3.The reduced potential causes hundreds of a) channels to open on that part of the cell membrane. The depolarization of the cell causes more b) channels to open in adjacent parts of the cell membrane. This begins the wave of c) moving down the axon. Depolarization begins at the d).

The reduced potential causes hundreds of voltage-gated sodium channels  to open on that part of the cell membrane. The depolarization of the cell causes more of voltage-gated sodium channels to open in adjacent parts of the cell membrane. This begins the wave of of depolarization  moving down the axon. Depolarization begins at the axon hillock.

Explanation:

When there is no neuron signaling it becomes polarized, termed as resting membrane potential (RMP) at a threshold voltage (around -55 mV), due to the action of the sodium-potassium pump and the potassium leak channels.  

When a change in the RMP occurs, depolarization takes place which causes the voltage-gated sodium channels to open and sodium ions rush into the nerve cell which in turn will increase the voltage threshold to nearly around +40 mV and also charges the neuron positive. This depolarization moves down the axon. This increase in threshold stops the sodium influx and opens the potassium channels to rush the potassium out of the cell.

All these actions decrease the membrane potential leading to a wave of depolarization and going back to resting state. Depolarization begins depending upon the potential gradient at the axon hillock.

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