Human Endocrine System

Cell function is broadly controlled by two mechanisms : nervous and endocrinal.
1. Neural control is due to the spread of depolarization through the nerve. It has rapid action (much shorter latent period) and it affects certain groups of cells for a short period.
2. Endocrinal control is by release of physiologically active substances called hormones directly into the blood stream. It has slower action (i.e. much longer latent period) and it affects number of cells over a longer period. The glands are called endocrine or ductless glands because they secrete physiologically active substances called hormones directly into the bloodstream.
The name hormone was first used by the English physiologists M. Bayliss and E. H. Starling in 1909. A hormone (Gr. hromao = to excite) may be defined as a specific organic product of an endocrine gland secreted into the blood which carries it to some part of the body (target organ) where it regulates a definite physiological effect.
The endocrine glands which secrete only hormones are called the holocrine glands {e.g., thyroid, parathyroids, ptuitary gland). The glands which have dual secretion of hormones and some other functions) the heterocrine glands (e.g., pancreas, testes, ovaries, etc).
Local chemical messengers are not generally considered as a part of the endocrine system, include autocrines, which act on the cells that secrete them, and paracrines, which act on a different cell type nearby.
When some hormones work together to control a process, this is called synergism, e.g., FSH and LH. When two hormones work against each other to control a process, this : is called antagonism, e.g. insulin and glucagon; and calcitonin and parathormone. The tissue on which the hormone acts is known as ‘target tissue’. A target cell responds to a hormone because it bears receptors for the hormone. Hormone receptors are found either exposed on the surface of the cell or within the cell (cytoplasm), depending on the type of hormone. In very basic terms, binding of hormone to receptor triggers a cascade of reactions within the cell that affects function.
Endocrine organs are activated to release their hormones by humoral, neural, or hormonal stimuli. The nervous system, acting through hypothalamic control, can in certain cases override or modulate hormonal effects.
The main endocrine glands include pituitary gland, pancreas, ovaries, testes, thyroid gland, and adrenal glands. The hypothalamus is a neuroendocrine organ.

Properties of hormones:
(i) They have low molecular weight.
(ii) They are soluble in water and blood.
(iii) They have no cumulative effect.
(iv) They can act in very low concentration.
(v) They are non-antigenic.
(vi) They are organic catalysts

Mechanism of action of hormones
Two mechanisms for hormone-receptor coupling’ operate:
1. cAMP mediated hormone activity and
2. Transcription and translation effect
1. cAMP mediated hormone activity

Most proteins and polypeptide hormones, many amino acid derivatives, specially catecholamines, and some of prostaglandins (because of their large size, cannot enter the cells); they act as the first messenger and exert their effect by combining with specific fixed receptor site over the outer surface of the cell membrane, thereby activate the enzyme adenylyl cyclase on the inner surface of the membrane. This increases formation of intracellular cAMP from ATP. cAMP, the second messenger, is a basic regulator of cell metabolism, it acts by conversion of inactive protein kinases to their active from. Protein kinases catalyse the phosphorylation of proteins thereby altering their form and activity. Adenylyl cyclase is stimulated by low [Ca2+] but inhibited by high [Ca2+]; therefore, [Ca2+] is an important regulator of cAMP concentration in the cell. cAMP transmits the signal of the hormone and may bring about different functions in different target cells such as:
(i) change in membrane permeability to different substances or ions.
(ii) activation or inactivation of rate limiting enzymes.
(iii) increased or decreased protein synthesis by action on ribosomes.
(iv) regulation of release of hormones from endocrine gland.

cAMP mediated Hormone response-hourlybook-com

2. Transcription and translation effect
Steroid hormones and thyroid hormones being lipid soluble can easily enter into the target cells and exert their effect by combining to a specific cytoplasmic receptor protein in a target cell i.e., cell that responds to the hormone. Each receptor molecule binds two molecules of hormone, forming a complex that enters the nucleus and becomes attached to the chromatin, the genetic material. The complex activates DNA, stimulates the transcription (i.e., formation of mRNA) of a particular gene, and specific messenger RNA (mRNA) synthesis increases.
The specific mRNA enters the cytoplasm, where it directs the ribosomes to synthesize specific proteins (translation). These proteins may be enzymes, structural proteins, receptor proteins or secretory proteins.


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