Neurons are a type of cell in our brains and spinal cord that serve to transmit and receive electrical signals (impulses) throughout our central nervous system. In the picture above, the neuron’s nucleus can be found on the left as the blue-colored mass with the black spots. The short arms or branches hanging off the nucleus on the left are known as “dendrites”. Dendrites carry impulses towards the nucleus. The long appendage to the right is an “axon”. Axons carry impulses away from the nucleus and towards the dendrite of another neuron. This axon is covered by five sections of “myelin”, which are colored green. You can see in the magnified box that the axon is a bundle of little fibers and the myelin is actually wrapped around it several times liked a rolled up carpet. Myelin serves some incredibly important functions. It protects the axon. It allows an impulse to get from one neuron to another, and it allows that impulse to move very, very fast.
Now, let’s say you have an idea in your head to move your right hand and pick up a pencil. That “idea” travels as an electrical impulse from your brain through your spinal cord (hopping neuron after neuron along the way), out to the peripheral nerves in your right arm and finally to your right hand, where motion finally occurs and you start to move your hand to grab the pencil.
Tragically, as the impulse tries to make its way across the axon, it leaks out where there is no myelin. This results in little, if any, impulse making it to the next neuron in the chain. And for a PMD patient, they’re lucky if they have neurons with even a little myelin. That’s what separates PMD from a disease like Multiple Sclerosis (MS). The PMD patient simply never forms any myelin on their axons, while the MS patient has healthy neurons but the immune system eventually attacks and destroys the myelin surrounding the axons.