6: Membrane Potential and the Passive Electrical Properties of the Neuron

Introduction

• The Resting Membrane Potential Results from the Separation of Charge Across the Cell Membrane

• The Resting Membrane Potential Is Determined by Nongated and Gated Ion Channels

  • Open Channels in Glial Cells Are Permeable to Potassium Only

  • Open Channels in Resting Nerve Cells Are Permeable to Several Ion Species

  • The Electrochemical Gradients of Sodium, Potassium, and Calcium Are Established by Active Transport of the Ions

  • Chloride Ions Are Also Actively Transported

• The Balance of Ion Fluxes That Maintains the Resting Membrane Potential Is Abolished During the Action Potential

• The Contributions of Different Ions to the Resting Membrane Potential Can Be Quantified by the Goldman Equation

• The Functional Properties of the Neuron Can Be Represented as an Electrical Equivalent Circuit

• The Passive Electrical Properties of the Neuron Affect Electrical Signaling

  • Membrane Capacitance Slows the Time Course of Electrical Signals

  • Membrane and Axoplasmic Resistance Affect the Efficiency of Signal Conduction

  • Large Axons Are More Easily Excited Than Small Axons

  • Passive Membrane Properties and Axon Diameter Affect the Velocity of Action Potential Propagation

• An Overall View

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 voltage but can 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 resting ion channels establish and maintain the resting potential 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 7 we shall 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 and receptor potentials are considered in Chapters 8, 9, 10 and 11 in the ...