Source: Comprehensive Study Guide on Neurons, APs, and Synaptic Transmission
Tags: action potential, AP, voltage-gated channels, sodium channels, potassium channels, threshold, depolarization, repolarization, refractory period, myelin sheath, nodes of Ranvier, saltatory conduction, propagation
An action potential is a rapid, all-or-nothing electrical signal that travels along a neuron's axon. It is triggered when a graded potential crosses the voltage threshold, opening voltage-gated Na+ channels. The signal propagates in one direction thanks to the refractory period, and myelin sheaths dramatically speed up conduction by allowing the impulse to jump between nodes of Ranvier.
Action potential (AP)
A brief, rapid reversal of membrane polarity that propagates along the axon. It is the primary long-distance signalling mechanism in neurons.
Voltage-gated Na+ channel
A sodium channel that opens in response to depolarization reaching the threshold voltage. Opens quickly, stays open for roughly 1 ms, then inactivates. Responsible for the rapid depolarization phase of the AP.
Voltage-gated K+ channel
A potassium channel that opens in response to depolarization but does so more slowly than the Na+ channel. Allows K+ outflow to repolarize the membrane. Also slow to close, which can cause a brief hyperpolarization overshoot.
Voltage threshold
The membrane potential (typically around -55 mV) at which voltage-gated Na+ channels open and an action potential is triggered. Below this level, the graded potential fades without producing an AP.
Graded potential
A local, variable-strength change in membrane potential (from synaptic input or sensory stimulation) that may or may not reach threshold. Contrasts with the all-or-nothing action potential.
Refractory period
The interval after an action potential during which the membrane cannot (absolute) or is less likely to (relative) fire again. Ensures one-way propagation of the signal down the axon.
Myelin sheath
An insulating fatty layer wrapped around axon segments by glial cells (oligodendrocytes in the CNS, Schwann cells in the PNS). Prevents ion leak and speeds up signal conduction.
Nodes of Ranvier
Gaps between myelin segments where voltage-gated ion channels are concentrated. Action potentials regenerate at these nodes.
Saltatory conduction
The process by which action potentials "jump" from one node of Ranvier to the next along a myelinated axon, greatly increasing conduction speed.
At rest: all voltage-gated channels are closed. Only leak channels are open, maintaining the resting potential (~-65 mV)
Step 1, threshold reached: a graded potential (from dendrites or the cell body) depolarizes the membrane to threshold (~-55 mV)
Step 2, Na+ influx (rapid depolarization): voltage-gated Na+ channels open, Na+ rushes into the cell, and the membrane potential shoots toward +30 to +40 mV
Step 3, Na+ channels inactivate: after approximately 1 ms, voltage-gated Na+ channels close (inactivate). No more Na+ enters
Step 4, K+ outflow (repolarization): voltage-gated K+ channels, which opened more slowly, now allow K+ to flow out of the cell. The membrane potential drops back toward resting
Step 5, return to rest: K+ channels eventually close. The membrane potential may briefly overshoot (hyperpolarize) before the Na+/K+ pump restores the precise resting value
When Na+ floods into one segment of the axon, the positive charge spreads to the neighbouring segment and depolarizes it to threshold
This triggers the next set of voltage-gated Na+ channels to open, creating a domino-like chain of depolarization down the axon
The refractory period in the segment that just fired prevents the signal from travelling backwards, so propagation is always one-directional (away from the cell body)
Myelin insulates the axon, preventing ion leak through the membrane between nodes
Because ions cannot escape through the myelinated sections, the electrical signal passes passively and very quickly to the next node of Ranvier
At each node, the action potential is regenerated by voltage-gated channels
This "jumping" from node to node (saltatory conduction) is much faster than continuous conduction along an unmyelinated axon
Diseases that damage myelin (e.g. multiple sclerosis) slow or block signal conduction
Typical threshold: ~-55 mV
Peak depolarization: ~+30 to +40 mV
Na+ channel open time: ~1 ms
Sequence summary: rest → threshold → Na+ in (depolarize) → Na+ channels close → K+ out (repolarize) → brief hyperpolarization → rest restored
⚠️ The action potential is all-or-nothing. Once threshold is reached, the AP fires at full strength regardless of stimulus intensity. Stimulus strength is coded by firing frequency, not AP amplitude.
⚠️ Na+ channels open fast and close fast. K+ channels open slow and close slow. This timing difference is what creates the shape of the AP waveform, and it is a very common exam point.
⚠️ The refractory period explains one-way propagation. The segment behind the AP cannot fire again immediately, so the signal can only move forward.
⚠️ Saltatory conduction is the reason myelinated neurons conduct so much faster. If asked to compare myelinated vs. unmyelinated conduction, the key difference is that myelin allows the signal to jump between nodes rather than travelling continuously.
Q: What are the key phases of an action potential, in order?
A: Resting state (all voltage-gated channels closed) → graded potential reaches threshold → voltage-gated Na+ channels open (rapid depolarization) → Na+ channels inactivate (~1 ms) → voltage-gated K+ channels allow K+ outflow (repolarization) → K+ channels close, membrane returns to resting potential (may briefly hyperpolarize before stabilising).
Q: Why does the action potential travel in only one direction along the axon?
A: Because of the refractory period. The segment of axon that has just fired cannot immediately fire again, so the depolarization wave can only move forward into segments that have not yet reached threshold.
Q: How do myelin sheaths affect the speed of action potential propagation?
A: Myelin insulates the axon and prevents ion leak between nodes of Ranvier. The signal passes passively through myelinated segments and regenerates at each node (saltatory conduction). This is significantly faster than continuous propagation along an unmyelinated axon.
Q: What would happen to neural signalling if voltage-gated K+ channels opened at the same speed as Na+ channels?
A: Repolarization would begin almost immediately after depolarization started, which would truncate or prevent the full depolarization phase. The characteristic shape (and function) of the action potential depends on the delay between Na+ channel opening and K+ channel opening.
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