Muscle relaxants

Muscle relaxants are the drugs that cause the effect of relaxing the muscle tissue. The use of these drugs can reduce pain caused by muscle spasms. An important property of muscle relaxants is their ability to block the reflex activity of the voluntary muscles, which is very useful in anesthesiology and surgery, as the muscle tone is often a hindrance during surgery and intubation.

Classification of muscle relaxants: relaxants involve the peripheral and central actions.

Muscle relaxants peripherally acting drugs include curariform directly effect on cholinergic receptors of skeletal muscles and disrupt neuromuscular transmission.

The second group (central action) muscle relaxants include drugs from various groups that have a depressant effect on the central structure of the regulation of muscle tone, along with other effects. Similar property is possessed by, for example, tranquilizers. Among tranquilizers, the most pronounced effect is the ability to cause muscle relaxation due to the presence of substance – benzodiazepines (such as hlozepid, diazepam, phenazepam). Tranquilizers of certain other chemical structure can cause muscle relaxation too – meprotan izoprotan and midokalm and baclofen.

Unlike curariform muscle relaxants, centrally acting muscle relaxants do not affect the transmission of impulses in the nerve-muscle synapses. They inhibit the activity of neurons of the spinal cord reflex paths and locate above the nervous system.

Curariform substances are used in surgical interventions, correcting dislocations, repositioning of bone fragments etc.

The side effects of muscle relaxants include drowsiness, dizziness, and lethargy. Prolonged use can develop addiction and drug dependence. Not recommended for use amongst those occupations that require considerable mental reaction and high coordination.

Muscle relaxants have become an important component of modern balanced anesthesia. Most modern muscle relaxants do not cause significant side effects (especially Vecuronium and Cisatracurium). The use of muscle relaxants involves some difficulties, including fluctuations in duration, accumulation of certain diseases, and inadequate termination. Some muscle relaxants cause side effects (histamine release). Their severity, however, is less than of most new drugs. Muscle relaxants are unique in the sense that direct and accurate monitoring of their action is possible. In conclusion, it should be recalled that the use of muscle relaxants masks important symptoms of inadequate anesthesia and increases the frequency of intraoperative recovery of consciousness. Therefore, muscle relaxants should be used only when it is absolutely necessary, and be limited to the minimum of effective dose.

A specific opioid antagonist naloxone has both therapeutic and adverse affect. It works within 1-2 min, but has a short duration of action. The duration of action of naloxone is usually shorter than the above introduced opioid. Consequently, after a while, the opioid’s action can finish, and then naloxone should be introduced again. Since naloxone eliminates the effects of opioids, it should be administered very slowly. With this approach, naloxone received a degree of analgesia. An alternative tactic involves the use of respiratory stimulants (eg, Doxapram), which are rapidly eliminated due to respiratory depression of opioids without compromising analgesia. Careful fractional administration of opioids to achieve the desired analgesia reduces the risk of overdose and the need for antagonists.

Cyclobenzaprine Hydrochloride

The following terms have been used as ‘street names’ or slang names for various forms of cyclobenzaprine hydrochloride: Cyclo; Cyclone. A white to off-white, odourless, crystalline powder. Freely soluble in water, in alcohol, and in methyl alcohol; sparingly soluble in isopropyl alcohol; slightly soluble in chloroform and in dichloromethane; insoluble in hydrocarbons.

Tizanidine Hydrochloride

Tizanidine hydrochloride may cause drowsiness; patients affected should not drive or operate machinery. Other adverse effects include dry mouth, fatigue, dizziness or vertigo, muscle pain and weakness, insomnia, anxiety, headache, bradycardia, nausea, and gastrointestinal disturbances.


Slightly soluble in water; very slightly soluble in alcohol; practically insoluble in acetone; dissolves in dilute mineral acids and in dilute solutions of alkali hydroxides. A white to off-white, odourless or practically odourless, crystalline powder.

Botulinum Toxins

Botulinum toxins A and B are neurotoxins produced by Clostridium botulinum. They are proteins comprising a heavy chain thought to be responsible for binding to the target cells and translocation of the toxin across the cell membrane, linked by a disulfide bond to a light chain responsible for the toxic activity.

Muscle Relaxants

The muscle relaxants included in this chapter are used in the management of musculoskeletal and neuromuscular disorders. Their mechanism of action may be due to their CNS-depressant activity.


A white or practically white, practically odourless, crystalline powder. Slightly soluble in water; sparingly soluble in alcohol, in isopropyl alcohol, and in methyl alcohol; soluble in solutions of alkali hydroxides and ammonia.


Very slightly soluble in water; freely soluble in alcohol, in acetone, and in dichloromethane. A white crystalline powder having a mild characteristic odour. Soluble 1 in 2083 of water, 1 in 2.5 of alcohol and of acetone, and 1 in 2.3 of chloroform.


A white powder, odourless or having a slight characteristic odour. M.p. about 94° or, if previously ground to a fine powder, about 90°. Soluble 1 in 40 of water at 20°; sparingly soluble in chloroform; soluble in alcohol only with heating; insoluble in n-hexane and in benzene.

Few Muscle Relaxants

Afloqualone is a centrally acting skeletal muscle relaxant that has been given orally for the treatment of muscle spasm associated with musculoskeletal conditions. Photosensitivity reactions have been reported.

Dantrolene Sodium

A yellowish-orange to orange crystalline powder. Very slightly soluble in water; slightly soluble in alcohol; practically insoluble in acetone; sparingly soluble in methyl alcohol. A fine orange to orange-brown powder.

Chlorphenesin Carbamate

Chlorphenesin carbamate produces drowsiness and dizziness. There may also be nausea, headache, weakness, confusion, agitation, and insomnia.