Biology: Class 11 — Chapter 19: Chemical Coordination and Integration

Located at the basal part of the diencephalon (forebrain), the hypothalamus contains groups of neurosecretory cells called nuclei that produce hormones regulating pituitary hormone synthesis and secretion.

• Releasing Hormones: Stimulate secretion of pituitary hormones. Example: Gonadotrophin releasing hormone (GnRH) stimulates the release of gonadotrophins.
• Inhibiting Hormones: Inhibit secretion of pituitary hormones. Example: Somatostatin inhibits the release of growth hormone.
• Pathway: Hypothalamic hormones reach the anterior pituitary through a portal circulatory system, while the posterior pituitary is under direct neural regulation.

Located in a bony cavity called sella tursica and attached to the hypothalamus by a stalk. It is divided anatomically into:

1. Adenohypophysis: Consists of two portions:
   • Pars Distalis (Anterior Pituitary): Produces Growth Hormone (GH), Prolactin (PRL), Thyroid Stimulating Hormone (TSH), Adrenocorticotrophic Hormone (ACTH), Luteinizing Hormone (LH), and Follicle Stimulating Hormone (FSH).
   • Pars Intermedia: Secretes Melanocyte Stimulating Hormone (MSH). In humans, it is almost merged with pars distalis.
2. Neurohypophysis (Pars Nervosa / Posterior Pituitary): Stores and releases Oxytocin and Vasopressin (synthesized by the hypothalamus and transported axonally).

Major Pituitary Hormones & Disorders:

• Growth Hormone (GH): Over-secretion leads to gigantism or Acromegaly (severe disfigurement in adults); low secretion causes pituitary dwarfism.
• Prolactin (PRL): Regulates mammary gland growth and milk formation.
• TSH: Stimulates thyroid hormone synthesis and secretion.
• ACTH: Stimulates synthesis of steroid hormones called glucocorticoids from the adrenal cortex.
• LH & FSH (Gonadotrophins): Stimulate gonadal activity. In males, LH stimulates androgen synthesis, and FSH regulates spermatogenesis. In females, LH induces ovulation and maintains the corpus luteum; FSH stimulates ovarian follicle development.
• MSH: Regulates skin pigmentation.
• Oxytocin: Stimulates smooth muscle contraction (uterus contraction during childbirth and milk ejection).
• Vasopressin / Anti-diuretic Hormone (ADH): Stimulates water and electrolyte resorption in renal distal tubules, reducing water loss. Deficiency causes Diabetes Insipidus.

Located on the dorsal side of the forebrain. It secretes melatonin, which regulates:

• The 24-hour (diurnal) rhythm of the body (sleep-wake cycle, body temperature).
• Metabolism, pigmentation, menstrual cycle, and defense capability.

Composed of two lobes on either side of the trachea, interconnected by a thin flap of connective tissue called the isthmus. It consists of follicles and stromal tissues.

• Hormones: Follicular cells synthesize tetraiodothyronine (thyroxine or $T_4$) and triiodothyronine ($T_3$). Iodine is essential for their synthesis.
• Thyrocalcitonin (TCT): A protein hormone secreted by the thyroid that regulates and lowers blood calcium levels.
• Hypothyroidism: Iodine deficiency leads to enlargement of the thyroid gland (goitre). During pregnancy, it causes cretinism (stunted growth, mental retardation, low IQ) in the baby. In adult women, it causes irregular menstrual cycles.
• Hyperthyroidism: Caused by thyroid cancer or nodules, leading to abnormally high hormone levels. Characterized by Exophthalmic goitre (Graves' disease), which involves thyroid enlargement, protrusion of eyeballs, increased BMR, and weight loss.

Four parathyroid glands are present on the back side of the thyroid gland (one pair in each lobe).

• Parathyroid Hormone (PTH): A peptide hormone that increases blood $Ca^{2+}$ levels (hypercalcemic hormone).
• Mechanism: Stimulates bone resorption (demineralisation), increases $Ca^{2+}$ reabsorption by renal tubules, and increases $Ca^{2+}$ absorption from digested food.

A lobular structure located between the lungs, behind the sternum on the ventral side of the aorta. It plays a major role in immune system development.

• Thymosins: Peptide hormones secreted by the thymus.
• Function: Promote differentiation of T-lymphocytes (providing cell-mediated immunity) and stimulate antibody production (providing humoral immunity).
• Aging: The thymus degenerates in old individuals, leading to weakened immune responses.

Located anteriorly on top of each kidney. It consists of two types of tissues:

1. Adrenal Medulla (Centrally located): Secretes catecholamines:
   • Adrenaline (epinephrine) and Noradrenaline (norepinephrine).
   • Known as emergency hormones or hormones of Fight or Flight.
   • Effects: Increase alertness, pupillary dilation, piloerection, sweating, heart rate, strength of heart contraction, respiration rate, and stimulate glycogen, lipid, and protein breakdown.
2. Adrenal Cortex (Outer layer): Divided into three layers: zona reticularis (inner), zona fasciculata (middle), and zona glomerulosa (outer). It secretes corticoids:
   • Glucocorticoids (mainly cortisol): Regulate carbohydrate metabolism, stimulate gluconeogenesis, lipolysis, proteolysis, and suppress immune responses.
   • Mineralocorticoids (mainly aldosterone): Regulate water and electrolyte balance, stimulating $Na^+$ and water reabsorption and $K^+$ excretion in renal tubules.
   • Androgenic steroids: Play a role in the growth of axial, pubic, and facial hair during puberty.

Disorder: Underproduction of cortical hormones alters carbohydrate metabolism, causing acute weakness and fatigue, known as Addison's disease.

A composite gland acting as both exocrine and endocrine. The endocrine part consists of Islets of Langerhans (1-2 million in a normal pancreas).

• $\alpha$-cells: Secrete glucagon, a hyperglycemic peptide hormone that stimulates glycogenolysis and gluconeogenesis in hepatocytes, increasing blood glucose.
• $\beta$-cells: Secrete insulin, a hypoglycemic peptide hormone that enhances cellular glucose uptake and utilization by hepatocytes and adipocytes, and stimulates glycogenesis.
• Diabetes Mellitus: Prolonged hyperglycemia leads to this disorder, characterized by loss of glucose in urine and formation of harmful ketone bodies. Treated with insulin therapy.

A pair of testes is located in the scrotal sac of males. It performs dual functions as a primary sex organ and an endocrine gland.

• Composed of seminiferous tubules and stromal/interstitial tissue.
• Leydig cells (Interstitial cells): Produce a group of hormones called androgens (mainly testosterone).
• Functions: Regulate male accessory sex organ development, stimulate muscular growth, facial/axillary hair, low pitch of voice, spermatogenesis, and male sexual behavior (libido).

A pair of ovaries is located in the abdomen of females. It is the primary female sex organ.

• Estrogen: Synthesized and secreted by growing ovarian follicles. Regulates female secondary sex characters, mammary gland development, and sexual behavior.
• Progesterone: Secreted by the corpus luteum (formed from ruptured follicles after ovulation). It supports pregnancy and stimulates the formation of alveoli (milk-storing structures) and milk secretion in mammary glands.