Action Potential Threshold- What Determines It?

What Is the Action Potential Threshold?

The action potential threshold is the minimum membrane potential a neuron must reach before an action potential fires. It's typically around -55 mV, but this number varies.

Below threshold? Nothing happens. The neuron sits quiet, processing tiny graded signals. Cross threshold? Bang — voltage-gated sodium channels snap open and the neuron fires.

Most textbooks give you a single number and move on. That's lazy. The threshold isn't fixed. It shifts based on several factors, and understanding those factors is what separates people who actually understand neuroscience from those who just memorize definitions.

Why the Threshold Exists

Your neurons don't fire at every whisper of input. That would be chaos. The threshold acts as a noise filter.

Small, random fluctuations in membrane potential happen constantly — thermal noise, neurotransmitter spillover, ion channel chatter. If neurons responded to all of it, your nervous system would be useless static.

The threshold ensures only meaningful signals — ones strong enough to push the membrane far enough — trigger an action potential. It's a decision point. A gate.

What Actually Determines the Threshold

Three main things set your threshold:

• Resting membrane potential — How polarized the neuron is at rest. More negative resting potential means you need more excitatory input to hit threshold.
• Input resistance — Higher resistance means smaller currents produce bigger voltage changes. More sensitive to inputs. Lower threshold, basically.
• Threshold voltage for sodium channel activation — Different sodium channel subtypes have different activation voltages. What voltage snaps them open? That sets the floor.

These aren't independent. They interact. Change one, and you shift the threshold.

The Sodium Channel Factor

Voltage-gated sodium channels are the real threshold enforcers. They're the ones that detect when the membrane has depolarized enough and flip into action.

Different sodium channel types open at different voltages:

• Nav1.1, Nav1.2, Nav1.6 — Common in most neurons. Activation starts around -55 to -50 mV.
• Nav1.7, Nav1.8 — Found in peripheral neurons. Nav1.8 activates at more positive potentials — higher threshold.

More sodium channels packed into your membrane? Lower threshold. Fewer channels? You need stronger input to reach the same voltage.

The Potassium Channel Influence

Potassium channels don't set the threshold directly, but they shape the depolarization curve. Fast-activating potassium currents (like Kv1.1 or Kv4.x) oppose depolarization. They make the membrane voltage climb more slowly.

The practical effect? A neuron with strong fast potassium currents needs more excitatory drive to reach threshold. The threshold effectively rises.

Block potassium channels with toxins or drugs, and threshold drops. The neuron becomes easier to fire. That's why potassium channel blockers often cause seizures — hyperexcitable neurons.

Factors That Shift the Threshold

Threshold isn't static. It moves based on conditions:

• Temperature — Warmer neurons have lower thresholds. They fire more easily. Fever makes neurons more excitable, which is part of why high fevers can cause seizures.
• pH changes — Acidosis (lower pH) raises threshold. Alkalosis lowers it. Ion channel kinetics change with hydrogen ion concentration.
• Neuromodulators — Substances like norepinephrine, serotonin, or dopamine shift threshold through G-protein coupled receptors. This is how your brain state affects excitability.
• Axon diameter — Larger diameter axons have lower internal resistance. Current spreads faster and further. Lower threshold.
• Myelination — Myelin increases conduction speed but also affects threshold at nodes of Ranvier. Nodes have high sodium channel density, so threshold is lower there.
• Neurological disease — Channelopathies (mutations in ion channel genes) directly alter threshold. Some epilepsy mutations raise threshold. Others lower it too much.

Comparing Factors That Determine Threshold

All-or-None: Why Threshold Doesn't Change the Output

Here's the weird part: once you cross threshold, the action potential amplitude is fixed. It doesn't matter if you barely crossed threshold or blew past it by 20 mV.

This is the "all-or-none" principle. The threshold is a gate, not a dial. Cross it, and sodium channels flood in until the membrane reverses polarity. The action potential size is determined by ion concentrations and membrane properties, not by how strong the triggering stimulus was.

Stronger stimuli don't produce bigger action potentials. They produce more frequent action potentials. Rate coding, not amplitude coding.

How to Think About Threshold Practically

Stop thinking of threshold as a single number. Think of it as a dynamic excitability state.

Your neurons are constantly adjusting their threshold based on:

• Prior activity (use-dependent changes)
• Neuromodulatory tone (what brain state are you in?)
• Metabolic state (ATP levels affect ion pumps)
• Pathological changes (ischemia, toxins, mutations)

This is why understanding threshold matters beyond textbook definitions. It's not just "what voltage triggers firing." It's "what determines whether this neuron fires in a given physiological context?"

When you're reading about epilepsy, channelopathies, anesthesia, or neuromodulation — you're reading about threshold manipulation. The drugs and conditions don't change some abstract property. They shift the voltage at which sodium channels open, or the balance of currents pushing the membrane toward or away from that voltage.

That's the actual picture. Everything else is detail.