Question 4

The graph shows the changes in membrane potential during an action potential in a mammalian neurone.

Action potential curve with labeled positions

Which row in the table correctly describes the states of the voltage-gated channels and the primary ionic movement occurring at the specified position on the graph?

	Position	Voltage-gated $Na+\text{Na}^+$ channels	Voltage-gated $K+\text{K}^+$ channels	Primary ionic movement across the membrane
A	W	Open	Closed	$Na+\text{Na}^+$ entering the axon via active transport
B	X	Open	Open	Equal diffusion of $Na+\text{Na}^+$ and $K+\text{K}^+$ in opposite directions
C	Y	Closed	Open	$K+\text{K}^+$ leaving the axon via facilitated diffusion
D	Z	Closed	Closed	$Na+\text{Na}^+$ leaving the axon via simple diffusion

Position W: Voltage-gated $Na+\text{Na}^+$ channels are Open, Voltage-gated $K+\text{K}^+$ channels are Closed, and $Na+\text{Na}^+$ enters the axon via active transport

Position X: Voltage-gated $Na+\text{Na}^+$ channels are Open, Voltage-gated $K+\text{K}^+$ channels are Open, with equal diffusion of $Na+\text{Na}^+$ and $K+\text{K}^+$ in opposite directions

Position Y: Voltage-gated $Na+\text{Na}^+$ channels are Closed, Voltage-gated $K+\text{K}^+$ channels are Open, and $K+\text{K}^+$ leaves the axon via facilitated diffusion

Position Z: Voltage-gated $Na+\text{Na}^+$ channels are Closed, Voltage-gated $K+\text{K}^+$ channels are Closed, and $Na+\text{Na}^+$ leaves the axon via simple diffusion