The effect

The effect of muscle relaxants also differs depending on the group of muscle relaxants. The peripheral muscle relaxants act on the skeletal muscles. Skeletal muscles are those muscles that can be moved at will – such as lifting an arm.

Peripheral muscle relaxants can in turn be divided into two classes. There are peripheral muscle relaxants that have a depolarizing effect. For this purpose, the drug binds to receptors of the motor end plate, a contact point between a motor neuron and a muscle cell.

This binding triggers a contraction, a depolarization, of the muscle cell. However, since the drug cannot be broken down by the enzymes, the muscle cell is not excitable and the cell becomes flaccid. The best known depolarizing drug is succinylcholine.

It is also the only drug in this class that is still used in human medicine. Succinylcholine unfolds its effect quite quickly, in about 40 – 60 seconds, and acts for about 5 – 10 minutes. Besides depolarizing muscle relaxants, there are also non-depolarizing muscle relaxants.

Their effect occurs within 2 – 5 minutes and can be reversed by special medication (e.g. neostigmine). They also bind to receptors of the motor end plate but do not lead to depolarization. Known depolarizing muscle relaxants are Mivacurium, Rocuronium and Atracurium.

Centrally acting muscle relaxants exert their effect in the brain and spinal cord.Different classes are available – not all central muscle relaxants act on the same nerve cells or through the same mechanism. However, most of them have a dampening effect on the brain, so that muscle tone is reduced. Some drugs, such as tizanidine, can also bind directly to receptors in the spinal cord and relieve muscle tension there.

The side effect

Depolarizing muscle relaxants, especially succinylcholine, can cause muscle cells to break down. In extreme cases, rhabdomyolysis can be triggered. Rhabdomyolysis describes a disintegration of the muscles with release of muscle proteins and other components.

This can lead to various complications; in extreme cases, kidney failure can occur. Furthermore, electrolyte disorders and consequently cardiac arrhythmia can be caused. Furthermore, succinylcholine is a trigger substance of malignant hyperthermia.

Malignant hyperthermia is a dreaded complication in anesthesia. It leads to cardiac arrhythmia, an increase in body temperature and further organ damage. Due to these side effects, succinylcholine is rarely used anymore.

Non-depolarizing muscle relaxants can lead to apnea, respiratory paralysis. Some drugs in this class, such as pancuronium, can cause cardiac arrhythmia. Mivacurium, on the other hand, can cause a release of histamine up to anaphylactic reactions.

Centrally acting muscle relaxants have different side effects depending on the drug. Due to their inhibitory effect on the nervous system, however, most drugs have a fatiguing effect. They can also affect concentration and coordination.

In the case of methocarbamol, however, this effect seems to be less pronounced. Tizanidine is known to cause blood pressure problems. Especially when changing from lying down to standing or during sudden exertion, blood pressure can drop.

Some muscle relaxants have a potential for dependence. Benzodiazepines are particularly affected. Other typical side effects of muscle relaxants are non-specific complaints such as dizziness, headaches or nausea.