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During the propagation of a nerve impulse, the action potential results from the movement of:
A
$K^+$ ions from intracellular fluid to extracellular fluid
B
$Na^+$ ions from extracellular fluid to intracellular fluid
C
$K^+$ ions from extracellular fluid to intracellular fluid
D
$Na^+$ ions from intracellular fluid to extracellular fluid
Detailed Solution
In the resting state the axon membrane is more permeable to $K^+$ and nearly impermeable to $Na^+$; the outer surface is positively charged and the inner surface is negatively charged (polarised membrane).
The concentration of $Na^+$ is high in the extracellular fluid and low in the axoplasm.
When a stimulus is applied, the membrane at that site becomes freely permeable to $Na^+$ because the voltage-gated $Na^+$ channels open.
This leads to a rapid influx of $Na^+$ from the extracellular fluid into the intracellular fluid.
The polarity of the membrane at the site is reversed: the outer surface becomes negatively charged and the inner surface becomes positively charged. The membrane is said to be depolarised.
The electrical potential difference across the membrane at this site is called the action potential, which is in fact termed the nerve impulse.
The outward movement of $K^+$ occurs afterwards and restores the resting potential (repolarisation).
So the action potential results from the movement of $Na^+$ ions from the extracellular fluid to the intracellular fluid.
The concentration of $Na^+$ is high in the extracellular fluid and low in the axoplasm.
When a stimulus is applied, the membrane at that site becomes freely permeable to $Na^+$ because the voltage-gated $Na^+$ channels open.
This leads to a rapid influx of $Na^+$ from the extracellular fluid into the intracellular fluid.
The polarity of the membrane at the site is reversed: the outer surface becomes negatively charged and the inner surface becomes positively charged. The membrane is said to be depolarised.
The electrical potential difference across the membrane at this site is called the action potential, which is in fact termed the nerve impulse.
The outward movement of $K^+$ occurs afterwards and restores the resting potential (repolarisation).
So the action potential results from the movement of $Na^+$ ions from the extracellular fluid to the intracellular fluid.