Hypothalamus‐Pituitary‐Thyroid Axis - Comprehensive Physiology
We'll go over the anatomy and function of the pituitary gland, the Your thyroid gland and the hormones it produces are crucial for metabolism. . intake; avoiding refined sugars; drinking at least four to six cups of water a day. Explain the anatomical and functional relationships of the hypothalamus and . This flow chart illustrates the hormone cascade that stimulates human growth. . The six anterior pituitary hormones are: growth hormone (GH), thyroid-stimulating . The hypothalamic–pituitary–thyroid axis is part of the neuroendocrine system responsible for . 27 (6 Suppl): – "Regulation of the pituitary-thyroid axis in man: Relationship of TSH concentration to "Thyrotropic Feedback Control: Evidence for an Additional Ultrashort Feedback Loop from Fractal Analysis".
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This positive feedback loop continues until birth. As the newborn begins suckling, sensory receptors in the nipples transmit signals to the hypothalamus. In response, oxytocin is secreted and released into the bloodstream.
Secondly, in both males and females, oxytocin is thought to contribute to parent—newborn bonding, known as attachment. Oxytocin is also thought to be involved in feelings of love and closeness, as well as in the sexual response. In the last examples oxytocin is functioning as a neurotransmitter in the brain.
Antidiuretic Hormone ADH The solute concentration of the blood, or blood osmolarity, may change in response to the consumption of certain foods and fluids, as well as in response to disease, injury, medications, or other factors. Blood osmolarity is constantly monitored by osmoreceptors—specialized cells within 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 following a very salty meal, stimulation of osmoreceptors causes neurons in the supraoptic nucleus to signal the posterior pituitary to release antidiuretic hormone ADH.
The target cells of ADH are located in the tubular cells of the kidneys. Its effect is to increase epithelial permeability to water, allowing increased water reabsorption. This increase in water reabsorption has the effect of making the blood more dilute and the urine more concentrated.
ADH is also known as vasopressin because in high concentrations, rarely seen except in cases of hemorrhage or shock, it causes constriction of blood vessels, which increases blood pressure by increasing peripheral resistance. The release of ADH is controlled by a negative feedback loop. As blood osmolarity decreases, the hypothalamic osmoreceptors sense the change prompting a corresponding decrease in the secretion of ADH.
As a result, less water is reabsorbed from the urine filtrate.
Some drugs can affect the secretion of ADH. For example, alcohol consumption inhibits the release of ADH, resulting in increased urine production that can eventually lead to the dehydration of a hangover. A disease called diabetes insipidus is characterized by chronic underproduction of ADH that causes chronic dehydration.
Because little ADH is produced and secreted, not enough water is reabsorbed by the kidneys. Electrolyte imbalances can occur in severe cases of diabetes insipidus. There are three regions: Recall that the posterior pituitary does not synthesize hormones, but merely stores them. In contrast, the anterior pituitary does manufacture hormones. Like the posterior pituitary the release of hormones from the anterior pituitary is controlled by the hypothalamus.
This control is mediated by secretion of releasing or inhibiting hormones into the blood. Within the infundibulum is a bridge of capillaries that connects the hypothalamus to the anterior pituitary. This portal system begins with a primary capillary plexus originating from the superior hypophyseal artery, a branches of the internal carotid artery. 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. Growth Hormone Growth hormone GHalso called somatotropin regulates the growth of the human body, protein synthesis, and cellular replication. Its primary function is anabolic; it promotes protein synthesis and tissue building through direct and indirect mechanisms Figure 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 released 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 caused by the hypersecretion of GH before the growth plates have closed resulting in excessive growth of all bones.
Abnormally low levels of GH in children can cause growth impairment—a disorder called pituitary dwarfism also known as growth hormone deficiency which affects all bones. Achondroplastic dwarfism affects only the bones with growth plates long bones resulting in short arms and legs with normal sized trunk and head.
Thyroid-Stimulating Hormone The activity of the thyroid gland is regulated by thyroid-stimulating hormone TSHalso called thyrotropin. TSH is released from the anterior pituitary in response to thyrotropin-releasing hormone TRH from the hypothalamus. As will be 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 TSH.
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The Pituitary Gland and Hypothalamus | Anatomy & Physiology
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