Generalities and classification
Muscle relaxants - as can be easily deduced from their name - are drugs used in a clinical setting to promote the relaxation of both skeletal and smooth muscles.
Muscle relaxants are available in the form of several 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 muscle release action:
- Central action muscle relaxants;
- Muscle relaxants with peripheral action.
Central action muscle relaxants
Centrally acting muscle relaxants perform 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 trauma, spinal cord pathologies of various origins and nature, brain pathologies, autoimmune pathologies (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.
Some of these drugs will be briefly described below.
Eperisone
Eperisone (Expose®) is a muscle relaxant used to counteract muscle spasticity resulting from degenerative, traumatic or neoplastic neurological pathologies.
The muscle relaxant action of eperisone is due to its ability to inhibit the spontaneous discharge of gamma motor neurons present in the spinal area.
Furthermore, eperisone is also capable of exerting analgesic and vasodilating actions.
The main undesirable effects of eperisone are: palpitations, dizziness, headache, tremors, fatigue, asthenia, gastrointestinal disturbances 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 conditions of the spine.
Thiocolchicoside exerts its activity through a complex mechanism of action, which involves antagonization of the GABA-A receptor.
Thiocolchicoside can cause gastrointestinal side effects, as well as it can promote drowsiness.
Baclofen
Baclofen (Lioresal®) is a muscle relaxant used in the treatment of spastic hypertonia associated with multiple sclerosis, spinal cord pathologies of a tumor, traumatic, infectious or degenerative nature (such as, for example, spastic spinal paralysis, transverse myelitis, traumatic paraplegia , amyotrophic lateral sclerosis, etc.) and brain pathologies, such as childhood encephalopathies, cerebral vasculopathies and tumors.
Baclofen carries out its activity as a muscle relaxant by exerting an "agonist action against the GABA-B receptor, thanks to which the release of some excitatory neurotransmitters is controlled, 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, insomnia, euphoria, confusion, gastrointestinal disturbances and tremors.
Tizanidine
Tizanidine (Sirdalud®) is a muscle relaxant used both in the treatment of painful muscle spasms associated with disorders of the spine and in surgery, and in the treatment of muscle spasticity caused by various pathologies.
Tizanidine is a central agonist of the α2 adrenergic receptor, therefore, it exerts its muscle relaxant action by inhibiting the release of noradrenaline from spinal interneurons.
The side effects that can occur following the intake of tizanidine consist of: bradycardia, hypotension, somnolence, dizziness, insomnia and sleep disturbances, fatigue and muscle weakness.
Muscle relaxants with peripheral action
Muscle relaxants with peripheral action are drugs that - as you can easily guess - exert their action at the level of the peripheral nervous system.
More specifically, these active principles exert a neuromuscular blocking action through the interaction with the 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.
Peripherally acting muscle relaxants can in turn be divided into two subgroups:
- Depolarizing muscle relaxants with peripheral action;
- Nondepolarizing muscle relaxants with peripheral action.
Depolarizing muscle relaxants with peripheral action
This kind of peripherally acting muscle relaxants binds to nicotinic receptors present in the neuromuscular plate, exerting on them an "agonist action that causes a depolarization of the plasma membrane of the nerve cells. This depolarization leads to a consequent muscle fasciculation. Later," the cell membrane repolarizes but is in a state of desensitization whose consequence is precisely the relaxation of the muscles.
Succinylcholine belongs to this category of muscle relaxants. This active principle has a short duration of action and is used in case it is necessary to induce a rapid neuromuscular block for relatively short periods. In fact, succinylcholine is often used to facilitate some types of endoscopic examinations.
In addition, this drug can also be used in combination with anesthetic drugs during surgery and to facilitate tracheal intubation.
Nondepolarizing muscle relaxants with peripheral action
These muscle relaxants do not depolarize the membrane of nerve cells, so they do not induce muscle fasciculation before producing the relaxing 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 plate.
These active ingredients are mostly used in surgery, in order to obtain adequate muscle relaxation during operations.
Active ingredients belong to this category of muscle relaxants, such as:
- Atracurium (Acurmil®, Tracrium®), with a duration of action slightly higher than that of succinylcholine;
- Vecuronium (Norcuron®), this active principle, on the other hand, has an intermediate duration of action. Furthermore, compared to other non-depolarizing muscle relaxants, it does not induce the release of histamine and has fewer 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. Similarly to vecuronium, rocuronium does not induce a significant release of histamine. .
Side effects of peripherally acting muscle relaxants
Naturally, the type of side effects that can occur following the use of peripheral muscle relaxants varies according to the active ingredient used and according to the sensitivity of each patient towards the drug itself.
However, many of these muscle relaxants - but not all - cause hypotension, heart disease, and bronchospasm.
Furthermore, as mentioned, some peripherally acting muscle relaxants are also able to promote the release of histamine, resulting in the onset of undesirable effects in the skin (itching, hives, wheals, erythema), in the respiratory tract (asthma, bronchospasm) and the cardiovascular system.
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