CHEMICAL COORDINATION AND INTEGRATION
NCERT Masterclass & NEET Biology Study Module
1. Endocrine Glands and Hormones
The neural system provides rapid but short-lived point-to-point coordination. The endocrine system provides slower, long-lasting chemical integration through hormones.
Current scientific definition of a Hormone: Hormones are non-nutrient chemicals which act as intercellular messengers and are produced in trace amounts.
2. Human Endocrine System
[Image of Human Endocrine System]A. Hypothalamus
The basal part of the diencephalon (forebrain). Contains groups of neurosecretory cells called nuclei which produce hormones. It regulates the synthesis and secretion of pituitary hormones.
- Releasing Hormones: Stimulate pituitary secretion (e.g., GnRH stimulates pituitary to release gonadotrophins).
- Inhibiting Hormones: Inhibit pituitary secretion (e.g., Somatostatin inhibits release of growth hormone).
- Note: Hypothalamic hormones reach the anterior pituitary through a portal circulatory system. The posterior pituitary is under the direct neural regulation of the hypothalamus.
B. Pituitary Gland
Located in a bony cavity called Sella tursica. Divided into:
Hormones of Pars distalis: Growth Hormone (GH), Prolactin (PRL), Thyroid Stimulating Hormone (TSH), Adrenocorticotrophic Hormone (ACTH), Luteinizing Hormone (LH), and Follicle Stimulating Hormone (FSH).
Hormone of Pars intermedia: Melanocyte Stimulating Hormone (MSH).
2. Neurohypophysis (Posterior Pituitary / Pars nervosa): It does not synthesize hormones. It only stores and releases hormones synthesized by the hypothalamus:
- Oxytocin: Vigorous contraction of uterus during childbirth and milk EJECTION from mammary glands.
- Vasopressin (ADH): Stimulates reabsorption of water and electrolytes by the distal tubules of the kidney. Deficiency causes Diabetes insipidus (water loss/dehydration).
C. Thyroid and Parathyroid Glands
- Thyroid Gland: Composed of follicles and stromal tissues. Follicular cells synthesize $T_4$ (Thyroxine) and $T_3$. Iodine is essential.
Hypothyroidism: Enlargement of gland (Goitre). During pregnancy, it causes defective development of the baby leading to stunted growth (Cretinism), mental retardation, deaf-mutism.
Hyperthyroidism: E.g., Exophthalmic goitre (Graves' disease) - enlargement of thyroid, protrusion of eyeballs, weight loss, increased BMR.
Also secretes a protein hormone called Thyrocalcitonin (TCT) which regulates (lowers) blood calcium levels. - Parathyroid Gland: Secretes Parathyroid hormone (PTH). It is a hypercalcemic hormone (increases blood $Ca^{2+}$ levels by bone dissolution/demineralisation and absorption from food/kidneys).
D. Thymus and Pineal Gland
Pineal Gland: Located on the dorsal side of the forebrain. Secretes Melatonin. Regulates the 24-hour diurnal rhythm (sleep-wake cycle, body temperature) and influences pigmentation and menstrual cycle.
Thymus: Located on the dorsal side of the heart/aorta. Major role in immune system development. Secretes Thymosins which promote differentiation of T-lymphocytes (cell-mediated immunity) and antibodies (humoral immunity). It degenerates in old individuals.
E. Adrenal Gland
- Adrenal Medulla: Secretes catecholamines: Adrenaline (Epinephrine) and Noradrenaline (Norepinephrine). These are "Fight or Flight" hormones. They increase heart beat, pupil dilation, piloerection, and glycogenolysis (increasing blood glucose).
- Adrenal Cortex: Divided into Zona reticularis (outer), fasciculata (middle), glomerulosa (inner).
- Glucocorticoids (Cortisol): Stimulate gluconeogenesis, lipolysis, proteolysis. Anti-inflammatory and suppresses immune response.
- Mineralocorticoids (Aldosterone): Regulates $Na^+$, $K^+$, water balance.
Disease: Underproduction of cortical hormones leads to weakness and fatigue, called Addison's disease.
F. Pancreas & Gonads
Pancreas: A composite (exocrine + endocrine) gland. The endocrine part consists of Islets of Langerhans (1-2 million).
- $\alpha$-cells secrete Glucagon (Hyperglycemic - increases blood sugar).
- $\beta$-cells secrete Insulin (Hypoglycemic - decreases blood sugar, promotes glycogenesis).
Prolonged hyperglycemia leads to Diabetes mellitus (loss of glucose in urine, ketone bodies).
Testis: Leydig cells secrete Androgens (Testosterone). Regulates male secondary sexual characters and stimulates spermatogenesis.
Ovary: Ovarian follicles secrete Estrogen (female secondary characters). Corpus luteum secretes Progesterone (supports pregnancy, stimulates milk secretion structures).
3. Hormones of Heart, Kidney and GI Tract
4. Mechanism of Hormone Action
Hormones bind to specific proteins called hormone receptors (located on cell membrane or inside the cell). Hormones are classified chemically into:
- Peptide, polypeptide, protein hormones: (Insulin, glucagon, pituitary/hypothalamic hormones).
- Steroids: (Cortisol, testosterone, estradiol, progesterone).
- Iodothyronines: (Thyroid hormones).
- Amino-acid derivatives: (Epinephrine).
Protein Hormones do not enter the target cell. They bind to membrane receptors and generate secondary messengers (e.g., cyclic AMP, $IP_3$, $Ca^{2+}$) which regulate cellular metabolism.
Steroid Hormones and Thyroid Hormones pass through the cell membrane and bind to intracellular receptors (mostly nuclear receptors). They regulate gene expression or chromosome function by the interaction of the hormone-receptor complex with the genome.
🚀 NEET ENDOCRINE SYSTEM MEGA QUIZ (100 MCQ)
Solve the 5 parts below to master Hormones, Glands, Disorders, and Mechanism of Action.
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