19.6

*Biomedical Applications of Electric Potential Differences

| Conduction of Electrical Signals in Neurons

The human nervous system is remarkable for its ability to transmit information in the form of electrical signals. These signals are carried by the nerves, and the concept of electric potential difference plays an important role in the process. For example, sensory information from our eyes and ears is carried to the brain by the optic nerve and auditory nerves, respectively. Other nerves transmit signals from the brain or spinal column to muscles, causing them to contract. Still other nerves carry signals within the brain.
A nerve consists of a bundle of axons, and each axon is one part of a nerve cell, or neuron. As Figure 19-23 illustrates, a typical neuron consists of a cell body with numerous extensions, called dendrites, and a single axon. The dendrites convert stimuli, such as pressure or heat, into electrical signals that travel through the neuron. The axon sends the signal to the nerve endings, which transmit the signal across a gap (called a synapse) to the next neuron or to a muscle.

Figure 19-23 The anatomy of a typical neuron.

The fluid inside a cell, the intracellular fluid, is quite different from that outside the cell, the extracellular fluid. Both fluids contain concentrations of positive and negative ions. However, the extracellular fluid is rich in sodium (Na+) and chlorine (Cl−) ions, whereas the intracellular fluid is rich in potassium (K+) ions and negatively charged proteins. These concentration differences between the fluids are extremely important to the life of the cell. If the cell membrane were freely permeable, the ions would diffuse across it until the concentrations on both sides were equal. (See Section 14.4 for a review of diffusion.) This does not happen, because a living cell has a selectively permeable membrane. Ions can enter or leave the cell only through membrane channels, and the permeability of the channels varies markedly from one ion to another. For example, it is much easier for K+ ions to diffuse out of the cell than it is for Na+ to enter the cell. As a result of selective membrane permeability, there is a small buildup of negative charges just on the inner side of the membrane and an equal amount of positive charges on the outer side (see Figure 19-24). The buildup of charge occurs very close to the membrane, so the membrane acts like a capacitor (see problems 36 and 37). Elsewhere in the intracellular and extracellular fluids, there are equal numbers of positive and negative ions, so the fluids are overall electrically neutral. Such a separation of positive and negative charges gives rise to an electric potential difference across the membrane, called the resting membrane potential. In neurons, the resting membrane potential ranges from −40 to −90 mV, with a typical value being −70 mV. The minus sign indicates that the inner side of the membrane is negative relative to the outer side.

Figure 19-24 Positive and negative charge layers form on the outside and inside surfaces of a membrane during its resting state.

The physics of an action potential. A “resting” neuron is one that is not conducting an electrical signal. The change in the resting membrane potential is the key factor in the initiation and conduction of a signal. When a sufficiently strong stimulus is applied to a given point on the neuron, “gates” in the membrane open and sodium ions flood into the cell, as Figure 19-25 illustrates. The sodium ions are driven into the cell by attraction to the negative ions on the inner side of the membrane as well as by the relatively high concentration of sodium ions outside the cell. The large influx of Na+ ions first neutralizes the negative ions on the interior of the membrane and then causes it to become positively charged. As a result, the membrane potential in this localized region goes from −70 mV, the resting potential, to about +30 mV in a very short time (see Figure 19-26). The sodium gates then close, and the cell membrane quickly returns to its normal resting potential. This change in potential, from −70 mV to +30 mV and back to −70 mV, is known as the action potential. The action potential lasts for a few milliseconds, and it is the electrical signal that propagates down the axon, typically at a speed of about 50 m/s, to the next neuron or to a muscle cell.

Figure 19-25 When a stimulus is applied to the cell, positive sodium ions (Na+) rush into the cell, causing the interior surface of the membrane to become momentarily positive.

Figure 19-26 The action potential is caused by the rush of positive sodium ions into the cell and a subsequent return of the cell to its resting potential.

| Medical Diagnostic Techniques

Several important medical diagnostic techniques depend on the fact that the surface of the human body is not an equipotential surface. Between various points on the body there are small potential differences (approximately ), which provide the basis for electrocardiography, electroencephalography, and electroretinography. The potential differences can be traced to the electrical characteristics of muscle cells and nerve cells. In carrying out their biological functions, these cells utilize positively charged sodium and potassium ions and negatively charged chlorine ions that exist within the cells and in the extracellular fluid. As a result of such charged particles, electric fields are generated that extend to the surface of the body and lead to small potential differences.
Figure 19-27 shows some locations on the body where electrodes are placed to measure potential differences in electrocardiography. The potential difference between two locations changes as the heart beats and forms a repetitive pattern. The recorded pattern of potential difference versus time is called an electrocardiogram (ECG or EKG), and its shape depends on which pair of points in the picture (A and B, B and C, etc.) is used to locate the electrodes. The figure also shows some EKGs and indicates the regions (P, Q, R, S, and T) that can be associated with specific parts of the heart’s beating cycle. The distinct differences between the EKGs of healthy (normal) and damaged (abnormal) hearts provide physicians with a valuable diagnostic tool.

Figure 19-27 The potential differences generated by heart muscle activity provide the basis for electrocardiography. The normal and abnormal EKG patterns correspond to one heartbeat.

The physics of electrocardiography.

In electroencephalography the electrodes are placed at specific locations on the head, as Figure 19-28 indicates, and they record the potential differences that characterize brain behavior. The graph of potential difference versus time is known as an electroencephalogram (EEG). The various parts of the patterns in an EEG are often referred to as “waves” or “rhythms.” The drawing shows an example of the main resting rhythm of the brain, the so-called alpha rhythm, and also illustrates the distinct differences that are found between the EEGs generated by healthy (normal) and diseased (abnormal) tissue.

Figure 19-28 In electroencephalography the potential differences created by the electrical activity of the brain are used for diagnosing abnormal behavior.

The physics of electroencephalography.

The electrical characteristics of the retina of the eye lead to the potential differences measured in electroretinography. Figure 19-29 shows a typical electrode placement used to record the pattern of potential difference versus time that occurs when the eye is stimulated by a flash of light. One electrode is mounted on a contact lens, while the other is often placed on the forehead. The recorded pattern is called an electroretinogram (ERG), parts of the pattern being referred to as the “A wave” and the “B wave.” As the graphs show, the ERGs of normal and diseased (abnormal) eyes can differ markedly.

Figure 19-29 The electrical activity of the retina of the eye generates the potential differences used in electroretinography.

The physics of electroretinography.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.