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

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

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!

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

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

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

Voltage-dependent Na+ and K+ channelsGeneral concept

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

General questions about ion channels

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

Types of ion channels

Further diversity gained through alternative splicing, editing, phosphorylation, mixing and matching of different subunit types

Functional diversity Example: K+ channels

Nearly 100 known

Examples of functional variations:

Molecular architecture of ion channels

X-ray crystallography reveals mechanisms of ion permeation, selectivityKCsA bacterial ion channel

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

Mechanism of voltage sensitivity

TM4 contains charged residues; these move in the membrane when membrane potential changes

Human neurological diseases are caused by ion channel mutations

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

Ion transporters:

Proteins that actively transport ions across membranes to establish concentration gradients

Na+ efflux from the squid giant axon:

Sensitive to removal of extracellular K+

Sensitive to block of intracellular ATP generation

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

Transporter structures

Na+/K+ ATPase, deduced by mutagenesis

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