Disruptions in the motor system are common in MS – they lead to muscle weakness of the limbs and thus to restrictions in mobility. Since MS is a disease that exclusively affects the central nervous system (CNS), not all parts of the motor system are affected – usually, there are disturbances in the central part of the motor system, which we also call the 1st motor neuron. Therefore, in MS patients – when an inflammatory lesion affects the motor system – so-called “central pareses” (= spastic pareses) are found, which clearly differ from “peripheral pareses”. Just to explain briefly: Neurologists use the term paresis to describe motor weakness of individual muscles or muscle groups, which can be classified according to their degree of severity (mild, moderate, severe paresis). “Paresis” could be translated into German as “paralysis”, although this is sometimes misleading, because many patients associate the term “paralysis” with a complete inability to use a muscle group or a limb. For this complete motor inability to use, neurologists use the term “plegia” – therefore, it makes sense to use the terms “paresis” and “plegia” even as a layperson and to know their respective meanings. I will also use the term “paresis” repeatedly in the following.
Now back to the motor system: Voluntary movements are stimulated in the motor neurons (= 1st motor neuron or upper motor neuron) in the cerebral cortex. The extensions (= axons) of these nerve cells run in long tracts to the spinal cord, where they transmit their impulses (= movement information) to other motor neurons (2nd motor neuron or lower motor neuron). In this process, the 1st motor neurons responsible for the arms transmit their information to motor neurons in the cervical spinal cord (= cervical cord), because the arms are set in motion from here. Similarly, the 1st motor neurons that control the movement of the legs send their information to the 2nd motor neurons in the lumbar spinal cord, because the legs are innervated from here. The extensions of the 2nd motor neurons leave the spinal cord – and thus the central nervous system – and bundle themselves in the periphery into peripheral nerve strands (which also have their own names such as the sciatic nerve), which then supply the muscles of the limbs. This part of the motor nervous system is, as mentioned above, not affected by MS, since the inflammation in MS exclusively affects the central nervous system (and thus the 1st motor neuron).
How do damages to the 1st and 2nd motor neurons differ? Let’s start with the damage to the 2nd motor neuron, i.e., the damage that is not found in MS patients. Damage to the 2nd motor neuron can occur, for example, if one sustains a cut injury to the arm and an arm nerve is severed in the process. This interrupts the direct contact between the nerve and the muscle. However, a muscle needs contact with a healthy nerve for its function and structure maintenance – if this contact is lost, the muscle atrophies (“muscle wasting”) and the affected limb has a flaccid muscle tone. Therefore, peripheral paresis, i.e., lesions of the 2nd motor neuron, is recognized by the fact that the muscle mass is reduced – only skin and bones can be seen on the affected limb.
In multiple sclerosis, the 1st motor neuron is damaged. The 2nd motor neuron remains intact, but is no longer properly controlled due to the damage to the 1st motor neuron. This means that the affected muscle groups can no longer be moved as usual, but the muscles themselves remain intact (because the connection between the 2nd motor neuron in the spinal cord and the muscle is still intact). That’s why patients with multiple sclerosis often show no significant muscle wasting (muscle atrophy) even with severe motor weakness. There is another peculiarity of central paresis that prevents atrophy. The loss of contact with the 1st motor neuron leads to a disinhibition of certain reflex pathways in the spinal cord, so that the peripheral musculature is more tonic (strengthened) – this phenomenon is also known as spastic increase in tone (or simply spasticity). This increased tonicity, which corresponds to an increased basic activity of the muscles, also prevents the atrophy of the affected muscles. The disadvantage, however, is that it leads to unpleasantly perceived muscle cramps and pains, or in worse cases to deformities and contractures.
However, central paresis always offers the chance that targeted physiotherapy can link other, undamaged central tracts with the still intact peripheral motor system, creating new functionality. This is also the reason why rehabilitation is such an important component of symptomatic MS therapy. The fact that the peripheral motor system in MS patients is healthy anticipates the answer to the question many MS patients ask – namely, whether they can “break” anything through increased motor activity/exertion. The answer is a clear “No”. Although exercises are difficult due to the changed controllability of the motor system, such exertion does not cause any damage, it rather promotes functionality.
The fact of an intact peripheral motor system is sometimes exploited in certain therapeutic concepts, for example when it comes to electrical stimulation treatments, external stimulation or so-called EMS (= Electrical Muscle Stimulation) training. Although there are different views on the benefits of such methods, they cannot cause any damage if they are performed under professional guidance.
