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Full curriculum

Full curriculum
  1. Voltage-gated ion channels and electrical excitability
  2. Action potentials, threshold and refractory periods
  3. Action-potential propagation along axons
  4. Myelin and saltatory conduction
  5. Chemical synapses and presynaptic-postsynaptic organization
  6. Calcium-triggered synaptic vesicle release
  7. Ionotropic and metabotropic neurotransmitter receptors
  8. Neurotransmitter clearance and termination of chemical synaptic signals

Unit content

Action potentials, threshold and refractory periods

An action potential is a rapid, regenerative change in membrane voltage produced by coordinated opening and closing of voltage-gated ion channels.

It is not simply a large passive depolarization. Once the membrane reaches a sufficiently depolarized threshold region, positive feedback through voltage-gated Na$^+$ channels drives a stereotyped electrical event.

A simplified action-potential cycle

  1. Resting state. Most voltage-gated Na$^+$ channels are closed but available to open; K$^+$ leak pathways help maintain a negative resting potential.
  2. Threshold and rising phase. Depolarization opens voltage-gated Na$^+$ channels. Na$^+$ entry depolarizes the membrane further, opening still more Na$^+$ channels.
  3. Peak. Na$^+$ channels begin to inactivate, so the regenerative inward current cannot continue indefinitely.
  4. Repolarization. Delayed voltage-gated K$^+$ channels are open. K$^+$ movement drives the membrane voltage back toward $E_K$.
  5. After-hyperpolarization. K$^+$ conductance can remain elevated briefly, making the voltage more negative than its resting value before channels return toward their resting state.

The voltage trajectory is therefore created by changing relative conductances, not by reversing the underlying Na$^+$ and K$^+$ concentration gradients on every spike.

Threshold produces an all-or-none event

A small depolarization below threshold decays without triggering the full regenerative cycle. A depolarization that recruits enough inward positive feedback produces a full action potential.

Stronger stimulation generally does not make individual action potentials proportionally taller. Instead, it can change when action potentials occur or how frequently they are generated.

Refractory periods

Immediately after an action potential, many Na$^+$ channels are inactivated. During this absolute refractory period, another normal action potential cannot be initiated because too few Na$^+$ channels are available.

As Na$^+$ channels recover but K$^+$ conductance remains elevated, the membrane enters a relative refractory period: another action potential is possible, but a stronger depolarizing input is required.

Refractoriness limits firing frequency and helps make action-potential propagation effectively directional along an axon.