What is a Muscle Relaxant?

Muscle relaxants are drugs or other substances used to reduce muscle tone, also called tonus or residual muscle tension, or to induce temporary immobility. Muscle tone is a continuous partial contraction of the muscles that keeps the muscles in a constant state of readiness for action. Muscle relaxants are used to treat muscle spasms and the symptoms of disorders affecting muscle tone and to induce paralysis in a patient during medical procedures. A muscle relaxant can be placed in one of two broad categories, spasmolytics and neuromuscular blocking drugs, though the term is often used to refer specifically to the former.

A spasmolytic, also sometimes called an antispasmodic, is a muscle relaxant used to control muscle spasms and spasticity. These drugs are often referred to as centrally acting muscle relaxants, meaning that they work by affecting cells in the central nervous system. This is not entirely accurate, as it has been discovered that some muscle relaxants actually do not produce their effects in the central nervous system, but the term centrally acting muscle relaxant is still often used as a blanket term for all spasmolytic drugs.

Spasms are abrupt, involuntary contractions in the skeletal muscles. Many spasms are commonplace and minor, such as cramps from intense exercise, but in more serious cases spasms can cause severe pain and significantly impaired mobility. Spasticity is a condition in which a person's skeletal muscles are in a constant state of increased tension, or hypertonia. This impairs flexibility and movement by making it harder for the muscles to relax and stretch, and over time this constant tension can affect the alignment of bones near the affected muscles. This can cause problems such as impaired mobility, reduced dexterity, or abnormal gait, and in some severe cases can result in total paralysis.

Spasmolytics work by either increasing inhibition or decreasing excitation in motor neurons, which inhibits generation of the electrical discharges that send signals from cell to cell. This reduces spasticity and spasms by causing the muscles to receive fewer signals telling them to contract. Commonly used spasmolytic drugs include diapezam (C₁₆H₁₃MClN₂O), dantrolene (C₁₄H₁₀N₄O₅), and clonazepam (C1₅H₁₀ClN₃O₃).

Spasmolytic drugs are often used to treat the symptoms of neurological disorders, such as the spasticity caused by conditions such as spastic diplegia, cerebral palsy, and multiple sclerosis or the severe muscle spasms caused by dystonia and fibromyalgia. Spasmolytics can also be used to treat other problems, such as severe lower back pain, which is usually caused by muscle injuries, and tension headaches. Spasmolytics affecting smooth muscle can be used for problems in the digestive system such as irritable bowel syndrome.

Muscle relaxants can have significant side effects. They frequently cause sedation and fatigue, and some have additional side effects that can include blurred vision, loss of coordination, and stomach problems such as stomachache and nausea. In rare cases, some muscle relaxants can cause heart or respiratory failure. Some are also potentially habit-forming after extended use.

The term muscle relaxant is also sometimes used to refer to a separate group of drugs called neuromuscular blocking drugs. These cause temporary paralysis in skeletal muscles by inhibiting the production, release, or reception of the neurotransmitter acetylcholine. They are given in combination with anesthetics during surgery to prevent spontaneous movement.