Neuroscience: Membrane Bioelectricity

Neuroscience: Membrane Bioelectricity

1. Basics of Electrical signalling in neurons

   • All electrical signalling in neurons involves movement of ions in solution
   • Ions directly involved are: Na+, K+, Ca++ and Cl-
   • All electrical signalling occurs across or near the cell membranes of neurons
   • Non-random movement of ions occurs because of
     • Concentration gradients, electrical gradients, membrane transporters or pumps

2. Structure of water

   • Water has a polarised structure - one side of molecule tends to have net negative charge

3. Cell membranes have 4 major components

   • Phospholipids
   • Cholesterol
   • glycolipids
   • Membrane proteins

4. Cell membrane: Cholesterol, phospholipid, glycolipid

   • Membrane lipids
     • Lipids (phospholipids) form a stable bilayered structure
     • Hydrophilic heads are directed to the exterior surfaces of membrane, to aqueous intracellular and extracellular fluids
     • Hydrophobic tails of lipids are directed towards the interior of membrane
   • Lipid layer is an excellent insulator, with electrical resistance about the same as glass
   • Polar molecules and (charged) ions cannot pass through the non-polar layer of the membrane. Impermeant to ions
   • Some lipid - soluble compounds pass readily through phospholipid membranes, these include: carbon dioxide, oxygen, steroid hormones, nitric oxide

5. Major functions of membrane proteins

   • Ion channels: Proteins form the walls of channels that allow movement of specific ions across cell membrane
   • Receptors: for neurotransmitters, hormones, antibodies, cytokines, neurotrophins, drugs
   • Transporters and pumps: membrane transport systems (e.g. enzyme Na+/K+ ATPase) allow particular ions and polar molecules to enter to leave the cell
   • Structural proteins: proteins provide rigidity to the otherwise amorphous, oily lipid components, providing anchor points so that cells can maintain shape and form stable tissues

6. Ion Channels

   • Opened by
     • Membrane potential: Usually open when membrane depolarises
       • Some open when membrane hyprpolarises
     • Neurotransmitters (extracellular ligands bind to channel)
     • Intracellular ligands
     • Phosphorylation of channel proteins under the influence of intracellular second messenger pathways

7. Cell membrane acts a resistor and a capacitor in parallel

   • Nernst Equation