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Action Potential
Resting, Depolarization, Repolarization
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An action potential is the rapid electrical signal that allows neurons, skeletal muscle cells, and cardiac cells to communicate and respond. It begins from a resting membrane potential and then moves through depolarization and repolarization in a precise sequence. Understanding this process is essential in medical science because it explains nerve signaling, muscle contraction, reflexes, and many drug effects. It also helps students connect cell physiology to clinical problems such as arrhythmias, seizures, and electrolyte disorders.
At rest, the cell membrane is polarized because ion concentrations differ across the membrane and selective channels allow more K+ movement than Na+ movement. During depolarization, voltage gated Na+ channels open and Na+ rushes into the cell, making the membrane potential less negative and then positive. During repolarization, Na+ channels inactivate and voltage gated K+ channels open, allowing K+ to leave the cell and restore a negative membrane potential. The Na+/K+ ATPase helps maintain the ion gradients over time by pumping 3 Na+ out and 2 K+ in.
Key Facts
- Typical resting membrane potential in neurons is about -70 mV.
- Threshold for triggering an action potential is often about -55 mV.
- Depolarization occurs when voltage gated Na+ channels open and Na+ enters the cell.
- Repolarization occurs when Na+ channels inactivate and voltage gated K+ channels open, causing K+ efflux.
- Na+/K+ ATPase: 3 Na+ out + 2 K+ in + 1 ATP used.
- Membrane potential can be written as Vm = Vin - Vout.
Vocabulary
- Resting membrane potential
- The stable negative voltage across the membrane of an unstimulated cell, usually around -70 mV in neurons.
- Depolarization
- The phase when the membrane potential becomes less negative, usually because Na+ enters the cell.
- Repolarization
- The phase when the membrane potential returns toward its resting negative value, mainly because K+ leaves the cell.
- Threshold
- The membrane potential that must be reached to open enough voltage gated Na+ channels to trigger an action potential.
- Refractory period
- The time after an action potential when the cell cannot fire again easily because ion channels have not fully reset.
Common Mistakes to Avoid
- Thinking the Na+/K+ pump directly causes the rapid upstroke of the action potential, which is wrong because the fast depolarization is mainly due to voltage gated Na+ channels opening. The pump mainly maintains long term ion gradients.
- Assuming repolarization happens because Na+ is pumped out immediately, which is wrong because repolarization mainly occurs when Na+ channels inactivate and K+ exits through voltage gated K+ channels. Pump activity is too slow to explain the rapid falling phase.
- Forgetting that threshold must be reached before a full action potential occurs, which is wrong because subthreshold stimuli usually produce only small local changes. Action potentials follow the all or none principle once threshold is crossed.
- Mixing up channel opening with channel inactivation, which is wrong because a Na+ channel can stop conducting even while the membrane is still depolarized. This inactivation is a key reason the refractory period occurs.
Practice Questions
- 1 A neuron starts at -70 mV and reaches threshold at -55 mV. By how many millivolts must the membrane potential change to reach threshold?
- 2 During one cycle of the Na+/K+ ATPase, 300 Na+ ions are pumped out of a cell. How many K+ ions are pumped into the cell during the same time?
- 3 A toxin prevents voltage gated K+ channels from opening normally. Explain how this would affect repolarization and the duration of the action potential.