Pharmacology of the non-depolarising muscle relaxants

Non-depolarising muscle relaxant drugs compete with acetylcholine (ACh) molecules released at the neuromuscular junction to bind with the ACh receptors on the post-synaptic membrane of the motor endplate. They therefore block the action of ACh and prevent depolarisation. Non-depolarising muscle relaxant drugs also act on presynaptic receptors interfering with the entry of calcium, which causes an inhibition of the release of ACh. Other drugs such as the aminoglycoside antibiotics (e.g. gentamicin) and volatile agents may also affect this mechanism and increase sensitivity to relaxants.

None of the drugs cross the blood-brain barrier, as they are water soluble, polar molecules and therefore have no effect on the central nervous system. All non-depolarising drugs should be used with care in patients suspected to be suffering with myasthenia gravis or myasthenic syndrome, as patients with these conditions are extremely sensitive to their effects.