This disease, affects one in 20,000 persons. It causes the person to become paralysed because of inability of neuromuscular junction to transmit signals from the nerve to muscle fibre. The muscles affected include those used in walking, breathing, chewing, and talking. A characteristic sign of the disease is drooping eyelids. Myasthenia gravis most often affects women between the ages of 20 and 30, but it can strike anyone after adolescence. It is believed that myasthenia gravis is an autoimmune disease in which patients have developed antibodies against their own acetylcholine activated ion channels. These antibodies are produced by thymus gland. If the disease gets intense enough, the patient dies of paralysis of respiratory muscles. The disease can be cured by removal of the thymus and by a blood-cleansing process, called plasmaphoresis, that removes the destroying antibodies. About 10 per cent of the victims die.
Types of muscle contraction :
There are 2 types of muscle contractions.
1. Isotonic contraction: It involves muscle shortening as the myofilaments slide. This contraction produces normal movements, such as bending the knee, rotating the arms and smiling.
2. Isometric contraction: It occurs when the myofilaments 'spin their wheels' without moving, causing tension in the muscle. This is due to the muscles that are pitted against some more or less immovable object.
Isometric and aerobic exercises
In isometric exercise, muscles are moved through a short distance against a high resistance, as in pushing or pulling an immovable object. Isometric exercise is best for developing large muscles, whereas isotonic exercise has beneficial effects on the cardiovascular system. Isometric exercise-increases the thickness of the muscle fibres and their ability to store glycogen.