Biomedical Engineering Reference
In-Depth Information
and nerves touched the muscular parts, contractions were excited. Any idea of a stimulus arising earlier from
the action of the salt, or from the impulse produced by the fall of the nerves, may be easily removed. Nothing
will be necessary but to apply the same nerves to the muscles of another prepared frog, not in a Galvanic
circle; for, in this case, neither the salt, nor the impulse even if more violent, will produce muscular motion.
The claims and counterclaims of Volta and Galvani developed rival camps of supporters
and detractors. Scientists swayed from one side to the other in their opinions and loyalties.
Although the subject was complex and not well understood, it was on the verge of an era of
revelation. The next great contribution to the field was made by Carlo Matteucci, who both
confirmed Galvani's third experiment and made a new discovery. Matteucci showed that
the action potential precedes the contraction of skeletal muscle. In confirming Galvani's
third experiment, which demonstrated the injury potential, Matteucci noted:
I injure the muscles of any living animal whatever, and into the interior of the wound I insert the nerve of
the leg, which I hold, insulated with glass tube. As I move this nervous filament in the interior of the wound,
I see immediately strong contractions in the leg. To always obtain them, it is necessary that one point of the
nervous filament touches the depths of the wound, and that another point of the same nerve touches the
edge of the wound.
By using a galvanometer, Matteucci found that the difference in potential between an
injured and uninjured area was diminished during a tetanic contraction. The study of this
phenomenon occupied the attention of all succeeding electrophysiologists. More than
this, however, Matteucci made another remarkable discovery: that a transient bioelectric
event, now designated the action potential, accompanies the contraction of intact skeletal
muscle. He demonstrated this by showing that a contracting muscle is able to stimulate a
nerve that, in turn, causes contraction of the muscle it innervates. The existence of a bioelec-
tric potential was established through the experiments of Galvani and Matteucci. Soon
thereafter, the presence of an action potential was discovered in cardiac muscle and nerves.
Volta, on the other hand, advocated that the source of the electricity was due to the con-
tact of the dissimilar metals only, with the animal tissue acting merely as the indicator. His
results differed substantially depending on the pairs of metals used. For example, Volta
found that the muscular reaction from dissimilar metals increased in vigor depending on
the metals that were used.
In an effort to obtain better quantitative measurements, Volta dispensed with the use of
muscles and nerves as indicators. He substituted instead his “condensing electroscope.” He
was fortunate in the availability of this superior instrument because the contact charge poten-
tial of the dissimilar metals was minute, far too small to be detected by the ordinary gold-leaf
electroscope. Volta's condensing electroscope used a stationary disk and a removable disk
separated by a thin insulating layer of shellac varnish. The thinness of this layer provided a
large capacity for accumulation of charge. When the upper disk was raised after being
charged, the condenser capacity was released to give a large deflection of the gold leaves.
Volta proceeded systematically to test the dissimilar metal contacts. He made disks of
various metals and measured the quantity of the charge on each disk combination by the
divergence of his gold foil condensing electroscope. He then determined whether the charge
was positive or negative by bringing a rubbed rod of glass or resin near the electroscope.
The effect of the rod on the divergence of the gold foil indicated the polarity of the charge.
