Overview
This lecture explains how action potentials are generated in skeletal muscle cells, describing the roles of ions, ion channels, and the sarcolemma during the process.
Structure of the Sarcolemma
- The sarcolemma is the plasma membrane of skeletal muscle cells.
- It separates the extracellular space from the intracellular space.
Ion Distribution at Rest
- Sodium ions (Na⁺) are concentrated outside the muscle cell.
- Potassium ions (K⁺) are concentrated inside the muscle cell.
Ion Channels and Pumps
- The sarcolemma contains voltage-gated sodium channels, voltage-gated potassium channels, and sodium-potassium ATPase pumps.
Initiation of Action Potential
- A neuron at the neuromuscular junction provides a stimulus to the sarcolemma.
- This stimulus causes a positive charge to reach voltage-gated sodium channels.
- The increased positive charge opens these sodium channels.
Depolarization
- Sodium ions enter the cell through open channels, making the inside of the membrane more positive.
- This increase in positive charge is called depolarization.
Repolarization
- The positive charge inside the membrane causes sodium channels to close and potassium channels to open.
- Potassium ions flow out of the cell, reducing the positive charge inside.
- This return to resting membrane potential is called repolarization.
- Potassium channels close once repolarization is complete.
Restoration of Ion Gradients
- The sodium-potassium pump uses ATP to restore normal Na⁺ and K⁺ concentration gradients.
Key Terms & Definitions
- Sarcolemma — plasma membrane of a skeletal muscle cell.
- Depolarization — the process where the inside of the sarcolemma becomes more positive due to Na⁺ influx.
- Repolarization — the return of the membrane to its negative resting potential by K⁺ efflux.
- Sodium-potassium pump — an enzyme that uses ATP to restore Na⁺ and K⁺ gradients.
Action Items / Next Steps
- Review the steps of action potential generation and restoration.
- Study the functions of each ion channel and pump in the sarcolemma.