Neuromuscular Diseases

When a single nerve impulse strikes a muscle it causes a "twitch," and ordinarily there is a brief refractory period of relaxation. If another impulse is received before relaxation is completed, the twitches can add up and cause a prolonged muscular spasm or tetany. Normally, muscles continue to function properly because the ACh transmitted down the nerve is enzymatically eliminated. Certain drugs such as neostigmine and physostigmine can block this action and paralyze a muscle. So can poisonous nerve gases and insecticides. In a neuromuscular disease called myasthenia gravis, antibodies can block the passage of ACh to the end plate creating a similar paralysis. Leg cramps at night, on the other hand, are due to sustained muscular contractions (200 per second). They can be relieved by quinine or diphenhydramine.

Paralysis can take place anytime there is a failure or interference in the transfer of biochemical impulses from nerve to muscle. On the other hand, hyperactivity of neuromuscular transmission can lead to minor twitches and cramps or to severe spasms as in tetanus (lockjaw) or amyotrophic lateral sclerosis (Lou Gehrig disease). There is still much to learn about both hyperactive and paralytic cases, but new research on DNA and immunology is proving helpful.

Abnormal levels of blood electrolytes such as sodium and potassium can also cause neuromuscular disturbances. When potassium is too high or too low, the muscles of the trunk, arms, and legs can be very weak, even to the point of paralysis. If the blood calcium is low (as in vitamin D deficiency or inadequate function of the parathyroid gland), twitching may occur. When blood calcium is too high, there may be profound weakness. Normal magnesium levels are also important for proper neuromuscular functioning.

Creatine is a nitrogenous organic acid normally present in muscle and other tissues. When muscle is injured, creatine leaks out and can be measured in the blood as creatine kinase (CK). Blood levels of CK are increased when heart muscle is damaged, but also in muscle trauma, polymyositis, rapidly worsening cases of muscular dystrophy, vigorous exercise, or for no apparent reason.