Questions: Action Potential: Generation and Propagation

Question 1

A neuron receives a stimulus at twice the threshold voltage. Compared to a stimulus exactly at threshold, what happens to the resulting action potential?

• A. It has twice the amplitude, reaching roughly +80 mV instead of +40 mV
• B. It has the same amplitude but propagates twice as fast along the axon
• C. It has the same amplitude and the same propagation speed
• D. It fails to propagate because excessive depolarization inactivates too many channels at once

Answer: (C)

This is the all-or-none principle: once the threshold is reached, the action potential goes to completion regardless of stimulus strength. The peak amplitude (~+40 mV) is determined by the Na⁺ equilibrium potential and channel properties, not stimulus size. A stimulus twice the threshold does not produce a larger or faster spike — it produces an identical one. Information about stimulus intensity is encoded in firing *frequency*, not spike amplitude.

Question 2

Why can't a second action potential be triggered during the absolute refractory period, even with a very strong stimulus?

• A. The membrane potential is too negative (hyperpolarized) for threshold to be reached
• B. Voltage-gated Na⁺ channels are in an inactivated state and cannot reopen regardless of membrane potential
• C. The Na⁺/K⁺-ATPase pump is actively hyperpolarizing the membrane
• D. Voltage-gated K⁺ channels are still open and prevent depolarization

Answer: (B)

The absolute refractory period is defined by Na⁺ channel inactivation. After depolarization, these channels enter a closed, inactivated state that differs from their resting closed state — they cannot be reopened by voltage until they have had time to recover. This is a physical constraint on the channel protein itself, not just a matter of voltage. Even if you artificially push the membrane potential back to −55 mV, no action potential can fire. This is why the absolute refractory period places an upper limit on firing frequency.

Question 3

A stronger stimulus to a sensory neuron produces a subjectively more intense sensation because it generates action potentials with larger amplitudes.

• T. True
• F. False

Answer: False

Action potentials obey the all-or-none principle — they are always the same amplitude. Stimulus intensity is encoded in the *frequency* and *pattern* of firing, not in spike size. A strong stimulus causes the neuron to fire more action potentials per second; a weak stimulus produces fewer. The brain reads frequency as intensity, not amplitude.

Question 4

During action potential propagation, the absolute refractory period in the previously fired axon segment ensures the signal travels in only one direction.

• T. True
• F. False

Answer: True

When a segment fires, depolarization spreads by local circuit currents to the adjacent resting membrane, triggering a new action potential there. But it also spreads back toward the segment that just fired — however, those Na⁺ channels are inactivated (absolutely refractory), so that segment cannot re-fire. The wave is therefore forced to advance only into previously resting tissue, producing unidirectional propagation from the initial stimulation site toward the axon terminal.

Question 5

Why does a neuron encode the strength of a stimulus through firing frequency rather than through varying action potential amplitude, and what structural feature makes this possible?

Model answer: Because action potentials are all-or-none events — channel kinetics and electrochemical gradients determine a fixed peak amplitude, not stimulus strength. The neuron instead varies how often it fires: a strong stimulus depolarizes the cell above threshold repeatedly, producing a high-frequency train of identical spikes. The relative refractory period (during which a stronger-than-normal stimulus can trigger a second spike) sets the range over which frequency can be modulated.

The all-or-none constraint is a feature, not a bug: it ensures that signals arrive at the synaptic terminal with the same waveform they started with, regardless of axon length. Analog encoding of intensity by amplitude would degrade over long distances. Frequency coding is distance-invariant — each spike is refreshed to full amplitude at every segment of the axon, preserving signal fidelity across the entire length of the neuron.