Membrane Potentials
Fred Rieke and Bertil Hille
Learning Objectives
1. Know how ion diffusion in ion channels establishes membrane potentials.
When an ion-selective ion channel opens in the membrane, specific ions can move down their electrochemical gradient (this gradient is a combination of electrical and concentration gradients). By moving charges from one side to the other, the resulting ion currents change the total charge in the cell and the electrical potential across the membrane. Without charge movement, there can be no membrane potential change.
2. Recognize that fluxes needed to make typical membrane potentials are extremely small: membrane capacitance.
Membrane capacitance is by definition the amount of charge you have to move across the membrane to change the membrane potential a certain amount. In cells, the membrane capacitance is small, so the quantity of ions that has to move for electrical signaling is tiny, depleting the existing gradient by only a little.
3. Explain with Na+, K+, Cl–, and Ca2+ what contribution each could make individually to electrical potential changes in the plasma membrane of excitable cells.
Na+, K+, Cl– and Ca2+ ions have different ion gradients and charge. Therefore, opening selective ion channels for each of them pulls the membrane potential towards a different final value. For example, open K+ channels produce a negative inside potential, and open Na+ channels produce a positive inside potential. Therefore electrical responses of cells depend on opening the right ion channels in the right sequence to shape the signal.
Standard laws of electricity govern how cells make electrical signals across their membranes. This chapter reviews these laws. The need to think about electricity may seem daunting to some biologists, but really it is not particularly complicated. Electricity obeys a few simple physical rules in a logical manner. You have studied them before. Comfort with the key concepts in this chapter will be very helpful in subsequent more specific and detailed study of electrophysiology. As we will see, if you have both (i) gradients of ions across a membrane and (ii) a membrane permeability to some of these ions by open ion channels, the necessary net flow of the charged ions in these channels forms an electric current and can change the membrane potential.
To motivate our study of principles, we look ahead at a nerve cell sending an electrical message, the spike-like action potential (we will examine this process in more detail later in the course). The action potential is a stereotyped, efficient, brief, electrical signal that can travel regeneratively at high speed along an axon. It is the long-distance, electrical message of the nervous system, the unit of information like the information bits in the wires of a computer. Figure 1 is an electrical recording from the inside of an active nerve cell. We will note just three typical properties for now. (i) This neuron starts with a NEGATIVE inside r