 |
| Image Copyright Alila Sao Mai, 2012
Used under license from Shutterstock.com |
Way back in the dim mists of this blog (just over a year ago), I described the basic mechanism of neurons, the building blocks of the brain and nervous system. The blog is here (http://teddysratlab.blogspot.com/2011/02/piece-of-action-potential.html) if you need to refresh your memory on the subject. One of the key features of neurons are their long axons which conduct an electrochemical signal over long distances - many meters in some cases. One of the means of speeding up the transmission is to insulate most of the axon, leaving periodic gaps for the chemical channels required to propagate the action potential.
That insulation is illustrated on a neuron in the figure abovet. The left neuron is normal and has intact myelin insulating its axon. The right neuron has damaged myelin, which slows and even stops the signal, due to the fact that once an axon is wrapped with myelin, it doesn't have ion channels under the insulation. Without ion channels covered the whole length of the (exposed) axon, an action potenial cannot keep occurring in sequence down the length of the axon. Without an action potential, the neuron cannot transfer information from brain to muscle or from sensory organs to the brain. Even inside the brain, axons that cover "long distances" (more than a few millimeters) are also insulated, thus MS can affect the normal function of information processing in the brain.
So - three questions come up: What is myelin? How does the degeneration of MS occur? How can it be treated?
Myelin is actually a special type of cell - called a "Schwann Cell" - that wraps itself around an axon many times. The normal cell membrane is supplemented with thick lipid (fat or oil) deposit within the layers of cell
surrounding the neuron, forming an electrical insulator - much the way non-conducting grease or transformer oil is used in high-voltage electrical installations. The insulation serves two purposes - it prevents the "leakage" of electrical charge from axons, and the thick blanket of Schwann Cell surrounding the axon prevents any of the fluid surrounding the neuron from making contact with the membrane (and any ion channels located there). The neuron adapts to the insulation by moving it's ion channels to the gaps between Schwann Cells. These gaps have the name "Nodes of Ranvier" and are a key component of the signal speed increase caused by myelination. With ion channels concentrated at the Nodes, the action potentials are actively generated only at the nodes, then passively conducted through the now very efficiently insulated spaces between the nodes. The speed of action potential propagation is at least ten times faster in myelinated vs. unmyelinated axons, which is very important for sending signals to distant muscles.
MS is an "autoimmune" disease, meaning that the bodies own antibodies attack the Schwann Cells and cause the damage to the myelin that is characteristic of MS. The location and extent of myelin damage controls the severity and type of symptom, which can range from dizziness, unusual sensations and muscle tremors at it's mildest, to muscle spasms, paralysis, blindness, at worst. All of the causative factors are not clear, but MS affects more women than men, runs in families and also is also more frequent in certain parts of the world.
Symptoms of MS can occur in multiple parts of the muscle and nervous system - hence its designation as a neuromuscular disease. Muscle symptoms include loss of balance, weakness, spasms, problems with coordination and/or tremor. MS can cause vision problems including double vision, sudden eye movement and blindness; it can cause hearing loss, speech problems, dizziness, depression, memory loss, decreased attention, difficulty reasoning and making decisions. MS is also associated with many internal issues as well, constipation, trouble swallowing, incontinence, erectile dysfunction. The most notable symptoms with the most difficult treatment are a burning/tingling/crawling sensation of the skin, painful spasms, facial pain and fatigue. MS is diagnosed via tests of nerve and muscle conduction speed, an MRI, spinal tap to look for inflammation proteins in the cerebrospinal fluid, and physical examination for changes in vision, hearing, sense of touch, pain sensation, balance, muscle coordination, etc. [As usual, the PubMed Health site on MS: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001747/ is an excellent place to start for more information.]
Since MS is an autoimmune disease, one course of treatment is to suppress inflammation and immune function using chemotherapy, interferon or antibodies to immunoglobin E (IgE) - the primary immune system
protein responsible for autoimmune reactions. However, these drugs will only slow the course of the disease, they cannot restore lost function. Given that damaged neurons will not regrow, it is possible to supplement the normal neuron function by replacing or reinforcing the action of certain neurotransmitters - such as GABA-like medicines for muscle spasms, acetylcholine for urinary problems or antidepressants for the mental changes.
This will seem a bit strange for those who know my stand on medical marijuana, but cannabinoids have been shown to reduce spasms, reduce pain and alleviate mood changes. I still maintain that these effects would be better served by a true medicinal cannabinoid with controlled formulation and dosing, but in the case of MS and cancer pain, smoked marijuana appears to be quite effective.
Patients with MS can live quite a long time (20+ years) with minimal symptoms, or may have periodic attacks with severe symptoms, but be symptom-free for months or even years in between. Keeping in mind that I am writing about neurological and neuromuscular disorders at fictional plot devices, MS is the sort of disorder that a person can have and show no overt symptoms, while the disease continues to affect neurons until a tipping point is reached in which the patient experiences loss of both voluntary and involuntary muscle control and requires assistance caring for themselves.
I highly recommend that anyone desiring more information visit the PubMed Health site and start there.
We will continue with the "Three M's" next time, with Myasthenia Gravis, a disease that can have many overlapping symptoms with MS, but with a totally different causative source.
