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Membrane Potentials
Physiology and Pharmacology
| Question | Answer |
|---|---|
| What is a membrane potential | An electrical potential/voltage difference between the inside of a cell and its surroundings This is usually -85 mV to -60 mV |
| How can membrane potentials be measured | Using glass microelectrodes |
| What are normal Na+ concentrations | Intracellular - 5-15 mM Extracellular - 140-145 mM |
| What are normal K+ concentrations | Intracellular - 140 mM Extracellular - 4.5 mM |
| What are normal Ca2+ concentrations | Intracellular - 100 nM Extracellular - 1.25 mM |
| What are normal Mg2+ concentrations | Intracellular - 1-2 mM Extracellular - 1-2 mM |
| What are normal H+ concentrations | Intracellular - 10^-7.2 M Extracellular - 10^-7.4 M |
| What are normal Cl- concentrations | Intracellular - 5-50 mM Extracellular - 120 mM |
| What are normal HCO3- concentrations | Intracellular - 15 mM Extracellular - 24 mM |
| What is diffusion | The movement of particles via random Brownian motion driven by thermal energy The distribution of particles becomes uniform |
| What is a diffusion potential | When equilibrium is established between chemical gradients and electrochemical gradients e.g. potassium is drawn into the cell down it electrochemical gradient but drawn out the cell down its chemical gradient Em = Ek = -90 mV |
| Nernst Equation | Ek = RT/zF In (conc of ion outside / conc of ion inside) R = gas constant T = temperature in kelvin F = faradays constant z = ion charge |
| What determines membrane permeability | Membrane potential is a balance between K+, Cl- and Na+ membrane fluxes. As the membrane at rest is far more permeable to K than Na, the potential is closer to Ek than Ena Ek = -90 mV Ena = +58 mV Ecl = -70 mV Em = -75 mV |
| Constant Field Equation | Em = 58 log10 ((pK{K+ outside}+pNa{Na+ outside}+pCl{Cl- inside})/(pK{K+ inside}+pNa{Na+ inside}+pCl{Cl- outside})) pCl, pNa and pK are permeability coefficients At rest pK>>>>pNa |
| Evidence for role of ions in membrane permeability | Hodgkin and Horowicz Measured Em in frog muscle at different {K+} At high K+ Em was predicted by the K+ Nernst equation At low K+ the graph shifted towards the Na Nernst equation,showing that Na plays a comparatively small role, but still has an effect |
| What are single Donnan equilibria | When the effects of an impermeable ion are only on one side of the membrane Electrochemical equlibrium can be established by permeable ions, but this does not give an osmotically balanced system |
| What are Double Donnan equilibria | When impermeable ions are on both sides of the membrane This assumes cells are impermeable to sodium K and Cl reach their electrochemical equilibrium, with the confinement of Na outside establishing osmotic balance with impermeable ions inside |
| The sodium potassium pump | Na and K diffuse across the membrane via leaky ion channels. The pump maintains asymmetric ion gradients via moving 2 K in and 3 Na out the cell. The rate of this pump acting is dependant on Na concentration inside the cell |
| Evidence for electrogenicity of the Na-K pump | Thomas When Na is injected into a snails neuron the membrane hyperpolarises - increased rate of removal by pump When inhibitor added before Na no hyperpolarisation is seen The inhibitor causes a small depolarisation as pump stops acting |
| What happens to membrane potential when we raise potassium concentration | This will depolarise the cell Hyperkalaemia (elevated plasma K+) is a dangerous clinical condition |
| Electrical signalling in membranes | Movement of ions across cell membranes generate electric currents Dynamic modulation of membrane potential is achieved by controlling this electric current across the membrane |
| Ohm's Law in Cells | Ik = (Em-Ek)gk (Em-Ek) = net driving force acting on K gk is the membrane conductance to K+ ions These can be used for other ions |
| What are the ionic equilibrium potentials for K+, Na+ and Cl- | Ena = +70 mV Ek = -95 mV Ecl = -40 mV |
| Conductance equation | Em = (Ek.gk+Ena.gna+Ecl.gcl) / (gk+gna+gcl) The electrical signalling occurs by changing conductance, not changing the ion concentrations. The membrane acts as a capacitor, changing the movement of ions changes the voltage |
| Charge stored on a membrane | Q = VC Q = charge V = voltage C = Capacitance (for a biological membrane 1uf.cm^-2) Moles of charge stored on a membrane at -100 mV = 1 x 10^-12 M/cm^2 |
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