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Motor neurone disease is an uncommon disease that causes a progressive degeneration of the motor system; the cells and nerves in the brain and spinal cord that control the muscles. Over time, this leads to weakness and wasting of the muscles. It is a slowly progressive disease - in other words, it tends to get steadily worse. Although much is known about MND, a really effective treatment has not yet been discovered.

What is the motor system?
The motor system controls all the movements we make with our bodies, right from nodding our heads or lifting a cup to our lips to walking or dancing. The main parts of the motor system are the motor neurones, a complex system of nerve cells in the brain and spinal cord that controls the action of the muscles. These are divided into upper and lower motor neurones (see figure 1 in printed bound version or pdf).

The lower motor neurones consist of the motor cells in the spinal cord and the motor nerves round the individual muscles in the arms, legs and torso. Motor cells in the brainstem are also part of the lower motor neurones. These cells, which are arranged into small groups called motor nuclei, control individual muscles in the throat, tongue, face and eyes.

The upper motor neurones are motor pathways situated in the brain itself. They integrate the activity of the lower motor neurones via the motor cells in the brainstem and spinal cord to produce patterns of movement. Some of these patterns are inbuilt or instinctive, like walking, while others are learned, like riding a bicycle.

Whilst many of the movements we make are repeated countless times, the brain is constantly processing information from the nervous system and adapting movements as necessary. This means, for example, that our bodies can adapt to walking downhill on an icy path or standing in a moving train.

In people with motor neurone disease, the motor system is affected but all the other systems in the brain are normal. The muscle weakness that results from the disease can therefore be well compensated for by the brain for a time, but eventually there will be increasing disability. This is due to a combination of weakness due to muscle wasting and damage to the motor control system in the brain itself, affecting patterns of movement like walking, and leading to clumsiness, stiffness and spontaneous twitching of groups of mucles fibres. These are known as fasciculations.

The extent to which the lower motor neurone system and the upper motor neurone system are affected determines the varying patterns of disability experienced by different individuals with the disease.