CoachA cell signals to another with a secreted molecule, a messenger or ligand, that binds a receptor on or in the target. By range: endocrine signalling, in which a hormone is released into the blood by a gland and reaches every cell of the body, acting only on those with the receptor, over minutes to hours (insulin, cortisol, adrenaline, thyroxine); paracrine signalling, in which the messenger diffuses to neighbouring cells within a millimetre and is destroyed on the way (growth factors, histamine, prostaglandins, the morphogens of Chapter 10); autocrine, the cell signalling to itself; and synaptic, in which a neuron releases a neurotransmitter across a gap of onto one target cell, in a millisecond (Chapter 20). By chemistry: water-soluble messengers — amino-acid derivatives such as adrenaline, peptides and proteins such as insulin — cannot cross the membrane and act on surface receptors; lipid-soluble messengers — the steroids (cortisol, oestradiol, testosterone), thyroxine, retinoic acid, nitric oxide — cross it and act on receptors inside. The endocrine and nervous systems are the two ways a body of cells coordinates itself: the one broadcasts slowly to everyone, the other wires quickly to someone.
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Example 19.8 (Speed, reach and cost of the two systems)
Adrenaline in the blood reaches every cell in about a minute (the circulation time) and acts for minutes; a nerve impulse reaches its target in milliseconds and acts for milliseconds. A hormone costs almost nothing per cell reached — a nanomole spread over the body — but cannot address one cell; a nerve addresses one cell exactly but must build and maintain a wire to it. The body uses both: the sympathetic nerves of Chapter 18 act on the heart and the arterioles in a second, and the adrenal medulla, itself a modified sympathetic ganglion, backs them up by pouring the same molecule into the blood for the whole body. And at the synapse the two designs meet, for a neurotransmitter is a messenger acting on a receptor by the same mechanisms — a channel that opens, a G protein that switches — only across twenty nanometres instead of a metre.