BIO178 Lecture 3 Summer24 PDF

Lecture 3: The Endocrine System Part 2  Outline of Today’s Lecture 1. The Anterior Pituitary 2. Prolactin 3. The Adrenal Glands and the Hypothalamic-Pituitary- Adrenal Axis 4. The Hypothalamic-Pituitary-Thyroid Axis 2 ...

Lecture 3: The Endocrine System Part 2  Outline of Today’s Lecture 1. The Anterior Pituitary 2. Prolactin 3. The Adrenal Glands and the Hypothalamic-Pituitary- Adrenal Axis 4. The Hypothalamic-Pituitary-Thyroid Axis 2  Learning Objectives By the end of today’s lecture, you should be able to… 1. Explain how the anterior pituitary communicates with and is controlled by the hypothalamus. 2. Compare and contrast the posterior and anterior pituitary with respect to structure, endocrine functions, and relationship to the hypothalamus. 3. Describe the major functions of prolactin and how its secretion is controlled. 4. Briefly describe the source, structure, and functions of adrenal and thyroid hormones. 5. Explain the endocrine pathways and regulation of hypothalamic-pituitary- adrenal axis and the hypothalamic-pituitary-thyroid axis. 3 The Anterior Pituitary Anterior Pituitary (adenohypophysis/pars distalis/pars anterior) Develops from outpocketing of roof of mouth (Rathke’s pouch; epithelial tissue) Glandular tissue (non-neural) Receives little neural input Synthesizes and secretes hormones in response to blood- borne hormones received from hypothalamus Receives direct blood circulation from hypothalamus (portal system) Nelson 2005 Fig. 2.6 5  Anterior Pituitary: Portal Blood System Hypothalamo-Hypophyseal Portal Circulation Regular Circulation: Heart → Arteries → Arterioles → Capillaries → Venules → Veins → Heart Portal Systems: Heart → Arteries → Arterioles → Capillaries → Veins → Capillaries → Portal blood vessels Venules → Veins → Heart **(Blood flows through 2 consecutive capillary beds before returning to the heart) Materials can enter and exit the blood only at the capillary beds. 6  Anterior Pituitary: Portal Blood System Hypothalamo-Hypophyseal Portal Circulation Capillary beds in hypothalamus and anterior pituitary connected by portal vein Closed, direct, one-way circulation from hypothalamus to anterior pituitary. 7  Anterior Pituitary 1. Releasing (or inhibiting) hormone is synthesized in neurosecretory cells (in hypothalamus) and… 2. Transported down the axons, to base of hypothalamus. 3. When neurosecretory cells are stimulated, they conduct action potentials… 4. Causing release of the releasing hormone. 8  Anterior Pituitary 5. Releasing hormone diffuses into portal blood vessels and travels directly to the anterior pit., where it… 6. Stimulates anterior pituitary cells to release pituitary hormone. 7. Pituitary hormone diffuses into capillaries and circulates throughout the body. 9  Prolactin Protein hormone from anterior pituitary Acts on breasts → stimulates milk synthesis Remember: milk let-down is due to oxytocin Many other functions: – Parental Behavior – Osmoregulation – Growth & development – Stress response 10  Prolactin Under mainly *inhibitory hypothalamic control by dopamine, or prolactin inhibiting hormone (PIH) *This means it’s usually suppressed Stimulated by suckling & hearing infant cries - Stimulates thyrotropin releasing hormone (TRH), which stimulates prolactin 11 Prolactin Pathway 12 The Hypothalamic-Pituitary Axes 13  The Adrenal Glands and Hypothalamic-Pituitary-Adrenal Axis Hypothalamic & Anterior Pituitary Hormones 15  Adrenal Glands Sit on top of kidneys like little party hats Comprise two distinct organs: – Adrenal Medulla (inner region) Secretes catecholamines as part of the sympathetic nervous system – Adrenal Cortex (outer region) Secretes steroids 16  Adrenal Cortex Glandular tissue; 3 zones of the cortex produce different steroid hormones (corticosteroids and androgens) a. Aldosterone (mineralocorticoid; C21) Mineral and water metabolism  retention of Na+, excretion of K+ b. Cortisol, Corticosterone (glucocorticoids; C21) Controlled by ACTH from anterior pituitary Glucose metabolism, stress c. Dehydroepiandrosterone (DHEA) (androgen; C19) Controlled by LH from anterior pituitary Weak androgen Might play a role in aging 17  21 CH3 Corticosteroids 20 C=O (C21 Steroids) 19 Pregnenolone (precursor) HO O CH2OH