Final answer:
Option D: During the depolarization phase of action potential development, voltage-gated sodium channels are open, allowing Na+ ions to enter the cell and initiate the action potential.
Step-by-step explanation:
During the depolarization phase of action potential development, voltage-gated sodium channels are open. This process starts when a stimulus depolarizes the neuron's membrane, increasing its voltage towards a critical threshold potential. If the threshold of -55 mV is reached, voltage-gated Na+ channels open, allowing Na+ ions to rush into the cell, which then causes the rapid depolarization phase of the action potential. This increases the membrane potential from -70 mV up to about +30 mV, leading to the generation of an action potential. The opening of these channels is the result of a positive feedback mechanism that further depolarizes the membrane. During repolarization, the Na+ channels close and cannot reopen due to the inactivation gate, leading to the restoration of the negative membrane potential.
During the depolarization phase of action potential development, voltage-gated sodium channels are open. Depolarization is the process of the membrane potential becoming more positive, and it is initiated when the threshold potential is reached. At the threshold, voltage-gated sodium channels open, allowing sodium ions to enter the cell and causing depolarization.