Motor neurone disease (MND) is a term used to cover a group of diseases where a particular nerve cell in the nervous system, called a motor neuron, becomes damaged.
MND is classed as a neurodegenerative disease which means that damage gets worse over time and eventually the motor neurons aren’t able to work as they should and they become damaged and eventually die. The death of these neurons means that muscles don’t get sent messages to move and they become weak. The muscles eventually become so weak that they are no longer able to function which leads to paralysis.
People with MND experience different symptoms and the disease progresses at different rates. Some of the symptoms that people may have range from muscle cramps and weakness, swallowing difficulties, speech and breathing problems to changes in thinking and behaviour.
What are motor neurons?
Motor neurons are a type of cell found in the brain and spinal cord and they send messages to muscles to tell them to move. You can find out more about these and how they send messages in the video below.
Motor neurons send signals to each other as well as to our muscles. The ends of motor neurons, called axon terminals, connect to the fibres that make up our muscles. The motor neurons pass messages to move from our brains to our muscle fibres. These fibres are responsible for helping our muscles to contract and move when we want them to.
How do motor neurons connect to muscles?
Motor neurons connect to our muscles using a structure called a neuromuscular junction. These specialised structures are how the messages are passed from the motor neuron to the muscle fibres.
When the chemical messages reach the end of the axon terminals, the bubbles of chemicals burst and the chemicals inside are released into the space between the end of the motor neuron and the surface of the muscle fibre.
The surface of the muscle fibre has receptors on it. These receptors are a bit like a lock and the chemicals that stick to them are like a key. When the chemicals stick to the receptors, an electrical signal is produced inside the muscle fibre and it’s this signal that tells the muscle fibres to contract to move the muscle.
What happens to motor neurons and neuromuscular junctions in MND?
Research has shown that many different areas of motor neurons are affected by the disease and contribute to the damage and death of neurons. The following are just some of the things that can go wrong in motor neurons in MND, and we still don’t know everything that happens to the cells during the disease. Click on the drop down menu to find out more about what can go wrong in the different parts of the motor neurons.
[toggle title=”Dendrites”] Dendrites are the branches that come from the cell body of the motor neuron. These branches usually receive chemical messages and start passing them through the neuron. In cells affected by MND, these dendrites can become shorter and the number of branches may reduce. This means that the messages may not be taken in by the neuron or that the signal is weaker than it should be. [/toggle] [toggle title=”Cell body”] There are several things that are thought to go wrong within the cell bodies of neurons. The cell body contains lots of different pieces of machinery that produce the substances, such as proteins and energy, which our cells need to function properly. In MND, some of these machines become faulty. One example is the tiny ‘batteries’ inside our cells, called mitochondria, which seem to work abnormally in MND and don’t produce enough energy. Another thing that goes wrong in the cell body is the build-up of toxic waste and proteins that are made incorrectly by the machinery. [/toggle] [toggle title=”Axon”] The axon is the long thin part of the motor neuron which signals travel down. Research has shown that travel down the axon can be disrupted in MND, meaning that messages are not sent to the next neuron or to muscles and this may play a role in the death of the neuron. The scaffolding that supports the axon’s shape can also start to break down in cells affected by the disease. Damage to this scaffolding can cause the axon to degrade and no longer work as it should. [/toggle] [toggle title=”Neuromuscular junctions”] The connections between motor neurons and muscle fibres can also become faulty in MND. The branches at the ends of the axon, called axon terminals, can shorten and the number of branches can reduce. This means that the signals being sent out to the muscle fibres may be weaker than usual, as less chemicals are released into this space. As well as this, the axon terminals becoming shorter means that the space between the end of the axon and the muscle fibres gets bigger so the axon terminals are further away from the muscles than they should be. This can mean that the messages being sent to the muscles to tell them to move are not as effective and may not be powerful enough to create the signal for muscles to contract inside the muscle fibre. [/toggle]
While we know some of the things that go wrong in motor neurons in MND, there are still many more to uncover and, for the things we do know about, we often don’t know what causes them. Understanding more about the biology behind motor neuron damage and what causes it could help to reveal ways to stop or reverse it.
We fund several research projects which aim to find out more about what goes wrong in motor neurons in the disease and identify things that could be targets for new treatments. If we know what goes wrong and what causes it to go wrong, then potential treatments can be developed which target the cause. These may help to reduce the damage to motor neurons and slow the progression of MND.
You can find out more about the projects we fund in this area on our website or read more about what causes MND in our next blog in the series.
