1. Resting Membrane Potential:
* Sodium-Potassium Pump: Muscle cells maintain a resting membrane potential of about -70mV. This is achieved primarily by the sodium-potassium pump, which actively pumps 3 sodium ions out of the cell for every 2 potassium ions pumped in. This creates a concentration gradient, with more sodium outside the cell and more potassium inside.
* Leak Channels: There are also leak channels that allow a small amount of sodium to leak into the cell and potassium to leak out. However, the pump maintains the overall negative charge inside the cell.
2. Depolarization:
* Stimulus: A stimulus, such as a signal from a nerve, triggers the opening of voltage-gated sodium channels. These channels are sensitive to changes in membrane potential.
* Sodium Influx: As the channels open, sodium ions rush into the cell, driven by both the concentration gradient and the electrical gradient.
* Rapid Depolarization: This influx of positive sodium ions causes the membrane potential to become less negative (depolarize). If the stimulus is strong enough, the membrane potential will reach a threshold level, typically around -55mV.
3. Action Potential:
* Positive Feedback: Once the threshold is reached, a positive feedback loop is initiated. More sodium channels open, allowing even more sodium to enter, further depolarizing the cell.
* Peak Potential: The membrane potential continues to rise rapidly, reaching a peak of about +30mV.
* Sodium Channel Inactivation: As the membrane potential reaches its peak, sodium channels begin to inactivate, reducing sodium influx.
4. Repolarization:
* Potassium Channel Activation: At the same time that sodium channels are inactivating, voltage-gated potassium channels are opening. This allows potassium ions to flow out of the cell, driven by both their concentration gradient and the now positive electrical gradient.
* Restoration of Polarity: The outflow of potassium ions rapidly repolarizes the membrane, returning it towards its resting potential.
5. Hyperpolarization:
* Potassium Channel Closure: Potassium channels remain open for a short time after the membrane potential returns to resting, leading to a brief hyperpolarization (more negative than resting potential).
6. Return to Resting Potential:
* Pump Activity: The sodium-potassium pump continues to operate, restoring the original ion concentration gradients and returning the membrane potential to its resting state.
Summary:
Muscle cells generate a potential difference by changing the permeability of their membranes to sodium and potassium ions. The influx of sodium during depolarization and the outflow of potassium during repolarization create a rapid change in membrane potential that is known as an action potential. This action potential travels along the muscle cell membrane and triggers the contraction of the muscle fiber.
