Exhaustive Guide to Neural Action Potentials

An action potential is defined as a brief electrical charge that travels down a neuron, specifically along the axon.
Commonly used synonyms in various textbooks include:
- Neural impulse
- Nerve impulse
- Spark
The primary function of an action potential is to enable neurons to communicate with one another, allowing messages to be transmitted throughout the nervous system.

To understand the action potential, one must identify the relevant parts of the neuron:
- Soma (Cell Body): The central part of the neuron that houses the nucleus.
- Dendrites: Branch-like structures that receive incoming messages from the sending neuron.
- Axon: A long, tube-like extension that carries the electrical charge (the action potential) away from the soma.
- Axon Terminal: The end of the axon which contains neurotransmitters used for signaling.
The neuron is enclosed by a protective membrane that regulates what enters and exits the cell through specialized protein structures called channels.

Neurons are surrounded by ions, which are charged particles or molecules. These can be categorized as positive or negative.
The two most critical ions in neural firing are:
- Sodium ( $Na^+$ ): Contains a positive charge.
- Potassium ( $K^+$ ): Contains a positive charge.
The Salty Banana Memory Technique:
- Think of a neuron like a banana.
- The inside of a banana is rich in potassium ( $K^+$ ). Thus, there is a high concentration of potassium inside the neuron.
- If you pour salt ( $NaCl$ ) all over the banana, the outside becomes covered in sodium ( $Na^+$ ). Thus, there is a high concentration of sodium outside the neuron.

Resting Potential: When a neuron is at rest and not actively firing, the internal voltage of the neuron is approximately $-70\,\text{mV}$ . The interior is considered very negative compared to the exterior environment.
Stimulation: Neurons can be stimulated by various factors, such as thinking, smelling, or physical sensations like cold water on the skin.
Example Scenario: Reaching for a glass of water.
- This action is sparked by neurotransmitters such as Acetylcholine ( $ACh$ ).
- Acetylcholine is a neurotransmitter responsible for controlling muscle movement.
- $ACh$ molecules bind to specific receptor sites on the dendrites.
- This binding triggers the opening of gates, allowing sodium ( $Na^+$ ) ions to begin flowing into the neuron, making the internal environment more positive.

As sodium enters the cell, the internal charge rises from $-70\,\text{mV}$ toward zero.
Threshold: The specific voltage level required to trigger an action potential is $-55\,\text{mV}$ .
The All-or-None Principle: If the neuron reaches the $-55\,\text{mV}$ threshold, the action potential will fire completely every time. If the stimulation is insufficient to reach this "magic number," the neuron will not fire. These insufficient stimulations are sometimes called "false alarms."

Once the $-55\,\text{mV}$ threshold is reached, voltage-gated sodium channels along the axon are triggered to open.
Depolarization Process:
- Sodium ( $Na^+$ ) rushes very quickly into the neuron through these opened gates.
- The influx of positive ions causes the internal charge to spike from $-55\,\text{mV}$ all the way up to $+30\,\text{mV}$ .
- During this phase, the inside of the neuron becomes positive while the outside becomes negative.
- This rapid change in voltage is the essence of the electrical charge traveling down the axon.

When the internal charge reaches the peak of $+30\,\text{mV}$ , the neuron must begin returning to its resting state.
Repolarization Process:
- The voltage-gated sodium ( $Na^+$ ) channels close, preventing further sodium from entering.
- Voltage-gated potassium ( $K^+$ ) channels open.
- Potassium ( $K^+$ ) ions rush out of the cell.
- Since positive ions are leaving the interior, the internal voltage drops rapidly back toward a negative state.

Hyperpolarization: As potassium rushes out, the voltage often drops below the original resting potential, reaching approximately $-90\,\text{mV}$ . This occurs because the potassium gates take a relatively long time to close, causing an "undershoot."
The Refractory Period: This is the phase where the neuron is recharging and cannot fire again immediately.
- Toilet Analogy: Like a toilet that has just been flushed, it cannot be flushed again until the tank refills with water. Similarly, a neuron must reset its ionic balance before another impulse can be generated.

After hyperpolarization, the neuron eventually stabilizes and returns to its Resting Potential of $-70\,\text{mV}$ .
In this final state, the neuron is considered Polarized once more.
The ionic balance returns to normal (high sodium outside, high potassium inside), and the neuron is ready to be stimulated and fire again.