Mechanism of Action

Mechanism of Action and Clinical Uses of Neuromuscular Blocking Agents

Mechanism of Action

Muscle relaxants work at the neuromuscular junction. They block transmission of nerve impulses, resulting in muscular relaxation and paralysis. During the course of normal muscle stimulation a wave of electrical impulses passes to a nerve ending in individual muscle fibers. At the neuromuscular junction the electrical impulse causes the release of acetylcholine from the nerve ending. Acetylcholine crosses the junction to continue the movement to occupy receptors at the motor end plate of the electrical impulse, resulting in muscle contraction. The released acetylcholine is hydrolyzed fairly quickly by cholinesterase found in the region of the motor end plate.

Neuromuscular blocking drugs (NMBDs) interrupt transmission of nerve impulses at the neuromuscular junction (NMJ) (Figure 3.1) and thereby produce paresis or paralysis of skeletal muscles. On the basis of electrophysiologic differences in their mechanisms of action and duration of action, these drugs can be classified as depolarizing NMBDs (mimic the actions of acetylcholine [ACh]) and nondepolarizing NMBDs (interfere with the actions of ACh), the latter of which are further subdivided into long-, intermediate-, and short-acting drugs. Succinylcholine (SCh) is the only depolarizing NMBD used clinically. It is also the only NMBD that has both a rapid onset and ultra short duration of action. Among the nondepolarizing NMBDs, rocuronium's onset time most closely resembles that of SCh. (insert the indication of each figure No on the description)

Skeletal muscle relaxation can be produced by deep inhalational anesthesia, regional nerve block, or neuromuscular blocking agents.

Clinical Uses

The principal clinical uses of NMBDs are to produce skeletal muscle relaxation for facilitation of tracheal intubation and to provide optimal surgical working conditions. Muscle relaxants do not affect the patient's consciousness or sensation. They should not be given to a conscious patient who is not under anesthesia. Muscle relaxants should not be given to any patient that you may not be able to ventilate or intubate. An inadequately anesthetized, but paralyzed patient is a major risk for awareness during general anesthesia.