Endocrine online notes. Chapter 18 The Endocrine System The nervous and endocrine systems act as a coordinated interlocking supersystem, the neuroendocrine.

Endocrine online notes

Chapter 18The Endocrine System

• The nervous and endocrine systems act as a coordinated interlocking supersystem, the neuroendocrine system.

• The endocrine system controls body activities by releasing mediator molecules called hormones.– hormones released into the bloodstream travel

throughout the body– results may take hours, but last longer

• The nervous system controls body actions through nerve impulses.– certain parts release hormones into blood– rest releases neurotransmitters excite or inhibit nerve,

muscle & gland cells– results in milliseconds, brief duration of effects

NERVOUS and ENDOCRINE SYSTEM

• The nervous system causes muscles to contract or glands to secrete. The endocrine system affects virtually all body tissues by altering metabolism, regulating growth and development, and influencing reproductive processes.

• Parts of the nervous system stimulate or inhibit the release of hormones.

• Hormones may promote or inhibit the generation of nerve impulses.

• Table 18.1 compares the characteristics of the nervous and endocrine systems.

General Functions of Hormones• Help regulate:

– extracellular fluid– metabolism– biological clock– contraction of cardiac &

smooth muscle– glandular secretion– some immune functions

• Growth & development• Reproduction

• Hormones have powerful effects when present in very low concentrations.

Endocrine Glands Defined• Exocrine glands

– secrete products into ducts which empty into body cavities or body surface

– sweat, oil, mucous, & digestive glands

• Endocrine glands– secrete products (hormones) into bloodstream– pituitary, thyroid, parathyroid, adrenal, pineal– other organs secrete hormones as a 2nd

function– hypothalamus, thymus,

pancreas,ovaries,testes, kidneys, stomach, liver, small intestine, skin, heart & placenta

Hormone Receptors

• Although hormones travel in blood throughout the body, they affect only specific target cells.– Target cells have specific protein or glycoprotein

receptors to which hormones bind.

• Receptors are constantly being synthesized and broken down.

• Synthetic hormones that block the receptors for particular naturally occurring hormones are available as drugs. (Clinical Application)

Circulating and Local Hormones

• Hormones that travel in blood and act on distant target cells are called circulating hormones or endocrines.

• Hormones that act locally without first entering the blood stream are called local hormones.– Those that act on neighboring cells are called

paracrines.

– Those that act on the same cell that secreted them are termed autocrines.

• Figure 18.2 compares the site of action of circulating and local hormones.

Chemical Classes of Hormones - Overview

• Table 18.2 provides a summary of the hormones.

• Lipid-soluble hormones include the steroids, thyroid hormones, and nitric oxide, which acts as a local hormone in several tissues.

• Water-soluble hormones include the amines; peptides, proteins, and glycoproteins; and eicosanoids.

Hormone Transport in Blood

• Protein hormones circulate in free form in blood• Steroid (lipid) & thyroid hormones must attach to

transport proteins synthesized by liver– improve transport by making them water-soluble– slow loss of hormone by filtration within kidney– create reserve of hormone

• only 0.1% to 10% of hormone is not bound to transport protein = free fraction

General Mechanisms of Hormone Action

• Hormone binds to cell surface or receptor inside target cell

• Cell may then– synthesize new molecules– change permeability of membrane– alter rates of reactions

• Each target cell responds to hormone differentlyAt liver cells---insulin stimulates glycogen synthesis

At adipocytes---insulin stimulates triglyceride synthesis

Second Messengers• Some hormones exert their influence by increasing the

synthesis of cAMP– ADH, TSH, ACTH, glucagon and epinephrine

• Some exert their influence by decreasing the level of cAMP– growth hormone inhibiting hormone

• Other substances can act as 2nd messengers– calcium ions– cGMP

• A hormone may use different 2nd messengers in different target cells

Hormonal Interactions

• The responsiveness of a target cell to a hormone depends on the hormone’s concentration, the abundance of the target cell’s hormone receptors, and influences exerted by other hormones.

• Three hormonal interactions are the – permissive effect– synergistic effect– antagonist effect

Hormonal Interactions• Permissive effect

– a second hormone, strengthens the effects of the first– thyroid strengthens epinephrine’s effect upon lipolysis

• Synergistic effect– two hormones acting together for greater effect– estrogen & LH are both needed for oocyte production

• Antagonistic effects– two hormones with opposite effects– insulin promotes glycogen formation & glucagon

stimulates glycogen breakdown

Control of Hormone Secretion

• Regulated by signals from nervous system, chemical changes in the blood or by other hormones

• Negative feedback control (most common)– decrease/increase in blood level is reversed

• Positive feedback control– the change produced by the hormone causes

more hormone to be released

• Disorders involve either hyposecretion or hypersecretion of a hormone

HYPOTHALAMUS AND PITUITARY GLAND

• The hypothalamus is the major integrating link between the nervous and endocrine systems.– Hypothalamus receives input from cortex, thalamus,

limbic system & internal organs

– Hypothalamus controls pituitary gland with 9 different releasing & inhibiting hormones

• The hypothalamus and the pituitary gland (hypophysis) regulate virtually all aspects of growth, development, metabolism, and homeostasis.

