen between people with untreated diabetes mellitus and those with excess GH. Blood sugar levels rise as a result of a combination of glucose-sparing and diabetogenic effects.

GH indirectly mediates growth and protein synthesis by stimulating the liver and other tissues to produce a group of proteins called GH.insulin-like growth factors (IGFs). These proteins promote cell proliferation and inhibit apoptosis or programmed cell death. IGFs stimulate cells to increase the absorption of amino acids from the blood for protein synthesis. Skeletal muscle and cartilage cells are particularly sensitive to IGF stimulation.

Dysfunction in the endocrine system's growth control can lead to various disorders. For example,gigantismis a disease in children caused by the secretion of abnormally large amounts of GH, leading to excessive growth. A similar condition in adults isAcromegaly, a condition that causes the bones of the face, hands, and feet to grow in response to excessive GH levels in people who have stopped growing. Abnormally low levels of GH in children can lead to growth retardation, a disorder called GHpituitary dwarfism(also known as growth hormone deficiency).

thyroid stimulating hormone

Thyroid activity is regulated byThyroid stimulating hormone (TSH), also called thyrotropin. TSH is released by the anterior pituitary in response to thyrotropin-releasing hormone (TRH) from the hypothalamus. As briefly discussed, it triggers the secretion of thyroid hormones from the thyroid gland. In a classic negative feedback loop, elevated blood levels of thyroid hormone trigger a drop in TRH and then TSH production.

Hormona adrenocorticotropa

HeHormona adrenocorticotropa (ACTH), also called corticotropin, stimulates the adrenal cortex (the most superficial "cortex" of the adrenal glands) to release corticosteroid hormones, such as cortisol. ACTH comes from a precursor molecule called pro-opiomelanotropin (POMC), which when cleaved produces a number of biologically active molecules, including ACTH, melanocyte-stimulating hormone, and brain opioid peptides known as endorphins.

ACTH release is regulated by corticotropin-releasing hormone (CRH) from the hypothalamus in response to normal physiological rhythms. A variety of stressors can also affect its release, and ACTH's role in the stress response is discussed later in this chapter.

Follicle stimulating hormone and luteinizing hormone

Endocrine glands secrete a variety of hormones that control the development and regulation of the reproductive system (these glands include the anterior pituitary gland, adrenal cortex, and gonads, testes in males, and ovaries in females). Much of the development of the reproductive system occurs during puberty and is characterized by the development of sex-specific characteristics in 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 secretegonadotropins—Hormones that regulate the function of the gonads. GnRH levels are regulated by a negative feedback loop; High levels of reproductive hormones inhibit the release of GnRH. Throughout life, gonadotropins regulate reproductive function and, in women, the onset and cessation of reproductive function.

Gonadotropins include two glycoprotein hormones:Follicle Stimulating Hormone (FSH)stimulates the production and maturation of sex cells or gametes, including eggs in women and sperm in men. FSH also promotes follicular growth; these follicles then release estrogen into the female ovaries.Luteinizing hormone (LH)triggers ovulation and the production of estrogen and progesterone by the ovaries in women. LH stimulates the production of testosterone by the male testicles.

prolactin

As the name implies,Prolactin (PRL)promotes lactation (milk production) in women. During pregnancy it contributes to the development of the mammary glands and after childbirth it stimulates the mammary glands to produce breast milk. However, the effects of prolactin are highly dependent on the permissive effects of estrogen, progesterone and other hormones. And as mentioned above, milk is released in response to oxytocin stimulation.

In a woman who is not pregnant, prolactin secretion is inhibited by prolactin-inhibiting hormone (PIH), which is actually the neurotransmitter dopamine and is released by neurons in the hypothalamus. Only during pregnancy do prolactin levels increase in response to prolactin-releasing hormone (PRH) from the hypothalamus.

Intermediate pituitary: melanocyte-stimulating hormone

Cells in the area between the lobes of the pituitary gland secrete a hormone known as melanocyte-stimulating hormone (MSH), which is produced by the cleavage of pro-opiomelanocortin (POMC) precursor protein. Local production of MSH in the skin is responsible for the production of melanin in response to exposure to ultraviolet light. The role of MSH by the pituitary is more complicated. For example, people with lighter skin often have the same amount of MSH as people with darker skin. However, this hormone is capable of darkening the skin by inducing the production of melanin in the melanocytes of the skin. Women also have increased MSH production during pregnancy; in combination with estrogens, darker pigmentation of the skin may occur, especially of the skin around the areolas and labia minora. Figure 5 is a summary of pituitary hormones and their main effects.

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chapter overview

The hypothalamus-pituitary complex is located in the diencephalon of the brain. The hypothalamus and pituitary gland are connected by a structure called the infundibulum, which contains nerve vessels and axons. The pituitary gland is divided into two distinct structures of different embryonic origins. The posterior lobe houses the axon terminals of neurons in the hypothalamus. It stores and releases two hypothalamic hormones into the bloodstream: oxytocin and antidiuretic hormone (ADH). The anterior lobe is connected to the hypothalamus by vessels in the infundibulum and produces and secretes six hormones. However, its secretion is regulated by the release and inhibition of hormones from the hypothalamus. The six anterior pituitary hormones are: growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL).

glossary

Acromegaly:A disorder in adults caused when abnormally high levels of GH trigger bone growth in the face, hands, and feet.

Hormona adrenocorticotropa (ACTH):An anterior pituitary hormone that stimulates the adrenal cortex to release corticosteroid hormones (also called corticotropin).

Antidiuretic hormone (ADH):Hypothalamic hormone stored by the posterior pituitary gland that signals the kidneys to reabsorb water

Follicle Stimulating Hormone (FSH):An anterior pituitary hormone that stimulates the production and maturation of sex cells

gigantism:Disorder in children caused when abnormally high levels of GH trigger excessive growth

Gonadotropins:Hormones that regulate the function of the gonads

Growth Hormone (GH):An anterior pituitary hormone that promotes tissue formation and affects nutrient metabolism (also called somatotropin)

pituitary portal system:Network of blood vessels that allow hypothalamic hormones to enter the anterior pituitary without entering the systemic circulation

hypothalamus:Midbrain region below the thalamus involved in neuronal and endocrine signaling

funnel:Rod containing vessels and nerve tissue that connects the pituitary gland to the hypothalamus (also called the pituitary rod)

insulin-like growth factors (IGFs):Protein that enhances cell proliferation, inhibits apoptosis, and stimulates cellular uptake of amino acids for protein synthesis

luteinizing hormone (LH):Anterior pituitary hormone that triggers ovulation and ovarian hormone production in women and testosterone production in men

Osmorreceptor:hypothalamic sensory receptor stimulated by changes in the concentration of solutes (osmotic pressure) in the blood

oxytocin:Hypothalamic hormone stored in the posterior pituitary gland that is important for stimulating uterine contractions during labor, milk ejection during lactation, and attachment (also produced in males)

Pituitary dwarfism:Disorder in children caused when abnormally low levels of GH cause growth retardation

Pituitary:bean-sized organ attached to the hypothalamus that produces, stores, and secretes hormones in response to hypothalamic stimulation (also called the pituitary gland)

Prolactin (PRL):An anterior pituitary hormone that promotes the development of mammary glands and the production of breast milk

Thyroid stimulating hormone (TSH):anterior pituitary hormone that causes the thyroid to release thyroid hormone (also called thyrotropin)