Miyastenia gravis II: Patofizyoloji

In myasthenia gravis, the immune system produces antibodies that block acetylcholine receptors on the postsynaptic membrane at the neuromuscular junction. These receptors normally receive signals that trigger muscle contraction. When antibodies block them, nerve signals become too weak to activate muscles effectively, causing progressive muscle weakness, especially after repeated use.

Antibodies targeting acetylcholine receptors not only block acetylcholine binding but also activate complement proteins, which damage the postsynaptic membrane and destroy receptor sites. Over time, the muscle end plate flattens and the number of functional receptors decreases by nearly 90%, reducing the strength of each nerve impulse and causing rapid muscle fatigue.

Some patients develop antibodies against muscle-specific kinase (MuSK) or lipoprotein-related protein 4 (LRP4), molecules that organize and stabilize acetylcholine receptors. When these proteins are attacked, receptors cannot cluster properly, disrupting nerve-muscle communication. Patients with these antibodies often experience prominent bulbar weakness, affecting swallowing and speech.

The thymus gland regulates immune cell development, but in myasthenia gravis it often becomes enlarged or contains a thymoma. Abnormal thymic tissue produces immune cells that mistakenly recognize acetylcholine receptor proteins as foreign, sustaining production of harmful antibodies. Removing the thymus often reduces symptoms or induces remission, especially in generalized cases.

Myasthenia gravis is an autoimmune disorder specifically targeting the neuromuscular junction, whereas conditions like alterations in muscle tone involve different mechanisms affecting motor control. Unlike degenerative disorders, myasthenia gravis involves immune-mediated damage to receptors and supporting proteins, causing reversible weakness that improves with rest.

Repeated nerve stimulation depletes acetylcholine stores and exhausts the remaining functional receptors. Since antibodies have already reduced receptor numbers by up to 90%, fewer receptors are available to respond to subsequent signals. Rest allows acetylcholine to replenish and receptors to recover, temporarily restoring muscle function until activity resumes.

When antibodies bind to acetylcholine receptors, they activate complement proteins, which directly damage the postsynaptic membrane and destroy receptor sites. This complement-mediated destruction accelerates the loss of functional receptors, contributing to the progressive weakening of neuromuscular transmission and the characteristic muscle fatigue seen in the disease.