This cheat sheet covers how neurons generate and transmit action potentials, the rapid electrical signals used for communication in the nervous system. Students need this reference to connect membrane voltage changes with ion movement, channel behavior, and signal direction. It is especially useful for reading action potential graphs and understanding how neurons encode information.
The focus is on clear stages, key voltages, and the roles of sodium and potassium ions.
Key Facts
- A typical resting membrane potential is about -70 mV, with the inside of the neuron more negative than the outside.
- Threshold is usually about -55 mV, and reaching it triggers an all-or-none action potential.
- Depolarization occurs when voltage-gated Na+ channels open and Na+ rushes into the neuron, making the membrane potential more positive.
- Repolarization occurs when voltage-gated Na+ channels inactivate and voltage-gated K+ channels open, allowing K+ to leave the neuron.
- Hyperpolarization occurs when K+ channels close slowly, causing the membrane potential to briefly become more negative than resting potential.
- The sodium-potassium pump helps maintain ion gradients by moving 3 Na+ out of the cell and 2 K+ into the cell using ATP.
- The absolute refractory period prevents a second action potential because voltage-gated Na+ channels are inactivated.
- Action potentials travel in one direction along an axon because the membrane behind the signal is temporarily refractory.
Vocabulary
- Resting membrane potential
- The voltage across a neuron's membrane when it is not sending an action potential, usually about -70 mV.
- Threshold
- The membrane voltage, usually about -55 mV, that must be reached to trigger an action potential.
- Depolarization
- The stage of an action potential when the inside of the neuron becomes more positive due to Na+ entering the cell.
- Repolarization
- The stage when the membrane potential becomes more negative again as K+ leaves the cell.
- Refractory period
- The short recovery time after an action potential when the neuron cannot fire or is harder to fire.
- Myelin sheath
- An insulating layer around some axons that speeds signal conduction by allowing impulses to jump between nodes.
Common Mistakes to Avoid
- Confusing depolarization with repolarization is wrong because depolarization is caused mainly by Na+ entering, while repolarization is caused mainly by K+ leaving.
- Thinking a stronger stimulus makes a larger action potential is wrong because action potentials are all-or-none and do not increase in height after threshold is reached.
- Ignoring the refractory period is wrong because it explains why action potentials travel forward and why neurons need time before firing again.
- Saying the sodium-potassium pump directly causes the spike is wrong because the spike is mainly produced by voltage-gated ion channels, while the pump maintains long-term ion gradients.
- Reading voltage values without signs is wrong because -70 mV and +30 mV represent very different membrane states and ion movements.
Practice Questions
- 1 A neuron has a resting potential of -70 mV and threshold of -55 mV. How many millivolts must the membrane potential increase to reach threshold?
- 2 During an action potential, the membrane voltage rises from -55 mV to +30 mV. What is the total change in membrane potential?
- 3 If one sodium-potassium pump cycle moves 3 Na+ out and 2 K+ in, how many Na+ ions move out after 10 cycles?
- 4 Explain why an action potential usually travels in only one direction along an axon.
Understanding Neuron Action Potential Reference
A neuron membrane acts like a thin barrier with carefully controlled leaks. Ions are not spread evenly on the two sides. This uneven distribution stores potential energy, much like water held behind a dam.
Proteins in the membrane decide which ions can cross and when they can cross. Some channels are open at rest, while others respond to a voltage change. The membrane voltage is a measurement of charge separation across an extremely thin layer.
Only a small fraction of the cell's total ions need to move to produce a large voltage change. This is why a neuron can signal rapidly without emptying its supply of sodium or potassium each time.
Voltage-gated channels have more than one moving part. A sodium channel has an activation gate that opens quickly when the voltage rises. It also has an inactivation gate that closes shortly afterward.
This timing explains the sharp upward and downward parts of an action potential graph. Potassium channels usually open more slowly, so their effect becomes strongest after sodium channels stop admitting ions. When reading a graph, focus on the sequence of channel states rather than memorising the curve shape.
A change in voltage causes channel movements, then ion flow changes the voltage further. This feedback makes the rising phase so fast.
An action potential moves because an active patch of membrane creates local electrical currents in the next patch of axon. These currents bring the nearby membrane toward threshold. The signal is regenerated at each new section, so it does not fade away like a sound becoming quieter over distance.
In a myelinated axon, insulation reduces ion leakage across most of the membrane. Ion exchange is concentrated at gaps called nodes of Ranvier. The signal appears to jump from node to node, which greatly increases speed and saves energy.
Neurons communicate stronger information mainly by changing how often they fire, not by making one action potential taller. Different groups of neurons can be recruited when a stimulus becomes stronger.
It helps to separate the fast electrical event from the slower work of restoring ion distributions. The sodium-potassium pump is essential over time, but it does not cause the rapid falling phase of one action potential. Channel opening and closing produce that immediate change.
This distinction is often tested in biology courses. Some poisons, medicines, and diseases affect nerve signals by changing channel function. Local anaesthetics reduce pain signals by blocking sodium channels.
Demyelinating diseases slow conduction because current escapes through poorly insulated membrane. When studying, practise tracing a graph from left to right and naming the dominant ion movement, channel condition, and membrane response at each point. This method makes unfamiliar graphs much easier to interpret.