Biological Underpinnings of Multiple Sclerosis and Conditions with Similar Presentations


Multiple sclerosis (MS) is a chronic demyelination disease, or an ongoing condition that causes damage to the myelin sheath of nerve cells in the brain and spinal cord.  Myelin coating is important for conducting signals in the form of action potentials and is thus essential to executing many of our basic bodily functions.  

In order for a neuron to fire an action potential, it must first detect an electrical signal that is strong enough.  If this received signal is above the threshold voltage, the action potential will be initiated and channels in a patch of the neuron’s axon will open in response.  The change in channel configuration allows positively charged sodium ions into the nerve cell for a brief period of time in a process known as depolarization.  This makes the inside of the cell very positive compared to the outside.  After that, a section of the membrane will become re-polarized as positive potassium ions leave the cell through channels.  As time progresses, this process of increasing and decreasing charge continues to occur down the length of the cell’s axon without losing its strength. The rate at which these action potentials travel is called conduction velocity.

The presence of myelin is the reason why we can have such thin nerve cells and still rapidly transmit signals.  Normally, the larger the diameter of our neurons, the higher the conduction velocity, or the faster the signal travels. However, myelin increases conduction velocity and causes a change in the way action potentials propagate. The myelin sheath doesn’t cover the entirety of the axon, but rather is broken up at points called nodes where the ion channels are concentrated.  Thus, the action potentials travel quickly through the myelinated internodes, essentially jumping from node to node in a process known as saltatory conduction. When there has been demyelination, the action potential has no issue traveling from a demyelinated section to a myelinated one, but it will have difficulty reaching the section that’s demyelinated, causing many complications.

In MS, the central nervous system (the brain and spinal cord) is specifically targeted.  Rather than impacting the myelin itself, the immune system destroys cells called oligodendrocytes which are responsible for synthesizing the myelin.  Thus, once they are destroyed by inflammation, symptoms begin to manifest.  Individuals may experience a loss of energy, vision, and motor control, and the severity is subject to fluctuation over time through periods of remission and relapse (Multiple Sclerosis…).  

When someone experiences MS-like symptoms and goes through remission in the absence of subsequent flare-ups, it is simply referred to as Clinically Isolated Syndrome (CIS).  As the name suggests, these are single isolated episodes, typically characterized by neurological symptoms that persist over 24 hours.  These may evolve into full-fledged MS down the line, but this is not guaranteed (Clinically Isolated…).  Acute Disseminated Encephalomyelitis (ADEM) is very similar in this sense, but it is most common in children after the onset of severe viral or bacterial infection whereas CIS is not usually associated with fever or infection (Moawad, 2019).

However, these are not the only conditions that involve demyelination.  For instance, Neuromyelitis Optica Spectrum Disorder (NMOSD) involves other cells called astrocytes which support the neuron and provide the lipids necessary to make myelin.  Despite the distinct pathology, myelin production is also being hampered in this case, thus producing effects that resemble the response to oligodendrocyte destruction.  Even deficiency in vitamin B12 can mimic the signs of MS, causing fatigue and numbness in the limbs due to its role in the breakdown of fatty acids which is necessary to get these lipid building blocks (Orenstein).

From a purely scientific standpoint, these diseases can be fascinating, as they showcase the complexity of myelin production and its integral role in basic physiological functioning.  However, the nuances of each of these conditions and the devastating impacts they have is heartbreaking when applied to real people and situations.  Trying to get a clear diagnosis and to understand what’s happening in one’s own body can be an unnerving experience.  Thus, to combat the added stress and confusion, it is imperative that there is dialogue about the biological underpinnings of demyelinating diseases so that the general public is informed about the symptoms to look out for.

References

Clinically isolated syndrome (CIS). National Multiple Sclerosis Society. (n.d.). Retrieved from https://www.nationalmssociety.org/Symptoms-Diagnosis/Clinically-Isolated-Syndrome-(CIS). 

Heidi Moawad, M. D. (2019, December 1). Adem can cause temporary weakness or paralysis. Verywell Health. Retrieved October 10, 2021, from https://www.verywellhealth.com/acute-disseminated-encephalomyelitis-adem-4691870. 

MediLexicon International. (n.d.). Multiple sclerosis (MS): Types, symptoms, and causes. Medical News Today. Retrieved October 10, 2021, from https://www.medicalnewstoday.com/articles/37556. 

MS and MOG antibody disease. Cleveland Clinic. (n.d.). Retrieved October 10, 2021, from https://my.clevelandclinic.org/departments/neurological/depts/multiple-sclerosis/ms-approaches/mog-antibody-disease. 

Orenstein, B. W., Gleason, T., Dunleavy, B. P., Bostick, M., & Sen, M. (n.d.). 16 conditions commonly mistaken for multiple sclerosis: Everyday Health. EverydayHealth.com. Retrieved October 10, 2021, from https://www.everydayhealth.com/multiple-sclerosis/symptoms/conditions-commonly-mistaken-multiple-sclerosis/.

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