Organization of the nervous system

The levels of the nervous system

The nervous system is very complicated, in terms of both its structure and its physiology. Fortunately, when things go wrong they can be categorized on the basis of a relatively simple scheme of neuroanatomy. The nervous system can be thought of as having different levels (Fig. 1). The distribution and type of the clinical problem will often point to the affected level. For example, a patient who is confused must have a disturbance affecting the cerebral hemispheres. There are some situations when the level cannot immediately be determined: for example, a patient with foot drop could have a problem in the peripheral nerve, nerve root, spinal cord or cerebral hemisphere. Terminology used to describe disturbances at different levels is given in Box 1.

Fig. 1 Levels of the nervous system.

Box 1 Terminology

Abnormalities in the different levels of the nervous system can be referred to according to different terms. One commonly used system is described here. The suffix -opathy can be replaced by -itis if there is thought to be an inflammatory basis to the disturbance.

Encephalopathy: abnormality of the brain (may be refined by terms such as focal, diffuse, metabolic or toxic).

Myelopathy: abnormality of the spinal cord.

Radiculopathy: abnormality of a single nerve root.

Polyradiculopathy: abnormality of many nerve roots.

Plexopathy: abnormality of a plexus (brachial or lumbar).

Polyneuropathy or neuropathy: abnormality of many nerves.

Mononeuropathy: abnormality of a single named nerve.

Mononeuritis multiplex: an abnormality of multiple named nerves.

Myopathy: abnormality of muscle (myositis or polymyositis – inflammation of muscle).

Meningitis: inflammation of the meninges.

These terms can be combined with one another or other qualifiers to produce descriptions such as focal encephalopathy, or meningoencephalomyelitis (inflammation of the meninges, brain and spinal cord).

The central nervous system