03-17-25

Overview of the Nervous System Exam

• Upcoming exams will be more challenging but students can perform just as well as previous ones with effort and help.

Membrane Potential

• Definition: Membrane potential is a phenomenon representing the voltage across a cell membrane, resulting from the distribution of ions.
• Comparison to Battery: Like a battery, the cell membrane separates charges, creating a potential difference, or voltage.

Understanding Voltage and Its Relation to Ions

• Charge Separation: Batteries have two ends: positive and negative, separated by charge. Similarly, cells have charges associated with their membranes, primarily ions (Na⁺, K⁺, Cl⁻).
• Types of Ions:
  • Sodium (Na⁺): Positively charged, high concentration outside the cell (140 mM) and low inside (15 mM).
  • Potassium (K⁺): Positively charged, high concentration inside the cell (135 mM) and low outside (5 mM).
  • Chloride (Cl⁻): Negatively charged, high concentration outside (100 mM) and low inside (10 mM).

Measurement of Membrane Potential

• Units: Measured in millivolts (mV), which reflects the small scale of ionic movement in cells compared to batteries.
• Calculation Example: Membrane potential (V_m) = (Charges on the inner membrane) - (Charges on the outer membrane).
• Typical Range: V_m is usually negative, between -40 mV and -90 mV depending on the cell type.

Importance of Ion Movement

• The movement of ions across the membrane is crucial for functions like action potentials in neurons.
• Ions contribute to resting membrane potential; the distribution of positive and negative charges leads to the overall membrane charge.

Permeability and Transport Mechanisms

• Permeability: Defined as the ability of ions to pass through the membrane, which influences the membrane potential.
  • Potassium permeability is highest and primarily affects resting potential, while sodium has lower permeability.
  • Resting State Transport: Potassium moves out, and sodium moves in, creating the gradient essential for electrical signaling.

Types of Membrane Potential Changes

• Resting Membrane Potential: The state of a cell at rest, where it is not actively transmitting signals.
• Changes in Membrane Potential:
  • Depolarization: Membrane potential becomes less negative (more positive).
  • Repolarization: Membrane potential returns towards resting levels after depolarization.
  • Hyperpolarization: Membrane potential becomes more negative than the resting level.

Importance of Sodium-Potassium Pump

• Maintains the concentration gradients of sodium and potassium across the cell membrane, crucial for generating membrane potential and facilitating electrical activity in cells.
• Note: The pump is not directly responsible for resting potential but is essential for maintaining the conditions that allow it to exist.

Summary

• Understanding membrane potential is pivotal as it forms the basis of neuron excitability, signaling, and many cell functions. Pay close attention to how ions and their transport mechanics contribute to electrical properties in cells.