Think back to a time when you were able to catch a falling object before it shattered
onto the ground.
Incredibly fast response times can only occur if neurons in our nervous system are rapidly
communicating with each other.
This speedy messaging is only possible because we have a layer of fat, called the myelin
sheath, wrapped around our neurons.
What happens when this myelin is damaged, such as in the disease known as multiple sclerosis?
To answer that question, we first need to understand how signals move within a neuron.
Communication between two neurons occurs when a signal in one neuron causes the release
of neurotransmitters, which bind to receptors on the next neuron, starting another signal.
But what exactly is this signal that moves within these neurons?
These are electrical currents, which are essentially movements of positive charge.
Let’s take a closer look at the membrane of the axon of an unmyelinated neuron to see
why myelin is crucial for the movement of electrical signals.
Membrane potential is a measure of how positive or negative the inside of the neuron is compared
to the outside.
At rest, a neuron is more negatively charged inside than outside. Therefore, its membrane
potential is negative.
Let’s say this neuron was just activated by an excitatory neurotransmitter, initiating
a wave of positive charge, indicated by this red shading, to move down the axon.
This positive charge spreads down the axon due to repulsion within the area of positive
charge and attraction towards the more negative surroundings.
However, as the positive charge moves down the axon, the amount of positive
charge decreases the further away it is from the starting point, as shown by the increasingly
more negative membrane potentials.
This is because some charge naturally leaks out of the neuron,
while the rest spreads out.
This is problematic because eventually there will be so little positive charge left by
the time the signal reaches the end of the axon. Essentially, the signal fades away.
Heating a metal rod is analogous to this process. If you briefly heat one end of the rod,
the nearby metal also heats up.
However, the further away from the heated end of the metal rod, the colder the rod is
because heat is lost to the environment
and there is not enough energy to heat the entire rod.
Thankfully, we have evolved amazing adaptations to solve the problem with decreasing charge
and signal.
By wrapping myelin around the axon, charge leakage is reduced because myelin acts as
an insulator, like wrapping duct tape around a leak.
Since less charge is lost, more charge is available to spread out.
Therefore, this increases the distance that the positive charge can spread to.
However, the signal strength will still eventually decrease to zero given enough distance.
The only way to solve this problem is by replenishing the positive charge.
There are small segments of the neuron called the Nodes of Ranvier that don’t have myelin,
but instead have many ion channels which open in response to an increased membrane potential.
These channels allow positively charged sodium ions to enter the neuron, which changes the
inside of the neuron from negative to positive, replenishing the positive charge so that there
is enough positive charge to spread to the next Node of Ranvier.
You may be wondering, why not just forget about myelin and just have these ion channels
throughout the entire axon?
If that’s the case, way more ion channels are needed along the neuron membrane to replenish
the positive charge lost from leakage, slowing down the signal significantly compared to
a neuron with myelin and a few ion channels at the nodes of Ranvier.
This is similar to driving an old car around that has a leaky gasoline tank.
You would run out of gas quicker and have to stop at gas stations more often, which
increases your travel time, compared to a brand new car that has no leaks.
Therefore, myelin is crucial in speeding up signals moving through our neurons.
In the brain and spinal cord, myelin is essentially extensions of special cells called oligodendrocytes.
Damage to oligodendrocytes and myelin result in signals that are slow or even stop completely.
This damage appears as multiple scars, or sclera, in the brain and spinal cord, which
is how multiple sclerosis got its name.
Loss of oligodendrocytes and myelin result in a variety of debilitating symptoms depending
on what parts of the brain were demyelinated.
This includes tingling and numbness, cognitive impairment, speech problems, visual problems,
and muscle weakness – all symptoms of MS.
How does this damage occur?
Normally, the blood-brain barrier, which is a layer of tightly packed cells on the walls
of blood vessels, only allows certain substances like sugars to enter the brain from the blood.
However, after an infection, the blood-brain barrier may become damaged, allowing immune
cells circulating in the blood to sneak into the brain.
These immune cells incorrectly recognize the myelin as a dangerous substance and begin
to wreak havoc by producing inflammatory substances that damage myelin and eventually kill the
oligodendrocytes, which is why MS is considered an autoimmune disease.
Though it is clear that damage to myelin by the immune system results in the symptoms
of MS, the reason why our immune system attacks myelin in the first place is still unknown.
Certain mutations seem to increase the risk of developing the disease, and having parents
with MS slightly increases the risk of developing the disease yourself.
Exposure to certain viruses, such as human herpesvirus 6 and Epstein-Barr virus may trigger
MS, but a concrete connection has yet to be shown.
Avoid smoking as well. Toxins in cigarettes have been linked to a higher risk of developing MS.
Women are twice as likely to suffer from MS than men, with the disease affecting people
between 15 and 50 years old.
MS is most common in people of Northern European descent, with around 1 in 500 people compared
to 1 in 10,000 for other ethnicities.
Over 2 million people suffer from MS worldwide, making it the most common autoimmune disorder
affecting the central nervous system.
Surprisingly, MS only decreases normal life expectancy by a few months.
However, it does decrease quality of life significantly due to its various debilitating
symptoms.
In most cases, symptoms happen in brief attacks, known as relapse-remitting MS, while in others,
symptoms are constantly present and progressively get worse, known as primary progressive MS.
70% of people who start off with relapse-remitting MS may eventually develop progressive MS,
known as secondary progressive MS.
Current treatment involves regulating or suppressing the immune system with a variety of different
drugs so it won’t attack oligodendrocytes and myelin.
However, prolonged usage decreases effectiveness and increases side effects.
As new research uncovers more and more about multiple sclerosis, one day we will find a
better treatment for this disease.
To speed up our progress towards a cure, just like how our myelin speeds up our electrical
signals, check out the links in the description below to see how you can help.
Thanks for watching, and see you next time for an explanation of another disease on Medicurio.