Membrane Potentials and Action Potentials

Membrane Potentials

• Definition: Membrane potential is the uneven ion distribution across the plasma membrane, creating an electrical gradient.

  • Charges are separated by the phospholipid bilayer.
  • All cells possess a membrane potential.

• Resting Membrane Potential: The difference in charges across the membrane of an unstimulated cell.

  • Each cell type has a characteristic resting potential.
  • Estimated values include:
  • Adipocytes: -40 mV
  • Thyroid cells: -50 mV
  • Neurons: -70 mV
  • Skeletal muscle cells: -85 mV
  • Cardiac muscle cells: -90 mV

Factors Contributing to Resting Membrane Potential

1. Unequal Ion Distribution
   • Membrane permeability varies for different ions due to the presence of ion channels (leak, gated, etc.) and active transport such as the Na+/K+ pump.
2. Negatively Charged Molecules Inside the Cell
   • Large molecules exert an attractive force on positively charged ions like K+.

Intracellular and Extracellular Ion Distribution

• Cation and Anion Distributions:
  • Intracellular Fluid (ICF): Rich in K+ and anions.
  • Interstitial Fluid (ISF): Rich in Na+ and Cl-.

Transport Channels Review

• Channel Gating: Most channels can open or close based on specific triggers.
  • Leak Channels: Always open (e.g., K+ leak channels).
  • Ligand-Gated Channels: Open in response to specific molecules (ligands).
  • Voltage-Gated Channels: Open or close in response to changes in membrane potential.
  • Mechanically-Gated Channels: Respond to physical stimuli like stretch.
  • Phosphorylation-Gated Channels: Depend on the phosphorylation state of the channel.

Forces Affecting Ion Movement

1. Concentration Gradient:
   • Ions move from areas of high to low concentration.
   • Example: K+ leaking out through channels creates a negative internal environment.
2. Electrical Gradient:
   • Differences in charge influence ion movement, attracting cations to negative regions.

Establishing Resting Membrane Potential

• Key Mechanisms:
  1. Na+/K+ pump maintains ion gradients.
  2. Large negatively charged molecules inside.
  3. Presence of specific ion channels for Na+ and K+.

Changes in Membrane Potential

• Depolarization: Membrane becomes less negative (more positive).
• Repolarization: Returns to resting values after depolarization.
• Hyperpolarization: Membrane becomes even more negative.

Action Potential

• Definition: Rapid changes in membrane potential that occur in excitable cells.
  • Associated with the opening and closing of ion channels, triggering depolarization and repolarization of adjacent membranes.

Neuromuscular Junction (NMJ)

• Definitions:
  • Synapse: Communication site between a neuron and another cell (muscle or gland).
  • NMJ for skeletal muscle is also termed a motor end-plate.

Process at the Neuromuscular Junction

• An action potential from a motor neuron triggers action potential in a muscle cell.
  • Involves voltage-gated Ca²+ channels, release of acetylcholine (ACh), and binding to nicotinic receptors on the muscle cell’s sarcolemma.
  • Summary of Events:
  1. Nerve Impulse travels down the axon to the terminal.
  2. Voltage-Gated Ca²+ Channels open, allowing Ca²+ influx.
  3. ACh released and binds to receptors, leading to Na+ influx and depolarization.