Nondepolarizing (Competitive) Neuromuscular Blockers: Pharmacological Actions

Nondepolarizing neuromuscular blockers inhibit muscle contraction by preventing the depolarization of muscle cell membranes. When administered at low doses, they compete with acetylcholine, a neurotransmitter, to bind to its receptor site. So they are also known as competitive blockers.

They are usually given with anesthetics during surgeries to produce complete muscle relaxation. Their collective action allows a lower dosage of anesthetics to achieve the desired effect, decreasing the risks associated with general anesthesia.

When a competitive blocker is injected, small, rapidly moving facial and hand muscles are paralyzed first, followed by the limb, neck and trunk muscles. Next, larger muscles, like the intercostal muscles, are affected. The last muscle to paralyze is the diaphragm.

When the drug administration is stopped, these muscles recover rapidly in the reverse order, with the diaphragm recovering first and facial muscles last.

Most competitive blockers cannot penetrate the blood-brain barrier but can affect the autonomic ganglia. Such drugs, like atracurium, release histamine into the systemic circulation, causing a drop in blood pressure, flushing, and bronchospasm.