Lecture Notes: Neuron Function and Ion Channels

Overview of Ion Channels

• Ion channels are critical for neuron function.
• Transport proteins facilitate movement across the cell membrane, especially for polar or charged molecules.
• Most substances cannot cross the lipid bilayer without these proteins.

Types of Ion Channels

Leakage Channels

• Definition: Transmembrane proteins with pores for ions.
• Function: Allow random movement of ions with their concentration gradient.
• Characteristics:
  • Alternate between open and closed states.
  • Sodium and potassium leakage channels exist.
  • Potassium channels are more numerous than sodium channels, allowing faster potassium diffusion.

Gated Channels

• Characteristics: Do not open/close randomly; require signals.
• Types:
  • Ligand Gated Channels:
    • Small molecules (neurotransmitters/hormones) bind to proteins, opening the pore for ion movement.
    • Highly specific to ions (e.g., sodium, potassium).
  • Mechanically Gated Channels:
    • Open due to physical movement (pressure, touch) in surrounding tissue.
  • Thermoreceptors:
    • Open due to local temperature changes.
  • Voltage Gated Channels:
    • Open when membrane voltage reaches a specific level.
    • Responsible for polarization (net negative inside, net positive outside).

Measuring Membrane Potential

• Method: Use electrodes on the inside and outside of the membrane.
• Purpose: Measure relational voltage difference.
• Resting Membrane Potential:
  • Typically around -70 millivolts in neurons and muscle cells.
  • Varies but -70 is a common estimate.

Factors Affecting Membrane Potential

• Unequal Ion Distribution:
  • Mediated by sodium-potassium pump (ATPase).
  • Pumps three sodium ions out and two potassium ions in.
  • Consumes energy, leading to positive outside, negative inside.
  • Anions (e.g., proteins) mostly stay inside, contributing to negative charge.