CH2OH C=O HO CH C=O HO OH Aldosterone Cortisol O O 18 Hypothalamic-Pituitary-Adrenal (HPA) Axis 19  Hypothalamic-Pituitary-Adrenal (HPA) Axis 1. Hypothalamic Hormone: Corticotropin-Releasing Hormone (CRH) Binds to receptors on anterior pituitary Stimulates anterior pituitary to synthesize and secrete adrenocorticotropic hormone 2. Anterior Pituitary Hormone: Adrenocorticotropic Hormone (ACTH) = Corticotropin Binds to receptors on adrenal cortex Stimulates growth of adrenal cortex Stimulates synthesis and secretion of glucocorticoids 3. Adrenocortical Hormones: Glucocorticoids (Cortisol, Corticosterone) Feed back to hypothalamus and pituitary to  CRH and  ACTH secretion Influence metabolism, stress response, etc. 20  Adrenal Medulla Neural - Part of the sympathetic branch of the autonomic nervous system Composed of chromaffin cells, which release: Epinephrine, Norepinephrine (catecholamines; water-soluble) Cardiovascular effects ( heart rate,  cardiac output, vasoconstriction/vasodilation) Glucose metabolism ( blood glucose) “Fight or flight” response 21  Adrenal Medulla Nelson 2005 *You should learn the sequence of events of catecholamine synthesis, but you do not have to memorize the structures or enzymes 22 Autonomic Nervous System Sends motor output to internal organs Sympathetic branch: “Fight or flight” Epinephrine/norepinephrine Parasympathetic branch: “Rest and digest” Acetylcholine 23 Autonomic Nervous System Parasympathetic Sympathetic 24 The Hypothalamic-Pituitary- Thyroid Axis Hypothalamic & Anterior Pituitary Hormones 26  Thyroid Silverthorn 2009 27 Thyroid 28  Hypothalamic-Pituitary-Thyroid Axis 1. Hypothalamic Hormone: Thyrotropin-Releasing hormone (TRH) Binds to receptors on anterior pituitary Stimulates ant. pituitary to produce thyroid-stimulating hormone 2. Anterior Pituitary Hormone: Thyroid-Stimulating Hormone (TSH) = Thyrotropin Binds to receptors on thyroid Stimulates growth of thyroid cells Stimulates synthesis and secretion of thyroid hormones 3. Thyroid Hormones: Triiodothyronine (T3), Tetraiodothyronine (T4; Thyroxine) Lipophilic amines: Bind to nuclear receptors and alter gene expression Act on most cells of the body Feed back to hypothalamus and pituitary to  TRH and TSH secretion 29  Thyroid Iodinated dimers of tyrosine – Tetraiodothyronine (T4; thyroxine) more abundant in circulation – Triiodothyronine (T3) higher bioactivity 30  Thyroid T3 and T4 (cont.) Stored as thyroglobulin in thyroid follicles Lipophilic; circulate bound to carrier proteins T4 is converted to T3 in target cells Bind to nuclear receptors or membrane receptors 31  Thyroid T3 and T4 (cont.) Functions in adults –  oxygen consumption in most tissues –  thermogenesis – Affect protein, carbohydrate, and fat metabolism – Permissive effects on reproduction Functions in children – Necessary for normal growth and development (esp. in nervous system – synapses, myelin) 32  Thyroid Problems Hypothyroidism Insufficient production of thyroid hormones Most common in women 40 and older (1 in 8 chance) Common causes: – Iodine deficiency – Autoimmune disease – Pituitary tumor – Some medications – Radiation therapy Goiter (enlarged thyroid) – Can occur with hypothyroidism (if low thyroid hormone levels lead to increased TSH secretion). 33  Thyroid Problems Hypothyroidism in adults –  metabolic rate; weight gain without change in diet/exercise –  thermogenesis → cold intolerance –  protein synthesis → brittle nails, thin hair & skin – Effects on nervous system → fatigue, slow reflexes, slow speech and thought processes Hypothyroidism in infants – Impaired physical growth & maturation – Mental impairments – Future Infertility 34  Thyroid Problems Hyperthyroidism Overproduction of thyroid hormones Common causes: – Thyroid tumors – Pituitary tumors – Autoimmune disease – Thyroiditis (inflammation of thyroid) Goiter (enlarged thyroid) - Can occur if thyroid is overstimulated by TSH or thyroid-stimulating-immunoglobulin (TSI) 35  Thyroid Problems Hyperthyroidism in adults –  metabolic rate; weight loss –  thermogenesis → heat intolerance –  protein catabolism → muscle weakness & loss –  adrenergic receptors in heart →  heartbeat,  force of contraction – Exophthalmus (bulging of eyes; Graves’disease) 36 End of Lecture 3 37