The motor neuron diseases (MNDs) are a group of severe disorders that attack the brain and spinal cord.
They destroy motor neurons, which are the nerve cells that control muscles used for essential activities such as walking, speaking, swallowing and breathing.
Other famous sufferers of motor neuron disease include American baseball legend Lou Gehrig, for whom the most common MND, Lou Gehrig's Disease, is named; the brilliant British physicist Stephen Hawking; and screen star David Niven.
How MNDs affect the body
Normally, messages from nerve cells in the brain (called the upper motor neurons) are transmitted to nerve cells in the spinal cord (the lower motor neurons) and from there to specific muscles. Upper motor neurons direct the lower motor neurons to produce movements such as walking or chewing. Lower motor neurons control movement in the arms, legs, chest, face, throat and tongue.
MNDs disrupt the signals from the motor neurons to the muscles; the result can be gradual weakening and wasting away of muscle and uncontrollable twitching. When upper motor neurons are affected, there can be spasticity or stiffness of limb muscles and over-active tendon reflexes, causing knee and ankle jerks. Eventually, the ability to control voluntary movement can be lost.
Types of motor neuron disease
With Lou Gehrig's Disease, also called amyotrophic lateral sclerosis (ALS) or classical motor neuron disease, both upper and lower motor neurons are affected, and the disease eventually disrupts signals to all voluntary muscles. Symptoms usually first appear in the arms, legs or swallowing muscles. Affected individuals lose strength and the ability to move their limbs and body, and most have difficulty talking. The disease does not usually affect thinking or personality, although some people may have problems with decision-making and memory.