In Motor Neurone Disease (MND), the damage is highly selective, targeting the nerves responsible for voluntary muscle control while leaving others entirely intact. These specialised cells, known as motor neurones, act as a bridge between your brain and your muscles. When they degenerate, the connection is lost, and the muscles they once controlled begin to weaken and waste away. However, it is important to note that MND does not affect every nerve in the body. Sensory nerves, which allow you to feel touch, pain, and temperature, are typically spared, as are the nerves that control automatic functions like heart rate and digestion.
The pattern of nerve involvement determines the symptoms an individual experiences and the specific subtype of MND they may have. By understanding which nerves are affected, clinicians can better predict the progression of the disease and tailor support to manage the physical impact. This article provides a detailed breakdown of the motor neurones targeted by the disease and explains the clinical difference between damage to the brain and damage to the spinal cord.
What We’ll Discuss In This Article
- The distinction between upper and lower motor neurones
- Cranial nerves and their role in bulbar onset symptoms
- Nerves that are typically spared in MND
- How nerve damage leads to specific symptoms like spasticity and muscle wasting
- The clinical testing used to identify nerve degeneration
- Emergency guidance for acute neurological changes
Upper and Lower Motor Neurones
Motor neurones are divided into two main groups based on where they are located in the body. MND can affect either or both of these groups.
Upper Motor Neurones (UMN)
These nerves originate in the motor cortex of the brain and travel down to the brainstem or spinal cord. Their job is to carry instructions from the brain to the lower motor neurones. When UMNs are damaged, the inhibitory signals that keep muscles relaxed are lost, leading to:
- Spasticity: Increased muscle tone and stiffness
- Hyperreflexia: Overactive or brisk reflexes
- Slowness of movement: Difficulty initiating voluntary actions
Lower Motor Neurones (LMN)
These nerves start in the brainstem and spinal cord and extend out to the muscles. They are the final link in the chain of command. When LMNs degenerate, the muscles lose their primary source of stimulation, resulting in:
- Muscle Wasting (Atrophy): The muscle shrinks due to lack of use
- Fasciculations: Small, involuntary muscle twitches or flickers
- Weakness: A loss of strength in the affected limbs or torso
Cranial Nerves and Bulbar Involvement
The nerves that emerge directly from the brainstem are called cranial nerves. In MND, the nerves that control the muscles of the face, throat, and tongue (specifically cranial nerves IX, X, XI, and XII) are often affected. This is known as bulbar involvement.
When these specific nerves degenerate, individuals experience difficulty with:
- Dysarthria: Slurred or nasal speech
- Dysphagia: Difficulty swallowing food or liquids
- Weakness of the tongue: Often accompanied by twitches on the surface of the tongue
Nerves That Are Typically Spared
One of the defining features of MND is the relative sparing of specific nerve groups. Even in advanced stages of the disease, the following are usually unaffected:
- Sensory Nerves: You will still be able to feel touch, temperature, and pain.
- Extraocular Nerves: The nerves that control eye movement (cranial nerves III, IV, and VI) are remarkably resistant to MND, allowing most people to maintain full control of their vision.
- Autonomic Nerves: The nerves regulating the heart, lungs (automatic breathing), and digestive system remain functional.
- Sphincter Muscles: Control over the bladder and bowel is typically preserved.
Testing for Nerve Damage
Because MND mimics other conditions, neurologists use specialised tests to confirm which nerves are degenerating.
- Electromyography (EMG): Tiny needles are used to record the electrical activity in muscles, identifying signs of lower motor neurone damage.
- Nerve Conduction Studies (NCS): These measure how fast electrical signals travel through the nerves, primarily used to rule out other conditions like peripheral neuropathy.
- Clinical Examination: Brisk reflexes or specific signs help identify upper motor neurone involvement.
Emergency Guidance
While MND is a progressive condition, certain symptoms require immediate medical intervention. Seek emergency care if you or someone you care for experience:
- A sudden and severe difficulty with breathing or a feeling of suffocation
- An acute episode of choking that cannot be cleared
- A sudden, profound loss of muscle strength resulting in a fall
- Rapid confusion or a sudden change in mental state
In these cases, call 999 or attend the nearest Accident and Emergency department immediately.
To Summarise
Motor Neurone Disease selectively targets the upper and lower motor neurones while sparing the sensory and autonomic systems. Damage to the upper motor neurones in the brain leads to stiffness and overactive reflexes, while damage to the lower motor neurones in the spinal cord causes muscle wasting and twitching. In bulbar onset cases, the cranial nerves controlling the face and throat are involved first. By identifying which nerves are affected through tests like EMG, clinicians can provide a precise diagnosis and plan for the specific needs of the patient. Despite the progression of the disease, the preservation of eye movement and sensation remains a key characteristic of MND.
Why are eye nerves not affected by MND?
Researchers believe that the motor neurones controlling eye muscles have a different biological makeup that makes them more resilient to the proteins and cellular stress that cause MND.
If I have tingling in my hands, is it MND?
MND does not typically affect sensory nerves, so tingling or numbness is more likely to be caused by other conditions, such as a trapped nerve or peripheral neuropathy.
Can MND affect my memory nerves?
MND primarily affects motor nerves, but about 15 percent of people may develop Frontotemporal Dementia (FTD), which affects the nerves in the brain responsible for behaviour and personality.
Is it possible for only upper motor neurones to be affected?
Yes, this is a rare form of MND called Primary Lateral Sclerosis (PLS). It causes stiffness and slowness but usually progresses more slowly than other types.
What is the difference between a nerve and a neurone?
A neurone is an individual nerve cell. A nerve is a bundle of many neurone axons that carry signals throughout the body.
Does Riluzole protect the nerves?
Riluzole works by reducing the levels of a chemical called glutamate, which can overstimulate and damage motor neurones, thereby slowing the progression of the disease.
Can nerves regenerate in MND?
Currently, there is no way to regrow motor neurones once they have died. This is why early diagnosis and proactive symptom management are so critical.
Authority Snapshot
This article was reviewed by Dr. Rebecca Fernandez, a UK-trained physician with an MBBS and extensive experience in internal medicine, surgery, and psychiatry. Dr. Fernandez has managed critically ill patients in intensive care and stabilized acute trauma cases. Her expertise in integrating digital health solutions and providing evidence based psychological support ensures that this guide provides a medically accurate and holistic overview of the neurological impact of MND.
