Final answer:
The voltage-gated sodium channel is responsible for initiating the depolarization of a neuron's membrane during an action potential, upon reaching the threshold potential.
Step-by-step explanation:
The channel that initiates the depolarization during the action potential, after reaching the threshold, is the voltage-gated sodium channel. When a neuron's membrane is stimulated and the membrane potential reaches approximately -55 mV, these sodium channels open, allowing Na+ ions to flood into the cell. This influx of positive ions causes the membrane potential to rise rapidly, initiating the action potential.
Following this influx, voltage-gated potassium channels open to allow K+ ions to flow out of the cell, which helps to repolarize the membrane and end the action potential. The entire process is vital for the conduction of electrical signals along neurons.