Introduction

Every living organism must constantly sense changes around it and respond in a coordinated way — the many organs of the body never work in isolation, but adjust and combine their individual activities so that a task is carried out smoothly. This working-together of organs for a shared purpose is what we call coordination.

A simple everyday example makes the idea concrete. During strenuous physical exercise, the muscles need extra energy and therefore extra oxygen. To supply this, the rate of breathing rises, the heart beats faster, and blood flow through the vessels increases. Once the exercise stops, the nerves, lungs, heart and kidneys gradually return to their resting state. At every stage, the muscles, lungs, heart, blood vessels, kidneys and other organs are acting together in a tightly linked way — this is coordination in action.

In the human body, two organ systems work jointly to bring about this coordination and integration of all the body's activities: the neural system and the endocrine system. The neural system sets up an organised network of point-to-point connections, so that its coordination is fast and precise. The endocrine system, in contrast, achieves coordination chemically, through hormones carried in the blood — a route that is generally slower but longer-lasting and more widespread in its effect.

This chapter looks at the neural system of the human body and the mechanisms by which it achieves coordination — in particular, how a nerve impulse is generated and transmitted along a neuron, and how it is passed on from one neuron to the next across a synapse.