Sonoma State University 2002
Department of Biology - Hanes
Neurophysiology

Chapter 3: Neuronal Membrane at Rest

Introduction

The nerve signals digitally with action potentials. The off signal is a "resting potential", the on signal is an "action potential" with voltages of around -80mV and +40 mV respectively. The voltages are created with ion concentrations different on the two sides of the cell membrane and changes in permeability to these ions.

Cast of Chemicals

Polar molecules readily make hydrogen bonds with water. Ions attract large numbers of water molecules that movewith them. The major cations of interest are Na+, K+, Ca++ and anions Cl- and protein-. The cell membrane is composed of a phospholipid bilayer which is hydrophobic and which contains proteins that are held in the membrane and are hydrophilic acting as water soluble holes in the membrane. Some of these proteins can control the entrance or exit rate of specific ions. Some of the proteins are called gates because they can allow certain ions to pass selectively. There are sodium gates, potassium gates, chloride ion gates, calcium gates, and ion gates that are named for their specificity or lack of it. Being a gate, they can also change their permeability to the ions they are specific for. The gates are controlled either by voltage (voltage controlled gates) or by chemicals as neurotransmitters (ligand controlled gates).

The ions are unequally distributed between the inside and outside of the cell due to the presence of negatively charged protein. If gates are opened to the ion, the voltage of the cell will move toward the equilibrium potential of the ion. The equilibriumk potential can be calculated via Nernst's Equation.

Resting Potential

Protein is negatively charged at physiological pH's and is trapped in side the cell.

Each negative charge inside the cell must be balanc ed with a positive charge.

Potassium is always free to penetrate the cell membrane and thus becomes the dominant intracellular monovalent cation.

The negative pull of the protein balances the tendency for K+ to diffuse down its concentration gradient.

Na+ and Ca++ do not penetrate the membrane well and there are also pumps to remove it when it does leak.

Usually chloride ion can penetrate the cell membrane and is not pumped leaving its concentration difference at that predicted by Nernst's Equation and it is also the resting potential of the cell.

The Goldman Equation, which takes into account the Chloride, Sodium and Potassium permeabilities and concentration differentials predicts very well the resting (and action) potential of the cell.

The Potassium Channel

This protein is characterized as a channel because it never closes completely, but it can change its permeability. It opens more widely when the cell is depolarized.

The potassium channel is located on most parts of the cell membrane of all cells including nerve cells.

There is a good characterization of this protein on pp 68-69.

Note the concentration differentials and equilibrium potentials on p 65 Fig 3.15. Know how to solve problems with the Nernst Equation and the Goldman Equation.

The Sodium Potassium pump.

The pumps activity is regulated by the concentration of Na+ inside the cell.

It pumps 3 Na+ out for every 2K+ it pumps in.

It is poisoned by Oubain.

If located only on the serosal side of a membrane, it will pump Na+ across cells into the animal.

It requires

ATP

Na+ inside the cell

K+ outside the cell