Questions: Absolute and Relative Refractory Periods: Neuronal Timing Constraints

Question 1

During the absolute refractory period of an action potential, no new action potential can be triggered. What is the direct cause of this impossibility?

• A. The membrane is hyperpolarized below resting potential, making it impossible to reach threshold
• B. Voltage-gated Na+ channels are in an inactivated state — their inactivation gates are closed — and cannot open regardless of how strong the stimulus is
• C. Voltage-gated K+ channels are fully open and are electrically shunting any incoming depolarization
• D. The neuron has depleted its intracellular Na+ supply and cannot sustain a depolarizing current

Answer: (B)

The absolute refractory period is caused by Na+ channel inactivation, not hyperpolarization. After opening during an action potential, voltage-gated Na+ channels transition to an inactivated state in which the inactivation gate (a 'ball and chain' structure) physically blocks the pore. In this state, the channel cannot reopen regardless of membrane voltage or stimulus strength — the inactivation gate must first be removed, which requires repolarization to allow the channel to return to the closed-but-ready state. Hyperpolarization during the relative refractory period is caused by K+ efflux, but that is a separate phenomenon and does not account for the absolute refractory period.

Question 2

A neuron is receiving a sustained strong stimulus and firing repeatedly. Why does increasing stimulus strength produce a higher firing rate rather than simply producing larger action potentials?

• A. Stronger stimuli produce larger action potentials with higher amplitude, which are counted as multiple spikes
• B. Stronger stimuli can exceed the elevated threshold during the relative refractory period, shortening the time between spikes and therefore increasing firing frequency
• C. Stronger stimuli suppress K+ channel opening, reducing the duration of each refractory period
• D. Stronger stimuli permanently inactivate fewer Na+ channels, making each action potential more efficient

Answer: (B)

Action potentials are all-or-nothing — stimulus strength does not change their amplitude. Frequency coding works through the relative refractory period: during this window, the membrane is hyperpolarized and Na+ channel availability is partial. A weak stimulus may not overcome the elevated threshold and fails to fire. A stronger stimulus can exceed the elevated threshold even early in the relative refractory period, generating a spike sooner. Because the interspike interval is shortened, the neuron fires more frequently. This is how stimulus intensity is translated into firing rate — the fundamental currency of neural coding.

Question 3

During the absolute refractory period, a stimulus strong enough — say 10× the normal threshold — can still trigger a new action potential.

• T. True
• F. False

Answer: False

The absolute refractory period is absolute precisely because Na+ channel inactivation cannot be overcome by stimulus strength. The inactivated channel is physically blocked by the inactivation gate and cannot reopen until that gate is removed by repolarization — regardless of the membrane voltage or the size of the depolarizing stimulus. This distinguishes the absolute from the relative refractory period: during the relative period, a suprathreshold stimulus can fire the neuron because some Na+ channels have recovered. During the absolute period, none have, and no stimulus works.

Question 4

Refractory periods ensure that action potentials travel only in one direction along an axon because the membrane behind the advancing wavefront is in a refractory state and cannot be re-excited.

• T. True
• F. False

Answer: True

As an action potential propagates along an axon, depolarization spreads forward into resting membrane (which can fire) and backward into membrane that was just depolarized (which is now refractory). Because the membrane behind the wavefront has Na+ channels in the inactivated state, the backward-traveling depolarization cannot re-excite it. The action potential can only advance forward into membrane that has not yet fired. This is what gives action potential propagation its directionality — without the refractory period, signals could bounce back and forth along the axon.

Question 5

Explain how the relative refractory period allows neurons to encode stimulus intensity as firing frequency (rate coding).

Model answer: After the absolute refractory period ends, Na+ channels progressively recover from inactivation, but the membrane remains hyperpolarized below resting potential due to ongoing K+ efflux. During this relative refractory period, the threshold for triggering a new action potential is elevated — a larger depolarization is needed. A weak stimulus cannot overcome this elevated threshold and fails to fire; a strong stimulus can. Crucially, a very strong stimulus can exceed the elevated threshold even early in the relative refractory period, while a moderate stimulus can only fire the neuron later, when the threshold has returned closer to normal. This means that stronger inputs produce shorter interspike intervals and therefore higher firing frequencies. Stimulus intensity is thus translated into spike rate — a continuous variable — rather than simply 'fire or not fire.'

The rate code is fundamental to how the nervous system represents graded quantities (light intensity, force, temperature) as patterns of discrete all-or-nothing spikes. The relative refractory period is the mechanism that makes this translation possible by creating a window during which threshold varies continuously with time since the last spike.