Chapter 12 - Membrane Potential and Action Potential

Abstract

It is now known that action potential generation in nearly all types of neurons and muscle cells is accomplished through mechanisms similar to those first detailed in the squid giant axon in early research by Hodgkin and Huxley. In this chapter, we consider the cellular mechanisms by which neurons and axons generate a resting membrane potential and how this membrane potential is briefly disrupted for the purpose of propagation of an electrical signal, the action potential. The membrane potential is generated by the unequal distribution of ions, particularly K+, Na+, and Cl−, across the plasma membrane. This unequal distribution of ions is maintained by ionic pumps and exchangers. K+ ions are concentrated inside the neuron and tend to flow down their concentration gradient, leading to a hyperpolarization of the cell. At the equilibrium potential, the tendency of K+ ions to flow out of the cell is exactly offset by the tendency of K+ ions to enter the cell owing to the attraction of the negative potential inside the cell. The resting membrane is also permeable to Na+ and Cl− and therefore the resting membrane potential is approximately −75 to −40Mv – in other words, substantially positive to EK. An action potential is generated by the rapid influx of Na+ ions followed by a slightly slower efflux of K+ ions. Although the generation of an action potential does not disrupt the concentration gradients of these ions across the membrane, the movement of charge is sufficient to generate a large and brief deviation in the membrane potential. Action potentials are typically initiated in the axon initial segment and the propagation of the action potential along the axon allows communication of the output of the cell to its distal synapses. Neurons possess many different types of ionic channels in their membranes, allowing complex patterns of action potentials to be generated and complex computations to occur within single neurons.