text: A depolarizing neuromuscular blocking agent is a form of neuromuscular blocker that depolarizes the motor end plate. An example is succinylcholine. Depolarizing blocking agents work by depolarizing the plasma membrane of the muscle fiber, similar to acetylcholine.  However, these agents are more resistant to degradation by acetylcholinesterase, the enzyme responsible for degrading acetylcholine, and can thus more persistently depolarize the muscle fibers. This differs from acetylcholine, which is rapidly degraded and only transiently depolarizes the muscle.
  There are two phases to the depolarizing block. During phase I (depolarizing phase), succinylcholine interacts with nicotinic receptor to open the channel and cause depolarization of the end plate, which later spread to and result in depolarization of adjacent membranes. As a result, there is disorganisation of contraction of muscle motor unit. This causes muscular fasciculations (muscle twitches) while they are depolarizing the muscle fibers.  Eventually, after sufficient depolarization has occurred, phase II (desensitizing phase) sets in and the muscle is no longer responsive to acetylcholine released by the motoneurons. At this point, full neuromuscular block has been achieved. Phase I block effect can be increased by cholinesterase inhibitors which further delay the action of metabolism and removal by cholinesterase.
  Under continuous exposure to succinylcholine, the initial end plate depolarization is reduced, and repolarisation process is initiated. As a result of the widespread sustained depolarization the synapses ultimately begin repolarization. Once repolarized, the membrane is still less susceptible to additional depolarization (phase II block).
  The prototypical depolarizing blocking drug is succinylcholine (suxamethonium). It is the only such drug used clinically. It has a rapid onset (30 seconds) but very short duration of action (5–10 minutes) because of hydrolysis by various cholinesterases (such as butyrylcholinesterase in the blood). The patient will experience fasciculation due to the depolarisation of muscle neurone fibres and seconds later, flaccid paralysis will occur. Succinylcholine was originally known as diacetylcholine because structurally it is composed of two acetylcholine molecules joined with a methyl group. Decamethonium is sometimes, but rarely, used in clinical practice.
  It is indicated for rapid sequence intubation.