Endocrine System

Overview Two major controlling systems in the body: nervous and endocrine. Nervous system regulates the activity of muscle and glands by means of electrochemical impulses delivered by neurons. The endocrine system influences the metabolic activities of cells by means of hormones (chemical messengers). The endocrine glands are ductless glands and include pituitary, thyroid, parathyroid, adrenal, pineal, and thymus. Organs that contain endocrine tissue include the pancreas, gonads (testes and ovaries) and digestive organs (stomach and intestines). Pituitary secretions regulated by a neuroendocrine gland, the hypothalamus.

Hormones Hormones - chemical substances secreted by cells into the extracellular fluids and regulate the metabolic function of other cells in the body Two chemical classes: Amino acid based - protein, peptide, and catecholamines (epinephrine and norepinephrine) Steroids - most hormones synthesized from cholesterol, lipid soluble passing through cell membranes (estrogen, testosterone)

Hormone Target Cell Specificity Hormone is received by a target cell and evokes a response Cell response is dependent upon protein receptors on the plasma membrane or receptors in the nucleus and if receptors present, will bind hormones complimentarily Extent of target-cell activation (response) by hormone-receptor interaction depends on.... blood levels of the hormone relative numbers of receptors for the specific hormone affinity for the hormone by receptor Regulation of cell receptors: Up regulation - increase in the number of receptors with increase in hormone concentration Down regulation - prolonged exposure, cells become desensitized and there is a loss of receptors

Mechanism of Hormone Action A hormonal stimulus typically produces one or more of the following changes: changes in plasma membrane permeability and/or electrical state (membrane potential) synthesis of proteins or certain regulatory molecules (such as enzymes) within the cell enzyme regulation (activation or deactivation) induce secretion stimulation of mitosis

Mechanism of Hormone Action Two major mechanisms leading to hormone action: 1. Second messenger systems (mediate the target cell's response to hormone) Hormone binds to a receptor on the cell membrane Receptor causes adenylate cyclase in the membrane to be active and convert ATP into cAMP cAMP activates protein kinases Protein kinases activates enzymes, stimulate cellular secretion, opens ion channels, etc..... A.C.

Mechanism of Hormone Action 2. Direct gene activation Steroid hormones diffuse easily into their target cells They bind to receptors within the nucleus The activated complex then interacts with the nuclear chromatin binding to a receptor protein Binding to chromatin "turns on" DNA Transcription of mRNA which is used to direct the synthesis of other proteins (e.g. enzymes).

Control of Hormones Synthesis and release of most hormones are regulated by some type of negative feedback The output of the system feeds back and decreases the input into the system Three types of stimuli that initiates/stimulates the manufacture and release of hormones: Hormonal - hypothalamus produces releasing and inhibiting hormones Humoral - changing in blood levels of ions and nutrients Neural - neuronal control (e.g. sympathetic nervous system release of catecholamines during stress)

Pituitary Gland (Hypophysis) – The Master Gland Housed in the sella turcica of the sphenoid bone Pituitary is connected the hypothalamus by the infundibulum and is divided into two lobes: Posterior Pituitary (neurohypophysis) - neuroglia and nerve fibers release neurohormones premade from hypothalamus Anterior Pituitary (adenohypophysis) - glandular tissue

Pituitary Gland (Hypophysis) – The Master Gland Pituitary-Hypothalamic Relationships Posterior Pituitary (neurohypophysis) -Connected with hypothalamus via a nerve bundle (hypothalamic-hypophyseal tract) which runs through the infundibulum ADH-anti-diuretic hormone, promotes reabsorption of water in kidney resulting in less urine volume Oxytocin- stimulates birth contractions in uterus and allows for milk ejection from breast

Anterior Pituitary (adenohypophysis) No neural connection, rather vascular connection Adenohypophyseal hormones (all second messenger hormones) - six distinct adenohypophyseal hormones: (GH) - growth hormone, increases growth and maintenance of organs (PRL) – prolactin, stimulates mammary development and milk Tropic (regulates the hormone function of other endocrine glands) (TSH) - thyroid-stimulating hormone, stimulates secretion of thyroid hormone from thyroid (ACTH) – adrenocorticotropic hormone, promotes growth of adrenal cortex and secretion of cortical hormones gonadotropic (regulate the function of gonads) (FSH) - follicle-stimulating hormone, stimulates ova to develop and secretion of estrogen; stimulates sperm production and section of testosterone (LH) – luteinizing hormone, stimulates corpus luteum after ovulation and secretion of progesterone; stimulates secretion of testosterone

Thyroid Butterfly-shaped and largest endocrine gland located in the anterior neck overlying the trachea just inferior to the larynx. The thyroid gland has two lateral lobes Synthesizes two thyroid hormones (thyroxin and triodothyronine, require iodine) which are collectively called thyroid hormone. Thyroxine (T4) and triodothyronine (T3) - thyroid hormone stimulates enzymes concerned with glucose oxidation and therefore increases basal metabolic rates and heat production (calorigenic effect) Calcitonin (thyrocalcitonin) - polypeptide hormone that lowers blood calcium and phosphate levels and is a direct antagonist of parathyroid hormone

Parathyroid Embedded on the posterior surface of the lobes of the thyroid gland (usually four). Secretes parathyroid hormone (PTH) PTH controls the calcium balance of the blood and is triggered by decreased calcium PTH stimulates three target organs: skeleton, kidneys, and intestines. PTH release results in... Activation of osteoclasts to digest bony matrix and release calcium ions and phosphate Enhanced resorption of calcium ions Increased absorption of calcium by intestinal mucosal cells

Adrenal Gland Pyramid-shaped organs located at superior border of each kidney and is surrounded by connective tissue and embedded in fat There two separate portions: inner medulla and an outer cortex. Adrenal Cortex Synthesizes steroid hormones (corticosteroids) from cholesterol Glucocorticoids- produce cortisol, released under stress and low blood sugar; and acts to increase blood sugar, suppress immune system and aid in metabolism. Mineralocorticoids- produce aldosterone causing sodium retention, water retention, and increase in blood pressure Gonadocorticoids- produces sex hormones, androgens (testosterone), some estrogens and progesterone.

Adrenal Gland Adrenal Medulla Secretes epinephrine (adrenaline) and norepinephrine (noradrenaline) The catechlolamines secreted increase heart rate and blood pressure (via vasoconstriction).

Pancreas Contains acinar cells that produce enzymes that are ducted into the small intestine, therefore is a exocrine gland Among the acinar cells are islets of Langerhans that produce pancreatic hormones, therefore endocrine gland Cells of islets of Langerhans: Alpha cells – make glucagon to promote raising blood sugar Beta cells – make insulin to promote lowering blood sugar

Gonads Produce gonadal sex hormones: Ovaries - estrogens and progesterone Testes - testosterone

Pineal Gland Roof of third ventricle in cranium and contains pinealocytes Secrete melatonin Involved in sleep-wake cycle

Thymus Gland Thymic epithelial cells secrete the peptide hormones thymopoeitin and thymosin used in the development of immunity