Muscle Relaxants are medications or therapies which used to relieve muscle pain and relaxing muscles that are abnormally contracted in spasm. Some muscle-relaxant drugs could affect the mind and some are habit-forming.

Spasm, involuntary and abnormal violent contraction of muscles or muscle tissue. Tonic spasm, or cramp, is characterized by an unusually prolonged and strong muscular contraction, with relaxation taking place slowly. The extreme example of tonic spasm is tetanus, in which the spasms are so violent and so enduring that they may paralyze breathing. In the other form of spasm, called clonic spasm, contractions of the affected muscles take place repeatedly, forcibly, and in quick succession, with equally sudden and frequent relaxations. The most typical examples of clonic spasm are epilepsy and convulsive hysteria.

Muscle, tissue or organ of the animal body characterized by the ability to contract, usually in response to a stimulus from the nervous system. The basic unit of all muscle is the myofibril, a minute, threadlike structure composed of complex proteins. Each muscle cell, or fiber, contains several myofibrils, which are composed of regularly arranged myofilaments of two types, thick and thin. Each thick myofilament contains several hundred molecules of the protein myosin. Thin filaments contain two strands of the protein actin. The myofibrils are made up of alternating rows of thick and thin myofilaments with their ends interleaved. During muscular contractions, these interdigitated rows of filaments slide along each other by means of cross bridges that act as ratchets. The energy for this motion is generated by densely packed mitochondria that surround the myofibrils.

The medicines used to counteract spasmodic muscular activity are termed antispasmodics. These are used in certain surgical procedures because they block transmission of nervous impulses to the muscles. Derivatives of hemlock and some 50 other plants have a relaxing effect on muscles.