Fisiologia do Potencial de Ação: Etapas da Propagação do Sinal Neuronal

An action potential (AP) is a brief, all-or-none reversal of membrane potential propagating along axons.

Phases

1. Resting Potential (~−70 mV): Maintained by Na⁺/K⁺-ATPase (3 Na⁺ out, 2 K⁺ in) and K⁺ leak channels. Inside negative.
2. Depolarization: Stimulus opens voltage-gated Na⁺ channels → Na⁺ influx → Vm → +30 mV.
3. Overshoot / Peak: Na⁺ channels inactivate; voltage-gated K⁺ channels open (delayed).
4. Repolarization: K⁺ efflux restores negative Vm.
5. After-hyperpolarization (AHP): K⁺ channels close slowly → Vm ~−90 mV.
6. Refractory Periods:
   • Absolute: Na⁺ channels inactivated; no AP possible.
   • Relative: K⁺ conductance elevated; stronger stimulus needed.

Propagation

Local current flow depolarizes adjacent membrane to threshold → sequential AP generation. Myelinated axons: saltatory conduction at nodes of Ranvier (100× faster).

Key Properties

• All-or-none (amplitude independent of stimulus strength).
• Non-decremental (amplitude maintained).
• Unidirectional (refractory period prevents back-propagation).

APs encode information in frequency, not amplitude.