22. Chemical Control and Coordination
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The hypothalamus produces both releasing and inhibiting hormones that regulate the pituitary gland. Releasing hormones stimulate pituitary hormone secretion while inhibiting hormones inhibit secretion. For example, GnRH stimulates gonadotropin release from the pituitary while somatostatin inhibits growth hormone release. The pituitary gland is divided into the adenohypophysis and neurohypophysis. The adenohypophysis secretes TSH, ACTH, LH, FSH, GH, prolactin and MSH which regulate other endocrine glands. The neurohypophysis stores and releases oxytocin and vasopressin from the hypothalamus.
22. Chemical Control and Coordination
- 4. HYPOTHALAMUS • It contains several groups of neurosecretory cells called nuclei which produce hormones. • These hormones regulate the synthesis and secretion of pituitary hormones. However, the hormones produced by hypothalamus are of two types, the releasing hormones (which stimulate secretion of pituitary hormones) and the inhibiting hormones (which inhibit secretions of pituitary hormones). • For example a hypothalamic hormone called Gonadotrophin releasing hormone (GnRH) stimulates the pituitary synthesis and release of gonadotrophins. • somatostatin from the hypothalamus inhibits the release of growth hormone from the pituitary. • These hormones originating in the hypothlamic neurons, pass through axons and are released from their nerve endings. These hormones reach the pituitary gland through a portal circulatory system and regulate the functions of the anterior pituitar • The posterior pituitary is under the direct neural regulation of the hypothalamus
- 5. PITUITARY GLAND • The pituitary gland is located in a bony cavity called sella tursica and is attached to hypothalamus by a stalk It is divided anatomically into an adenohypophysis and a neurohypophysis. • Adenohypophysis consists of two portions, pars distalis and pars intermedia. The pars distalis region of pituitary, commonly called 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 only one hormone called melanocyte stimulating hormone (MSH). However, in humans, the pars intermedia is almost merged with pars distalis. • Neurohypophysis (pars nervosa) also known as posterior pituitary, stores and releases two hormones called oxytocin and vasopressin, which are actually synthesised by the hypothalamus and are transported axonally to neurohypophysis
- 9. THE PINEAL GLAND • The pineal gland is located on the dorsal side of forebrain. Pineal secretes a hormone called melatonin. Melatonin plays a very important role in the regulation of a 24-hour (diurnal) rhythm of our body. For example, it helps in maintaining the normal rhythms of sleep-wake cycle, body temperature. In addition, melatonin also influences metabolism, pigmentation, the menstrual cycle as well as our defense capability.
- 11. THYMUS GLAND
- 14. Thyroid gland • The thyroid gland is composed of two lobes which are located on either side of the trachea (Figure 22.3). Both the lobes are interconnected with a thin flap of connective tissue called isthmus. • The thyroid gland is composed of follicles and stromal tissues. Each thyroid follicle is composed of follicular cells, enclosing a cavity. These follicular cells synthesise two hormones, tetraiodothyronine or thyroxine (T4) and triiodothyronine (T3). Iodine is essential for the normal rate of hormone synthesis in the thyroid. Deficiency of iodine in our diet results in hypothyroidism and enlargement of the thyroid gland, commonly called goitre. Hypothyroidism during pregnancy causes defective development and maturation of the growing baby leading to stunted growth (cretinism), mental retardation, low intelligence quotient, abnormal skin, deaf-mutism, etc. • . Due to cancer of the thyroid gland or due to development of nodules of the thyroid glands, the rate of synthesis and secretion of the thyroid hormones is increased to abnormal high levels leading to a condition called hyperthyroidism which adversely affects the body physiology.
- 17. Thymus gland • The thymus gland is a lobular structure located on the dorsal side of the heart and the aorta. The thymus plays a major role in the development of the immune system. This gland secretes the peptide hormones called thymosins. • Thymosins play a major role in the differentiation of T- lymphocytes, which provide cell-mediated immunity. In addition, thymosins also promote production of antibodies to provide humoral immunity. • Thymus is degenerated in old individuals resulting in a decreased production of thymosins. As a result, the immune responses of old persons become weak.
