Non depolarising agents occupy the nicotinic receptors at the motor end plate, hence prevent Ach from binding to the receptor. Their effect can be overcomed by increasing Ach, eg by using cholinesterase inhibitors.
Depolarising blockers act in two phases:
phase one: they act like nicotinic agonists, depolarising the motor end plate. (this is also responsible for the flickering and twitching observed with these agents).As these agents are not effectively metabolised at the synapse, the membrane remains depolarised and thus unresponsive to further impulses. Finally this phase results in flaccid paralysis b/c muscle tension can not be sustained without repolarisation. This phase is not reversed by cholinesterase inh, infact its augmented.
phase two:this phase is similar to non depolarising blockade. The membrane gradually repolarises, but is refractory to stimulation b/c its desensitised. And the blockade can be reversed by cholinesterase inh.