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Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Chapter Contents Endocrine system is a system of isolated glands which secrete chemicals called ‘hormones’ directly into lymph or venous blood for transport to target tissues, often located away from the site of secretion. Hormones are defined as non-nutrient chemicals which act as intercellular messengers that are produced in trace amounts. Differences between Nervous and Endocrine System S. No. Nervous System Endocrine System 1. It receives and sends the information in the form of electrical impulses. It receives and sends the information in the form of chemicals called hormones. 2. The system has direct connection with every part under its control. The system is not directly connected with parts under its control. 3. Transmission of information is rapid. Transmission of information is comparatively slower. 4. The response is very quick. The response is comparatively slower. 5. Response is limited to only those cells which are innervated by neurons. Response is produced by all the cells of target tissue. 6. Response lasts for short duration. Response lasts for longer duration. Types of Glands 1. Exocrine glands: The glands which release their secretions through a duct are called exocrine glands e.g., liver, intestinal glands, gastric glands. 2. Endocrine glands: The glands which lack ducts and discharge their secretions into the blood stream are called endocrine glands or ductless glands. The chemical secretions of endocrine glands are called hormones and are popularly called the chemical messengers of the body. They affect definite parts of the body and prepare them for specific functions. The part of the body where a hormone produces its effect is called the target. Endocrine secretion – Target organ usually lies at a distance from the site of origin of hormone. Human endocrine system Mechanism of hormone action Chapter 9 Chemical Coordination and Integration
82 Chemical Coordination and Integration NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Paracrine secretion – Act on cells in near vicinity. Autocrine secretion – Act on same cell which secretes it. Heterocrine glands are those glands which are exocrine as well as endocrine in function e.g., testes, ovaries. Hormones are required in specific amounts. Their hyposecretion (secretion in lesser amount than normal), as well as hypersecretion (secretion in amount larger than normal) produce serious physiological disturbances in the body. Radioimmunoassay is a technique to measure the hormones, their precursor and their metabolic end products quantitatively in a living body. Biochemical Classification of Hormones : Depending upon their chemical structure, hormones may be classified as follows: (i) Phenolic hormones : e.g. Thyroxine, Adrenaline and Noradrenaline etc. (ii) Proteinaceous or Polypeptide hormones : e.g. Oxytocin, Vasopressin, Parathormone, Prolactin, Somatotropin, Insulin, Glucagon, Secretin, Relaxin etc. (iii) Glycoproteinaceous hormones : e.g., Thyrotropin, Follicle stimulating hormone, Luteinizing hormone etc. (iv) Steroid hormones : e.g., Estradiol, Progesterone, Testosterone, Cortisone, Aldosterone, Estrogen etc. HUMAN ENDOCRINE SYSTEM The endocrine glands and hormone producing diffused tissues/cells located in different parts of our body constitute the endocrine system. It includes pituitary gland, pineal gland, thyroid gland, adrenal glands, pancreas, parathyroid glands, thymus and gonads (testis in males and ovary in females). In addition to these glands, certain other organs, e.g., gastrointestinal tract, liver, kidney, heart also produce hormones. Fig. : Location of endocrine glands 1. Hypothalamus Hypothalamus is an anatomical and physiological connection between the nervous system and the endocrine system. Hypothalamus is connected to the pituitary gland via infundibulum. Hypothalamus is the basal part of diencephalon in the forebrain. Hypothalamus contains several groups of neurosecretory cells called nuclei which produce hormones for regulating wide spectrum of body functions. There are several hormones released by neurosecretory cells of the hypothalamus which regulate the synthesis and secretion of anterior pituitary hormones, i.e., hormones secreted by pituitary gland. These neurohormones are the releasing and inhibiting hormones acting on anterior pituitary.
