A neuron does not fire every time it receives input. Only when the membrane potential at the axon hillock reaches a critical value—typically around –55 mV—do voltage-gated sodium channels open regeneratively and produce an action potential. That critical value is the threshold.

Subthreshold inputs produce local depolarizations that decay; suprathreshold inputs trigger the full spike. Threshold is not absolutely fixed: it can shift with recent activity, neuromodulation, or changes in channel density and resting potential. The concept also applies at the systems level—to the stimulus intensity required to elicit a percept or a behavioral response.

Understanding threshold is essential for interpreting neuronal integration and for designing stimuli in experiments. Drugs and toxins that alter threshold change excitability and can be therapeutic or pathological. In clinical neurophysiology, measures related to threshold help characterize nerve and muscle disorders.

• Critical membrane potential for firing
• Typically near –55 mV at the axon hillock
• Separates subthreshold from suprathreshold inputs
• Can be modulated by experience and neuromodulators