The majority of people with myasthenia gravis produce antibodies against a self-protein called the acetylcholine receptor (AChR). This is found at the junction between the nerve and the muscle (the neuromuscular junction (see figs 1 and 2). It acts as a ‘receiver’ for the chemical signal, acetylcholine that is released from the nerve when we want to use a muscle. The antibodies bind to the acetylcholine receptors on the muscle membrane and greatly reduce their ability to receive the chemical signal. As a result the patient experiences muscle weakness which becomes worse as they repeatedly try to use the same muscle. Although we now understand how antibodies to the acetylcholine receptor cause muscle weakness, we do not know why patients with myasthenia gravis develop these particular antibodies. In some cases, the thymus gland in the chest appears to be important in triggering the abnormal immune response.
Fig. 1: The normal neuromuscular junction
Fig. 2 The neuromuscular junction in myasthenia gravis
The diagram shows the chemical signal, acetylcholine, and the receivers, acetylcholine receptors. The inverted ‘Y’ shaped molecules are antibodies binding to the acetylcholine receptors and preventing them from working.