Endocrine System

Cheatsheet Content

### Introduction to Endocrine System - **Definition:** A system of ductless glands that secrete chemical messengers called hormones directly into the bloodstream. - **Hormones:** Non-nutrient chemicals acting as intercellular messengers, produced in trace amounts. They regulate physiological processes. - **Role of Hormones:** Act as messengers and regulators, controlling growth, metabolism, reproduction, and mood. ### Major Endocrine Glands & Location - **Hypothalamus:** Basal part of diencephalon, forebrain. - **Pituitary Gland:** Located in a bony cavity called sella turcica, attached to hypothalamus. - **Pineal Gland:** Dorsal side of forebrain. - **Thyroid Gland:** Located on either side of the trachea, anterior neck region. - **Parathyroid Gland:** Four small glands on the posterior surface of the thyroid gland. - **Thymus:** Between the lungs, behind the sternum on the ventral side of the aorta. - **Adrenal Gland:** On the anterior part of each kidney. - **Pancreas:** Located behind the stomach, a composite gland. - **Gonads:** - **Testes (males):** Within the scrotum. - **Ovaries (females):** In the abdomen. - **Gastrointestinal (GI) Tract:** Wall of the digestive tract. ### Hypothalamus: The Master Controller - Contains neurosecretory cells that produce hormones. - **Role:** Regulates the synthesis and secretion of pituitary hormones. - **Hormones:** - **Releasing Hormones:** Stimulate pituitary hormone secretion (e.g., GnRH, GHRH, TRH). - **Inhibiting Hormones:** Inhibit pituitary hormone secretion (e.g., Somatostatin (GHIH), PIH). - Hormones are transported to the anterior pituitary via a portal system and directly released into the posterior pituitary. ### Pituitary Gland (Hypophysis) - Divided into two main parts: Adenohypophysis (anterior pituitary) and Neurohypophysis (posterior pituitary). #### 1. Adenohypophysis (Anterior Pituitary) - **Pars Distalis (Anterior Pituitary proper):** - **Growth Hormone (GH):** - **Functions:** Promotes growth of body tissues. - **Hypersecretion:** Gigantism (childhood), Acromegaly (adulthood - severe disfigurement, arthritis, premature death). - **Hyposecretion:** Dwarfism (childhood - stunted growth). - **Prolactin (PRL):** - **Functions:** Regulates growth of mammary glands and milk production. - **Thyroid Stimulating Hormone (TSH):** - **Functions:** Stimulates synthesis and secretion of thyroid hormones from the thyroid gland. - **Adrenocorticotrophic Hormone (ACTH):** - **Functions:** Stimulates synthesis and secretion of steroid hormones (glucocorticoids) from the adrenal cortex. - **Luteinizing Hormone (LH):** - **Functions (Males):** Stimulates synthesis and secretion of androgens from testes. - **Functions (Females):** Induces ovulation, maintains corpus luteum. - **Follicle Stimulating Hormone (FSH):** - **Functions (Males):** Along with androgens, regulates spermatogenesis. - **Functions (Females):** Stimulates growth and development of ovarian follicles. - **Pars Intermedia:** - **Melanocyte Stimulating Hormone (MSH):** - **Functions:** Acts on melanocytes to regulate skin pigmentation. #### 2. Neurohypophysis (Posterior Pituitary / Pars Nervosa) - Stores and releases hormones produced by the hypothalamus. - **Oxytocin:** - **Functions:** Stimulates uterine contractions during childbirth and milk ejection from mammary glands. - **Vasopressin (Antidiuretic Hormone - ADH):** - **Functions:** Acts mainly on kidneys, stimulates reabsorption of water and electrolytes, reducing water loss through urine. - **Hyposecretion:** Diabetes Insipidus (excessive water loss, thirst). #### Feedback Control of Tropic Hormones (Example) - **Hypothalamus** releases **TRH** (Thyrotropin-releasing hormone). - **TRH** stimulates the **anterior pituitary** to release **TSH** (Thyroid-stimulating hormone). - **TSH** stimulates the **thyroid gland** to release **Thyroid hormones (T3, T4)**. - High levels of **T3/T4** in the blood then **inhibit** the release of **TRH** from the hypothalamus and **TSH** from the pituitary (Negative Feedback). ### Pineal Gland - **Hormone:** **Melatonin**. - **Functions:** Regulates 24-hour (diurnal) rhythm of the body, sleep-wake cycle, body temperature, metabolism, pigmentation, and menstrual cycle. ### Thyroid Gland - Composed of follicles and stromal tissues. Follicular cells synthesize thyroid hormones. - **Hormones:** - **Thyroxine (T4) & Triiodothyronine (T3):** Iodine is essential for their synthesis. - **Functions:** Regulate Basal Metabolic Rate (BMR), support RBC formation, control metabolism of carbohydrates, proteins, and fats, maintain water and electrolyte balance. - **Thyrocalcitonin (TCT):** (Secreted by parafollicular cells) - **Functions:** Lowers blood calcium levels (hypocalcemic hormone), works antagonistically with PTH. #### Disorders of Thyroid - **Hypothyroidism:** - **Deficiency:** Goitre (enlargement of thyroid due to iodine deficiency), Cretinism (childhood - stunted growth, mental retardation, low intelligence, abnormal skin, deaf-mutism), Myxedema (adults - low BMR, lethargy, weight gain, puffy appearance, irregular menstrual cycles). - **Hyperthyroidism:** (Excess secretion) - **Grave's Disease (Exophthalmic Goitre):** Enlargement of thyroid, protrusion of eyeballs, increased BMR, weight loss, nervousness. ### Parathyroid Gland - **Hormone:** **Parathyroid Hormone (PTH)**. - **Functions:** Increases blood calcium levels (hypercalcemic hormone). - Acts on bones