# Action Potential

Psychology I · Behavioral Biology · https://tryals.app/learn/psychology-i/action-potential

## From Threshold to Spike

If enough excitatory input pushes a neuron from its **resting potential** ($V_{rest} \approx -70$ mV) up to about $-55$ mV, it crosses **threshold** and fires an **action potential**: a rapid voltage spike peaking at $V_{peak} \approx +40$ mV. 

The **all-or-none law** states that once threshold is crossed, the spike always reaches the same peak regardless of how much threshold was exceeded. A sub-threshold stimulus produces nothing.

| Phase | What happens |
| --- | --- |
| Depolarization | Voltage-gated Na+ channels open; Na+ rushes in, driving voltage toward $+40$ mV |
| Repolarization | Na+ channels inactivate; voltage-gated K+ channels open, K+ rushes out |
| Hyperpolarization | K+ channels close slowly, briefly overshooting below $-70$ mV |
| Return to rest | The Na+/K+ pump and leak channels restore resting potential |

## Refractory Periods & Speed

During the **absolute refractory period** (when Na+ channels are inactivated), no stimulus can trigger a second spike, enforcing one-way propagation. During the **relative refractory period**, only an unusually strong stimulus can fire a spike.

**Myelin** speeds conduction dramatically. The impulse jumps between **Nodes of Ranvier** via **saltatory conduction**, reaching up to 120 m/s compared to 1 m/s in unmyelinated axons.

> **Common pitfall:** Thinking a stronger stimulus produces a bigger action potential. Intensity is coded by firing **rate**, not spike size.

## Practice questions

4 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Put the phases of a single action potential in the correct order.

**Answer:**

1. The membrane reaches threshold; voltage-gated Na+ channels open and Na+ rushes in.
2. The membrane depolarizes rapidly toward its +40 mV peak.
3. Na+ channels inactivate and voltage-gated K+ channels open, so K+ flows out.
4. K+ channels are slow to close, briefly overshooting below -70 mV (hyperpolarization).
5. Leak channels and the Na/K pump restore the resting potential.

**Why:** Na+ influx depolarizes to the peak, K+ efflux repolarizes, sluggish K+ channels overshoot into hyperpolarization, and the pump plus leaks reset rest. Getting the ion for each phase right is the point - Na+ up, K+ down.

Page: https://tryals.app/practice/psychology-i/action-potential/put-the-phases-of-a-single-action-potential-in-the-correct-order

### 2. Select every statement that is TRUE about action potentials.

A. A stimulus twice as strong (both suprathreshold) makes a spike twice as tall.
B. Neurons signal a more intense stimulus by firing more frequently.
C. The peak voltage is roughly the same for every spike in a given axon.
D. Myelination lowers conduction speed by insulating the axon.
E. During the absolute refractory period, even a very strong stimulus cannot trigger a spike.

**Answer:** B. Neurons signal a more intense stimulus by firing more frequently.; C. The peak voltage is roughly the same for every spike in a given axon.; E. During the absolute refractory period, even a very strong stimulus cannot trigger a spike.

**Why:** Fixed peak, absolute refractoriness, and rate-coding of intensity are all true. A stronger stimulus does not make a taller spike (all-or-none), and myelin RAISES conduction speed via saltatory conduction, so those two are false.

Page: https://tryals.app/practice/psychology-i/action-potential/select-every-statement-that-is-true-about-action-potentials

### 3. During the absolute refractory period no stimulus will fire a second action potential, however strong. Why does raising the stimulus strength not help here, when it is exactly what triggers a spike at rest?

A. Because the pump must first restore the ion concentrations.
B. Because the Na+ channels are inactivated, not merely closed.
C. Because the axon has exhausted its supply of sodium ions.
D. Because the membrane is hyperpolarised, so a strong stimulus fires.

**Answer:** B. Because the Na+ channels are inactivated, not merely closed.

**Why:** Na+ channels have two gates, and that is the whole answer: the stimulus acts on the activation gate, which is no longer the obstacle once the inactivation gate has swung shut. Only repolarisation resets it. A single spike barely dents the ion gradients, so the pump has nothing to do with the timing.

Page: https://tryals.app/practice/psychology-i/action-potential/during-the-absolute-refractory-period-no-stimulus-will-fire-a-second

### 4. In multiple sclerosis the immune system strips myelin from axons. Signals slow dramatically and often fail entirely. Why does losing the *insulation* stop the signal rather than merely weakening it?

A. Because the spike is regenerated only at the gaps in the sheath
B. Because the exposed axon is attacked directly by the immune system and dies
C. Because myelin carries the signal and the axon underneath is structurally inert
D. Because myelin generates the action potential, so without it no spike is produced

**Answer:** A. Because the spike is regenerated only at the gaps in the sheath

**Why:** Myelin does not carry the signal, it forces the spike to leap between nodes, and each landing must still be strong enough to fire the next. Strip it and the current bleeds out across the bare membrane, so the relay fails rather than merely slows. Insulation was doing load-bearing work.

Page: https://tryals.app/practice/psychology-i/action-potential/in-multiple-sclerosis-the-immune-system-strips-myelin-from-axons