Resting Membrane Potential as the Basis of Excitation
The action potential is a brief change in the cell membrane potential from negative to positive in response to a stimulus. For it to occur, the cell must initially be at the resting membrane potential — a negative membrane charge of about −70 mV.
The maintenance of the resting membrane potential is ensured by three key mechanisms:
- Passive efflux of K⁺ ions from the cell through resting potassium channels: the departure of positive charges makes the internal environment more negative.
- Operation of the Na⁺/K⁺-ATPase: each cycle pumps out 3 Na⁺ ions and pumps in 2 K⁺ ions, which establishes and maintains ionic gradients.
- Formation of a near-surface layer of K⁺ ions on the outer side of the membrane, which electrostatically prevents the entry of other cations.
Stimulus and Critical Level of Depolarization
Any effect on a cell is called a stimulus. By strength, stimuli are divided into two categories:
- Subthreshold — do not reach the critical level of depolarization; they cause only a local response without a full action potential.
- Threshold — sufficient for the membrane potential to reach the critical level of depolarization (CLD), usually around −50 mV.
Upon reaching the CLD, voltage-gated sodium channels open, which triggers the full action potential.
Phases of the Action Potential
The development of an action potential includes several sequential phases:
- Local response — a small shift of the membrane potential toward the CLD under the action of a weak stimulus.
- Depolarization — an avalanche-like influx of Na⁺ ions through open voltage-gated sodium channels; the membrane charge rapidly becomes positive.
- Sodium channel inactivation — a built-in channel "sensor" closes it upon reaching a positive level, stopping the influx of Na⁺.
- Repolarization — opening of voltage-gated potassium channels, efflux of K⁺ from the cell, and rapid restoration of the negative charge.
- Afterpotentials — a slow return to the baseline level due to the operation of the Na⁺/K⁺-ATPase; hyperpolarization often occurs, where the membrane charge temporarily becomes more negative than the resting potential.
Having arisen at one point, the action potential propagates along the entire membrane, ensuring cell excitation and signal transmission to neighboring structures.
Graphical Components of the Action Potential
The following characteristic sections are distinguished on the action potential graph:
- Pre-spike (pacemaker potential / foot) — the slow approach of the membrane potential to the CLD.
- Spike — a sharp peak consisting of ascending (depolarization) and descending (repolarization) parts.
- Negative afterpotential — the continuation of repolarization below the baseline level.
- Positive afterpotential — the gradual return to the resting potential from below.
The terms depolarization, repolarization, and hyperpolarization describe the direction of charge change: from minus to plus, back, and beyond the initial negative level, respectively.
Excitability Dynamics and Refractory Periods
Excitability — the ability of a cell to respond to stimulation by generating an action potential. It is higher the closer the membrane potential is to the CLD, and changes systematically throughout the cycle:
- Supernormal excitability (exaltation) — during the phase of the local response and pre-spike, as well as during the negative afterpotential; the cell is easily excited even by a weak stimulus.
- Absolute refractory period — corresponds to the ascending part of the spike; all sodium channels are open or inactivated, and the cell does not respond to any stimulus.
- Relative refractory period — the descending part of the spike; a response is possible only under the action of a very strong stimulus.
- Subnormal excitability — the phase of the positive afterpotential; the membrane is farther from the CLD, requiring a stronger stimulus than normal to excite.
Key points
- The resting potential of about −70 mV is maintained by K⁺ efflux, Na⁺/K⁺-ATPase activity, and the near-surface layer of cations.
- An action potential occurs only with a threshold stimulus that brings the membrane to the critical level of depolarization.
- Depolarization is caused by an avalanche-like influx of Na⁺, and repolarization by the efflux of K⁺.
- Sodium channels spontaneously inactivate upon reaching a positive membrane charge.
- Hyperpolarization is a temporary deepening of the negative charge below the resting level.
- Excitability is maximal near the CLD and minimal during the absolute refractory phase.
- Refractory periods are divided into absolute (spike) and relative (descending part of the spike).
- The action potential propagates along the entire membrane, ensuring the conduction of excitation.