- 20. ADRENAL MEDULLA • The adrenal medulla secretes two hormones called adrenaline or epinephrine and noradrenaline or norepinephrine. • These are commonly called as catecholamines. • Adrenaline and noradrenaline are rapidly secreted in response to stress of any kind and during emergency situations and are called emergency hormones or hormones of Fight or Flight. • These hormones increase alertness, pupilary dilation, piloerection (raising of hairs), sweating etc. Both the hormones increase the heart beat, the strength of heart contraction and the rate of respiration. Catecholamines also stimulate the breakdown of glycogen resulting in an increased concentration of glucose in blood. In addition, they also stimulate the breakdown of lipids and proteins
- 21. Adrenal cortex • The adrenal cortex can be divided into three layers, called zona reticularis (inner layer), zona fasciculata (middle layer) and zona glomerulosa (outer layer). • The adrenal cortex secretes many hormones, commonly called as corticoids. • The corticoids, which are involved in carbohydrate metabolism are called glucocorticoids. In our body, cortisol is the main glucocorticoid. • Corticoids, which regulate the balance of water and electrolytes in our body are called mineralocorticoids. Aldosterone is the main mineralocorticoid in our body.
- 22. GLlucocorticoids • Glucocorticoids stimulate, gluconeogenesis, lipolysis and proteolysis; and inhibit cellular uptake and utilisation of amino acids. Cortisol is also involved in maintaining the cardio- vascular system as well as the kidney functions. Glucocorticoids, particularly cortisol, produces antiinflamatory reactions and suppresses the immune response. Cortisol stimulates the RBC production.
- 23. Aldosterone • Aldosterone acts mainly at the renal tubules and stimulates the reabsorption of Na+ and water and excretion of K+ and phosphate ions. Thus, aldosterone helps in the maintenance of electrolytes, body fluid volume, osmotic pressure and blood pressur
- 24. ANDROGENIC CORTICOIDS • Small amounts of androgenic steroids are also secreted by the adrenal cortex which play a role in the growth of axial hair, pubic hair and facial hair during puberty
- 25. HORMONES AND FACTS • Now you know about the endocrine glands and their hormones. However, as mentioned earlier, hormones are also secreted by some tissues which are not endocrine glands. For example, the atrial wall of our heart secretes a very important peptide hormone called atrial natriuretic factor (ANF), which decreases blood pressure. When blood pressure is increased, ANF is secreted which causes dilation of the blood vessels. This reduces the blood pressure. • The juxtaglomerular cells of kidney produce a peptide hormone called erythropoietin which stimulates erythropoiesis (formation of RBC). • Endocrine cells present in different parts of the gastro-intestinal tract secrete four major peptide hormones, namely gastrin, secretin, cholecystokinin (CCK) and gastric inhibitory peptide (GIP). • Gastrin acts on the gastric glands and stimulates the secretion of hydrochloric acid and pepsinogen. • Secretin acts on the exocrine pancreas and stimulates secretion of water and bicarbonate ions. • CCK acts on both pancreas and gall bladder and stimulates the secretion of pancreatic enzymes and bile juice, respectively. • GIP inhibits gastric secretion and motility. Several other non-endocrine tissues secrete hormones called growth factors. These factors are essential for the normal growth of tissues and their repairing/regeneration.
- 26. L.S. of ovary
- 28. L.S. of testes
- 29. LEYDIG CELLS
- 32. Types of hormones • (i) peptide, polypeptide, protein hormones (e.g., insulin, glucagon, pituitary hormones, hypothalamic hormones, etc.) (ii) steroids (e.g., cortisol, testosterone, estradiol and progesterone) (iii) iodothyronines (thyroid hormones) (iv) amino-acid derivatives (e.g., epinephrine).