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Physiology of endocrine system. Possible fates and actions of a hormone following its secretion by an endocrine cell. Not all paths apply to all hormones.
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Possible fates and actionsof a hormone followingitssecretion by anendocrine cell. Not allpaths apply to allhormones.
Pathways by which the nervous system influences hormone secretion.(a) Certain neurons in the hypothalamus, some ofwhich terminate in the posterior pituitary, secrete hormones. The secretion of hypothalamic hormones from the posteriorpituitary and the effects of other hypothalamic hormones on the anterior pituitaryare described later in this chapter. (b) Theautonomic nervous system controls hormone secretion by the adrenal medulla and many other endocrine glands.
Hypophysiotropic Hormones • Secretionof the anterior pituitary hormones is largelyregulated by hormones produced by the hypothalamusand collectively called hypophysiotropic hormones. • These hormones are secreted by neurons thatoriginate in diverse areas of the hypothalamus and terminatein the median eminence around the capillariesthat are the origins of the hypothalamo-pituitary portalvessels. • The generation of action potentials in theseneurons causes them to release their hormones, whichenter the capillaries and are carried by the hypothalamopituitaryportal vessels to the anterior pituitary. There they act upon the various anteriorpituitary cells to control their hormone secretions.
Hypophysiotropic hormones • Each hypophysiotropic hormone is named for theanterior pituitary hormone whose secretion it controls. • Thus, secretion of ACTH (corticotropin) is stimulatedby corticotropin releasing hormone (CRH), secretionof growth hormone is stimulated by growth hormonereleasing hormone (GHRH), secretion of thyroidstimulatinghormone (thyrotropin) is stimulated bythyrotropin releasing hormone (TRH), and secretionof both luteinizing hormone and follicle-stimulatinghormone (the gonadotropins) is stimulated by gonadotropinreleasing hormone (GnRH).
Neural Control of Hypophysiotropic Hormones • Neurons of the hypothalamus receive synaptic input,both stimulatory and inhibitory, from virtually all areasof the central nervous system, and specific neural pathwaysinfluence secretion of the individual hypophysiotropichormones. • A large number of neurotransmitters(for example, the catecholamines and serotonin) arereleased at the synapses on the hormone-secretinghypothalamic neurons, and this explains why the secretionof the hypophysiotropic hormones can be alteredby drugs that influence these neurotransmitters.
Anterior Pituitary Hormones • The anterior pituitary secretes at least eight hormones,but only six have well-established functions. • All peptides, these six “classical” hormones are folliclestimulatinghormone (FSH), luteinizing hormone(LH), growth hormone (GH), thyroid-stimulatinghormone (TSH, thyrotropin), prolactin, and adrenocorticotropichormone (ACTH, corticotropin). Each ofthe last four is probably secreted by a distinct cell typein the anterior pituitary, whereas FSH and LH, collectivelytermed gonadotropic hormones (or gonadotropins)because they stimulate the gonads, areboth secreted by the same cells.
Section through an adrenal gland showing both the medulla and cortex, as well as the hormones they secrete.
Hormonal Regulation of T3 & T4 Secretion 1. Thyroid-releasing hormone (TRH) is released from neurons inthe hypothalamus and travels in the blood to the anteriorpituitary gland. 2. TRH stimulates the release of thyroid-stimulating hormone(TSH) from the anterior pituitary gland. TSH travels in theblood to the thyroid gland. 3. TSH stimulates the secretion of thyroid hormones (T3 and T4)from the thyroid gland into the blood. 4. Thyroid hormones act on tissues to produce responses. 5. Thyroid hormones also have a negative-feedback effect on thehypothalamus and the anterior pituitary to inhibit both TRHsecretion and TSH secretion. The negative feedback helpskeep blood thyroid hormone levels within a narrow range.
Positive feedback regulation of estrogen 1. During the menstrual cycle, before ovulation, smallamounts of estrogen are secreted from the ovary. 2. Estrogen stimulates the release of gonadotropin-releasinghormone (GnRH) from the hypothalamus and luteinizinghormone (LH) from the anterior pituitary. 3. GnRH also stimulates the release of LH from the anteriorpituitary. 4. LH causes the release of additional estrogen from theovary. The GnRH and LH levels in the blood increasebecause of this positive-feedback effect.
Negative feedback regulation of estrogen 1. During the menstrual cycle, after ovulation, the ovarybegins to secrete progesterone in response to LH. 2. Progesterone inhibits the release of GnRH from thehypothalamus and LH from the anterior pituitary. 3. Decreased GnRH release from the hypothalamus reducesLH secretion from the anterior pituitary. GnRH and LHlevels in the blood decrease because of this negativefeedbackeffect.
Gonadal production of steroids. Only the ovaries have highconcentrations of the enzymes (aromatase) required toproduce the estrogens estrone and estradiol.
Notion about hormones • The endocrine system is composed of glands that secretechemical signals into the circulatory system. • The secretory products of endocrine glands arecalled hormones (hoЇrmoЇnz), a term derived from the Greek wordhormon, meaning to set into motion. • Traditionally, a hormone is definedas a chemical signal, or ligand, that (1) is produced in minuteamounts by a collection of cells; (2) is secreted into the interstitialspaces; (3) enters the circulatory system, where it is transportedsome distance; and (4) acts on specific tissues called target tissues atanother site in the body to influence the activity of those tissues in aspecific fashion. All hormones exhibit most components of this definition,but some components don’t apply to every hormone.
Hormone molecules diffuse from the blood through the walls of the capillariesinto the interstitial spaces. Once within the interstitial spaces, they diffuse tothe target cells. As the concentration of free hormone molecules increasesin the blood, more molecules diffuse from the capillary to the target cells. As the concentration of free hormone molecules decreases in the blood,fewer diffuse from the capillary to the target cells. Hormone Concentrations at the Target Cell
Effect of Changes in Plasma ProteinConcentration on the Concentrationof Free Hormone • An equilibrium exists between free hormone molecules and hormonemolecules bound to plasma proteins. • The free hormone molecules can diffusefrom the capillaries to the interstitial spaces.
Effect of decrease in plasma proteinconcentration • A decrease in plasma proteinconcentration reduces the number of hormone molecules bound to plasmaproteins. • This increases the rate at which free hormone molecules diffuse fromthe capillaries. More importantly, hormones that diffuse from capillaries areeliminated from the blood by the kidney and liver. • The rapid loss of hormonefrom the circulatory system reduces the hormone concentration in the bodyand fewer hormone molecules are available to bind to receptors.
Response of Target Cells to Hormones • (a) Down-regulation occurs when the number of receptors for a hormonedecreases within target cells. For example, gonadotropin-releasing hormone(GnRH) released from the hypothalamus binds to GnRH receptors in theanterior pituitary. GnRH bound to its receptors causes down-regulation of theGnRH receptors so that eventually the target cells become less sensitive to theGnRH. • (b) Up-regulation occurs when some stimulus causes the number ofreceptors for a hormone to increase within a target cell. For example, FSH actson cells of the ovary to up-regulate the number of receptors for LH. Thus theovary becomes more sensitive to the effect of LH.
The Cascade EffectThe combination of a hormone with a membrane-bound receptor activates several G proteins. The G proteins, in turn, activate adenylyl cyclase enzymes, whichcause the synthesis of a large number of cAMP molecules. The cAMP molecules, in turn, activate many protein kinase enzymes, which produce a rapid andamplified response.
