# The Neuron and Action Potentials

Psychology I · Psychophysiology · https://tryals.app/learn/psychology-i/the-neuron-and-action-potentials

## Anatomy and Resting State

The **neuron** is the basic signalling cell of the nervous system, driven by three core structures. **Dendrites** receive incoming signals, the **soma** integrates them, and the **axon** carries outgoing signals away.

Axons are often wrapped in a **myelin sheath**, speeding up signals across its gaps. At rest, the neuron maintains a **resting potential** of $-70$ mV, meaning the inside is negatively charged relative to the outside.

When signals push the membrane to its **threshold**, positive ions rush in. This electrical spike is the **action potential**, which sweeps down the axon.

## Firing Rules and Limits

The **all-or-none principle** dictates that an action potential either fires completely or not at all. A stronger stimulus never creates a bigger spike; it increases the **frequency** of firing instead.

| Stimulus Property | What Changes? | Why? |
| --- | --- | --- |
| **Intensity** | Firing frequency | All-or-none law |
| **Spike Size** | Stays constant | Fixed amplitude |

> **Pitfall:** Do not confuse stimulus strength with spike size. After firing, a **refractory period** must pass before the neuron can fire again.

## Practice questions

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

### 1. Order the path a signal travels through a single neuron.

**Answer:**

1. Dendrites receive incoming signals from other neurons.
2. The soma integrates those incoming signals.
3. If threshold is reached, an action potential begins at the start of the axon.
4. The action potential travels down the axon, leaping between the nodes of Ranvier.
5. The signal reaches the axon terminal to be passed to the next cell.

**Why:** Dendrites receive, the soma integrates, the axon fires and conducts (leaping between nodes), and the terminal hands off the signal. Receiving precedes integrating precedes sending - the neuron's fixed one-way flow.

Page: https://tryals.app/practice/psychology-i/the-neuron-and-action-potentials/order-the-path-a-signal-travels-through-a-single-neuron

### 2. Neurons cannot fire faster than roughly 500–1000 times a second, however intense the stimulus. What sets that ceiling?

A. The speed at which the axon can conduct the signal to its terminal
B. The rate at which the sodium-potassium pump can restore ion concentrations
C. The maximum rate at which neurotransmitter can be synthesised
D. The refractory period, since sodium channels need time to reset

**Answer:** D. The refractory period, since sodium channels need time to reset

**Why:** The channels set the tempo. Inactivation gates must reset before another spike can fire, so the ceiling is roughly one per millisecond no matter how hard you push, and since intensity is coded by rate, that ceiling is also why perceived intensity saturates rather than rising without limit.

Page: https://tryals.app/practice/psychology-i/the-neuron-and-action-potentials/neurons-cannot-fire-faster-than-roughly-500-1000-times-a-second

### 3. Sort each statement by whether it describes the neuron at rest or during the action potential.

**Answer:**

- Resting neuron: Inside is about -70 mV, negative relative to outside, A charged battery waiting to discharge
- During the action potential: Positive ions rush in and the membrane briefly reverses to positive, Voltage-gated channels snap open once threshold is crossed

**Why:** The steady -70 mV and the 'loaded battery' describe rest; the influx of positive ions and voltage-gated channels opening describe the firing event. Rest is a stable baseline; the action potential is the transient discharge.

Page: https://tryals.app/practice/psychology-i/the-neuron-and-action-potentials/sort-each-statement-by-whether-it-describes-the-neuron-at-rest-or

### 4. Doubling the intensity of a stimulus (with both levels above threshold) roughly doubles the SIZE of each resulting action potential.

**Answer:** False

**Why:** False. By the all-or-none principle, every action potential in a neuron is the same size; a stronger stimulus increases the RATE of firing, not the height of each spike. Amplitude coding of intensity is the named misconception.

Page: https://tryals.app/practice/psychology-i/the-neuron-and-action-potentials/doubling-the-intensity-of-a-stimulus-with-both-levels-above

### 5. Select every statement that is TRUE about neurons and action potentials.

A. A stronger stimulus produces a bigger action potential.
B. The resting potential is about +40 mV.
C. The nodes of Ranvier in the myelin sheath let the signal leap forward faster.
D. After firing, a refractory period must pass before the neuron can fire again.
E. Dendrites receive signals while the axon carries them away.

**Answer:** C. The nodes of Ranvier in the myelin sheath let the signal leap forward faster.; D. After firing, a refractory period must pass before the neuron can fire again.; E. Dendrites receive signals while the axon carries them away.

**Why:** Dendrites-in/axon-out, faster leaping via the nodes, and the refractory period are all true. A stronger stimulus does not enlarge the spike (all-or-none), and the resting potential is about -70 mV, so those two are false.

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

### 6. Action potentials are all-or-none, so every spike is the same size. How does the nervous system represent the difference between a gentle touch and a hard press?

A. By releasing a different neurotransmitter at the synapse for stronger stimuli.
B. By producing larger action potentials, since the all-or-none law applies only to sensory neurons.
C. By conducting the same spikes faster along the axon, so the signal arrives sooner.
D. By firing rate and by the number of neurons recruited.

**Answer:** D. By firing rate and by the number of neurons recruited.

**Why:** The all-or-none law removes amplitude as a variable, so the code moves to rate and recruitment. That is a design rather than a limitation: a signal carrying information in its size loses it as it decays, where a spike either arrives or does not. It also explains why perceived intensity is compressive, a receptor already at maximum cannot report that the stimulus grew.

Page: https://tryals.app/practice/psychology-i/the-neuron-and-action-potentials/action-potentials-are-all-or-none-so-every-spike-is-the-same-size