The Movement of Na+ ions from Extracellular Fluid to Intracellular Fluid

The correct answer for the movement of ions during the propagation of a nerve impulse is option 'B', which states that the action potential results from the movement of Na+ ions from the extracellular fluid to the intracellular fluid. Let's understand this process in detail:

Resting Membrane Potential
- The resting membrane potential of a neuron refers to its electrical charge when it is not conducting an impulse.
- At rest, the inside of the neuron is negatively charged compared to the outside due to an uneven distribution of ions.
- The main ions involved in the resting membrane potential are sodium (Na+), potassium (K+), and chloride (Cl-).

Depolarization
- When a neuron receives a stimulus, the permeability of the cell membrane changes, leading to the depolarization of the neuron.
- Depolarization occurs when the voltage-gated sodium channels on the cell membrane open, allowing Na+ ions to enter the neuron.
- This movement of Na+ ions from the extracellular fluid to the intracellular fluid results in a change in the electrical charge across the membrane.
- The inside of the neuron becomes less negative, reaching a positive charge.

Action Potential
- The depolarization of the neuron triggers the generation of an action potential, which is a rapid change in the electrical charge of the neuron.
- The opening of voltage-gated sodium channels causes an influx of Na+ ions into the neuron, leading to further depolarization.
- This influx of Na+ ions creates a positive feedback loop, opening more sodium channels and allowing more Na+ ions to enter the neuron.
- As a result, the electrical charge inside the neuron rapidly becomes positive, reaching its peak value.

Repolarization
- After reaching its peak, the voltage-gated sodium channels close, and the voltage-gated potassium channels open.
- The opening of potassium channels allows K+ ions to exit the neuron, leading to repolarization.
- The movement of K+ ions from the intracellular fluid to the extracellular fluid restores the negative charge inside the neuron.

Hyperpolarization and Refractory Period
- During repolarization, the voltage-gated potassium channels remain open for a short period, causing an excessive efflux of K+ ions.
- This efflux of K+ ions briefly hyperpolarizes the neuron, making the inside of the neuron more negative than the resting membrane potential.
- This hyperpolarization creates a refractory period during which the neuron is unable to generate another action potential.

In summary, during the propagation of a nerve impulse, the action potential results from the movement of Na+ ions from the extracellular fluid to the intracellular fluid. This movement of Na+ ions leads to depolarization and the generation of an action potential, allowing the nerve impulse to travel along the neuron.