Final answer:
In the propagation of the action potential, the depolarization of the membrane provides the stimulus to open the next channel.
Step-by-step explanation:
In the propagation of the action potential, the stimulus to open the next channel is provided by the depolarization of the membrane.
Once the membrane reaches the threshold voltage of -55 mV, voltage-gated Na+ channels open and allow the entry of sodium ions into the cell. This causes further depolarization and initiates the action potential.