# Resting Potential

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

## The Loaded Spring

Every neuron holds a static electrical charge across its membrane even when completely silent. This baseline, the **resting potential**, sits at about $-70\text{ mV}$ (inside negative relative to outside) and acts as a loaded spring ready to fire.

Two forces maintain this state. First, **ion gradients** create concentration differences. Second, **selective permeability** lets potassium ($K^+$) leak out far more easily than sodium ($Na^+$) leaks in, pulling the voltage close to the potassium equilibrium.

| Ion | Concentration | Resting permeability |
|---|---|---|
| $Na^+$ | High outside | Low |
| $K^+$ | High inside | High |
| $Cl^-$ | High outside | Moderate |
| $A^-$ (anions) | High inside | Impermeant |

> **Common pitfall:** Treating $-70\text{ mV}$ as a neutral state. It is not zero; it is a heavily defended, polarized baseline powered by continuous cellular work.

## Keeping the Charge

Left alone, leaking ions would eventually destroy the gradient. The **sodium-potassium pump** prevents collapse by spending one ATP to export three $Na^+$ ions while importing two $K^+$ ions.

Because it moves three positive charges out for every two in, the pump is **electrogenic**, adding a small direct negativity on top of the diffusion potential.

$$V_{rest} \approx -70\text{ mV}$$

If ATP supply stops, the pump halts. The resting potential does not vanish instantly, but it decays steadily as passive leaks drain the ion gradients.

## Practice questions

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

### 1. Sort each ion by where it is more concentrated across the membrane of a resting neuron.

**Answer:**

- More concentrated OUTSIDE: Na+ (sodium), Cl- (chloride)
- More concentrated INSIDE: K+ (potassium), A- (large organic anions)

**Why:** Na+ and Cl- are concentrated outside; K+ and the impermeant organic anions (A-) are concentrated inside. These standing gradients are the stored energy the neuron later spends when it fires - not an accident but an actively held arrangement.

Page: https://tryals.app/practice/psychology-i/resting-potential/sort-each-ion-by-where-it-is-more-concentrated-across-the-membrane-of

### 2. A neuron at rest sits at about −70 mV. Why is the membrane NOT electrically neutral at rest?

A. Because the pump adds charge continuously without removing any
B. Because the intracellular fluid contains no positive ions at all
C. Because the potential is a separation of charge across the membrane
D. Because neutrality would require the membrane to be impermeable to every ion

**Answer:** C. Because the potential is a separation of charge across the membrane

**Why:** The resting potential IS a charge separation, maintained at metabolic cost. The cell is a charged capacitor waiting to discharge, and calling it neutral removes the mechanism.

Page: https://tryals.app/practice/psychology-i/resting-potential/a-neuron-at-rest-sits-at-about-70-mv-why-is-the-membrane-not

### 3. Trace WHY the inside of the resting neuron ends up negative. Put the causal steps in order.

**Answer:**

1. Potassium (K+) is far more concentrated inside the neuron than outside.
2. At rest, open K+ channels vastly outnumber open Na+ channels.
3. K+ diffuses out of the cell, down its concentration gradient.
4. Each K+ leaving carries positive charge out, making the inside more negative.
5. The membrane potential settles near the K+ equilibrium value, close to -70 mV.

**Why:** Because the membrane leaks K+ readily and K+ is high inside, K+ exits and drags positive charge out, leaving the interior negative until the potential rests near the K+ equilibrium. This is why it settles near E_K rather than midway between the ions.

Page: https://tryals.app/practice/psychology-i/resting-potential/trace-why-the-inside-of-the-resting-neuron-ends-up-negative-put-the

### 4. A neuron's potassium equilibrium potential is about −90 mV, yet the membrane actually rests at about −70 mV. Why does the resting potential fall short of the potassium equilibrium value instead of sitting on it?

A. Because the sodium-potassium pump clamps the membrane at −70 mV directly, overriding the potassium gradient.
B. Because K+ has not finished leaving yet. Left undisturbed for long enough, a resting neuron would settle at −90 mV.
C. Because the equilibrium potential is a theoretical figure with no real bearing on the membrane potential a cell actually holds.
D. Because the membrane is not perfectly selective; Na+ leaks inward.

**Answer:** D. Because the membrane is not perfectly selective; Na+ leaks inward.

**Why:** The resting potential is a compromise, and the −20 mV gap is where you see it happening: potassium dominates but a small Na+ leak drags the membrane up from E_K. The pump is not the source, it contributes a few millivolts of its own, and blocking it lets the potential decay over minutes rather than milliseconds.

Page: https://tryals.app/practice/psychology-i/resting-potential/a-neurons-potassium-equilibrium-potential-is-about-90-mv-yet-the

### 5. Select every statement that is TRUE of the resting potential.

A. It is maintained by continuous active transport using ATP.
B. The membrane is equally permeable to Na+ and K+ at rest.
C. The inside of the cell is negative relative to the outside.
D. The Na/K pump moves equal numbers of Na+ and K+, so it adds no charge.
E. It sits close to the equilibrium potential for K+.

**Answer:** A. It is maintained by continuous active transport using ATP.; C. The inside of the cell is negative relative to the outside.; E. It sits close to the equilibrium potential for K+.

**Why:** Inside is negative, upkeep is active (ATP), and rest sits near E_K - all true. The membrane is NOT equally permeable (K+ leaks far more), and the pump's 3-out/2-in ratio DOES add charge (electrogenic), so those two are false.

Page: https://tryals.app/practice/psychology-i/resting-potential/select-every-statement-that-is-true-of-the-resting-potential

### 6. The sodium-potassium pump runs constantly in every neuron, and the brain spends a large share of its entire energy budget on it. What is all that ATP buying?

A. The action potentials themselves, each of which is directly powered by the pump
B. Nothing useful at rest, the cost is only incurred when the neuron is active
C. Protection against ion leakage, which would otherwise destroy the membrane
D. A charged battery, whose gradients the action potential spends

**Answer:** D. A charged battery, whose gradients the action potential spends

**Why:** Rest is not idling, it is holding a charged battery. The pump pays in advance for gradients the spike then spends in a millisecond, which is why the brain burns roughly a fifth of the body's energy while apparently doing nothing. Readiness has a standing price.

Page: https://tryals.app/practice/psychology-i/resting-potential/the-sodium-potassium-pump-runs-constantly-in-every-neuron-and-the