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Neuron Structure
& Action Potentials

Explore how neurons are built, how they generate electrical signals, and why the action potential is an all-or-nothing event.

Anatomy & Physiology

What Is a Neuron?

A neuron is the fundamental functional unit of the nervous system. Neurons are specialized cells that receive, process, and transmit information using electrical and chemical signals.

Parts of a Neuron

Dendrites Cell Body (Soma) Nucleus Hillock Myelin Sheath Node Node Node Axon Axon Terminals Signal Direction →

Key Structures Explained

Dendrites
Branch-like extensions that receive signals from other neurons or sensory receptors.
Soma
The cell body containing the nucleus; integrates incoming signals.
Myelin
Fatty insulation that speeds up signal transmission via saltatory conduction.
Terminals
Release neurotransmitters into the synaptic cleft to signal the next cell.

Resting Membrane Potential

At rest, a neuron maintains a voltage difference across its membrane called the resting membrane potential, approximately \( -70 \text{ mV} \). The inside of the cell is negatively charged relative to the outside.

Cell Membrane Outside (Extracellular)
\( \text{Na}^+ \)
\( \text{Na}^+ \)
\( \text{Na}^+ \)
 + + + Inside (Intracellular)
\( \text{K}^+ \)
\( \text{A}^- \)
\( \text{K}^+ \)
 - - - -70 mV

The Nernst Equation

The equilibrium potential for a single ion can be calculated using the Nernst equation:

\( R \)
Gas constant: \( 8.314 \text{ J/(mol·K)} \)
\( T \)
Temperature in Kelvin (body temp: \( 310 \text{ K} \))
\( z \)
Valence of the ion (\( +1 \) for Na⁺, K⁺)
\( F \)
Faraday constant: \( 96{,}485 \text{ C/mol} \)

The Action Potential

An action potential is a rapid, temporary reversal of the membrane potential that travels along the axon. It is the neuron's way of sending a signal over long distances. Once triggered, it follows an all-or-nothing principle.

+30 0 -55 -70 -90 Voltage (mV) Time (ms) Threshold Resting Depol. Peak Repol. Hyperpol. Resting

Phases of the Action Potential

1
Resting State: Voltage-gated channels closed. Membrane at \( -70 \text{ mV} \).
2
Depolarization: Stimulus reaches threshold (\( -55 \text{ mV} \)). Na⁺ channels open, Na⁺ rushes in. Voltage rises toward \( +30 \text{ mV} \).
3
Repolarization: Na⁺ channels inactivate, K⁺ channels open. K⁺ flows out, restoring negative charge.
4
Hyperpolarization: K⁺ channels slow to close: voltage briefly dips below \( -70 \text{ mV} \) (undershoot to ~\( -90 \text{ mV} \)).
5
Return to Rest: Na⁺/K⁺ pump restores ion gradients. Channels reset to resting conformation.

The All-or-Nothing Principle

An action potential either fires completely or not at all. There is no partial or graded action potential. The strength of a stimulus is encoded not by amplitude, but by frequency of firing.

Subthreshold
Stimulus below \( -55 \text{ mV} \): no action potential. The membrane returns to rest.
Threshold
At \( -55 \text{ mV} \): voltage-gated Na⁺ channels open and the full action potential fires.
Suprathreshold
Stimulus above threshold: same amplitude AP, but may increase firing rate.

Graded vs. Action Potentials

Graded Potential Action Potential
Amplitude Variable (proportional to stimulus) Fixed (\( +30 \text{ mV} \) peak)
Propagation Decremental (fades with distance) Non-decremental (regenerated)
Direction Bidirectional Unidirectional (axon hillock → terminals)
Summation Temporal & spatial All-or-nothing: no summation
Location Dendrites & cell body Axon

Refractory Periods

After an action potential, the neuron enters a brief period where it is less responsive or completely unresponsive to new stimuli. This ensures unidirectional signal propagation.

Absolute Refractory
Na⁺ channels are inactivated. No stimulus, regardless of strength, can trigger another AP.
Relative Refractory
Some Na⁺ channels have reset. A stronger-than-normal stimulus can trigger a new AP (during hyperpolarization).

Saltatory Conduction

In myelinated neurons, action potentials "jump" between the Nodes of Ranvier: gaps in the myelin sheath where ion channels are concentrated. This dramatically increases conduction speed.

Myelin Node Node Node Signal "Jumps"

Synaptic Transmission

When the action potential reaches the axon terminal, it triggers a chemical signaling process across the synapse to communicate with the next cell.

1
AP arrives: Depolarization opens voltage-gated \( \text{Ca}^{2+} \) channels in the terminal.
2
\( \text{Ca}^{2+} \) influx: Calcium causes synaptic vesicles to fuse with the membrane (exocytosis).
3
Neurotransmitter release: Molecules diffuse across the synaptic cleft (\( \sim 20 \text{ nm} \) gap).
4
Receptor binding: Neurotransmitters bind receptors on the postsynaptic membrane, opening ion channels.

Key Neurotransmitters

Acetylcholine
Muscle contraction, memory, learning. Found at neuromuscular junctions.
Dopamine
Reward, motivation, motor control. Linked to Parkinson's and addiction.
Serotonin
Mood, sleep, appetite. Target of many antidepressant medications.
GABA
Primary inhibitory neurotransmitter. Reduces neuronal excitability in the CNS.

Tracing a Neural Signal

Follow these steps to trace a signal from stimulus to response:

1
Stimulus detected: Receptor or dendrite receives input (chemical, mechanical, or electrical).
2
Graded potential: Local depolarization travels toward the axon hillock, decaying with distance.
3
Threshold check: If summed input at the hillock reaches \( -55 \text{ mV} \), an AP fires.
4
AP propagation: The signal regenerates along the axon (saltatory in myelinated neurons).
5
Synaptic transmission: Neurotransmitters cross the synapse to excite or inhibit the next cell.

Common Mistakes

✕ APs Get Weaker
Action potentials do not lose strength over distance. They are regenerated at each point along the axon: non-decremental.
✕ Stronger = Bigger AP
A stronger stimulus does not make a bigger AP. It increases firing frequency, not amplitude (all-or-nothing).
✕ Na⁺ Moves Out
During depolarization, \( \text{Na}^+ \) moves into the cell, not out. \( \text{K}^+ \) moves out during repolarization.

Summary Table

Phase Ion Movement Voltage Change Channel State
Resting \( \text{K}^+ \) leak out \( -70 \text{ mV} \) Closed (gated)
Depolarization \( \text{Na}^+ \) rushes in \( -70 \to +30 \text{ mV} \) \( \text{Na}^+ \) channels open
Repolarization \( \text{K}^+ \) flows out \( +30 \to -70 \text{ mV} \) \( \text{Na}^+ \) inactivated, \( \text{K}^+ \) open
Hyperpolarization \( \text{K}^+ \) continues out Below \( -70 \text{ mV} \) \( \text{K}^+ \) slow to close
Return to Rest \( \text{Na}^+/\text{K}^+ \) pump active Back to \( -70 \text{ mV} \) All channels reset

Question 1 of 5

What is the approximate resting membrane potential of a typical neuron?

Question 2 of 5

During depolarization, which ion primarily moves into the neuron?

Question 3 of 5

What is saltatory conduction?

Question 4 of 5

During the absolute refractory period, a neuron:

Question 5 of 5

The "all-or-nothing" principle means that a stronger stimulus produces: