Marker. 1  Noun. A line of credit extended by a casino or cardroom. 2  Noun. A small object placed on top of a player's hand for the purpose of preventing it from being mucked or fouled. 3  Noun. A plastic device used to denote who is in the dealer's position.
Myasthenia gravis is caused by a defect in the transmission of nerve impulses to muscles. It occurs when normal communication between the nerve and muscle is interrupted at the neuromuscular junction -- the place where nerve cells connect with the muscles they control.
What Is It? Myasthenia gravis is a chronic (long-lasting) and rare disease that affects the way muscles respond to signals from nerves, leading to muscle weakness. The disease can occur at any age, but it mainly affects women between ages 20 and 40. After age 50, men are more likely to get the disease.