Question

When voltage-gated Na+ channels in a nerve cell open, what happens to the axonal membrane?

A. No change in the membrane potential occurs.
B. It becomes electrically charged.
C. It depolarizes.
D. The membrane potential rises to 0 mV and stays there.
E. The nerve cell becomes more negatively charged inside than outside.

Answer 1

When voltage-gated Na+ channels open, the axonal membrane depolarizes. Na+ ions enter the neuron, causing the membrane potential to become less negative, moving towards and surpassing zero to about +30 mV. Subsequently, the Na+ channels close and K+ channels open to repolarize the membrane and restore the resting potential.

Step-by-step explanation:

When voltage-gated Na+ channels in a nerve cell open, the axonal membrane depolarizes. This means that Na+ ions rush into the neuron due to the concentration gradient, as there is a higher concentration of Na+ outside the cell than inside. The influx of these positively charged ions causes the membrane potential to become more positive, moving from the resting potential of approximately -70 mV closer to zero and eventually reaching about +30 mV, known as the peak of the action potential. This is a part of the all-or-nothing event where the neuron fires, leading to the transmission of the nerve impulse.

Following the depolarization, the membrane needs to reset its voltage to the resting potential, and to accomplish this, the Na+ channels close and the K+ channels open, allowing K+ to exit the cell and bring the membrane potential back to a negative value. This is known as repolarization. This sequence of opening and closing ion channels and the resulting changes in membrane potential is what constitutes the action potential and allows for the propagation of the nerve impulse down the axon.