Generalities and classification
The muscle relaxants - as can easily be deduced from their name - are drugs used in the clinical setting to promote relaxation of the musculature, both skeletal and smooth.
Muscle relaxants are available in the form of various pharmaceutical formulations for oral, parenteral and even topical use.
Basically, muscle relaxants can be divided into two large groups, depending on the mechanism through which they exert the musculature release action:
- Central action muscle relaxants;
- Peripheral muscle relaxants.
Central action muscle relaxants
The centrally acting muscle relaxants exert their activity by acting directly at the level of the central nervous system.
Generally, this type of muscle relaxants is used for the treatment of muscle contractures and spasticity, which can be associated with traumas, spinal cord diseases of various origins and nature, cerebral pathologies, autoimmune diseases (such as, for example, multiple sclerosis), spinal disorders, degenerative diseases and tumors.
The active ingredients belonging to this category of muscle relaxants are different and have different mechanisms of action.
Below, some of these drugs will be briefly described.
eperisone
Eperisone (Expose®) is a muscle relaxant used to counteract muscle spasticity resulting from degenerative, traumatic or neoplastic neurological disorders.
The muscle relaxant action of the eperisone is due to its ability to inhibit the spontaneous discharge of the gamma motor neurons present at the spinal level.
Furthermore, the eperisone is also able to exert analgesic and vasodilatory actions.
The main undesirable effects given by eperisone are: palpitations, dizziness, headache, tremors, fatigue, asthenia, gastrointestinal disorders and skin rashes.
Thiocolchicoside
Thiocolchicoside (Muscoril®, Miotens®) is a centrally acting muscle relaxant that is mainly used in the treatment of painful muscle contractures associated with acute spinal conditions.
Thiocolchicoside exerts its activity through a complex mechanism of action, which involves the antagonization of the GABA-A receptor.
Thiocolchicoside can cause side effects on the gastrointestinal tract, as it may promote the onset of drowsiness.
Baclofen
Baclofen (Lioresal®) is a muscle relaxant used in the treatment of spastic hypertonia associated with multiple sclerosis, to spinal cord tumors, traumatic, infectious or degenerative (such as, for example, spastic spinal paralysis, transverse myelitis, traumatic paraplegia, amyotrophic lateral sclerosis, etc.) and to cerebral pathologies, such as childhood encephalopathies, cerebral vasculopathies and tumors.
Baclofen performs its muscle relaxant activity by exerting an agonist action against the GABA-B receptor, thanks to which control of the release of some excitatory neurotransmitters is obtained, thus causing a decrease in muscle tone.
The main undesirable effects that may arise following the use of baclofen are: sedation, drowsiness, respiratory depression, headache, euphoria insomnia, confusion, gastrointestinal disorders and tremors.
Zanaflex
Tizanidine (Sirdalud®) is a muscle relaxant used both in the treatment of painful muscle spasms associated with disorders of the spine and surgery, and in the treatment of muscle spasticity caused by diseases of different nature.
Tizanidine is a central agonist of the α2 adrenergic receptor, therefore, it exerts its myorelaxant action through the inhibition of noradrenaline release from spinal interneurons.
The side effects that can occur after taking tizanidine consist of: bradycardia, hypotension, drowsiness, dizziness, insomnia and sleep disorders, fatigue and muscle weakness.
Peripheral muscle relaxants
Peripheral muscle relaxants are drugs that - as one can easily guess - exert their action at the level of the peripheral nervous system.
More specifically, these active ingredients exert a neuromuscular blocking action through interaction with nicotinic acetylcholine receptors.
Generally, these muscle relaxants are used as adjuvants in the anesthetic field, to facilitate surgical procedures, but also to facilitate the execution of some types of invasive diagnostic tests.
Peripheral action muscle relaxants can in turn be divided into two subgroups:
- Depolarising peripheral muscle relaxants;
- Non-depolarising peripheral muscle relaxants.
Depolarising peripheral muscle relaxants
This genus of peripheral action muscle relaxants binds to the nicotinic receptors present at the level of the neuromuscular plaque, exerting on them an agonist action that causes a depolarization of the plasma membrane of nerve cells. This depolarization leads to a consequent muscle collation. Subsequently, the cellular membrane is repolarized but is in a state of desensitization, the consequence of which is relaxation of the musculature.
Succinylcholine belongs to this category of muscle relaxants. This active ingredient has a short duration of action and is used in cases where it is necessary to induce rapid neuromuscular blockade for relatively short periods. In fact, succinylcholine is often used to facilitate certain types of endoscopic examinations.
Furthermore, this drug can also be used in combination with anesthetic drugs during surgery and to facilitate tracheal intubation.
Non-depolarising peripheral muscle relaxants
These muscle relaxants do not depolarize nerve cell membranes, so they do not induce muscle fasciculation before producing the relaxation effect on smooth muscle. In fact, these molecules - unlike depolarizing muscle relaxants - exert a competitive antagonist action against the nicotinic receptors present at the level of the neuromuscular plaque.
These active ingredients are mainly used in the surgical field, in order to obtain an adequate relaxation of the muscles during the operations.
Active ingredients belonging to this category of muscle relaxants, such as:
- Atracurium (Acurmil®, Tracrium®), with a slightly longer duration of action than that of succinylcholine;
- Vecuronium (Norcuron®), this active ingredient, on the other hand, has an intermediate duration of action. Furthermore, compared to other non-depolarising muscle relaxants, it does not induce histamine release and has less cardiovascular effects;
- Rocuronium (Esmeron®), this muscle relaxant has a duration of action similar to that of vecuronium but, compared to the latter, it has a more rapid onset of action. Like vecuronium, rocuronium does not induce significant histamine release.
Side effects of peripheral action muscle relaxants
Naturally, the type of side effects that may occur following the use of peripheral action muscle relaxants varies depending on the active ingredient used and on the sensitivity of each patient to the drug itself.
However, many of these muscle relaxants - but not all - cause hypotension, heart disease and bronchospasm.
Furthermore, as mentioned, some peripheral muscle relaxants are also able to promote the release of histamine, with consequent occurrence of undesirable effects on the skin (itching, hives, wheals, erythema), of the respiratory tract (asthma, bronchospasm) and the cardiovascular system.
