Pt2 flash card

Muscle Contraction and Membrane Potentials

• Muscle Fibers:

  • Electrically excitable cells that respond to signals to initiate contraction.

  • Understanding of membrane potentials is crucial for grasping contraction mechanisms.

• Membrane Potential:

  • Caused by an unequal distribution of ions across the plasma membrane.

  • Negative ions are located on the cytosolic side, and positive ions are on the extracellular side, creating a potential difference.

• Phospholipid Bilayer:

  • Composed of two layers of phospholipids, preventing direct ion passage and maintaining ion separation.

  • Results in a gradient: Electrical gradient—negative inside and positive outside, contributing to potential energy.

• Potential Energy:

  • Electrical potential represents stored energy and is transformed into kinetic energy during diffusion when channels open.

  • Positive ions outside the cell move into the cell due to attraction to negative charges inside.

• Concentration Gradient:

  • Ions move from areas of higher concentration to areas of lower concentration.

  • For example, higher sodium (Na+) outside the cell moves into lower concentration areas inside the cell.

• Resting Membrane Potential:

  • Defined as the electrical potential across the plasma membrane when cells are at rest, measured at -90 millivolts for skeletal muscle cells.

  • This negative value indicates a higher concentration of negative ions inside the cell compared to the outside.

• Ion Channels and Sodium-Potassium Pump:

  • Channels allow ions to diffuse across the membrane; key ions include sodium (Na+) and potassium (K+).

  • The sodium-potassium pump actively moves 3 Na+ ions out and 2 K+ ions into the cell, maintaining concentration gradients:

    • High Na+ outside, low Na+ inside.

    • High K+ inside, low K+ outside.

• Action Potential:

  • A rapid change in membrane potential, transitioning from -90 mV (rest) to approximately +30 mV and back to -90 mV.

  • Triggered by the opening and closing of gated ion channels:

    • Ligand-gated channels open in response to specific molecules binding to receptors.

    • Voltage-gated channels open and close according to changes in membrane voltage.

• Stages of Action Potential:

  • Resting Membrane Potential: -90 mV with closed channels.

  • Depolarization:

    • Triggered by the opening of voltage-gated Na+ channels.

    • Na+ flows into the cell, making it less negative (more positive).

    • Rapid increase in potential to +30 mV.

  • Repolarization:

    • At +30 mV, voltage-gated Na+ channels close and voltage-gated K+ channels open.

    • K+ ions exit the cell, moving down their concentration gradient, causing membrane potential to decrease (becomes more negative).

    • Cell returns towards -90 mV as K+ continues to leave.

• Restoration of Resting Potential:

  • Membrane returns to resting state (-90 mV).

  • At this point, both sodium and potassium channels are closed.

• Understanding Movement:

  • Consider viewing an animation for visual aids to better grasp ion movements and their roles in establishing potentials.