90
The Body Electric
result from adding up a lot of action potentials all going in the same
direction and arriving in the same place at the same time?
Some answers were provided by a series of beautiful experiments by
Ling, Gerard, and Benjamin Libet at the University of Chicago. Work-
ing on frogs, they studied areas of the cortex where the neuron layer was
only one cell thick and the cells were arranged side by side like soldiers
on review, all pointing in the same direction. In such areas they found a
negative potential on the dendrites (the short incoming fibers) and a
positive potential at the ends of the axons (the longer outgoing fibers).
This indicated a steady direct current along the normal direction of im-
pulse transmission. The entire nerve cell was electrically polarized.
POLARIZED NERVE CELLS LINED UP IN CEREBRAL CORTEX
In another series of experiments, on brains removed from frogs and
kept alive in culture, the Chicago group found that direct currents swept
across the surface of the cortex in very slow waves, which could be pro-
duced experimentally by applying chemicals such as caffeine to a single
spot on the surface. When they made a cut on the brain, severing groups
of nerve fibers, these DC traveling waves would still cross the cut if the
two surfaces were in direct contact. If the researchers held the cut open and
filled it with a saline solution that matched body fluids, then the waves
couldn't cross the gap. These were particularly important observations.
They indicated that the current was transmitted by structures outside the
neurons; it crossed the cut when the edges touched, but the microscopic
parts of the severed neurons wouldn't have rejoined so easily. The results
also showed that the current was not a flow of ions; otherwise it would
have been able to cross the gap through the salt water.
Studying intact brains in living frogs, the lame group found a poten-
tial between the front and back of the brain. The olfactory (frontal) lobes
were several milivolts negative with respect to the occipital (rear) lobe,