Neurophysiology: The Action Potential

The Action Potential

• Action potential is generated when the membrane potential rises above threshold either spontaneously or because of depolarizing input. During the action potential, the membrane potential briefly rises to about +10 mV and then repolarizes to the resting membrane potential.
• Voltage-gated Na⁺ Channel: Sodium channels are opened when the potential rises (i.e., depolarizes) from -65mV to -55mV. They close again when it further rises to +10mV. Action potentials depend on a transient inward current of cations (Na⁺, Ca²⁺) followed by an outward current (K⁺).
• Axonal Propagation of Action Potentials: passive spread of depolarization opens Na⁺ channels in neighboring areas of membrane
• Repolarization: return to resting potential
• Refractory period follows the action potential during which time the threshold for firing is higher than normal.

The Duration of an action potential is 1-2ms in Vertebrates and 1-100ms in Invertebrates. Frequency of firing ranges from <1 to about 100/sec (100Hz). The Amplitude ranges between 70-80mV when recorded intracellularly and 5-200µV when recorded extracellularly.