Atlas MCAT Course
Systems & Physiology → The Action Potential
The Action Potential
Turning gradients into digital signals.
01The idea
Graded electrical changes decay with distance, are noisy, and depend on local conditions. That’s unacceptable for long axons, precise timing, and reliable communication. The solution is a regenerative, all-or-none signal.
Nothing new is created — gradients are spent.
THREAD · Gradients and flow
02Build the model
AThreshold: the decision point
Neurons constantly experience small voltage changes. Most of these do nothing. An action potential occurs only when membrane depolarization crosses a threshold.
Why threshold exists: voltage-gated channels respond nonlinearly. Below threshold, there is minimal channel opening; above threshold, runaway positive feedback.
BVoltage-gated channels: time matters
| Voltage-gated Na⁺ channels | Voltage-gated K⁺ channels |
|---|---|
| Open rapidly with depolarization | Open more slowly |
| Inactivate shortly after opening | Restore membrane potential |
CThe rising phase: positive feedback
Once threshold is crossed, Na⁺ channels open, Na⁺ rushes in (a strong electrochemical gradient), the membrane depolarizes further, and more Na⁺ channels open. This is positive feedback.
DThe peak and inactivation: why it stops
The action potential stops rising because Na⁺ channels inactivate and K⁺ channels open. This halts Na⁺ entry even though the gradient still favors it.
ERepolarization: spending stored energy
As K⁺ exits, membrane potential returns toward rest, and electrochemical gradients are partially dissipated. The Na⁺/K⁺ ATPase later restores gradients and resets readiness.
FHyperpolarization: overshoot, not error
Because K⁺ channels close slowly, membrane potential may become more negative than rest. This does not mean failure. It reflects continued K⁺ permeability and a temporary shift in balance.
GRefractory periods: enforcing direction and frequency
| Absolute refractory period | Relative refractory period |
|---|---|
| No new action potential possible | Stronger stimulus required |
| Na⁺ channels inactivated | K⁺ conductance still high |
This is fundamental to information encoding.
HPropagation: why signals don’t fade
Action potentials propagate because local depolarization triggers neighboring channels, and the signal regenerates at every segment. Unlike diffusion, amplitude does not decay and distance does not matter.
IWhat information is encoded
Action potentials do not encode information in size. They encode information in frequency, timing, and pattern. This sets up synaptic integration and neural computation.
03See the connections
The lesson names these links itself. Read the map left to right.
Trigger
- Depolarization past threshold
Shape
- Na⁺ in (fast, then inactivates)
- K⁺ out (slow)
- Refractory periods
Result
- Regenerates along the axon
- Codes by frequency and timing
- Pump restores gradients later
Next: synaptic transmission.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A stimulus is made twice as strong. What changes about the action potentials?
Think first.
Why does the action potential stop rising, even though the Na⁺ gradient still favors entry?
Think first.
Why can’t an action potential travel backward along the axon?
Think first.
A toxin blocks the Na⁺/K⁺ ATPase. What happens to the very next action potential?
05MCAT lens
The MCAT emphasizes
Threshold is a system property, not a number to memorize
Timing differences between Na⁺ and K⁺ channels shape the spike
Channel state, not the gradient, ends the spike
Refractory periods enforce direction and limit firing rate
Action potentials are digital: identical at every point
06Common traps
Trap 1: Larger stimuli produce larger action potentials
Misconception
Size is fixed.
Correct model
Frequency changes.
Trap 2: Pumps cause depolarization
Misconception
Channels do.
Correct model
Pumps maintain readiness.
Trap 3: Ions “want” to move
Misconception
Gradients + permeability.
Correct model
Physics, not intent.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
Why threshold exists
Check against 02 · A: nonlinear channel opening tips into positive feedback.
Why action potentials are all-or-none
Check against 02 · A and C: past threshold, the rise runs away on its own.
Why timing of channels matters
Check against 02 · B and D: fast Na⁺ then inactivation; slow K⁺.
Why propagation is reliable
Check against 02 · H: the signal regenerates at every segment.
If something feels unclear, the missing idea is usually thisAction potentials convert stored electrochemical gradients into repeatable, directional information pulses.
08Remember
REMEMBER · if you keep only a few things
- An action potential spends stored gradients; nothing new is created.
- Threshold → Na⁺ channels open → positive feedback.
- Na⁺ inactivation and K⁺ opening end the spike.
- K⁺ channels close slowly → brief overshoot below rest.
- Refractory periods: one-way travel, capped frequency.
- Fixed size; information is in frequency and timing.
09Forward hook
Now that signals can travel, the next problem is:
How do neurons communicate with each other across gaps?
That leads directly to synapses, neurotransmitters, and excitation vs inhibition.
Next lessonSynaptic Transmission→