Chemical substances secreted into extracellular fluids  regulate function of other cells (target cells).

3 mains chemical groups:
• amino acid based
• steroid (synthesised from cholesterol)
• eicosanoids (paracrines – local hormones, affecting nearby cells; biologically active lipids made from arachidonic acid; eg leukotrienes, prostaglandins)

All main hormones circulate to almost all tissues, but only can influence the target cells that have specific protein receptors.
How much the target cell is activated will depend on the serum levels of the hormone, the relative number of receptors for the specific hormone on the target cell and the affinity of the bond between the hormone and receptor.

Target cells can undergo ‘up-regulation’ – form more receptors in response to higher levels of specific hormones, or ‘down-regulation’ – the loss of receptors that prevents target cells from over responding to high levels of hormone.

Hormones affect cell activity by a number of possible mechanisms - changing plasma membrane permeability; opening or closing ion channels  changes membrane potential; stimulates synthesis of proteins/enzymes; activates or inactivates enzymes; increases secretory activity; stimulates mitosis.

Hormones can not penetrate plasma membrane  effect is exerted by second messengers.

Control of release of hormones:
Most are regulated by a negative feedback system – generally, rising levels of a hormone inhibits further release.

3 different mechanisms of endocrine gland stimulation:
1) Humoral stimuli:
• secretion of hormone is in direct response to changing serum levels of specific nutrients or ions
• eg calcium levels and parathyroid hormone; glucose levels and insulin from pancreas
2) Neural stimuli:
• neural fibre directly stimulate the release of hormone
• eg stimulation of adrenal medulla to release catecholamines
3) Hormonal stimuli:
• hormone is secreted in response to another hormone
• eg anterior pituitary hormones is stimulated by hormones from hypothalamus

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