What is motor neurone disease?|
This condition describes a malfunction of the nerves that elicit movement of muscles. The nerve and muscles it supplies is called a motor unit. Motor units consist of a single motor neurone and all of the muscle fibres it supplies through its extensive branching. Disorders of motor units can affect the muscles and / or the neuro-muscular junctions, in which case they are called myopathies; or the motor neurones, in which case they are called neuropathies.
Motor neurone diseases are now regarded as multisystem diseases in which the motor neurones tend to be affected earliest and most severely, however other parts of the nervous system may also be affected.
A neuropathy is the term for damage to the nerve cells. Damage to motor neurones can occur at any point in the cell, that is, at the cell body, along the axon of the neurone, or at the axon terminals (peripherally). An axon is usually a single, long process of a nerve cell which extends to make contact with the target cell it affects, or another neurone. It propagates electrical impulses and is used to transport materials through the neurone.
Injury to axons (axotomy)
This is damage to the neurone either due to a lack of oxygen perhaps because of restriction or lack of blood supply, or mechanical damage eg. resulting from a broken back or a "slipped disc". In the latter case the soft central part of the intervertebral disc protrudes into an adjacent vertebral disc, compressing the spinal cord and/or spinal nerves. This may lead to sectioning of the axon of a motor neurone.
What follows in either case, is known as a Wallerian degeneration of the distal segment. This involves the breakdown of the axon and its protective myelin sheath, and it starts terminally at the synaptic boutons (slight, rounded enlargement of the axon before it terminates at a synapse. A synapse is the connection between two neurones or between a neurone and the cell it affects, in this case a muscle fibre. The synapse between a motor neurone and a muscle fibre it supplies is called a neuro-muscular junction).
There is also chromatolysis of the cell body, which is the breakdown of a particular cell structure involved in the export of proteins from cells (called the rough endoplasmic reticulum). This is a sign of the metabolic preparation of the neurone for axon regrowth. Soon axonal regeneration may occur, which is facilitated and guided by arrays of Schwann cells, these cells produce myelin that covers and protects nerves, and aids in the propagation of electrical signals. Some axons reach their target muscle fibres again and there is some functional recovery from the re-innervation of the muscle.
The most common form is demyelinating neuropathies where the myelin sheaths (fatty tissue layers that coat a nerve acting as insulators) of neurones are lost. An example of this is diabetic neuropathy where there can be motor or sensory symptoms (loss of sensation in hands and feet) with a loss of ankle jerk reflexes. If it is solely a motor neuropathy, there is asymmetric wasting of the quadriceps muscles with a decrease or loss of the knee jerk reflex.
Another example is Guillain-Barre syndrome where 1-4 weeks following a viral infection there is an autoimmune reaction that results in peripheral demyelination in segments. Back pain is common, and tingling sensations that start from the ends of limbs. Weakness of muscles affects central parts of the body, especially the face, and there is widespread loss of reflexes.
Diseases directly affecting the cell bodies of motor neurones
These disorders affect the cell body of a motor neurone thus they result in the rapid de-innervation of all of the muscle fibres in a motor unit supplied by the affected motor neurone. An example of this is amyotrophic lateral sclerosis (ALS). The incidence of ALS is about one or two pe