• The pituitary gland is located in the sella turcica of the sphenoid bone and is differentiated into the anterior pituitary (adenohypophysis), the posterior pituitary (neurohypophysis), and pars intermedia (avascular zone in between (Figures 18.5 and 18.21b).

• Pea-shaped, 1/2 inch gland found in sella turcica of sphenoid– Infundibulum attaches it to brain

• Anterior lobe = 75% – develops from roof of mouth

• Posterior lobe = 25%– ends of axons of 10,000 neurons found in hypothalamus– neuroglial cells called pituicytes

Anatomy of Pituitary Gland

Anterior Pituitary Gland (Adenohypophysis)• The blood supply to the anterior pituitary is from

the superior hypophyseal arteries.• Hormones of the anterior pituitary and the cells

that produce the:– Human growth hormone (hGH) is secreted by somatotrophs.– Thyroid-stimulating hormone (TSH) is secreted by

thyrotrophs.– Follicle-stimulating hormone (FSH) and luteinizing hormone

(LH) are secreted by gonadotrophs.– Prolactin (PRL) is secreted by lactrotrophs.– Adrenocorticotrophic hormone (ACTH) and melanocyte-

stimulating hormone (MSH) are secreted by corticotrophs.

Feedback

• Secretion of anterior pituitary gland hormones is regulated by hypothalamic regulating hormones and by negative feedback mechanisms (Figure 18.6, Table 18.3).

Human Growth Hormone and

Insulin-like Growth Factors• Human growth hormone (hGH) is the most plentiful

anterior pituitary hormone.• It acts indirectly on tissues by promoting the

synthesis and secretion of small protein hormones called insulin-like growth factors (IGFs).– IGFs stimulate general body growth and regulate various

aspects of metabolism.– Various stimuli promote and inhibit hGH production

(Figure 18.7).– One symptom of excess hGH is hyperglycemia. (Clinical

Application)

Human Growth Hormone• Produced by somatotrophs• target cells synthesize insulinlike

growth – common target cells are liver, skeletal

muscle, cartilage and bone– increases cell growth & cell division by

increasing their uptake of amino acids & synthesis of proteins

– stimulate lipolysis in adipose so fatty acids used for ATP

– retard use of glucose for ATP production so blood glucose levels remain high enough to supply brain

Thyroid Stimulating Hormone (TSH)• Hypothalamus regulates thyrotroph cells

• Thyrotroph cells produce TSH

• TSH stimulates the synthesis & secretion of T3 and T4

• Metabolic rate stimulated

Follicle Stimulating Hormone (FSH)

• Releasing hormone from hypothalamus controls gonadotrophs

• Gonadotrophs release follicle stimulating hormone

• FSH functions – initiates the formation of follicles

within the ovary

– stimulates follicle cells to secrete estrogen

– stimulates sperm production in testes

Luteinizing Hormone (LH)

• Releasing hormones from hypothalamus stimulate gonadotrophs

• Gonadotrophs produce LH

• In females, LH stimulates– secretion of estrogen– ovulation of 2nd oocyte from ovary– formation of corpus luteum– secretion of progesterone

• In males, LH stimulates the interstitial cells of the testes to secrete testosterone.

Prolactin (PRL)• Prolactin (PRL), together with other

hormones, initiates and maintains milk secretion by the mammary glands.– Hypothalamus regulates

lactotroph cells

– Lactotrophs produce prolactin

– Under right conditions, prolactin causes milk production

• Suckling reduces levels of hypothalamic inhibition and prolactin levels rise along with milk production

Adrenocorticotrophic Hormone

• Adrenocorticotrophic hormone (ACTH) controls the production and secretion of hormones called glucocorticoids by the cortex of the adrenal gland. – Hypothalamus releasing

hormones stimulate corticotrophs– Corticotrophs secrete ACTH &

MSH– ACTH stimulates cells of the

adrenal cortex that produce glucocorticoids

Melanocyte-Stimulating Hormone

• Melanocyte-stimulating hormone (MSH) increases skin pigmentation although its exact role in humans is unknown.– Releasing hormone from hypothalamus

increases MSH release from the anterior pituitary

– Secreted by corticotroph cells

• Function not certain in humans (increase skin pigmentation in frogs )

Posterior Pituitary Gland (Neurohypophysis)

• Although the posterior pituitary gland does not synthesize hormones, it does store and release two hormones.– Hormones made by the hypothalamus and

stored in the posterior pituitary are oxytocin (OT) and antidiuretic hormone (ADH).

– The neural connection between the hypothalamus and the neurohypophysis is via the hypothalamohypophyseal tract (Figure 18.8).

Oxytocin• Two target tissues both involved in

neuroendocrine reflexes• During delivery

– baby’s head stretches cervix– hormone release enhances uterine

muscle contraction– baby & placenta are delivered

• After delivery– Oxytocin stimulates contraction of the

uterus and ejection (let-down) of milk from the breasts.

