Learning Objective

By the end of this section, the learner will be able to explain the events triggered by a threshold stimulus, describe the ionic basis of the upstroke, repolarization, and after-hyperpolarization of the action potential, understand why the action potential is all-or-none, and recognize how it propagates along the axon without decrement.

Threshold Stimulus

A threshold stimulus is any depolarizing input sufficient to bring the membrane potential to threshold, triggering an action potential.

• Critical concept: Once the threshold is reached, a positive-feedback cycle opens a critical mass of fast Na⁺ channels, producing rapid depolarization.

Ionic Events During the Action Potential

Depolarization / Upstroke

• Threshold depolarization opens fast Na⁺ channels.
• Na⁺ conductance (gNa) rises sharply, driving Em toward ENa (~ +70 mV).
• Further Na⁺ influx opens additional Na⁺ channels → positive-feedback loop.

Peak and Na⁺ Inactivation

• As Em becomes positive, fast Na⁺ channels inactivate → gNa decreases rapidly.
• Inactivation prevents reopening until repolarization occurs.

Repolarization

• Voltage-gated K⁺ channels open in response to depolarization.
• K⁺ conductance (gK) rises more slowly than gNa, initially allowing depolarization to dominate.
• Once gK is high and gNa is low, Em is driven toward EK (~ −95 mV) → rapid repolarization.

After-Hyperpolarization

• K⁺ channels close slowly → gK remains elevated briefly.
• Membrane potential transiently becomes more negative than the resting potential.
• Eventually, gK returns to baseline → membrane potential stabilizes at resting Em.

Key Points

• Upstroke: Mediated by fast Na⁺ influx.
• Repolarization: Dominated by K⁺ efflux; Na⁺ inactivation contributes but is secondary.
• All-or-None: Action potential occurs only if the threshold is reached; subthreshold stimuli fail.
• No Summation: Action potentials do not summate.
• Propagation: Action potential regenerates along the axon; magnitude remains constant.

Conductance Changes