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The Purpose of Muscle Relaxants
Designed to calm down skeletal muscles accountable for voluntary movement, muscle relaxants are used to reduce muscle tone and relax muscles, treating symptoms for pain and muscle spasms. While neuromuscular-blocking drugs stop nervous signals, which can cause paralysis to muscles, other types of muscle relaxants such as methocarbamol and benzodiapines work in a smaller degree for managing muscle spasms and seizures. Although they don't heal injuries, muscle relaxants reduce discomfort.
Types of Muscles
Excluding the heart muscle, muscles are either voluntary or involuntary. Skeletal muscles such as those in the arms and legs are voluntary, while smooth muscles covering the digestive organs are involuntary in that they don't move under conscious control. Generally it's the skeletal or voluntary muscles that need help for pain.
How Relaxants Control Muscles
Large spinal cord nerves control skeletal muscles. Although the nerve cells known as the neuron is a part of the spinal cord, its projections go outward to join with muscle cells. The nerve axon in projections senses the current condition of muscle cells, while the dendrites are fibers that transfer instructions to change its position to the muscle fiber. The neuromuscular junction is where the muscle and nerve connect. Here a chemical known as a neurotransmitter is released that runs across the tiny area between the muscle and the nerve, causing the desired response. Although there are five recognized neurotransmitters, only three are known for their functions. The neurotransmitters glycine and GABA reduce muscle activity, while acetylcholine stimulates it.
Centrally Acting Relaxants
Muscle relaxants can either act directly on a muscle (peripheral) or centrally. Most drugs are centrally acting; however, it's not totally clear how they do their job. The drugs act directly on a muscle, but they don't act on it directly to relax the muscle. In other words, they don't stop the release or crossing of the neurotransmitter. In some uncertain way they depress the central nervous system, creating a sedative effect.
Dantrolene Sodium: The Only Direct Acting Drug
Dantrolene Sodium is the only drug that is peripheral, acting directly on muscles. It has less side effects than the drugs that are centrally acting and its peak effect isn't felt for about five hours after it's entered the bloodstream. Because the drug may seem to have a small effect, patients may want to increase the dosage, but should resist the temptation to do so for safety reasons.