Nerve Physiology: Genesis of the Resting Membrane Potential PDF

NERVE PHYSIOLOGY GENESIS OF THE RESTING MEMBRANE POTENTIAL OBJECTIVES State the role of the resting membrane potential (RMP) Derive the negative RMP of the nerve Explain the role of protein channels and carriers in the genesis of the RMP Compare differences between the Nernst and Goldman-Constant Fi...

NERVE PHYSIOLOGY GENESIS OF THE RESTING MEMBRANE POTENTIAL OBJECTIVES State the role of the resting membrane potential (RMP) Derive the negative RMP of the nerve Explain the role of protein channels and carriers in the genesis of the RMP Compare differences between the Nernst and Goldman-Constant Field Equations The Resting Membrane Potential All excitable tissues have a RMP Nerve cells: RMP = -70mV Cardiac & Skeletal muscle cells: RMP= ~ -85 to -90mV GENESIS OF THE RMP 1. Due to the uneven distribution of electrolytes inside and outside the cell membrane Na+ K+ Cl- OUTSIDE INSIDE 140 mEq/L 14 mEq/L 4 mEq/L 140 mEq/L 125 mEq/L ~ 9 mEq/L 2. An excess of nondiffusible protein ions and other anions make the inside of the nerve membrane more negative than the outside + Na Na+ + K K+ P Cl- Cl 3. Two transport proteins are primarily responsible: 1. K+ leaky channels 2. Na+/K+ ATPase pump + Na ATPase Cl - Na+ + K P Cl K+ Na+/K+ ATPase PUMP An electrogenic pump extrudes 3Na + for every 2K + ions Contributes only a few mV to the RMP Development of the RMP Due largely to DIFFUSION forces: Concentration and electrical gradients exist for most ions across the cell membrane has an outward concentration gradient but an inward electrical gradient + Na K+ Na+ has both gradients directed inwards Cl- tends to move in along its concentration and out along its electrical gradient ATPase Cl - Na+ + K P Cl K + At rest, Cl- influx = Cl- efflux The membrane potential at which this equilibrium exists is called the Equilibrium Potential NERNST EQUATION EM = RT Ln [X]0 FZ [X]i R T F Z = Universal Gas constant = Absolute temperature = Faraday constant = valency of the ion Em = 61log[X]0 [X]i ECl- = -61 log (125/9) = +61 log (9/125) = +61 (-1.143) ✓ECl = - 70mV ENa+ = + 61 log (140/14) ENa+ = + 61 mV There are less Na+ in the cell than we can account for by simple diffusion EK + = + 61 log (4/140) EK + = - 94.1 mV There are more K+ in the cell than we account for by simple diffusion ASSUMPTIONS OF THE NERNST EQUATION 1. The membrane is selectively and totally permeable to each ion 2. Each ion is in electrochemical equilibrium across the membrane 3. The ion that is diffusing is isolated from all other ions IONIC PERMEABILITY OF THE MEMBRANE At rest: impermeable to Na+ ions permeable to K+ partially permeable to Cl- pK+ : pNa+ : pCl- = 1 : 0.04 : 0.45 GOLDMAN-CONSTANT FIELD EQUATION V = RT Ln PK+ [K+]0 + P Na+ [Na+]0 + PCl-[Cl-]i zF PK+ [K+]i + P Na+ [Na+ ]i + PCl-[Cl-]o V = +61log PK+ [K+]0 + P Na+ [Na+]0 + PCl-[Cl-]i PK+ [K+]i + P Na+ [Na+]i + PCl-[Cl-]o [K+]o = Vm A decrease in Vm leads to increased nerve excitability e.g. hyperkalemia Resting Membrane Potential = state of the nerve cell membrane that allows it to fire an electrical impulse or action potential leading to contraction of muscles or secretion from tissues/glands