Key Review Points:

1. Key Review Points: 1. Electrical signaling depends on the motion of ions across neuronal membranes 2. Na+, K+, Cl- and Ca++ ions are distributed unequally across neuronal membranes 3. At rest, diffusion of these ions creates the membrane potential 4. Rapid changes in ionic permeability cause transient, self-regenerating changes in the membrane potential known as action potentials, which carry information

2. Today’s Lecture: Ion channels: proteins that form pores in the membrane to permit ions to cross Ion transporters: proteins that actively transport ions across membranes to establish concentration gradients

3. New technology: The patch clamp technique The voltage clamp technique shown before was adequate for large currents, but produced large ‘background noise’ ‘Patch clamp’ technique has superior signal-to-noise ratio, so very small currents can be measured, even down to the current passed through a single ion channel!

4. Early sodium current during the action potential is due to the aggregate action of many individual sodium channels

5. Later potassium current during the action potential is due to the aggregate action of many individual potassium channels

6. Voltage dependence of open Na+ and K+ channel open probabilities mirrors the voltage dependence of Na+ and K+ conductances

7. Voltage-dependent Na+ and K+ channels General concept

8. General questions about ion channels How can a protein sense voltage? How does it respond respond with the appropriate timing? How does it permit some ions to cross the membrane while excluding others? How does it inactivate? --> Functional studies of ion channel proteins

9. Need to express ion channels in cells, in isolation from other channels: The Xenopus oocyte electrophysiology technique

10. Types of ion channels Further diversity gained through alternative splicing, editing, phosphorylation, mixing and matching of different subunit types

11. Functional diversity Example: K+ channels Nearly 100 known Examples of functional variations:

12. Molecular architecture of ion channels

13. X-ray crystallography reveals mechanisms of ion permeation, selectivity KCsA bacterial ion channel

14. Geometry of negative charges, pore size, and ion hydration work together to provide K+ selectivity, excluding Na+

15. Mechanism of voltage sensitivity TM4 contains charged residues; these move in the membrane when membrane potential changes

17. Human neurological diseases are caused by ion channel mutations

18. Kinetic properties of ion channels are finely-tuned, alteration of them causes disease

19. Ion transporters: Proteins that actively transport ions across membranes to establish concentration gradients

22. Na+ efflux from the squid giant axon: Sensitive to removal of extracellular K+ Sensitive to block of intracellular ATP generation

23. Usually, the Na+/K+ ATPase has only a small direct effect on membrane potential, (<1 mV) because it is very slow compared to ion flux through ion channels However, it can have a larger effect if in small-diameter axons, where the ratio of surface-area to cytoplasm volume is small and ion concentrations change appreciably

24. Transporter structures Na+/K+ ATPase, deduced by mutagenesis

25. The Ca++ pump: a more structure-based view