Neural Signaling The Membrane Potential Lesson 9 Membrane

Neural Signaling: The Membrane Potential Lesson 9

Membrane Structure Barrier l Compartmentalization n Semipermeable l selectively leaky n Fluid Mosaic Model l Phospholipids l Proteins ~ n

Phospholipid Bilayer Hydrophilic heads (phosphate) Hydrophobic tails (lipid)

Membrane Proteins Channels n Pumps l active transport n Receptor protein sites l bind messenger molecules n Transducer proteins: l 2 d messenger systems n Structural proteins l form junctions with other neurons ~ n

Membrane Proteins: Ionophores Ion Channels n Non-gated l always open n Gated l chemically-gated l electrically-gated l mechanically-gated ~ n

Chemically-Gated Channels ligand-gated n Ionotropic l receptor protein = channel l direct control ---> fast n Metabotropic l second messenger system l indirect ---> slow ~ n

Membrane Proteins OUTSIDE INSIDE

Metabolic pumps: Active Transport Membrane proteins n Pump ions l require energy l Na+ - K+ l Ca++ (calcium) n Also various molecules l nutrients l neurotransmitters ~ n

Biolelectric Potential Communication within neuron l electrical signal n electric current = movement of electrons n Bioelectric: movement of ions ~ n

Ion Distribution Particles / molecules l electrically charged n Anions l negatively charged n Cations l positively charged ~ n

Ion Distribution Anions (-) l Large intracellular proteins l Chloride ions Cln Cations (+) l Sodium Na+ l Potassium K+ ~ n

Resting Membrane Potential + + + K+ + + outside Cl + + Na + + + + Membrane - - Cl- - - + K - - Na+ - - A - - - - inside

Membrane is polarized more negative particles in than out n Bioelectric Potential l like a battery l Potential for ion movement n • current ~

Bioelectric Potential OUTSIDE POS INSIDE NEG

Forces That Move Ions Concentration (C) l particles in fluid move from area of high to area of low concentration l diffusion, random movement n Electrostatic (E) l ions = charged particles l like charges repel l opposite charges attract ~ n

Equilibrium Potential Also called reversal potential n Distribution of single ion across membrane l e. g. , EK+, ENa+, ECln Potential for movement of ion if channel opens l units millivolts (m. V) l Potential outside = 0, by convention ~ n

Equilibrium Potential R = gas constant n F = Faraday constant n T = temperature (K) n Z = valence (charge) of ion ~ n

Equilibrium Potential K+: z = +1 Cl-: z = -1 Mg++: z = +2

Equilibrium Potential n Constants never change n Assume 25 o. C (298 o. K) n Use log 10 ~

Equilibrium Potential

Membrane Potential Net bioelectric potential l for all ions l units = millivolts (m. V) n Balance of both gradients l concentration & electrostatic n Vm = -65 m. V l given by Goldman equation ~ n

Membrane Potential: Goldman Equation P = permeability l at rest: PK: PNa: PCl = 1. 0 : 0. 04 : 0. 45 n Net potential movement for all ions n known Vm: Can predict direction of movement of any ion ~ n

Organic anions Membrane impermeable Opposing electrical force not required Vm = -65 m. V A C

Chloride ion Cl C Vm = -65 m. V Concentration gradient equal to electrostatic gradient. n Leaks out neuron n ECl- = - 65 m. V ~ n E

Potassium ion E Vm = -65 m. V + K C Concentration gradient greater than electrostatic gradient. n Leaks out neuron n EK = - 75 m. V ~ n

Sodium ion + Na C Vm = -65 m. V Concentration gradient and electrostatic gradient into neuron. n ENa+ = +55 m. V ~ n E

Metabolic Pumps Active Transport mechanisms l Require energy n Move materials against gradient l Na+ - K+ l Calcium - Ca++ l Nutrients, etc. ~ n

Na+ - K+ Pump Moves ions against gradients l Pumps 3 Na+ out of cell l 2 K+ into cell n Maintains gradients at rest l no active role in signalling l Energy = ATP ~ n

Inside Outside Na+ Na + N a+ Na+ K+ ATP K+ K+ K+

Inside Outside + a N K+ K+ K+