1 / 29

Announcements:

Announcements:. Last lecture Organization of the nervous system Introduction to the neuron Today – electrical potential Generating membrane potential Nernst equation Goldman equation Maintaining ionic distributions. Neural Signaling. A Simple Circuit. Between neurons. Within

liza
Download Presentation

Announcements:

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Announcements:

  2. Last lecture • Organization of the nervous system • Introduction to the neuron • Today – electrical potential • Generating membrane potential • Nernst equation • Goldman equation • Maintaining ionic distributions

  3. Neural Signaling A Simple Circuit Between neurons Within neurons chemical & electrical electrical

  4. Bioelectric Potentials • Neurons have an electrical potential (voltage) across the cell membrane • The inside of the cell is more negative than the outside • called the Resting Membrane Potential

  5. Resting potential Measuring Membrane Potential amplifier microelectrode Reference electrode 0 mV cell -80 mV time Bathing solution

  6. Electrophysiology techniques Silver / Silver chloride wire electrode Amplifier output Reference electrode 3M KCl solution Very tiny hole (<<0.1m) Glass micropipette

  7. Resting Membrane Potential • How is it generated? • differential distribution of ions inside and outside the cell • Selective Permeability of the membrane to some ions

  8. How does unequal concentration of ions give rise to membrane potential ?

  9. Cl- Cl- Cl- Cl- Equal concentrations of ions 0 volts Artificial ion selective membrane (only K+, not Cl-) voltmeter K+ I II K+ 0.01 M KCL 0.01 M KCL K+ K+ No net movement

  10. Cl- Cl- Cl- Cl- Cl- Cl- Unequal concentrations of ions volts - + Ion selective membrane (only K+, not Cl-) K+ I II K+ K+ K+ 0.1 M KCL 0.01 M KCL K+ K+ K+ K+ concentration gradient

  11. K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Cl- Initial New Equilibrium + + + + + + CHEMICAL CHEMICAL ELECTRICAL

  12. 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

  13. Equilibrium Potential • As IIbecomes +, movement of K+ is repelled • Every K+ near the membrane has two opposing forces acting on it: • Chemical gradient • Electrical gradient • These two forces exactly balance each other • Called the electrochemical equilibrium

  14. 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: • only a very small number of ions actually contribute to the electrical potential • the overall concentrations of K and Cl in solution do not change.

  15. To calculate the equilibrium potential of any ion (eg. K, Na, Ca,) at any concentration • we use the Nernst Equation:

  16. Nernst Equation Ion Concentration I Temp (K) Gas Constant Equilibrium Potential of X ion (eg. K+) in Volts Ion Concentration II Valence of ion (-1, +1, +2) Faraday constant

  17. Nernst Equation • At 18C, for a monovalent ion, and converting to log10 ,the equation simplifies to:

  18. By convention electrical potential inside of cells is expressed relative to the outside of the cell

  19. in out 0.1 M KCL 0.02 M KCL Example: K+ = -0.040 Volts = - 40 mV

  20. 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

  21. in out Na+ Na+ Na+ Na+ Na+ Na+ 0.1 M KCl 0.02 M NaCl 0.01 M KCl 0.2 M NaCl K+ K+ K+ K+ K+ K+ What if there is more than one permeable ion? Permeable to K+ and Na+, but not Cl-

  22. To calculate the overall potential of multiple ions • use the Goldman Equation • Considers the permeability of ions and their concentrations

  23. Goldman equation Voltage Ion concentration Permeability Because Cl is negative

  24. Goldman equation • Example, typical mammalian cell: • Assume permeability for Na is 1/100 of permeability for K, and permeability of Cl is 0 • Assume [K]in= 140, [K]out=5 [Na]in =10, [Na]out=120

  25. Goldman equation • The resting membrane potential of most cells is predicted by the Goldman equation

  26. Summary & Key Concepts • Unequal distributions of an ion across a selective membrane • causes an electrochemical potential called the equilibrium potential • Two opposing forces act on ions at the membrane • A chemical force down the concentration gradient • An opposing electrical force

  27. Summary & Key Concepts • The equilibrium potential for an ion is described by the Nernst equation • Cell membranes are permeable to more than one ion • the membrane electrical potential is described by the Goldman equation

  28. So What??? • Everythingthe nervous system and muscles do depends on the resting membrane potential

  29. Sample question • If two concentrations of KCl solution across a membrane give an equilibrium potential for K+ of -60 mV, what will the equilibrium potential be if the concentrations on each side are reversed • -120 mV • 0 • +60 mV • -30 mV

More Related