Biomedical Engineering Reference
In-Depth Information
Volta's experiments led him toward the idea of an electric force or electrical “potential.”
This, he assumed, resided in contact between the dissimilar metals. As Volta experimented
with additional combinations, he found that an electrical potential also existed when there
was contact between the metals and some fluids. As a result, Volta added liquids, such
as brine and dilute acids, to his conducting system and classified the metal contacts as
“electrifiers of the first class” and the liquids as electrifiers of the “second class.”
Volta found that there was only momentary movement of electricity in a circuit com-
posed entirely of dissimilar metals. However, when he put two dissimilar metals in contact
with a separator soaked with a saline or acidified solution, there was a steady indication of
potential. In essence, Volta was assembling the basic elements of an electric battery: two dis-
similar metals and a liquid separator. Furthermore, he found that the overall electric effect
could be enlarged by multiplying the elements. Thus, by stacking metal disks and the
moistened separators vertically, he constructed an “electric pile,” the first electric battery.
This was the most practical discovery of his career.
12.2.5 The Final Result
Considerable time passed before true explanations became available for what Galvani
and Volta had done. Clearly, both demonstrated the existence of a difference in electric
potential, but what had produced it eluded them. The potential difference present in the
experiments carried out by both investigators is now clearly understood. Although Galvani
thought that he had initiated muscular contractions by discharging animal electricity resi-
dent in a physiological capacitor consisting of the nerve (inner conductor) and muscle sur-
face (outer conductor), it is now known that the stimulus consists of an action potential that
in turn causes muscular contractions.
It is interesting to note that the fundamental unit of the nervous system—the neuron—
has an electric potential between the inside and outside of the cell, even at rest. This
membrane-resting potential is continually affected by various inputs to the cell. When a
certain potential is reached, an action potential is generated along its axon to all of its dis-
tant connections. This process underlies the communication mechanisms of the nervous
system. Volta's discovery of the electrical battery provided the scientific community with
the first steady source of electrical potential, which when connected in an electric circuit
consisting of conducting materials or liquids, results in the flow of electrical charge—that
is, electrical current. This device launched the field of electrical engineering.
12.3 NEURONS
A reasonable estimate of the human brain is that it contains about 10
12
neurons parti-
tioned into fewer than 1,000 different types in an organized structure of rather uniform
appearance. While not important in this chapter, it is important to note that there are two
classes of neuron: the nerve cell and the neuroglial cell. Even though there are 10 to 50 times
as many neuroglial cells as nerve cells in the brain, attention is focused here on the nerve cell,
since the neuroglial cells are not involved in signaling and primarily provide a support func-
tion for the nerve cell. Therefore, the terms
neuron
and
nerve cell
are used interchangeably,