to stimulate bone resorption/demineralization. - Stimulates reabsorption of Ca2+ by renal tubules. - Increases Ca2+ absorption from digested food in the intestine. - Works antagonistically with TCT. ### Thymus - **Hormone:** **Thymosins**. - **Functions:** Major role in the development of the immune system. Differentiation of T-lymphocytes (cell-mediated immunity) and promotes antibody production (humoral immunity). - Degenerates with age, leading to decreased immune responses in elderly. ### Adrenal Gland - Composed of outer Adrenal Cortex and inner Adrenal Medulla. #### 1. Adrenal Medulla - **Hormones:** **Adrenaline (Epinephrine)** and **Noradrenaline (Norepinephrine)** (Catecholamines). - **Functions:** "Fight or flight" hormones (emergency hormones). Increase heart rate, strength of heart contraction, respiration rate, alertness, pupillary dilation, piloerection. Stimulate breakdown of glycogen, lipids, and proteins. #### 2. Adrenal Cortex - Three layers: Zona reticularis (inner), Zona fasciculata (middle), Zona glomerulosa (outer). - **Hormones:** - **Glucocorticoids (e.g., Cortisol - from zona fasciculata):** - **Functions:** Regulate carbohydrate, protein, and fat metabolism. Stimulate gluconeogenesis, lipolysis, proteolysis. Anti-inflammatory reactions, suppress immune response. Maintain cardiovascular system and kidney function. - **Hyposecretion:** Addison's Disease (weakness, fatigue, bronzing of skin, gastrointestinal disturbances). - **Hypersecretion:** Cushing's Syndrome (moon face, buffalo hump, hyperglycemia, protein catabolism, hypertension). - **Mineralocorticoids (e.g., Aldosterone - from zona glomerulosa):** - **Functions:** Regulate water and electrolyte balance. Acts on renal tubules to reabsorb Na+ and water, excrete K+ and phosphate ions. - **Adrenal Androgens (from zona reticularis):** - **Functions:** Small amounts, play a role in axial hair, pubic hair, and facial hair growth during puberty. ### Pancreas - Both exocrine and endocrine gland. Endocrine part consists of Islets of Langerhans. - **Hormones:** - **Glucagon (from alpha cells):** - **Functions:** **Hyperglycemic hormone**. Increases blood glucose levels by stimulating glycogenolysis and gluconeogenesis in the liver. - **Insulin (from beta cells):** - **Functions:** **Hypoglycemic hormone**. Decreases blood glucose levels by promoting glucose uptake by cells and stimulating glycogenesis in the liver and muscles. - **Hyposecretion/Insulin Resistance:** Diabetes Mellitus (prolonged hyperglycemia, loss of glucose through urine, accumulation of ketone bodies, increased thirst and urination). ### Gonads #### Testes (Males) - **Hormones:** **Androgens (e.g., Testosterone)**. - **Functions:** Regulate development, maturation, and function of male accessory sex organs. Stimulate spermatogenesis, influence male sexual behavior (libido), promote development of secondary sexual characters, and have anabolic effects on protein and carbohydrate metabolism. #### Ovaries (Females) - **Hormones:** - **Estrogen:** - **Functions:** Synthesized by growing ovarian follicles. Stimulates growth and activities of female secondary sex organs, development of growing ovarian follicles, and appearance of female secondary sexual characters. - **Progesterone:** - **Functions:** Secreted by corpus luteum. Supports pregnancy, acts on mammary glands to stimulate formation of alveoli and milk secretion. ### Hormones of the Gastrointestinal Tract - Produces several peptide hormones that regulate digestive processes. - **Gastrin:** Stimulates secretion of HCl and pepsinogen in the stomach. - **Secretin:** Acts on the exocrine pancreas to secrete water and bicarbonate ions. - **Cholecystokinin (CCK-PZ):** Acts on the pancreas to secrete digestive enzymes and on the gallbladder to release bile. - **Gastric Inhibitory Peptide (GIP):** Inhibits gastric secretion and motility. ### Mechanism of Hormone Action (Elementary Idea) - Hormones exert their effects by binding to specific **hormone receptors** on target cells. Receptors are protein molecules located either on the cell membrane or inside the cell. #### 1. Hormones Binding to Membrane-Bound Receptors (e.g., Protein Hormones, Catecholamines) - Hormones that are water-soluble typically do not enter the target cell. - They bind to extracellular receptors on the cell membrane. - This binding leads to the formation of a **hormone-receptor complex**. - This complex activates intracellular biochemical changes, often involving the generation of **second messengers** (like cyclic AMP - cAMP). - **Mechanism via cAMP:** 1. Hormone binds to receptor. 2. Activates an enzyme (adenylate cyclase). 3. Adenylate cyclase converts ATP to cAMP. 4. cAMP acts as a second messenger, activating protein kinases. 5. Protein kinases phosphorylate other enzymes, leading to a cascade of biochemical reactions and the physiological response. #### 2. Hormones Binding to Intracellular Receptors (e.g., Steroid Hormones, Thyroid Hormones) - Lipid-soluble hormones can easily pass through the cell membrane. - They bind to intracellular receptors (present in the cytoplasm or nucleus). - The **hormone-receptor complex** then enters the nucleus (if not already there). - This complex binds to specific regions on the DNA (genome). - This binding regulates gene expression, leading to the formation of new proteins (enzymes or structural proteins). - These proteins bring about the physiological and developmental effects of the hormone.