Muscle relaxation and paralysis can theoretically occur by interrupting function at several sites, including the central nervous system, myelinated somatic nerves, unmyelinated motor nerve terminals, nicotinic acetylcholine receptors, the motor end plate, and the muscle membrane or contractile apparatus. Most neuromuscular blockers function by blocking transmission at the end plate of the neuromuscular junction. Normally, a nerve impulse arrives at the motor nerve terminal, initiating an influx of calcium ions which causes the exocytosis of synaptic vesicles containing acetylcholine. Acetylcholine then diffuses across the synaptic cleft. It may be hydrolysed by Acetylcholine esterase (AchE) or bind to the nicotinic receptors located on the motor end plate. The binding of two acetylcholine molecules results in a conformational change in the receptor that opens the sodium-potassium channel of the nicotinic receptor. This allows 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. Following depolarization, the acetylcholine molecules are then removed from the end plate region and enzymatically hydrolysed by acetylcholinesterase.
Normal end plate function can be blocked by two mechanisms. Nondepolarizing agents like tubocurarine block the agonist, acetylcholine, from binding nicotinic receptors and activating them, thereby preventing depolarization. Alternatively, depolarizing agents such as succinylcholine are nicotinic receptor agonists which mimic Ach, 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.These neuromuscular blocking drugs are structurally similar to acetylcholine, the endogenous ligand, in many cases containing two acetylcholine molecules linked end-to-end by a rigid carbon ring system, as in pancuronium..
The generation of the neuronal signals in motor neurons that cause muscle contractions are dependent on the balance of synaptic excitation and inhibition that the motor neuron receives. Spasmolytic agents generally work by either enhancing the level of inhibition, or reducing the level of excitation. Inhibition is enhanced by mimicking or enhancing the actions of endogenous inhibitory substances, such as GABA.
Most sources still use the term "centrally acting muscle relaxant". According to MeSH, dantroline is usually classified as a centrally acting muscle relaxant. The World Health Organization, in its ATC, uses the term "centrally acting agents", but adds a distinct category of "directly acting agents", for dantrolene. Use of this terminology dates back to at least 1973.
The term "spasmolytic" is also considered a synonym for antispasmodic.
Muscle relaxants are thought to be useful in painful disorders based on the theory that pain induces spasm and spasm causes pain. However, there is considerable evidence to contradict this theory.
The benzodiazepines, such as diazepam, interact with the GABAA receptor in the central nervous system. While it can be used in patients with muscle spasm of almost any origin, it produces sedation in most individuals at the doses required to reduce muscle tone.
Baclofen is considered to be at least as effective as diazepam in reducing spasticity, and causes much less sedation. It acts as a GABA agonist at GABAB receptors in the brain and spinal cord, resulting in hyperpolarization of neurons expressing this receptor, most likely due to increased potassium ion conductance. Baclofen also inhibits neural function presynaptically, by reducing calcium ion influx, and thereby reducing the release of excitatory neurotransmitters in both the brain and spinal cord. It may also reduce pain in patients by inhibiting the release of substance P in the spinal cord as well.
Clonidine and other imidazoline compounds have also been shown to reduce muscle spasms by their central nervous system activity. Tizanidine is perhaps the most thoroughly studied clonidine analog, and is an agonist at α2-adrenergic receptors, but reduces spasticity at doses that result in significantly less hypotension than clonidine. Neurophysiologic studies show that it depresses excitatory feedback from muscles that would normally increase muscle tone, therefore minimizing spasticity. Furthermore, several clinical trials indicate that tizanidine has a similar efficacy to other spasmolytic agents, such as diazepam and baclofen, with a different spectrum of adverse effects.
The hydantoin-derivative dantrolene is a spasmolytic agent with a unique mechanism of action outside of the CNS. Dantrolene reduces skeletal muscle strength by inhibiting the excitation-contraction coupling in the muscle fiber. In normal muscle contraction, calcium is released from the sarcoplasmic reticulum through the ryanodine receptor channel, which causes the tension-generating interaction of actin and myosin. Dantrolene interferes with the release of calcium by binding to the ryanodine receptor and blocking the endogenous ligand ryanodine by competitive inhibition. Muscle that contracts more rapidly is more sensitive to dantrolene than muscle that contracts slowly, although cardiac muscle and smooth muscle are depressed only slightly, most likely because the release of calcium by their sarcoplasmic reticulum involves a slightly different process. Major adverse effects of dantrolene include general muscle weakness, sedation, and occasionally hepatitis.