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Neural Signaling: The Membrane Potential. Lesson 9. Membrane Structure. Barrier Compartmentalization Semipermeable selectively leaky Fluid Mosaic Model Phospholipids Proteins ~. Phospholipid Bilayer. Hydrophilic heads (phosphate) Hydrophobic tails (lipid). Membrane Proteins.
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Membrane Structure • Barrier • Compartmentalization • Semipermeable • selectively leaky • Fluid Mosaic Model • Phospholipids • Proteins ~
Phospholipid Bilayer Hydrophilic heads (phosphate) Hydrophobic tails (lipid)
Membrane Proteins • Channels • Pumps • active transport • Receptor protein sites • bind messenger molecules • Transducer proteins: • 2d messenger systems • Structural proteins • form junctions with other neurons ~
Membrane Proteins: Ionophores • Ion Channels • Non-gated • always open • Gated • chemically-gated • electrically-gated • mechanically-gated ~
Chemically-Gated Channels • ligand-gated • Ionotropic • receptor protein = channel • direct control ---> fast • Metabotropic • second messenger system • indirect ---> slow ~
Membrane Proteins OUTSIDE INSIDE
Metabolic pumps: Active Transport • Membrane proteins • Pump ions • require energy • Na+ - K+ • Ca++ (calcium) • Also various molecules • nutrients • neurotransmitters ~
Biolelectric Potential • Communication within neuron • electrical signal • electric current = movement of electrons • Bioelectric: movement of ions ~
Ion Distribution • Particles / molecules • electrically charged • Anions • negatively charged • Cations • positively charged ~
Ion Distribution • Anions (-) • Large intracellular proteins • Chloride ions Cl- • Cations (+) • Sodium Na+ • Potassium K+ ~
Cl- Na+ + + + + + + + + + + + + + + K+ + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - K+ Cl- Na+ Resting Membrane Potential outside Membrane A- inside
Membrane is polarized • more negative particles in than out • Bioelectric Potential • like a battery • Potential for ion movement • current ~
Bioelectric Potential POS NEG OUTSIDE INSIDE
Forces That Move Ions • Concentration (C) • particles in fluid move from area of high to area of low concentration • diffusion, random movement • Electrostatic (E) • ions = charged particles • like charges repel • opposite charges attract ~
Equilibrium Potential • Also called reversal potential • Distribution of single ion across membrane • e.g., EK+, ENa+, ECl- • Potential for movement of ion if channel opens • units millivolts (mV) • Potential outside = 0, by convention ~
Equilibrium Potential • R = gas constant • F = Faraday constant • T = temperature (K) • Z = valence (charge) of ion ~
K+: z = +1 Cl-: z = -1 Mg++: z = +2 Equilibrium Potential
Equilibrium Potential • Constants never change • Assume 25 oC (298 oK) • Use log10 ~
Membrane Potential • Net bioelectric potential • for all ions • units = millivolts (mV) • Balance of both gradients • concentration & electrostatic • Vm = -65 mV • given by Goldman equation ~
Membrane Potential: Goldman Equation • P = permeability • at rest: PK: PNa: PCl = 1.0 : 0.04 : 0.45 • Net potential movement for all ions • known Vm:Can predict direction of movement of any ion ~
Organic anions - Membrane impermeable Opposing electrical force not required Vm = -65 mV A- C
Chloride ion Cl- C • Concentration gradient equal to electrostatic gradient. • Leaks out neuron • ECl- = - 65 mV ~ Vm = -65 mV E
Potassium ion E • Concentration gradient greater than electrostatic gradient. • Leaks out neuron • EK = - 75 mV ~ Vm = -65 mV K+ C
Na+ Sodium ion C E • Concentration gradientandelectrostatic gradient into neuron. • ENa+ = +55 mV ~ Vm = -65 mV
Metabolic Pumps • Active Transport mechanisms • Require energy • Move materials against gradient • Na+ - K+ • Calcium - Ca++ • Nutrients, etc.~
Na+ - K+ Pump • Moves ions against gradients • Pumps 3 Na+ out of cell • 2 K+ into cell • Maintains gradients at rest • no active role in signalling • Energy = ATP ~
Na+ Na+ Na+ K+ K+ Inside Outside Na+ Na+ Na+ K+ K+ ATP
Na+ Na+ Na+ Inside Outside K+ K+ K+ K+