Question:
The magnitude of an action potential (AP) is measured at the initial segment using an electrode and then measured again 2 mm farther down the axon. Compared to the AP measured at the initial segment, the AP 2 mm farther down the axon is:
- smaller
- of the same magnitude
- completely nullified
- larger
Answer:
Correct Option: of the same magnitude
Detailed Explanation:
To understand why the magnitude remains the same, we must explore how action potentials propagate along the axon of a neuron.
Key Principles of Action Potential Propagation:
- All-or-None Principle: An action potential either occurs fully or not at all. Once triggered, it proceeds completely without partial activation.
- Active Regeneration: The action potential is regenerated at each segment of the axon using voltage-gated ion channels. This ensures that the signal does not weaken over distance.
- Constant Magnitude: Unlike passive electrical signals (like EPSPs or IPSPs) that fade with distance, action potentials maintain their magnitude due to this regenerative mechanism.
Why It Remains the Same:
When an action potential begins at the axon initial segment, it travels down the axon. Along the way, voltage-gated Na⁺ and K⁺ channels sequentially open and close, creating a full-strength action potential at each new segment.
This process ensures that the action potential is not simply a one-time passive current that decays, but rather an actively propagated electrical signal that maintains its full strength regardless of how far it travels, as long as the axon is intact.
Conclusion:
Therefore, the magnitude of the action potential measured 2 mm down the axon will be of the same magnitude as the one measured at the initial segment.