Action Potential Formula
Reference for the Hodgkin-Huxley equations describing neural action potentials, the electrical impulses that carry signals through the nervous system.
Key Notes
- Phases: resting (−70 mV) → depolarization → repolarization → hyperpolarization → recovery: The resting potential (−70 mV) is maintained by the Na⁺/K⁺-ATPase pump and selective channel permeability. Depolarization: Na⁺ channels open, Na⁺ rushes in → membrane reaches +30 mV. Repolarization: K⁺ channels open, K⁺ exits → voltage falls.
- All-or-nothing principle: Once membrane voltage reaches threshold (~−55 mV), a full action potential fires — no partial responses. Stimulus strength above threshold does not increase the action potential's size or duration; it increases the firing frequency (rate coding).
- Refractory periods: Absolute refractory period: no second action potential possible (Na⁺ channels inactivated). Relative refractory period: a second AP can fire but requires a stronger-than-normal stimulus (K⁺ channels still partially open, membrane slightly hyperpolarized). These periods limit maximum firing rate.
- Conduction velocity: Myelinated axons: saltatory conduction (signal jumps between nodes of Ranvier at 70–120 m/s). Unmyelinated C fibers: continuous conduction at 0.5–2 m/s. The ~60× speed difference explains why sharp (myelinated Aδ) pain is felt before dull (unmyelinated C) pain after an injury.
- Applications: Action potential dynamics underlie cardiac electrophysiology (EKG waveforms reflect ventricular action potentials), neuromuscular junction triggering (muscle contraction), local anesthesia (sodium channel blockers like lidocaine stop AP propagation), neurological disorder treatment, and neural interface/brain-computer interface design.