• Nursing a baby after delivery stimulates oxytocin release, promoting uterine contractions and the expulsion of the placenta (Clinical Application).

• suckling & hearing baby’s cry stimulates milk ejection

ADH

• Antidiuretic hormone stimulates water reabsorption by the kidneys and arteriolar constriction.

• The effect of ADH is to decrease urine volume and conserve body water.

• ADH is controlled primarily by osmotic pressure of the blood (Figure 18.9).

THYROID GLAND - Overview

• The thyroid gland is located just below the larynx and has right and left lateral lobes (Figure 18.10a).

• Histologically, the thyroid consists of the thyroid follicles composed of follicular cells, which secrete the thyroid hormones thyroxine (T4) and triiodothyronine (T3), and parafollicular cells, which secrete calcitonin (CT) (Figures 18.10b and 18.13c).

Formation, Storage, and Release of Thyroid Hormones

• Thyroid hormones are synthesized from iodine and tyrosine within a large glycoprotein molecule called thyroglobulin (TGB) and are transported in the blood by plasma proteins, mostly thyroxine-binding globulin (TBG).

• The formation, storage, and release steps include – iodide trapping,

– synthesis of thyroglobulin,

– oxidation of iodide,

– iodination of tyrosine,

– coupling of T1 and T2,

– pinocytosis and digestion of colloid,

– secretion of thyroid hormones, and transport in blood (Figure 18.11).

PARATHYROID GLANDS• The parathyroid glands are embedded on the posterior surfaces of the lateral lobes of the thyroid – principal cells produce parathyroid hormone– oxyphil cells … function is unknown (Figure 18.13).

• Parathyroid hormone (PTH) regulates the homeostasis of calcium and phosphate

• increase blood calcium level• decrease blood phosphate level

– increases the number and activity of osteoclasts– increases the rate of Ca+2 and Mg+2 from reabsorption from

urine and inhibits the reabsorption of HPO4-2 so more is

secreted in the urine– promotes formation of calcitriol, which increases the

absorption of Ca+2, Mg+2,and HPO4-2 from the GI tract

Blood Calcium

• Blood calcium level directly controls the secretion of calcitonin and parathyroid hormone via negative feedback loops that do not involve the pituitary gland (Figure 18.14).

• Table 18.7 summarizes the principal actions and control of secretion of parathyroid hormone.

Adrenal Cortex

• The adrenal cortex is divided into three zones, each of which secretes different hormones (Figure 18.15).– The zona glomerulosa (outer zone)

• secretes mineralocorticoids.

– The zona fasciculata (middle zone) • secretes glucocorticoids.

– The zona reticularis (inner zone) • secretes androgens.

Mineralocorticoids

• 95% of hormonal activity due to aldosterone• Functions

– increase reabsorption of Na+ with Cl- , bicarbonate and water following it

– promotes excretion of K+ and H+

• Hypersecretion = tumor producing aldosteronism– high blood pressure caused by retention of Na+ and

water in blood

Glucocorticoids• 95% of hormonal activity is due to cortisol• Functions = help regulate metabolism

– increase rate of protein catabolism & lipolysis– conversion of amino acids to glucose– stimulate lipolysis– provide resistance to stress by making

nutrients available for ATP production– raise BP by vasoconstriction– anti-inflammatory effects reduced (skin cream)

• reduce release of histamine from mast cells• decrease capillary permeability• depress phagocytosis

Androgens from Zona Reticularis

• Small amount of male hormone produced– insignificant in males– may contribute to sex drive in females– is converted to estrogen in postmenopausal

females

Adrenal Medulla

• Chromaffin cells receive direct innervation from sympathetic nervous system– develop from same tissue as postganglionic

neurons

• Produce epinephrine & norepinephrine

• Hormones are sympathomimetic– effects mimic those of sympathetic NS– cause fight-flight behavior

• Acetylcholine increase hormone secretion by adrenal medulla

PANCREATIC ISLETS

• The pancreas is a flattened organ located posterior and slightly inferior to the stomach and can be classified as both an endocrine and an exocrine gland (Figure 18.18).

• Histologically, it consists of pancreatic islets or islets of Langerhans (Figure 18.19) and clusters of cells (acini) (enzyme-producing exocrine cells).

Cell Types in the Pancreatic Islets

• Alpha cells (20%) produce glucagon

• Beta cells (70%) produce insulin

• Delta cells (5%) produce somatostatin

• F cells produce pancreatic polypeptide

Ovaries and Testes

• Ovaries– estrogen, progesterone, relaxin & inhibin– regulate reproductive cycle, maintain pregnancy &

prepare mammary glands for lactation

• Testes– produce testosterone– regulate sperm production & 2nd sexual characteristics

• Table 18.10 summarizes the hormones produced by the ovaries and testes and their principal actions.

Pineal Gland

• Small gland attached to 3rd ventricle of brain

• Consists of pinealocytes & neuroglia

• Melatonin responsible for setting of biological clock

• Jet lag & SAD treatment is bright light