Muscle relaxants

A drug that affects skeletal muscle function and decreases the muscle tone.

Used to alleviate symptoms such as muscle spasms, pain, and hyperreflexia.

Two major therapeutic groups: neuromuscular blockers and spasmolytics.

Neuromuscular blockers act by interfering with transmission at the neuromuscular end plate and have no central nervous system activity.

Neuromuscular blockers are often used during surgical procedures and in intensive care and emergency medicine to cause temporary paralysis.

Spasmolytics, are centrally acting muscle relaxants, used to alleviate musculoskeletal pain and spasms and to reduce spasticity in a variety of neurological conditions.

Muscle relaxant term is commonly used to refer to spasmolytics only.

Other skeletal muscle relaxants from a number of drug categories and other drugs used primarily for this indication include orphenadrine (anticholinergic), chlorzoxazone, tizanidine (clonidine relative), diazepam, tetrazepam and other benzodiazepines, mephenoxalone, methocarbamol, dantrolene, and baclofen,

Most neuromuscular blockers function by blocking transmission at the end plate of the neuromuscular junction.

When a nerve impulse arrives at the motor nerve terminal, it initiates an influx of calcium ions, which causes the exocytosis of synaptic vesicles containing acetylcholine.

Acetylcholine then diffuses across the synaptic cleft.

Acetylcholine may be hydrolyzed by acetylcholine esterase or bind to the nicotinic receptors located on the motor end plate.

When two acetylcholine molecules bind a conformational change in the receptor that opens the sodium-potassium channel of the nicotinic receptor, allowing Na+and Ca2+ions to enter the cell and K+ions to leave the cell, causing a depolarization of the end plate, resulting in muscle contraction.

Then acetylcholinesterase hydrolyzes acetylcholine molecules from the end plate region.

End plate function can be blocked by two mechanisms:

Nondepolarizing agents, such as tubocurarine, block the agonist, acetylcholine, from binding to nicotinic receptors and activating them, thereby preventing depolarization and, depolarizing agents, such as succinylcholine, are nicotinic receptor agonists which mimic acetylcholine, block muscle contraction by depolarizing to such an extent that it desensitizes the receptor and it can no longer initiate an action potential and cause muscle contraction.

Both classes of neuromuscular blocking drugs are structurally similar to acetylcholine, the endogenous ligand.

Neuronal signals in motor neurons that cause muscle contractions are dependent on the balance of synaptic excitation and inhibition the motor neuron receives.

Spasmolytic agents either enhance the level of inhibition, or reduce the level of excitation.

Inhibition is enhanced by mimicking or enhancing the endogenous inhibitory substances, such as GABA.

Inhibitors may act at the level of the cortex, brain stem or spinal cord, or all three areas

Inhibitors act centrally as muscle relaxants.

Spasmolytics such as carisoprodol, cyclobenzaprine, metaxalone, and methocarbamol are commonly prescribed for low back pain or neck pain, fibromyalgia, tension headaches and myofascial pain syndrome.

Spasmolytics are not recommended as first-line agents; in acute low back pain, they are not more effective than paracetamol or nonsteroidal anti-inflammatory drugs.

Most spasmolytic agents have the side effects of sedation, and may cause dependence with long-term use.

Several spasmolytic agents agents also have abuse potential.

Spasmolytic agents prescriptions are strictly controlled.

Spasmolytics are not more effective than antidepressants in fibromyalgia.

Evidence exists to suggest muscle relaxants can add benefit to treatment with NSAIDs.

In general, no high-quality evidence support the use of muscle relaxants.

No muscle relaxant has been shown to be better than another.

All muscle relaxants have adverse effects., but particularly dizziness and drowsiness.

Concerns about abuse and interaction with other drugs, especially if increased sedation is a risk, further limit their use.

Muscle relaxants are advised for for neurological conditions such as spasticity in cerebral palsy and multiple sclerosis.

Dantrolene, although thought of primarily as a peripherally acting agent, is associated with CNS effects, whereas baclofen activity is strictly associated with the CNS.

Muscle relaxants are thought to be useful in painful disorders based on the theory that pain induces spasm and spasm causes pain, however, evidence contradicts this theory.

Not approved for long-term use.

Cyclobenzaprine prescribed on a daily basis to increase stage 4 sleep, and is beneficial for patients who have fibromyalgia.

Muscle relaxants are often prescribed in the treatment of tension headaches.

Diazepam and carisoprodol are not recommended for older adults, pregnant women, or patients who suffer depression or have a history of drug or alcohol addiction.


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