Because electrical signals are the basis of information transfer in the nervous system, it is essential to understand how these signals arise. The use of electrical signals—as in sending electricity over wires to provide power or information—presents a series of problems in electrical engineering. A fundamental problem for neurons is that their axons, which can be quite long (remember that a spinal motor neuron can extend for a meter or more), are not good electrical conductors. Although neurons and wires are both capable of passively conducting electricity, the electrical properties of neurons compare poorly to even the most ordinary wire. To compensate for this deficiency, neurons have evolved a “booster system” that allows them to conduct electrical signals over great distances despite their intrinsically poor electrical characteristics. The electrical signals produced by this booster system are called action potentials (which are also referred to as “spikes” or “impulses”).

The best way to observe an action potential is to use an intracellular microelectrode to record directly the electrical potential across the neuronal plasma membrane (Figure 2.1). A typical microelectrode is a piece of glass tubing pulled to a very fine point (with an opening of less than 1 μm diameter) and filled with a good electrical conductor, such as a concentrated salt solution. This conductive core can then be connected to a voltmeter, such as an oscilloscope, to record the transmembrane voltage of the nerve cell. When a microelectrode is inserted through the membrane of the neuron, it records a negative potential, indicating that the cell has a means of generating a constant voltage across its membrane when it is at rest. This voltage, called the resting membrane potential, depends on the type of neuron being examined, but it is always a fraction of a volt (typically -40 to -90 mV).

Figure 2.1

Recording passive and active electrical signals in a nerve cell. (A) Two microelectrodes are inserted into a neuron; one of these measures membrane potential while the other injects current into the neuron. (B) Inserting the voltage-measuring microelectrode (more...)

Action potentials represent transient changes in the resting membrane potential of neurons. One way to elicit an action potential is to pass electrical current across the membrane of the neuron. In normal circumstances, this current would be generated by the action of neurotransmitters released by other neurons, or by the transduction of an external stimulus at specialized regions of sensory neurons (sensory receptors in the skin, for example; see Unit II). In the laboratory, however, electrical current suitable for initiating an action potential can be readily produced by inserting a second microelectrode into the same neuron and then connecting the electrode to a battery. If the current delivered in this way is such as to make the membrane potential more negative (hyperpolarization), nothing very dramatic happens. The membrane potential simply changes in proportion to the magnitude of the injected current. Such hyperpolarizing responses do not require any unique property of neurons and are therefore called passive electrical responses. A much more interesting phenomenon is seen if current of the opposite polarity is delivered, so that the membrane potential of the nerve cell becomes more positive than the resting potential (depolarization). In this case, at a certain level of membrane potential called the threshold potential, an action potential occurs (see Figure 2.1B).

The action potential, which is an active response generated by the neuron, appears on an oscilloscope as a brief (about 1 ms) change from negative to positive in the transmembrane potential. Importantly, the amplitude of the action potential is independent of the magnitude of the current used to evoke it; that is, larger currents do not elicit larger action potentials. The action potentials of a given neuron are therefore said to be all-or-none, because they occur fully or not at all. If the amplitude or duration of the stimulus current is increased sufficiently, multiple action potentials occur, as can be seen in the responses to the three different current intensities shown at the right of Figure 2.1B. It follows, therefore, that the intensity of a stimulus is encoded in the frequency of action potentials rather than in their amplitude.

This chapter addresses the underlying question of how nerve cells can generate electrical potentials by distributing ions across the neuronal membrane. Chapter 3 explores more specifically the means by which action potentials are produced and how these signals solve the problem of long-distance electrical conduction within nerve cells. Chapter 4 examines the properties of membrane molecules responsible for producing action potentials. Finally, Chapters 5–8 consider how electrical signals are transmitted from one nerve cell to another at synaptic contacts.