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Electric Activities of the Cell
mixture of live and dead cells. Dead cells do not properly conduct electricity since
the dead cells have no cytoplasm and no charge carriers as such. In addition, the
skin may excrete fat, which is also a poor conductor. Sweating, on the other hand,
improves conduction because of the salt content, i.e., the ions Na + , K + , and Cl − . These
issues can be handled by typical skin preparation and treatments such as degreasing
of the skin and exfoliating the skin.
Degreasing the skin can lower the skin–electrode impedance by a factor of 30,
and applying a conducting impedance matching layer between the electrode and the
skin can lower the impedance by approximately a factor of 60. Exfoliation will result
in impedance lowering by a factor of 250. All measures combined can bring the
transfer impedance down by a factor of approximately 2000.
In addition to the epidermis, the dermis can form a capacitive element. This capacitance is formed by the poor conduction conditions of the epidermis, the abundant supply of ions in the dermis, and the ample supply of free electrons in the electrode. This
means that the transfer impedance is not just resistive but also capacitive. There can be
a small contribution of inductive impedance based on the underlying tissue structure.
An ideal electrode needs to be easy to apply and maintain electric stability to
ensure reproducible measurement. In addition, an electrode needs to be able to conduct an alternating signal; this does not imply that it needs to be able to measure a
steady-state potential.
Additional considerations are that electrodes need to be made of metals that will
not dissolve, e.g., gold, silver, or platinum. The electrodes will require a large surface
area to limit current density in addition to a connection to a high-impedance amplifier
input to curb total current.
8.7 SUMMARY
In this chapter, we described the electrochemical phenomena causing action potentials as well as cell membrane potential. We also presented a mathematical model of
the cell membrane’s electric activities called Hodgkin–Huxley model. This model
is often used to relate the intercellular and extracellular ion concentrations to the
overall cell membrane as well as action potentials. Finally, we briefly reviewed the
structure and applications of some commonly used biomedical electrodes.
PROBLEMS
8.1 I mport the data in the file “p_8_1.xls” in MATLAB ® and plot the signal. In order to
do so, use File/Import Data … on the main MATLAB menu and follow the steps in
loading and naming of the data. The file contains the data from a surface electrode
measuring a nerve impulse. Sample frequency 1000 Hz.*
a. Use discrete Fourier transform (DFT) to describe the signals in the frequency
domain. Determine the dominant frequency.
b. Measure the duration of an entire pulse and comment on the results.
* N.M. Maurits, PhD, Department of Clinical Neurophysiology, Groningen University Medical Center
(GUMC), Groningen, the Netherlands.
Electric Activities of the Cell
mixture of live and dead cells. Dead cells do not properly conduct electricity since
the dead cells have no cytoplasm and no charge carriers as such. In addition, the
skin may excrete fat, which is also a poor conductor. Sweating, on the other hand,
improves conduction because of the salt content, i.e., the ions Na + , K + , and Cl − . These
issues can be handled by typical skin preparation and treatments such as degreasing
of the skin and exfoliating the skin.
Degreasing the skin can lower the skin–electrode impedance by a factor of 30,
and applying a conducting impedance matching layer between the electrode and the
skin can lower the impedance by approximately a factor of 60. Exfoliation will result
in impedance lowering by a factor of 250. All measures combined can bring the
transfer impedance down by a factor of approximately 2000.
In addition to the epidermis, the dermis can form a capacitive element. This capacitance is formed by the poor conduction conditions of the epidermis, the abundant supply of ions in the dermis, and the ample supply of free electrons in the electrode. This
means that the transfer impedance is not just resistive but also capacitive. There can be
a small contribution of inductive impedance based on the underlying tissue structure.
An ideal electrode needs to be easy to apply and maintain electric stability to
ensure reproducible measurement. In addition, an electrode needs to be able to conduct an alternating signal; this does not imply that it needs to be able to measure a
steady-state potential.
Additional considerations are that electrodes need to be made of metals that will
not dissolve, e.g., gold, silver, or platinum. The electrodes will require a large surface
area to limit current density in addition to a connection to a high-impedance amplifier
input to curb total current.
8.7 SUMMARY
In this chapter, we described the electrochemical phenomena causing action potentials as well as cell membrane potential. We also presented a mathematical model of
the cell membrane’s electric activities called Hodgkin–Huxley model. This model
is often used to relate the intercellular and extracellular ion concentrations to the
overall cell membrane as well as action potentials. Finally, we briefly reviewed the
structure and applications of some commonly used biomedical electrodes.
PROBLEMS
8.1 I mport the data in the file “p_8_1.xls” in MATLAB ® and plot the signal. In order to
do so, use File/Import Data … on the main MATLAB menu and follow the steps in
loading and naming of the data. The file contains the data from a surface electrode
measuring a nerve impulse. Sample frequency 1000 Hz.*
a. Use discrete Fourier transform (DFT) to describe the signals in the frequency
domain. Determine the dominant frequency.
b. Measure the duration of an entire pulse and comment on the results.
* N.M. Maurits, PhD, Department of Clinical Neurophysiology, Groningen University Medical Center
(GUMC), Groningen, the Netherlands.