The threshold potential of a neuron is the minimum voltage the cell membrane must reach to trigger an action potential, the electrical impulse neurons use to communicate. In most mammalian central neurons, this threshold falls between roughly −55 mV and −40 mV, with a typical value around −53 mV. Since neurons sit at a resting potential of about −70 to −80 mV, the membrane needs to depolarize (become less negative) by approximately 15 to 30 millivolts before firing becomes inevitable.
Why the Threshold Exists
A neuron’s membrane is studded with voltage-sensitive sodium channels that stay closed at rest. As the interior of the cell becomes slightly less negative, a few of these channels begin to open, letting positively charged sodium ions flow inward. At voltages below threshold, potassium and other “leak” currents flowing outward are strong enough to counteract that sodium influx, so the membrane voltage settles back down.
Threshold is the tipping point where inward sodium current overtakes outward potassium and leak currents. Once the membrane reaches that voltage, every additional sodium channel that opens drives the cell even more positive, which opens still more sodium channels. This positive feedback loop is why threshold acts like a trigger: below it, nothing happens; above it, the neuron fires a full action potential every time.
The All-or-None Rule
Once the membrane voltage crosses threshold, the action potential that follows is always the same size regardless of how strong the original stimulus was. A whisper and a shout may differ in how many neurons fire or how frequently they fire, but each individual spike is identical. This is the all-or-none principle. The neuron either reaches threshold and produces a complete action potential, or it doesn’t reach threshold and produces nothing. There is no half-sized spike.
Where Firing Starts in the Neuron
Not every part of a neuron is equally easy to excite. For decades, researchers assumed the axon hillock, the junction where the cell body meets the axon, had the lowest threshold because it was packed with sodium channels. More recent patch-clamp recordings from pyramidal neurons tell a slightly different story: sodium channel density in the axon hillock and the cell body is actually similar, around 3 to 4 picoamps per square micrometer. When researchers selectively blocked sodium channels at different locations, they found that the site with the true minimum threshold sits in the axon just beyond the initial segment, not at the hillock itself.
In practical terms, though, the region from the axon hillock through the first stretch of axon still functions as the spike initiation zone. Signals arriving from the dendrites converge here, and this is where the decision to fire or not is made.
How Signals Add Up to Reach Threshold
A single synapse rarely delivers enough voltage change to reach threshold on its own. Instead, a neuron collects small excitatory inputs from many synapses simultaneously and combines them. This process works in two ways. Spatial summation occurs when signals from multiple synapses at different locations on the dendrites arrive close enough in time that their effects overlap. Temporal summation occurs when the same synapse fires repeatedly in quick succession, stacking small voltage bumps before the previous one fades.
If the combined depolarization at the spike initiation zone crosses threshold, the neuron fires. If it falls short, the voltage decays back to rest. Inhibitory inputs from other neurons work against this process by making the membrane more negative, pushing it further from threshold and making firing less likely.
Threshold Is Not Fixed
The specific voltage at which a neuron fires can shift depending on its recent activity and its chemical environment.
Right after firing an action potential, a neuron enters a brief absolute refractory period during which no stimulus of any strength can trigger another spike, because the sodium channels are temporarily locked in an inactive state. This is followed by a relative refractory period, where firing is possible but requires a stronger stimulus than usual. During this phase, potassium channels remain open longer than normal, driving the membrane to a more negative voltage than its typical resting state. That extra negativity means the cell has farther to go before reaching threshold, effectively raising the bar for the next spike.
Sensory vs. Motor Neurons
Threshold differs between neuron types. Sensory axons generally require less current to activate than motor axons. This is partly because sensory neurons have more of a particular type of sodium channel that stays open near resting voltage, along with greater activity of channels that resist hyperpolarization. Both factors keep sensory axons sitting at a slightly less negative resting potential, closer to their firing threshold. On a stimulus-response curve, sensory axons are shifted to the left compared with motor axons, meaning they respond to weaker stimuli.
How Calcium Levels Shift the Threshold
Extracellular calcium concentration has a counterintuitive effect on neuronal excitability. Unlike sodium and potassium, where lower concentrations outside the cell make neurons less excitable, lower calcium outside the cell makes neurons more excitable. Calcium ions normally sit on the outer surface of the membrane and screen its negative charges. When calcium drops, that screening effect weakens, and the voltage-sensing parts of sodium channels behave as though the membrane is more depolarized than it actually is. Their activation shifts toward more negative voltages, meaning threshold is effectively lowered.
In hippocampal neurons, reducing extracellular calcium from the normal 1.2 millimolar to just 0.5 millimolar noticeably increased firing rates. Dropping it further to 0.1 millimolar depolarized the membrane by about 15 millivolts on top of boosting firing frequency. This is why people with abnormally low blood calcium can experience muscle twitching, tingling, and even seizures: their neurons fire too easily because threshold has shifted downward.
Threshold in Simple Terms
Think of threshold potential as a dam holding back a flood. Small trickles of water (subthreshold inputs) flow in but drain away before anything spills over. Once enough water accumulates to overtop the dam, the entire reservoir pours through in a rush that is always the same volume. The dam’s height can change with conditions (calcium levels, recent activity, neuron type), but the basic mechanism is constant: reach the top and the flood is guaranteed, fall short and nothing happens.