Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Cell encapsulation wikipedia , lookup
Node of Ranvier wikipedia , lookup
Organ-on-a-chip wikipedia , lookup
Cytokinesis wikipedia , lookup
Signal transduction wikipedia , lookup
Chemical synapse wikipedia , lookup
Endomembrane system wikipedia , lookup
List of types of proteins wikipedia , lookup
Mechanosensitive channels wikipedia , lookup
Cell membrane wikipedia , lookup
Measuring Membrane Potential amplifier microelectrode Reference electrode Tutorial Membrane potentials and ion channels Resting potential 0 mV cell -80 mV Bathing solution time Resting Membrane Potential • How is it generated? 1. differential distribution of ions inside and outside the cell 1. How does unequal concentration of ions give rise to membrane potential ? 2. Selective Permeability of the membrane to some ions 1 Equal concentrations of ions Unequal concentrations of ions 0 volts Artificial ion selective voltmeter - membrane (only K+, not Cl-) I Cl- II K+ 0.01 M KCL + Ion selective membrane (only K+, not Cl-) I Cl- K+ Cl- K+ Cl- II 0.1 M KCL 0.01 M KCL Cl- Cl- K+ Initial K+ ClCl- K+ Cl- New Equilibrium Cl- K+ Cl- K+ Cl- Cl- K+ Cl- K+ Cl- K+ K+ CHEMICAL Cl- K+ Cl- Cl- K+ ClCl- +K+ + + K+ + Cl- ClCl- + +K +K + Cl- CHEMICAL Cl- K+ K+ K+ No net movement K+ K+ Cl- K+ 0.01 M KCL volts ClCl- K+ K+ concentration gradient Unequal concentrations of ions • Initial diffusion of K+ down concentration gradient from I to II • This causes + charge to accumulate in II because + and - charges are separated – Remember that Cl- can’t cross the membrane ! • Therefore II becomes positive relative to I ELECTRICAL 2 Equilibrium Potential • As II becomes +, movement of K+ is repelled • • Every K+ near the membrane has two opposing forces acting on it: • 1. Chemical gradient 2. Electrical gradient • These two forces exactly balance each other • Called the electrochemical equilibrium • The electrical potential that develops is called the equilibrium potential for the ion. Electrical potential at which there is no net movement of the ion Note: 1. only a very small number of ions actually contribute to the electrical potential 2. the overall concentrations of K and Cl in solution do not change. Nernst Equation • To calculate the equilibrium potential of any ion (eg. K, Na, Ca,) at any concentration – we use the Nernst Equation: Gas Constant Ex = Temp (°K) Ion Concentration I RT [ X ]I ln zF [ X ]II Equilibrium Potential of X ion (eg. K+) in Volts Valence of ion (-1, +1, +2) Ion Concentration II Faraday constant 3 Nernst Equation • At 18°C, for a monovalent ion, and converting to log10 ,the equation simplifies to: 0.058 [ X[ X ]I ] I 0.058 log[ X ]II EXEx== z log z [ X ] II • By convention electrical potential inside of cells is expressed relative to the outside of the cell Ex = 0.058 [ X ]outside log z [ X ]inside Example: K+ in out 0.058 [X ]I EK = log z [ X ] II 0.1 M KCL 0.02 M KCL EK = 0.058 [ X ]out log z [ X ]in EK = 0.058log 0.02 0.1 • Therefore, – initial movement of K+ down concentration gradient – When electrical potential of -40 mV develops, there will be no net movement of K+ – Thus K+ is in electrochemical equilibrium = -0.040 Volts = - 40 mV 4 What if there is more than one permeable ion? in out 0.1 M KCl 0.02 M NaCl K+ Na+ 0.01 M KCl 0.2 M NaCl Na+ K+ K+ K+ Na+ Na+ K+ Na+ K+ Na+ • To calculate the overall potential of multiple ions • use the Goldman Equation • Considers the permeability of ions and their concentrations Permeable to K+ and Na+, but not Cl- Goldman equation • Voltage Vm = 0.058log Goldman equation PK[ K ]outside + PNa[ Na ]outside + PCl[Cl ]inside PK[ K ]inside + PNa[ Na ]inside + PCl[Cl ]outside Permeability Example, typical mammalian cell: 1. Assume permeability for Na is 1/100 of permeability for K, and permeability of Cl is 0 2. Assume [K]in= 140, [K]out=5 [Na]in =10, [Na]out=120 Vm = 0.058log Ion concentration 1[5] + 0.01[120] + 0 1[140] + 0.01[10] + 0 Because Cl is negative Vm = −78mV 5 What confers selective permeability? Goldman equation • The resting membrane potential of most cells is predicted by the Goldman equation Ion channels • At rest: – Leakage channels – Open at rest – allow K+ to flow out along its concentration gradient K+ – Voltage gated channels Summary 1. At rest PK>>PNa, PCl, PCa 2. Therefore, at rest, the membrane potential is close to EK • Whether ions will flow depends on 1. Permeability 2. Electrochemical driving force 3. In general, the membrane potential will be dominated by the equilibrium (Nernst) potential of the most permeable ion 6 • Scenario – Cell at rest Vm = -90 mV – EK = -90 mV – ENa = +50 mV • If a K+ channel opens no ions flow since Vm = EK • If a Na+ channel open, Na+ will flow into the cell because Vm≠ ENa – +’ve charge flows into cell – depolarizes it – When Vm= ENa Na stops going into cell 7