Vitals and Bits #12: The Myelin Sheath
Most neurons (or nerve cells in the brain, spinal cord, and peripheral nervous system) are shaped kinda like a sunflower. In general, neurons have a cell body (the center of the sunflower), projections off the cell body called dendrites (the petals of the sunflower), and one long tail called the axon (or the sunflower’s stem). In order to “talk” to one another, nerve cells pass electrical impulses down their axons and spread the impulse to the dendrites of other nerve cells.
Some nerve axons can be incredibly long. The longest axons in the human body are over a meter in length. Wow! And as we all know, size matters when it comes to neural excitation. The nervous system depends on lightning fast communication to function properly, and we can’t have electrical impulses puttering along limp axons. So in order spread electrical signals quickly, nerve cells have a special insulating substance covering their axons. This insulating substance, known as myelin, is a fatty material that coats the axon and keeps the electrical impulse from dissipating. The coating of myelin around the axon is called the myelin sheath. If it helps you to think of myelin as an electrical lubricant, be my guest.
Myelin is white in color, thus imparting the characteristic color (or lack thereof) to the brain’s white matter. White matter can be found on the interior of the brain, and comprises trillions of myelinated axons linking all the various parts of the brain.
Myelin is made by a special type of neural support cell called a glial cell. Glial cells and neurons are like BFFs. Glial cells are the Paul Rudd to the neuron’s Jason Segal. They’re the Nicole Richie to the neuron’s Paris Hilton. Like Harold and Maude, or Fred and Ginger, or King Arthur and Sir Lancelot they are. Do you like how my pop culture references just get more and more outdated? Shut up. I’m in grad school. I have no time to keep up with the latest Bromance movies, okay? I don’t even have time to feed myself dinner. I just slurp down old cans of cat food I find in the alley behind my house.
Anyhow, so special glial cells make myelin and deposit it along the axons of neurons, and everyone’s happy because now electrical signals can zip through the brain and body in milliseconds.
Unless, of course, you suffer from a demyelinating disorder like transverse myelitis or Guillain-Barre syndrome. Then your neural transmission slows to a crawl and you find yourself having a tough time, say, moving your limbs or the muscles that allow you to breathe.
The poster child of all demyelinating disorders is Multiple Sclerosis (MS), an autoimmune disorder in which the body’s own immune cells periodically “attack” the myelin sheaths of neurons. More specifically, the body attacks the glial cells that maintain the myelin sheath. After an attack, glial cells can recover and help repair the damaged myelin sheaths up to a certain point. However, the disease progresses when destruction of myelin outpaces repair. The repair process can also generate scarring (or sclerosis). Scar tissue is non-functional, and patients with MS will have characteristic scars or tangled myelin plaques on the white matter of their brains.
MS is a sad, difficult, frustrating disease. Unfortunately, it’s not terribly uncommon, and it’s more likely to occur in women. Before scientists discovered the physiologic underpinnings of the disease, some dismissed it as a psychosomatic manifestation in fragile or hysterical women. You know. Just women being dramatic. Not walking. Not moving. Not breathing. Darn women and their emotions!
Now we know much more about MS, and a slew of new information is on the horizon. You can bet I’ll take time out of my busy grad school schedule to blog about advances in MS therapies when they arise. Until then, put your myelin to use and go make me some dinner or something.