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.
Understanding Action Potential
The membrane acts like a thin barrier with controlled doors. These doors are ion channels, and they do not all behave the same way. Some leak ions slowly all the time.
Others open only when the voltage changes enough. A small local stimulus may fade away because leak channels counter it. Once the membrane reaches threshold, many sodium channels open in a self-reinforcing cycle.
Sodium entry makes the inside less negative, which opens even more sodium channels. This explains the all or none rule. A triggered action potential has a fairly fixed size, while a stronger stimulus usually makes a neuron fire more often rather than produce a larger signal.
The signal moves because one active patch of membrane affects the next patch. Positive charge entering one area spreads inside the cell for a short distance. The nearby membrane then reaches threshold and generates its own action potential.
In this way, the signal travels along an axon without the original ions needing to move from one end to the other. The area just behind the moving signal cannot fire immediately. Sodium channels there are inactivated for a brief time.
This is the absolute refractory period. A later phase, called the relative refractory period, needs a stronger than usual stimulus because potassium channels may still be open. These periods help make one-way travel possible and limit the maximum firing rate.
Myelin changes the speed of conduction. It is a fatty insulating layer wrapped around many axons by support cells. Myelin reduces ion movement across most of the membrane.
Voltage gated channels are concentrated at small uncovered gaps called nodes. The electrical change appears to jump from node to node, a process called saltatory conduction. This is faster and uses less energy than activating every part of the membrane.
Multiple sclerosis damages myelin in the central nervous system. Signals may then slow down, become unreliable, or fail to arrive. This can cause weakness, altered sensation, poor coordination, or vision problems.
Action potentials are not identical in every cell type. Neurons use them to carry information to synapses, where chemical neurotransmitters can affect another cell. Skeletal muscle fibers use them to start the calcium release that leads to contraction.
Heart muscle cells have a longer action potential with a plateau phase. Calcium entry during this plateau helps coordinate a strong heartbeat and prevents the heart from contracting again too soon. When learning this topic, track three things carefully.
Identify which ion channel opens, note the direction each ion moves, and connect that movement to the voltage change. It is useful to separate the fast signal itself from the slower work of restoring ion gradients afterward. Many medicines, toxins, and electrolyte imbalances affect action potentials by changing channel activity or changing the ion gradients that channels depend on.
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.