Question
Changes in membrane potential during nerve transmission along an axon involve the movement of sodium and potassium ions across the axon membrane.
What explains the membrane potential at Y in the trace?
A. Potassium channels open and allow potassium ions to move to the outside of the axon membrane.
B. Potassium channels allow potassium ions to enter the axon.
C. When the threshold voltage is reached, the sodium pump begins to pump sodium ions to the outside of the axon membrane.
D. Potassium and sodium channels are closed, so there is no movement of ions across the axon membrane.
▶️Answer/Explanation
Correct Answer: A. Potassium channels open and allow potassium ions to move to the outside of the axon membrane.
Explanation:
In the graph, point Y represents the repolarization phase of the action potential, which occurs after the membrane has reached its peak depolarization (+40 mV).
What happens at point Y?
- After an action potential peak, voltage-gated sodium (Na⁺) channels close.
- Voltage-gated potassium (K⁺) channels open, allowing K⁺ ions to exit the axon.
- This outward movement of positively charged K⁺ ions causes the membrane potential to become more negative, returning toward the resting potential — this is repolarization.
Why the other options are incorrect:
B. Potassium channels allow potassium ions to enter the axon → Incorrect: K⁺ ions move out, not in.
C. Sodium pump begins to pump sodium ions out → The sodium-potassium pump helps maintain resting potential over time, but it is not responsible for the rapid change in voltage at Y.
D. Both channels are closed → Incorrect: Potassium channels are open at Y; this is what drives repolarization.
Question
The graph shows changes in the membrane potential in an action potential.
What is the approximate value of the threshold potential?
A. −88 mV
B. −70 mV
C. −50 mV
D. +45 mV
▶️Answer/Explanation
Answer: C. −50 mV
Explanation:
The threshold potential is the critical level to which the membrane potential must be depolarized to initiate an action potential.
In the graph:
- The resting potential appears to be around −70 mV.
- The curve begins to rise steeply (depolarization) around −50 mV, which marks the threshold potential — the point where voltage-gated sodium channels open, causing a rapid influx of Na⁺ and the spike in membrane potential.
Evaluation of Incorrect Options:
A. −88 mV
Too negative — this is more hyperpolarized than the resting state.
B. −70 mV
This is the resting potential, not the threshold.
D. +45 mV
This is close to the peak of the action potential, not the threshold.
Question
The image shows a neuron.
What is the function of X?
A. Increases the speed of transmission along the axon
B. Increases the rate of exchange of sodium and potassium ions
C. Holds bundles of neurons together to form a nerve
D. Determines the direction of the action potential
▶️Answer/Explanation
Answer: A. Increases the speed of transmission along the axon
Explanation:
In the diagram, X is pointing to a myelin sheath, which wraps around the axon of the neuron in segments, leaving small gaps called nodes of Ranvier.
Function of the Myelin Sheath (X):
- Acts as an electrical insulator
- Enables saltatory conduction: the action potential jumps from one node of Ranvier to the next
- Significantly increases the speed of electrical impulse transmission along the axon
Why the other options are incorrect:
B. Increases the rate of exchange of sodium and potassium ions
Ion exchange happens at the nodes of Ranvier, not where the myelin sheath is.
C. Holds bundles of neurons together to form a nerve
That’s the role of connective tissue, not the myelin sheath.
D. Determines the direction of the action potential
The axon hillock and synaptic structure determine direction; myelin affects speed, not direction.
Question
How does potassium move across the membrane of a neuron during repolarization?
A. Simple diffusion
B. Facilitated diffusion
C. Endocytosis
D. Active transport
▶️Answer/Explanation
Answer: B. Facilitated diffusion
Explanation:
During repolarization, potassium ions (K⁺) move out of the neuron through potassium ion channels. This movement is driven by the concentration gradient, so potassium moves from inside the neuron (where K⁺ concentration is high) to outside (where it is lower). Since the ions pass through specific protein channels, this is called facilitated diffusion. It does not require energy (ATP) because it moves down the concentration gradient.
Options Evaluation:
A. Incorrect – Simple diffusion refers to movement directly through the lipid bilayer without protein channels, but potassium ions are charged and require channels to pass.
B. Correct – Facilitated diffusion occurs through ion channels without energy use.
C. Incorrect – Endocytosis is a process of engulfing substances by the membrane, not ion movement.
D. Incorrect – Active transport requires energy and moves ions against the concentration gradient, which is not what happens during repolarization.