Intercellular communication is the basic machinery through which homeostatic control is applied. Cells almost always uses a chemical messenger to communicate with target cells, which may be local or remote in location. One group of chemical messengers is formed by the neurotransmitters, which are secreted by one neurone to target a neighbouring neurone just a synapse away. On the other hand, a hormone travels in the circulation to target multiple tissues remote from the secreting cells. In general terms, chemical messengers (ligands) are usually received by cell membrane proteins (receptors) and this interaction triggers the required cellular response by various mechanisms.
Chemical messengers that reach a target cell in the extracellular fluid, whether via the circulation or through the interstitium, are referred to as first chemical messengers. The actions of a first chemical messenger often results in the intracellular release of an active ligand, which is termed a second messenger. Various groups of first messengers are recognized such as neurotransmitters, neuromuscular transmitters, hormones, paracrine agents and autocrine agents.
These are differentiated by their origin, the route travelled by the messengers and the cells targeted. Some examples are shown in Figure PG.24.
All chemical messengers or ligands interact with their target cells by binding to receptors and therefore have common properties that determine their performance.
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