Extracellular Recording Of The Nerve Action Potential

The existence of''animal electricity" had been known for more than 200 years, but the first direct experimental evidence for it was not provided until the development of electronic amplifiers and oscilloscopes. Figure 1 illustrates one of the earliest recordings that demonstrated the ability of nerve cells to alter their electrical activity for the purpose of coding and transmitting information. In this experiment, performed in 1934 by Hartline, extracellular recordings were made from the optic nerve of an invertebrate eye. Details of the techniques and interpretation of these extracellular recordings are described in Chapter 5, but for now it is sufficient to know that it is possible to place an electrode on the surface of a nerve axon and record electrical events that are associated with potential changes taking place across the axonal membrane. In the experiment illustrated in Fig. 1, light flashes of different intensities were delivered to the eye. With a very weak intensity light flash, there was no change in the baseline electrical activity. When the intensity of the

Nerve Activity—

Light Stimulus—^ I Dim

FIGURE 1 Action potentials recorded from an invertebrate optic nerve in response to light flashes of different intensities. With dim illumination no action potentials are recorded, but with more intense illumination the number and frequency of action potentials increase. (Modified from Hartline HK. J Cell Comp Physiol 1934; 5:229.)

light flash was increased, however, small spike-like transient events associated with the onset of the light were observed. Increasing the intensity of the light flash produced an increase in the rate of these spike-like events. These spike-like events are known as nerve action potentials, impulses, or, simply, spikes.

Even though this experiment was performed more than 60 years ago, it nonetheless illustrates three basic properties of nerve action potentials and how they are used by the nervous system to encode information. First, nerve action potentials are very brief, having a duration of only about 1 msec (1 msec = 10—3 sec). Second, action potentials are initiated in an all-or-nothing manner. Note that the amplitude of the action potentials does not vary during a sustained light flash. Third, and related to the above, with increasing stimulus intensity it is not the size of action potentials that varies but rather their number or frequency. This is the general means by which intensity information is coded in the nervous system, and it is true for a variety of peripheral receptors. Specifically, the greater the intensity of a physical stimulus (whether it be a stimulus to a photoreceptor, a stimulus to a mechanoreceptor in the skin, or a stimulus to a muscle stretch receptor), the greater is the frequency of nerve action potentials. This finding has given rise to the notion of the frequency code for stimulus intensity in the nervous system.

Most of the information transmitted to the central nervous system from the periphery is mediated by nerve action potentials. Moreover, all the motor commands initiated in the central nervous system are propagated to the periphery by nerve action potentials, and action potentials produced in muscle cells are the first step in the initiation of muscular contraction (see Chapter 6). Action potentials are therefore quite important, not only for the functioning of the nervous system, but also for the functioning of muscle cells, and for this reason it is important to understand the ionic mechanisms that underlie the action potential and its propagation.

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