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.

Pathways by which the nervous system influences hormone secretion Pathways by which the nervous system influences hormone secretion. (a) Certain neurons in the hypothalamus, some of which terminate in the posterior pituitary, secrete hormones. The secretion of hypothalamic hormones from the posterior pituitary and the effects of other hypothalamic hormones on the anterior pituitary are described later in this chapter. (b) The autonomic nervous system controls hormone secretion by the adrenal medulla and many other endocrine glands.

Hypophysiotropic Hormones Secretion of the anterior pituitary hormones is largely regulated by hormones produced by the hypothalamus and collectively called hypophysiotropic hormones. These hormones are secreted by neurons that originate in diverse areas of the hypothalamus and terminate in the median eminence around the capillaries that are the origins of the hypothalamo-pituitary portal vessels. The generation of action potentials in these neurons causes them to release their hormones, which enter the capillaries and are carried by the hypothalamopituitary portal vessels to the anterior pituitary. There they act upon the various anterior pituitary cells to control their hormone secretions. Hypophysiotropic Hormones

Hypophysiotropic hormones Each hypophysiotropic hormone is named for the anterior pituitary hormone whose secretion it controls. Thus, secretion of ACTH (corticotropin) is stimulated by corticotropin releasing hormone (CRH), secretion of growth hormone is stimulated by growth hormone releasing hormone (GHRH), secretion of thyroidstimulating hormone (thyrotropin) is stimulated by thyrotropin releasing hormone (TRH), and secretion of both luteinizing hormone and follicle-stimulating hormone (the gonadotropins) is stimulated by gonadotropin releasing hormone (GnRH).

Neural Control of Hypophysiotropic Hormones Neurons of the hypothalamus receive synaptic input, both stimulatory and inhibitory, from virtually all areas of the central nervous system, and specific neural pathways influence secretion of the individual hypophysiotropic hormones. A large number of neurotransmitters (for example, the catecholamines and serotonin) are released at the synapses on the hormone-secreting hypothalamic neurons, and this explains why the secretion of the hypophysiotropic hormones can be altered by 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 folliclestimulating hormone (FSH), luteinizing hormone (LH), growth hormone (GH), thyroid-stimulating hormone (TSH, thyrotropin), prolactin, and adrenocorticotropic hormone (ACTH, corticotropin). Each of the last four is probably secreted by a distinct cell type in the anterior pituitary, whereas FSH and LH, collectively termed gonadotropic hormones (or gonadotropins) because they stimulate the gonads, are both secreted by the same cells.

Activation of punishment center

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 in the hypothalamus and travels in the blood to the anterior pituitary gland. 2. TRH stimulates the release of thyroid-stimulating hormone (TSH) from the anterior pituitary gland. TSH travels in the blood 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 the hypothalamus and the anterior pituitary to inhibit both TRH secretion and TSH secretion. The negative feedback helps keep blood thyroid hormone levels within a narrow range. Hormonal Regulation of T3 & T4 Secretion

Positive feedback regulation of estrogen 1. During the menstrual cycle, before ovulation, small amounts of estrogen are secreted from the ovary. 2. Estrogen stimulates the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus and luteinizing hormone (LH) from the anterior pituitary. 3. GnRH also stimulates the release of LH from the anterior pituitary. 4. LH causes the release of additional estrogen from the ovary. The GnRH and LH levels in the blood increase because of this positive-feedback effect. Positive feedback regulation of estrogen

Negative feedback regulation of estrogen 1. During the menstrual cycle, after ovulation, the ovary begins to secrete progesterone in response to LH. 2. Progesterone inhibits the release of GnRH from the hypothalamus and LH from the anterior pituitary. 3. Decreased GnRH release from the hypothalamus reduces LH secretion from the anterior pituitary. GnRH and LH levels in the blood decrease because of this negativefeedback effect. Negative feedback regulation of estrogen

Gonadal production of steroids Gonadal production of steroids. Only the ovaries have high concentrations of the enzymes (aromatase) required to produce the estrogens estrone and estradiol.

Aldosterone action

Notion about hormones The endocrine system is composed of glands that secrete chemical signals into the circulatory system. The secretory products of endocrine glands are called hormones (hoЇrmoЇnz), a term derived from the Greek word hormon, meaning to set into motion. Traditionally, a hormone is defined as a chemical signal, or ligand, that (1) is produced in minute amounts by a collection of cells; (2) is secreted into the interstitial spaces; (3) enters the circulatory system, where it is transported some distance; and (4) acts on specific tissues called target tissues at another site in the body to influence the activity of those tissues in a specific fashion. All hormones exhibit most components of this definition, but some components don’t apply to every hormone.

Hormone Concentrations at the Target Cell Hormone molecules diffuse from the blood through the walls of the capillaries into the interstitial spaces. Once within the interstitial spaces, they diffuse to the target cells. As the concentration of free hormone molecules increases in 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.

An equilibrium exists between free hormone molecules and hormone molecules bound to plasma proteins. The free hormone molecules can diffuse from the capillaries to the interstitial spaces. Effect of Changes in Plasma Protein Concentration on the Concentration of Free Hormone

Effect of decrease in plasma protein concentration A decrease in plasma protein concentration reduces the number of hormone molecules bound to plasma proteins. This increases the rate at which free hormone molecules diffuse from the capillaries. More importantly, hormones that diffuse from capillaries are eliminated from the blood by the kidney and liver. The rapid loss of hormone from the circulatory system reduces the hormone concentration in the body and 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 hormone decreases within target cells. For example, gonadotropin-releasing hormone (GnRH) released from the hypothalamus binds to GnRH receptors in the anterior pituitary. GnRH bound to its receptors causes down-regulation of the GnRH receptors so that eventually the target cells become less sensitive to the GnRH. (b) Up-regulation occurs when some stimulus causes the number of receptors for a hormone to increase within a target cell. For example, FSH acts on cells of the ovary to up-regulate the number of receptors for LH. Thus the ovary becomes more sensitive to the effect of LH.

The Cascade Effect The combination of a hormone with a membrane-bound receptor activates several G proteins. The G proteins, in turn, activate adenylyl cyclase enzymes, which cause the synthesis of a large number of cAMP molecules. The cAMP molecules, in turn, activate many protein kinase enzymes, which produce a rapid and amplified response.

The endocrine system