INFORMATION IS CARRIED WITHIN neurons and from neurons to their target cells by electrical and chemical signals. Transient electrical signals are particularly important for carrying time-sensitive information rapidly and over long distances. These transient electrical signals—receptor potentials, synaptic potentials, and action potentials—are all produced by temporary changes in the electric current into and out of the cell, changes that drive the electrical potential across the cell membrane away from its resting value. This current represents the flow of negative and positive ions through ion channels in the cell membrane.
Two types of ion channels—resting and gated—have distinctive roles in neuronal signaling. Resting channels are primarily important in maintaining the resting membrane potential, the electrical potential across the membrane in the absence of signaling. Some types of resting channels are constitutively open and are not gated by changes in membrane voltage; other types are gated by changes in voltage but are also open at the negative resting potential of neurons. Most voltage-gated channels, in contrast, are closed when the membrane is at rest and require membrane depolarization to open.
In this and the next several chapters, we consider how transient electrical signals are generated in the neuron. We begin by discussing how particular ion channels establish and maintain the membrane potential when the membrane is at rest and briefly describe the mechanism by which the resting potential can be perturbed, giving rise to transient electrical signals such as the action potential. We then consider how the passive electrical properties of neurons—their resistive and capacitive characteristics—contribute to the integration and local propagation of synaptic and receptor potentials within the neuron. In Chapter 10 we examine the detailed mechanisms by which voltage-gated Na+, K+, and Ca2+ channels generate the action potential, the electrical signal conveyed along the axon. Synaptic potentials are considered in Chapters 11 to 14, and receptor potentials are discussed in Part IV in connection with the actions of sensory receptors.
The Resting Membrane Potential Results From the Separation of Charge Across the Cell Membrane
The neuron’s cell membrane has thin clouds of positive and negative ions spread over its inner and outer surfaces. At rest, the extracellular surface of the membrane has an excess of positive charge and the cytoplasmic surface an excess of negative charge (Figure 9–1). This separation of charge is maintained because the lipid bilayer of the membrane is a barrier to the diffusion of ions (Chapter 8). The charge separation gives rise to the membrane potential (Vm), a difference of electrical potential, or voltage, across the membrane defined as
where Vin is the potential on the inside of the cell and Vout the potential on the outside.