Action Potential Propagation Along the Axon — Comprehensive Notes

Resting state of the axon

When a neuron is not firing, the axon is in a resting state.
Resting neurons are not electrically neutral.
Intracellular fluid (inside the axon) has a negative electrical charge compared with the extracellular fluid (outside).
The electrical imbalance results from the presence of large negatively charged proteins inside the neuron.
Extracellular fluid contains a relatively higher concentration of positively charged sodium ions, which adds to the electrical imbalance between the intracellular and extracellular environments.
The axon membrane contains channels that, when opened, allow sodium or potassium ions to flow into or out of the axon.
These channels are special proteins that can change from a closed state to an open state to regulate the flow of ions.
Some channels are stimulated to open or close by chemicals including neurotransmitters, hormones, and some psychoactive drugs.
Other ion channels open or close depending on the electrical charge or voltage at that location on the axon membrane.
In this context, a small change in the electrical charge within the axon causes a few sodium channels to open.
Because the concentration of sodium is much higher in extracellular fluid, sodium ions rush into the axon through these open channels.
This movement is produced by two driving forces:
- Concentration gradient: particles flow from high concentration to low concentration.
- Electrical gradient: positively charged sodium ions are attracted to the negatively charged intracellular fluid.
When the action potential arrives at this section of the axon, sodium channels open, and sodium ions flow into the axon, making this section electrically positive for a brief moment.
The change from a negative to a positive charge triggers the opening of voltage-sensitive sodium channels and more sodium ions flow into the cell.
This chain reaction of more and more sodium channels opening in sequence transmits the signal, the action potential, along the cell membrane to the axon terminal.
As the positive charge inside the cell membrane reaches a high level, it triggers the opening of a separate type of channel that allows potassium ions to move through the membrane.
At about the same time, the sodium channels close.
When potassium channels open, potassium ions move out of the cell because of the same two forces that moved sodium ions into the cell initially:
- Concentration gradient: there are more potassium ions inside than outside, so diffusion pushes them out.
- Electrical gradient: the inflow of sodium ions has created a brief localized positive charge inside the axon, and the relatively negative charge outside helps pull potassium ions outward.
When enough potassium is expelled to return the local intracellular fluid to a negative charge, voltage-sensitive potassium channels close.
After the action potential moves past this section of the axon, the sodium and potassium channels return to their closed state, but the concentrations near these channels are reversed from their normal state: too many sodium ions inside the axon, and too many potassium ions outside.
To restore balance, the neuron uses sodium-potassium pumps, which are specialized proteins designed to pump sodium ions out of the axon while pumping potassium ions back inside.
The sodium-potassium pumps rapidly restore the proper balance in the concentrations of sodium and potassium ions.
This portion of the axon has returned fully to its resting state, ready for the next action potential.

Initiation and propagation of the action potential

External signals stimulate dendrites, triggering the neuron to fire an action potential.
The initial small electrical change causes a few sodium channels to open.
Sodium ions rush into the axon due to the concentration gradient (high outside) and electrical gradient (attracted to negative interior).
The influx of Na+ makes the local region briefly positive, which in turn triggers more voltage-sensitive sodium channels to open in sequence.
This sequential opening propagates the action potential along the axon membrane toward the axon terminal in a wave-like fashion.

Repolarization and return to resting state

As the local interior charge becomes highly positive, voltage-sensitive potassium channels open.
Sodium channels close around the same time.
Potassium ions move out of the cell, driven by:
- Concentration gradient: higher internal K+ pushes outward.
- Electrical gradient: the transient positive interior and relatively negative exterior pull K+ outward.
When enough potassium has exited to restore a negative interior, the potassium channels close.
After the wave passes, the sodium and potassium channels return to their closed state, but the local ion concentrations near these channels are temporarily reversed (more Na+ inside, more K+ outside).

Restoring ionic balance with the Na+/K+ pumps

Sodium-potassium pumps are specialized proteins that actively move ions to restore resting conditions.
They pump sodium ions out of the axon and pumping potassium ions back inside.
These pumps rapidly restore the proper balance in the concentrations of sodium and potassium ions.
The affected region returns to the resting state, ready for the next action potential.

Connections, implications, and broader context

The described process is the fundamental mechanism of neural signaling and communication within the nervous system.
Ion channel dynamics (chemical-gated and voltage-gated) illustrate how chemical signals and electrical states interact to propagate information.
The sequence of ion movements—Na+ influx for depolarization, followed by K+ efflux for repolarization—is essential for the propagation of action potentials along the axon.
The Na+/K+ pumps are critical for restoring and maintaining ionic gradients, enabling neurons to fire repeatedly.
Real-world relevance: understanding this mechanism underpins neuroscience, pharmacology (drugs that affect channels or receptors), and medical approaches to neurological disorders.