Medical applications of capacitors and potential difference

Medical and Biomedical Applications
of Capacitors and Potential Difference
The physics of a Defibrillator
During a heart attack, the heart produces a rapid, unregulated pattern of beats, a
condition known as cardiac fibrillation. Cardiac fibrillation can often be stopped by
sending a very fast discharge of electrical energy through the heart. For this purpose,
emergency medical personnel use defibrillators, such as the one shown in Figure 1. A
paddle is connected to each plate of a large capacitor, and the paddles are placed on the
chest near the heart. The capacitor is charged to a potential difference of about a
thousand volts. The capacitor is then discharged in a few thousandths of a second; the
discharge current passes through a paddle, the heart, and the other paddle. Within a
few seconds, the heart often returns to its normal beating pattern.

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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 nerves 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 2 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.
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 and chlorine ions, whereas the
intracellular fluid is rich in potassium 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. 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 ions to diffuse out of the cell than it is for 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 3). The buildup of charge occurs very close to the membrane, so the
membrane acts like a capacitor. 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
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potential. In neurons, the resting membrane potential ranges from -40 to -90 mV, with a
typical value of -70 mV. The minus sign indicates that the inner side of the membrane is
negative relative to the outer side.

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
4 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 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
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resting potential, to about -30 mV in a very short
time (see Figure 5). 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.

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 30 – 500 V), 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 the small potential
differences.

Figure 6 shows electrodes placed on the body to measure potential differences in
electrocardiography. The potential difference between two locations changes as the
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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)
associated with specific parts of the heart’s beating cycle. The differences between the
EKGs of normal and abnormal hearts provide physicians with a valuable diagnostic tool.

In electroencephalography the electrodes are placed at specific locations on the
head, as Figure 7 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. The electrical characteristics of the retina of the eye lead to
the potential differences measured in electroretinography.

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Figure 8 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), and parts of the pattern are
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.

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