7 Electrical Signaling by Neurons
Neurons share many properties with other cells, including their complement of organelles, an electrical potential across their surface membranes, and an ability to secrete various substances. What distinguishes neurons is the ways in which they have adapted these common properties for their roles as information-processing and information-conveying devices. For example, neurons have specialized configurations of organelles to support their extended anatomy (see Chapter 1). Similarly, they have adapted secretory processes to communicate with each other rapidly and precisely (see Chapter 8), and individual neurons use alterations in their membrane potentials to convey information between their various parts (this chapter).
A Lipid/Protein Membrane Separates Intracellular and Extracellular Fluids
The surface membrane of neurons, like that of other cells, is a double layer of lipid molecules with proteins embedded in it. Just as oil and water don’t mix very well, the lipid part of the membrane is impermeable to water-soluble substances, prominently including the ions whose movement is central to electrical signaling. Subsets of the proteins embedded in the lipid bilayer are specialized to allow or even facilitate the movement of ions across the membrane. Some are hollow ion channels with a central, aqueous pore whose size and charged lining determines which kinds of ions can pass through; others are ion pumps that use metabolic energy to move specific ions across the membrane.
The Resting Membrane Potential of Typical Neurons Is Heavily Influenced, but Not Completely Determined, by the Potassium Concentration Gradient
The K+ concentration inside neurons is much higher than that outside (because of ion pumps described a little later), and their surface membranes contain numerous K+ channels that are usually in the open state. If these were the only ion channels in the membrane, the following scenario would develop. The concentration gradient would drive K+ ions outward through the channels, creating a deficit of positive charges inside the cell. Opposite charges attract each other, so after a very small number of K+ ions had left, the resulting intracellular negativity would pull K+ ions back into the cell. At some point the concentration gradient and the electrical gradient would exactly counterbalance each other, and K+ ions would enter and leave at equal rates (Fig. 7-1). The system would be in equilibrium, with no net movement of K+ in either direction and no energy requirement to stay that way. The transmembrane potential at which this occurs, the potassium equilibrium potential or VK, is a logarithmic function of the concentration gradient and is specified by the Nernst equation (THB6 Appendix 7B, p. 176). At body temperature, a tenfold change in the K+ concentration on one side of the membrane causes a 62 mv change in VK.
In reality, however, the K+ channels are not 100% selective for K+, and not all the channels in a resting membrane are K+ channels. As a result, Na+ ions are also able to flow across the membrane. The Na+ concentration outside neurons is much higher than that inside, so Na+ moves into the cell because of both the concentration gradient and the intracellular negativity. The result is competing ion flows (Fig. 7-2)—inward Na+ flow trying to move the membrane potential to VNa and outward K+ flow trying to move the membrane potential to VK—and a steady state
