However, a given hormone usually affects only a limited number of cells, which are called target cells where it regulates a definite physiological effect.
A target cell responds to a hormone because it bears receptors for the hormone. In other words, a particular cell is a target cell for a hormone if it contains functional receptors for that hormone, and cells which do not have such a receptor cannot be influenced directly by that 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. The endocrine secretions are induced by a stimulus and have activating influences on the neighbouring or distant tissues. I.
Types of action of the following secretion:
The hormone is distributed in blood and binds to distant target cells.
The hormone acts locally by diffusing from its source to target cells in the neighbourhood. Eg. (i) The release of cytokines that cause an inflammatory response in the area. (ii) The release of neurotransmitters at synapses in the nervous system.
The hormone acts on the same cell that produced it. Autocrine signalling can occur (i) Solely within the cytoplasm of the cell (ii) By a secreted chemical interacting with the receptors on the surface of the same cell. Both autocrine and paracrine secretions are local regulators, they do not travel in the blood. II. Mechanisms controlling release of hormones:
Eg. Excess glucose in the blood causes the release of insulin from the pancreas which lowers the blood glucose level.
Eg. -Stimulating hormones released from the anterior pituitary gland cause the release of other hormones from other glands in the body.
Eg. Adrenaline and noradrenaline are released from the cells of adrenal medulla by the arrival of nerve impulse in situations of anxiety, stress and danger. III. Properties of hormones: The hormones have the following properties: (1) They have low molecular weight (2) They are soluble in water and blood (3) They have no cumulative effect (4) They can act in very low concentration (5) They are non-antigenic (6) They are organic catalysts (7) They may act slowly or quickly (8) Hormone controlled reactions are not reversible (9) Hormones are produced in inactive form called prohormones.
Eg. Proinsulin —> Insulin (active form) (10) It is also called messenger because it has effect at a site different from the site where it is made.