NEET Chemical Coordination and Integration 83 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Releasing or Inhibiting Hormones of Hypothalamus and their Roles Releasing or Inhibiting Hormone Thyrotropin releasing hormone (TRH) Control and Regulation of Specific Hormone Secretion Stimulates thyrotropin stimulating hormone release of Growth hormone releasing hormone (GHRH) ( ) Somatocrinin Stimulates release of growth hormone Growth hormone inhibiting hormone (GHIH) ( ) Somatostatin Inhibits release of growth hormone Gonadotropin releasing hormone (GnRH) Stimulates release of follicle stimulating hormone and luteinising hormone Prolactin releasing hormone (PRH) Stimulates release of prolactin Prolactin inhibiting hormone (PIH)/Dopamine (Produced by tubero-infundibular neurons) Inhibits release of prolactin Adrenocorticotropic hormone releasing hormone (CRH) Stimulates release of adrenocorticotropic hormone Melanocyte stimulating hormone releasing hormone (MRH) Stimulates release of melanocyte stimulating hormone Melanocyte stimulating hormone inhibiting hormone (MIH) Inhibits release of melanocyte stimulating hormone The hypothalamus is connected to the anterior lobe of the pituitary gland by hypophyseal portal veins and to the posterior lobe of the pituitary gland by the axons of neurosecretory cells. Unmyelinated nerve fibres originating from neuronal cell bodies located within SUPRAOPTIC and PARAVENTRICULAR NUCLEI of the hypothalamus form the hypothalamo-hypophyseal tract within the pituitary stalk. These neurons predominantly synthesise two nanopeptide hormones: vasopressin and oxytocin, respectively. These are then transported as neurophysin-protein bound secretory granules down the nerve fibres and stored in posterior lobe of pituitary in neuro secretory terminals called Herring bodies. 2. Pituitary Gland (Hypophysis) Posterior pituitary Anterior pituitary Hypothalamus Hypothalamic neurons Portal circulation Fig. : Diagrammatic representation of pituitary and its relationship with hypothalamus Pituitary gland is located in a bony cavity of the sphenoid bone of the skull called sella turcica. It is attached to the hypothalamus (inferior surface) by a stalk called infundibulum. Pituitary gland is ectodermal in origin. Anatomically, the pituitary gland (hypophysis) is divided into adenohypophysis and neurohypophysis. Adenohypophysis It comprises of 75% part of pituitary gland. It is highly cellular & vascular. It consists of two portions – pars distalis and pars intermedia. However, in humans, the pars intermedia is almost merged with the pars distalis. The pars distalis region of pituitary is commonly called anterior pituitary. Pars intermedia releases only one hormone called melanocyte stimulating hormones (MSH). Neurohypophysis It is the posterior lobe of pituitary and therefore commonly called posterior pituitary. It is primarily composed of pars nervosa and is the storage site of oxytocin and vasopressin.
84 Chemical Coordination and Integration NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Table : Hormones of Pituitary Gland and their Action on Target Organs Oxytocin (OT) Luteinising hormone (LH) Stimulates contraction of uterine muscles during birth; initiates ejection of milk In females, together with FSH, it triggers ovulation, stimulates conversion of ovarian follicles into corpus luteum Mammary glands Gonads No hormones synthesised here. Its hormones are synthesised in hypothalamus Neuro- hypophysis Antidiuretic hormone (ADH) or Vasopressin Stimulates reabsorption of water and reduction of urine secretion; stimulates constriction of blood vessels and thus increases blood pressure Kidneys Part of Pituitary Adeno- hypophysis Principal Cell Type Hormones Principal Action Target Organs Somatotroph Human growth hormone (hGH) Growth of body cells specially of bones of limbs, stimulates protein synthesis and inhibits protein breakdown; hydrolysis of fats, retards use of blood glucose for ATP production ( ) diabetogenic effect General Thyrotroph Thyroid stimulating hormone (TSH) (Thyrotropin) Controls secretion of thyroid hormones Thyroid gland Corticotroph Adrenocorti- cotropic hormone (ACTH) Controls secretion of adrenal cortex hormones Adrenal cortex Melanocyte stimulating hormone (MSH) Stimulates cutaneous pigmentation by dispersion of melanin granules Melano- cytes in skin Prolactin (PRL) Follicle stimulating hormone (FSH) Interstitial cell stimulating hormone (ICSH) Stimulates milk production and secretion, participates in control of reproduction, osmoregulation, growth and metabolisim In males, stimulates spermatogenesis. In females, growth of ovarian follicles In males, secretion of testosterone Mammary glands Gonads Gonads Lactotroph Gonadotroph Note: Vasopressin is an anti-diuretic which means it reduces loss of water through urine (diuresis). Hyposecretion of ADH secretion leads to a condition called diabetes insipidus characterised by polyuria (increased urine output). This leads to severe water loss and dehydration due to diminished utility of kidney of conserve water. Disorders due to growth hormone imbalance: Hyposecretion of GH Dwarfism : Low secretion or under secretion of growth hormone before puberty i.e., during childhood causes pituitary dwarfism. It leads to stunted growth due to premature stoppage of body growth and reduced development of skeleton. This form of dwarfism is called Ateleiosis and the dwarfs are called midgets.