Touching Lives - March 2009
The **adrenal glands** sit on top of the kidneys and produce a variety of substances called hormones (chemical messengers), which, having entered the blood stream, produce their effect at distant parts of the body.
If the nervous system recognises a situation where fight or flight might be appropriate, adrenaline makes the heart beat faster, prepares the muscles for action by diverting blood supply to them and even makes blood clot faster. Other parts of the gland make hormones to control blood pressure and steroids of various kinds which affect responses to physical and mental stress and the metabolism of minerals.
In evolutionary terms it is plausible that chemical messaging existed before the nerves became the main communication system. Although the conduction of an impulse is accompanied by electrical events, nerves do not work like metallic wires. When no impulse is passing there is a concentration difference of sodium across the membrane of the nerve fibre, which gives rise to a voltage difference. The impulse is propagated by special **ion channels** in the membrane opening, allowing sodium ions to rush through the membrane, temporarily abolishing this voltage difference. A small current flows and this opens up the ion channels in the next section and so on.
There are other ion channels in cell membranes, which are selective for different ions like calcium, and may be opened by chemicals rather than electrical events.