1. Ion Pumps and Ion Channels CHAPTER 48 SECTION 2

2. Overview  All cells have membrane potential across their plasma membrane  Membrane potential is the difference in electrical charge between the inside and the outside of a membrane  The membrane potential of a resting neuron—one that is not sending signals—is its resting potential

3. Formation of the Resting Potential  Potassium ions and sodium ions play critical roles in the formation of the resting potential. For each there is a concentration gradient across the plasma membrane of a neuron.  In mammalian neurons there is more potassium inside the cell and more sodium outside the cell.  Sodium-potassium pumps maintain these gradients  These pumps are used for ATP hydrolysis  These pumps use active transport  These pumps are ion channels

4. Ion Channels  Ion Channels - pores formed by clusters of specialized proteins that span the membrane  The concentration gradients represent a chemical form of potential energy for the neuron  In order to change the chemical potential into electrical potential ion channels are needed  As the ions diffuse through the channels they carry with them an electrical charge  The net movement of positive or negative charge will generate a voltage, or potential, across the membrane

6. Ion channels continued  Ion channels have selective permeability, meaning that they allow only certain ions to pass  A resting neuron has a lot of open potassium channels but few open sodium channels  The K + ions can flow out of the cell but the ion channels do not allow anions such as Cl - into the cell, therefore there is a negative buildup inside the cell causing membrane potential  The negative charge keeps the positive ions inside the cell

7. Modeling of Resting Potential  When the electrical gradient exactly balances the chemical gradient the model neuron has reached equilibrium  An ion’s equilibrium potential is the magnitude of the membrane voltage at equilibrium for that particular ion  To calculate equilibrium potential we use the Nernst equation  E ion = 62mV(log([ion outside ]/[ion inside ]))