Each set of Na channels that opens causes the opening of a nearby channel, and this event spreads down the length of the nerve. The nerve must repolarize after it has depolarized, in order for it to conduct another impulse. The Na and K contents have to be returned to normal by the pump. This requires some time, and a nerve cannot transmit two impulses in very fast succession. The time required is called the refractory period.
Refractory refers to the time the nerve is unresponsive.
We will study this part of the process as well. In the tip of the pre-synaptic axon the transmitter is synthesized and placed into vesicles. The vesicles wait in the axon tip until an impulse arrives. At this, the contents of the vesicle are released to the synapse. This causes depolarization and firing of the cell.