Summary

The body's many organs cannot work in isolation; their activities must be coordinated so that they act as one integrated whole. In animals, two systems share this task of coordination — the neural system, which works through fast electrical impulses, and the endocrine system, which works more slowly through chemical messengers called hormones. Together they keep the body's functions, along with its metabolic and homeostatic activities, properly integrated.

The basic building block of the neural system is the neuron, its structural and functional unit. A neuron is an excitable cell: because different ions are unevenly distributed across its membrane, a difference in electrical charge is set up between the inside and the outside. A typical neuron consists of a cell body, branching dendrites that receive signals, and a long axon that carries signals away to other cells.

In humans the neural system is divided into two parts. The central neural system (CNS) is made of the brain and the spinal cord and is the main site of information processing and control. The peripheral neural system (PNS) consists of the nerves that link the CNS with the rest of the body. Its fibres are of two kinds: afferent fibres that bring sensory information towards the CNS, and efferent fibres that carry commands away from the CNS to the organs. The PNS is further grouped into the somatic system, which relays impulses to skeletal muscles, and the autonomic system, which controls involuntary organs. The autonomic system in turn has two divisions with generally opposing effects — the sympathetic and the parasympathetic.

A resting, undisturbed neuron is said to be polarised: its membrane maintains a steady charge difference known as the resting potential, kept up by the uneven distribution of sodium and potassium ions and by a sodium–potassium pump that actively moves these ions across the membrane. When the neuron is stimulated, the membrane at that point suddenly becomes permeable to sodium; the charge reverses in a change called depolarisation, producing an action potential — the nerve impulse. This impulse then travels along the axon as a moving wave of depolarisation followed by repolarisation, so that each region recovers as the impulse moves on.

When the impulse reaches the end of one neuron it must be passed to the next cell across a junction called a synapse, formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron. At an electrical synapse the two membranes lie very close, and the impulse crosses directly and rapidly. At a chemical synapse the two membranes are separated by a tiny gap, the synaptic cleft; here the arriving impulse triggers the release of chemical messengers called neurotransmitters, which cross the gap and pass the signal to the next neuron.

The brain, the master organiser of the whole system, is divided into three major regions — the forebrain, the midbrain and the hindbrain.

The forebrain contains the cerebrum, the thalamus and the hypothalamus. The cerebrum is split lengthwise into two halves joined by a band of fibres, the corpus callosum. The thalamus relays sensory and motor signals, while the hypothalamus regulates body temperature and the urges to eat and drink, among other things. Deep within the cerebral hemispheres, certain inner structures form the limbic system, which is involved in the sense of smell, autonomic responses, sexual behaviour, emotional expression and motivation. …