Chemical messengers and the cell membrane

Higher organisms are composed of a multitude of cell types; each possessing specialized physiological functions. Their harmonious co-existence is only possible if they can communicate with each other; i.e. if they can exchange information concerning their respective needs. Cell signalling can result either from direct interaction of a cell with its neighbour (juxtacrine signalling) or from the exchange of small molecules, i.e. 'chemical messengers'.

These chemical messengers will only induce physiological responses in those cells which contain specific recognition proteins, i.e. 'receptors'. As a general definition, receptors carry out two distinct functions:

• First they recognize the structure of one or several chemical messengers, which results in messenger-receptor binding.

• Next the messenger-receptor complex is able to generate a 'signal', which modulates specific metabolic pathways in the cell, resulting in a physiological response such as contraction or relaxation of muscle cells.

Based on their mode of transportation and their range of action, the chemical messengers can be divided into three major classes: hormones, neurotransmitters and local chemical mediators (Figure 1).

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