|
Myasthenia Gravis (MG) is all about
muscles failing to contract.
Every time your body moves, whether
you're walking, talking or breathing, a muscle contracts. There
are three general types of muscles:
- smooth muscle - these are muscles that are not
under conscious control, such as the muscles in the walls of the
digestive tract;
- skeletal muscle - also known as striated or voluntary
muscles, these are the muscles that are attached to parts of the
skeleton and are under our voluntary control - we can send messages
from the brain to tell the muscle what to do - eg walking and
talking; and
- cardiac muscle - the muscles of the heart.
The muscles work by transforming chemical
energy into mechanical energy, which moves the human body. In summary,
for a muscle to contract, the following must happen:
- an electrical impulse travels from the brain,
through the spinal cord down a nerve (the nerves that command
the muscle are called motorneurons)
- the nerve ending releases a neurotransmitter
substance called acetylcholine (ACh)
- the acetylcholine travels through a small gap
between the nerve and the muscle (at the neuromuscular junction)
and binds to a protein (receptor) on the surface of the muscle
(the muscle membrane) to which the nerve is attached
- resulting in the contraction of that muscle.
It is at the neuromuscular junction
(NMJ) where MG does its damage.
We wish to avoid overly techinical
explanations, but an understanding of how the NMJ works and what
makes up the acetylcholine receptor may help you in understanding
MG. The website of Washington University School of Medicine, St
Louis, MO – Neuromuscular Disease Center has an excellent
piece on the anatomy of the neuromuscular junction at: www.neuro.wustl.edu/neuromuscular/synmg.html.
For a detailed description of the ACh Receptor, go to: www.macalester.edu/~psych/whathap/UBNRP/Gravis/ACH_receptor.html
.
In MG, the receptors at the muscle
surface are destroyed or deformed by antibodies that prevent a normal
musclar reaction from occurring. Antibodies are proteins produced
by the immune system to fight infection and disease. With autoimmune
diseases such as MG, the body mistakenly sends out antibodies to
attack healthy tissue. In MG specifically, the immune system gets
triggered to attack an otherwise healthy neuromuscular junction.
The antibodies bind to the muscle's membrane and initiate a series
of events that destroy the membrane and prevent ACh from binding.
ACh plays a critical role in muscle contraction. When a nerve sends
a message telling a muscle to contract, a large amount of ACh is
released. If ACh can't bind to the muscle, the muscle won't contract.
Nobody quite knows what triggers MG,
but around 70% of all myasthenics show evidence of a particular
kind of abnormal immune response that is believed to be caused by
an abnormal thymus gland (an organ in the chest that plays an essential
role in the development of the body's immune system). Around 15%
of all myasthenics have tumours in their thymus , although the tumours
are usually benign.
Model of normal neuromuscular
junction on the left,
compared with myasthenia neuromuscular junction on the right.
Next...
|
