Now that you have a good understanding of what the main components of the nervous system are and what the nervous system does from lesson 1, lets go into a bit more detail. Today I want to go over nerves since they are what gets damaged by MS. I guess you could say MS really gets on our nerves!
Nerves are responsible for carrying signals between central nervous system (CNS), which remember consists of the brain and spinal cord, and the peripheral nervous system (PNS). There are 3 things all normal nerves have in common:
1. They are excitable! Sensory nerves recognize things like heat and hunger. The nerve then gets excited, creates an electrical current, and uses it to quickly run the signal to the CNS so action can be taken. The action may be a reflex that pulls your hand away from a fire, or your brain telling you something like “I’m starving!” or “I really have to pee!”.
2. They are conductive. An electrical cord’s job is to get electricity from a wall socket to a TV so that it will turn on. Similarly a nerve takes electrical signals and conducts them from cell to cell in order to complete thousands of tasks that we need in order to survive like breathing, digesting food, moving muscles, and keeping your heart beating.
3. They release chemicals. All nerves end and have to somehow get their electrical signal to the next nerve. They do this by releasing chemicals known asneurotransmitters. Neurotransmitters leap across the gap between nerves and keep the electrical signal going forward. In the picture below you can see how the first nerve sends it’s electrical signal to the second nerve by releasing chemical neurotransmitters.You may have heard of some common neurotransmitters such as seratonin and dopamine.
In the illustration above the yellow ovals covering the long part of the nerve represent myelin. Myelin is made of protein and fat which form a protective coating around the nerve and act like the rubber insulation around electrical wires. Myelin is made by a special type of cell called an oligodendrocyte. The oligodendrocytes put together fats and proteins and wrap the nerve in myelin. They have also been shown to be capable of putting new myelin around nerves damaged by MS. MS targets both oligodendrocytes and myelin, which is a double whammy to the nerves because it keeps myelin from being made, and it damages existing myelin.
Now I want you to imagine a frayed wire. Maybe your cat gnawed through it, or maybe your phone charger finally frayed after months of being bent and abused. You have to get rid of it because in all likelihood it doesn’t work reliably anymore and/or you don’t want to burn down your house. MS is the same type of situation. The myelin is damaged leaving the inner wiring of the nerve exposed. Some electrical signals may go through, but the wire is overall frayed and unreliable. Scars then form around the damage which makes it even harder to conduct a signal. The lesions seen on MRI are the scar tissue that has formed where myelin was damaged.
So now you know the basics of the nervous system, nerves, and nerve conduction. For lesson 3 I’ll be putting everything together to teach you how MS damage causes common symptoms like walking difficulty, vision problems, bladder dysfunction, and dizziness.