Muscle Relaxants

Muscle relaxants are drugs that are administered to relax muscles. They are given to relieve the discomfort of muscle spasm or involuntary muscle contracture and also in cases of surgery to relax muscles and provide easier access for the surgeon. Some nonprescription drugs are available to combat painful contraction of the uterus during a woman's menstrual period.

Muscles can be divided into two classes, the voluntary or skeletal muscles and the involuntary or smooth muscles. The heart muscle, the myocardium, is a unique type of muscle that does not fit into either category. Skeletal muscles are those that are under voluntary control. The muscles that move the arms, that move the legs when you walk or run, or those that are involved in chewing are all skeletal muscles. They come into play only when you will them. Smooth muscles are those that are not under conscious control. The muscles in the digestive organs are smooth muscles.

Usually it is the skeletal or striated muscles that will require therapy for painful spasm or will need to be relaxed to allow the surgeon to gain access to the abdomen easily. Muscle spasm may be associated with a trauma or may be brought on by multiple sclerosis, cerebral palsy, stroke, or an injury to the spinal cord. Severe cold, an interruption of blood supply to a muscle, or overexertion of the muscle also can lead to spasms. A muscle spasm actually is an increase in muscle tone brought on by an abnormality in motor control by the spinal nerves.

Skeletal muscles are controlled by large nerves in the spinal cord. The nerve cell or neuron is part of the spinal cord, but its projections, the axon and the many dendrites course outward to connect to muscle cells. The nerve axon is a sensory device that senses the muscle cells' current condition. The dendrites are motor fibers that deliver the instructions to change its state to the muscle fiber. The area at which the muscle and nerve connect is called the neuromuscular junction. It is here that the end releases a chemical called a neurotransmitter that crosses the microscopic space between the nerve and muscle and causes the desired response. Five such neurotransmitters have been described: acetylcholine, serotonin, norepinephrine, glycine, and gamma-amminobutyric acid or GABA. Of these, the functions of three are known. Acetylcholine excites muscle activity and glycine and GABA inhibit it.

Muscle relaxants may act either peripherally, that is directly on the muscle, or centrally, in the spinal cord. Most such drugs act centrally, though how they perform their task is not understood. These drugs do not act directly on the muscle to relax it, they do not interfere with conduction along the nerve fiber, they do not stop the neurotransmitter from being released or crossing the nerve-muscle junction, and they do not alter the ability of the muscle to respond to the neurotransmitter. Somehow they act centrally to depress the central nervous system and may have a sedative effect.

The dosage of any muscle relaxing agent must be carefully tailored to achieve the desired result without overdosing the patient. Because these drugs have a sedative action the patient should be warned not to drive, operate machinery or perform any other task that requires wakefulness. An overdose of the drugs can put the patient to sleep and may well depress muscle function to the extent that the bladder will not contract normally and urine retention will occur. Also, some drugs will produce a change in the color of the urine of which the patient should be made aware.

The centrally acting muscle relaxants have an onset of action ranging from 30 minutes to an hour. They reach their maximum concentration in the blood between 1-8

hours, and their effects last for 4-24 hours. Dosages are designed to allow some overlap to maintain the desired blood concentration of the drug for maximum benefit.

Only one drug is classified as a peripherally acting muscle relaxant. This agent, dantrolene sodium, acts directly on the muscle, so it has fewer side effects than do the centrally acting drugs. Dantrolene is marketed as Dantrium by its manufacturer. It is a slow-acting drug whose peak effect is not felt for some five hours after it has been taken. The peak effect of a single dose, however, does not constitute a therapeutic dose. The therapeutic effects of the drug may not be seen for up to a week or longer. The patient may be tempted to increase the dosage of the drug because it seems to have little effect in the beginning stages of therapy. Of course, this could be a critically unwise procedure that would be seen once the therapeutic dosage is built up.

Dantrolene seems to work directly on the muscle cell, affecting the sarcoplasmic reticulum within the cell. The sarcoplasmic reticulum in a muscle cell stores calcium. When the electrical signal through the nerve releases an excitatory neurotransmitter the sarcoplasmic reticulum releases calcium, which brings on the contraction of the muscle. Dantrolene seems to prevent the release of calcium, and thus short-circuits the muscle contraction.

Baclofen (marketed as Llorosal) is another muscle relaxant that acts on the spinal cord to lessen muscle spasms especially in patients with multiple sclerosis or spinal cord injuries. The chemical structure of baclofen is similar to that of the neurotransmitter GABA, which is an inhibitory chemical. Baclofen seems to inhibit neuron activity, which reduces muscle tone and reduces spasm. Its primary usage is in patients with spinal cord injury that has resulted in paralysis. The drug relaxes tightly flexed leg muscles, improves bowel and bladder control, and reduces muscle spasms both in number and severity. Baclofen builds up slowly and may require a month or two of therapy before the optimal dose is achieved. It does not have a high degree of sedative effect, so it may be preferable for many patients.

Another drug used to treat muscle spasms, diazepam, commonly known by its trade name, Valium, is a tranquilizer or antianxiety and anticonvulsant drug. Still, it has found a niche as a muscle relaxing drug in cases of acute, painful muscle spasm. Diazepam appears to work by enhancing the effects of the inhibitory neurotransmitter GABA. Only small doses of diazepam can be given as a muscle relaxant because of its sedative effects. For the patient who is not sensitive to the sedation often brought about by diazepam, the drug can serve both to relieve muscle spasm and to relieve anxiety about that situation.

All muscle relaxing agents should be given with caution to anyone who is taking other drugs. The muscle relaxers will enhance the sedative effects of many other classes of drugs such as antihistamines, antidepressants, sedatives, other muscle relaxants, and tranquilizers.

In addition, they should not be suddenly discontinued. Stopping the drugs suddenly can lead to a return of symptoms to an even more painful degree. After the optimal clinical dosage is reached the physician will provide the drugs until the muscle spasm has passed and then wean the patient off the drug. In some cases, as in paraplegia, the drug may be given for years to maintain the patient's comfort.