Tags: action potential, resting potential, sodium-potassium pump, Na+/K+ ATPase, depolarization, repolarization, hyperpolarization, voltage-gated channels, threshold, axon hillock, refractory period, saltatory conduction, nodes of Ranvier, myelin, ion channels
Neurons communicate via action potentials, rapid all-or-none changes in membrane voltage that travel along the axon. The resting potential (~-65 mV) is maintained by the Na+/K+ pump. When depolarisation reaches threshold (~-50 mV), voltage-gated Na+ channels open, triggering an action potential. Repolarisation follows via K+ efflux. In myelinated axons, the signal jumps between nodes of Ranvier (saltatory conduction), making transmission fast and energy-efficient.
Resting membrane potential
The voltage across a neuron's membrane at rest, approximately -65 mV (inside negative relative to outside). Maintained primarily by the Na+/K+ ATPase pump and potassium leak channels.
Na+/K+ ATPase (sodium-potassium pump)
An active transporter that moves 3 Na+ ions out of the cell and 2 K+ ions into the cell per ATP molecule consumed. This creates and maintains the electrochemical gradient essential for signalling.
Firing threshold
The membrane potential (approximately -50 mV) at which voltage-gated Na+ channels open and an action potential is triggered. Reached when excitatory inputs summate at the axon hillock.
Axon hillock
The region where the axon emerges from the cell body. It has a high density of voltage-gated Na+ channels and is where the decision to fire an action potential is made.
Depolarisation
The phase of the action potential in which the membrane potential becomes more positive (less negative). Caused by the rapid influx of Na+ through voltage-gated Na+ channels.
Repolarisation
The phase in which the membrane potential returns toward resting levels. Caused by voltage-gated K+ channels opening and K+ flowing out of the cell, while Na+ channels close or inactivate.
Hyperpolarisation (undershoot)
A brief period during which the membrane potential dips below resting level (more negative than -65 mV) before returning to rest. Results from the slight delay in K+ channel closing.
All-or-none principle
Once threshold is reached, the action potential fires at full amplitude. There is no "partial" action potential. Signal strength is encoded by firing frequency, not by the size of each individual action potential.
Refractory period
A brief interval after an action potential during which the neuron cannot fire again (absolute refractory period) or requires a stronger-than-normal stimulus (relative refractory period). This prevents backward propagation and ensures the signal travels in one direction.
Voltage-gated Na+ channels
Membrane proteins that open in response to depolarisation, allowing Na+ to rush into the cell. Responsible for the rising phase of the action potential.
Voltage-gated K+ channels
Membrane proteins that open slightly after Na+ channels, allowing K+ to flow out of the cell. Responsible for repolarisation.
Nodes of Ranvier
Gaps in the myelin sheath along a myelinated axon where ion channels are concentrated. Action potentials regenerate at these nodes.
Saltatory conduction
The mode of action potential propagation in myelinated axons. The signal "jumps" from one node of Ranvier to the next, which is much faster and more energy-efficient than continuous conduction in unmyelinated axons.
The Na+/K+ pump uses one molecule of ATP to move 3 Na+ out and 2 K+ in.
This net export of positive charge, combined with potassium leak channels and the selective permeability of the membrane, keeps the inside of the neuron at approximately -65 mV.
The resulting electrochemical gradient is the stored energy that powers action potentials.
Rest: membrane at ~-65 mV.
Stimulus and summation: excitatory postsynaptic potentials (EPSPs) arrive and summate at the axon hillock.
Threshold (~-50 mV): voltage-gated Na+ channels open.
Depolarisation: Na+ rushes in, membrane potential shoots toward +30 to +40 mV.
Na+ channel inactivation: Na+ channels close or enter an inactivated state.
Repolarisation: voltage-gated K+ channels open, K+ flows out, membrane potential drops back.
Hyperpolarisation (undershoot): K+ channels close with a slight delay, so the potential briefly dips below -65 mV.
Return to rest: the Na+/K+ pump and leak channels restore the resting potential.
The influx of Na+ at one point depolarises the adjacent membrane segment, bringing it to threshold.
This chain reaction moves the action potential along the axon.
The refractory period behind the active zone ensures unidirectional travel (no back-propagation).
Myelin insulates the axon, preventing ion flow through the insulated segments.
Ion channels are concentrated at the nodes of Ranvier (the gaps between myelin segments).
The action potential regenerates at each node and current flows passively through the myelinated internodes, making the signal appear to "jump."
This is faster (up to 100x) and more energy-efficient than continuous conduction.
Na+/K+ ATPase stoichiometry
3 Na+ out : 2 K+ in : 1 ATP consumed
Key voltages
Resting potential: ~-65 mV Firing threshold: ~-50 mV Peak depolarisation: ~+30 to +40 mV
⚠️ The pump ratio is 3 Na+ out, 2 K+ in. The exam tests this exact ratio. Do not reverse the ions or the direction.
⚠️ At threshold, voltage-gated Na+ channels open (not Cl- channels, not the pump stopping). This is a common distractor.
⚠️ Saltatory conduction is made possible by nodes of Ranvier along myelinated axons. The exam tests this specific mechanism, not gap junctions or extracellular K+.
⚠️ The refractory period prevents backward propagation. It does not mean the neuron "enters the refractory period immediately" at threshold; the refractory period follows the action potential.
Q: How many ions does the Na+/K+ ATPase move per ATP molecule, and in which direction?
A: 3 Na+ out of the cell and 2 K+ into the cell.
Q: What happens when a neuron's membrane potential reaches -50 mV?
A: Voltage-gated Na+ channels open, initiating an action potential. Na+ rushes into the cell, causing rapid depolarisation.
Q: What makes saltatory conduction possible?
A: Nodes of Ranvier along myelinated axons. Ion channels are concentrated at these unmyelinated gaps, so the action potential regenerates at each node and jumps between them.
Q: Why does the action potential travel in only one direction along the axon?
A: The refractory period. After an action potential passes a given point, the Na+ channels there are inactivated and cannot be immediately re-triggered, preventing backward propagation.
Q: Describe the ion movements during depolarisation and repolarisation.
A: Depolarisation: voltage-gated Na+ channels open, Na+ flows into the cell. Repolarisation: Na+ channels close/inactivate, voltage-gated K+ channels open, K+ flows out of the cell.
action potential, resting potential, membrane potential, sodium-potassium pump, Na+/K+ ATPase, depolarisation, repolarisation, hyperpolarisation, voltage-gated sodium channels, voltage-gated potassium channels, threshold potential, axon hillock, all-or-none, refractory period, saltatory conduction, nodes of Ranvier, myelin sheath, continuous conduction, EPSP, electrochemical gradient, MCB C61, Brain-Mind Odyssey