Motor neurons are cells that control the movement of the body. Pathologies related to this part of the body therefore lead to loss of motor skills. There is currently no treatment for these diseases.

The motor neuron, also called a motor neuron, is a nerve cell that allows all movement to be controlled. It is present in the spinal cord and controls the muscles through the peripheral nerve.

The motor neuron is therefore used to receive movement information and retransmit it to a muscle. We are talking about a nervous message. When the motor neuron is impaired, this message no longer reaches the muscle. The person affected by the pathology therefore develops a more or less severe paralysis.

Motor neurons innervate skeletal muscle fibers. When they arrive in the muscle, their axons – the extension of the neuron – branch out to establish contact with several muscle fibers. In contrast, a muscle fiber is innervated by only one motor neuron. The whole consisting of a motor neuron, its axon and the muscle fibers it innervates constitutes a motor unit. The motor unit is the smallest contracting element that the nervous system can bring into play.

Motor neurons are located in the gray anterior horn of the spinal cord (trunk and limb musculature) or in the motor nuclei of the cranial nerves of the brainstem (neck and head musculature).

The larger the size of a motor neuron, the larger the diameter of its axon and the greater the number of muscle fibers it innervates. There are thus motor units of different sizes.

All the diseases caused by the breakdown of the nerve cells that control muscles are grouped together under the name of motor neuron disease (MMN).

About 10% of MMNs are passed from parents to children. The causes of the majority of MMNs (90%) are still unknown.

Amyotrophic lateral sclerosis or Charcot’s disease

ALS is a neuromuscular disease that gradually attacks motor neurons. When these motor neurons die, the brain can no longer control the muscles, in the later stages of the disease patients are paralyzed. Patients often die 3 to 5 years after diagnosis. For 5 to 10% of patients the disease is transmitted genetically. For the remaining 90%, the cause of the pathology is still unknown. The symptoms of the disease are:

• Muscle cramps and small contractions called muscle twitches.
• The patient drops objects and feels his arms and legs tire quickly.
• Difficulty speaking, walking and using the hands in daily activities, such as washing or dressing.
• As the weakness and paralysis spreads to the trunk muscles, difficulty swallowing, chewing, then breathing. When the respiratory muscles are affected, assisted ventilation is essential.

Primary lateral sclerosis

PLS is a non-inherited neurological disease. The first symptoms of the disease do not appear until adulthood, around 50 or 55 years old. Moreover, the disease is disabling without being fatal. Due to its slow progress, people with an advanced stage of the disease are very rare. SLP is distinguished from other motor neuron diseases by:

• rapid posture change,
• loss of delayed walking
• upper motor neuron degeneration with no significant complications
• slower speech loss
• absence of mental alteration

Kennedy syndrome

Kennedy syndrome is a condition linked to the X chromosome. Therefore, the inherited disease affects only men, but women can be healthy carriers of the gene involved. This disease affects peripheral motor neurons (in the spinal cord and brainstem) while PLS only affects motor neurons in the cerebral cortex.

Over the years, the patient will present with neurological disorders such as cramps, hand tremors and disappearance of patellar reflexes. Different hormonal disorders and related symptoms also appear in the subject (gynecomastia – a development of the mammary glands – or infertility).

There is currently no treatment for motor neuron diseases, but much research is converging in particular on the solution of stem cells. They could thus help regulate harmful immune responses, or even produce growth factors that would allow neurons to survive and repair themselves.

The majority of treatments therefore focus on relieving symptoms to improve the quality of life of patients. Thus, physiotherapy and rehabilitation exercises are used in the treatment of the subject suffering from motor neuron diseases.

The diagnosis of motor neuron diseases is therefore mainly based on clinical arguments and observation of symptoms.

After observing the symptoms, it is possible to do an electromyogram (EMG) to confirm that there is indeed damage to the motor neurons of the anterior horn of the spinal cord and to assess its importance and extent.

New imaging methods are being developed aimed at confirming damage to motor neurons, in particular of the cerebral cortex, such as MRI, spectro MRI, magnetic stimulation.

It is also important to note that no other system, particularly sensitive, visual or auditory, is supposed to be affected by these pathologies. Any contrary observation should redirect the diagnosis to another pathology.