During the depolarization phase of an action potential, the membrane potential overshoots 0 mV and reaches approximately +40 mV. What drives this overshoot?
AThe Na⁺/K⁺-ATPase temporarily reverses direction, pumping Na⁺ into the cell
BVoltage-gated Na⁺ channels open and Na⁺ rushes in down both its concentration and electrical gradients
CVoltage-gated K⁺ channels open, allowing K⁺ to rush into the cell along its electrical gradient
DCl⁻ channels open and Cl⁻ exits the cell, making the interior more positive
At rest, Na⁺ is both more concentrated outside the cell and electrically attracted inward by the negative interior. When voltage-gated Na⁺ channels open at threshold, both driving forces combine to produce a large inward Na⁺ current that rapidly depolarizes the membrane toward the Na⁺ equilibrium potential (~+60 mV). The membrane doesn't fully reach this value because the channels begin to inactivate, halting the process around +40 mV.
Question 2 True / False
Stimulus intensity is encoded as action potential amplitude — a stronger stimulus produces a larger action potential.
TTrue
FFalse
Answer: False
The action potential is all-or-none: once threshold is reached, the amplitude is fixed regardless of stimulus strength. A neuron cannot produce a 'half' action potential. Instead, stimulus intensity is encoded by firing frequency (rate coding) — a stronger stimulus causes the neuron to fire more action potentials per second, not larger ones.
Question 3 Short Answer
What prevents an action potential from traveling backward along the axon after it is generated?
Think about your answer, then reveal below.
Model answer: The absolute refractory period. Immediately after depolarization, voltage-gated Na⁺ channels in the just-fired segment undergo inactivation — a conformational change that prevents them from reopening regardless of voltage. Because the membrane behind the advancing wavefront is always in this refractory state, the action potential can only propagate forward into the next unexcited patch of membrane.
Na⁺ channel inactivation is distinct from simply closing: an inactivated channel cannot reopen until the membrane repolarizes, which takes time. This creates a temporal window during which no stimulus can retriger an action potential in that segment. The directional propagation of the action potential is thus a direct consequence of channel kinetics, not a special property of the axon's geometry.