104
The Body Electric
Charlie gave me a lesson in basic electronics. A voltage entering a
wire will decrease as the current travels along, so there'll be a uniform
voltage drop in each unit of length. All you need to do is put both
electrodes along the conductor, with the reference electrode closer to
what you think is the source. If you use a standard distance between the
electrodes, you can compare the voltage drop along various wires and
measure changes in the whole system from any segment of it.
All I had to do was the surgery. I decided to work on bullfrogs,
whose hind legs were long and contained a nice big sciatic nerve. It was
easy to find and could be exposed with just a little careful dissection,
going between muscles instead of cutting through them. I was able to
isolate over an inch of nerve with no bleeding or tissue damage, slipping
a plastic sheet underneath so as not to pick up readings from the sur-
rounding muscles. We measured the voltage gradient over a standard
distance of 1 centimeter. It was the same from one frog to the next. In
deep anesthesia it was absent or pretty small; as the anesthesia wore off,
it became a constant drop of about 4 millivolts per centimeter, always
gradually positive toward the spinal cord and negative toward the toes.
In some frogs we cut the nerve above the measuring site, whereupon
the voltages disappeared—another indication that the current was actu-
ally in the nerve. Voltages returned a little later, but they weren't the
same as before. We figured these secondary voltages were probably an
artifact—a spurious measurement produced by extraneous factors—
caused by currents of injury from the cut nerve itself or from the other
tissues where I'd made the incision to cut the nerve.
Charlie then suggested that we make measurements on a longer sec-
tion of nerve, and that was when we ran into a puzzle. The nicely re-
producible voltages we'd found before couldn't be duplicated when we
extended our measurement distance to 2 centimeters, close to the knee.
We expected double the potential we'd found over the 1-centimeter dis-
tance, but often it was lower or higher than it should have been. I
insisted that my dissection was producing local currents of injury that
made our readings unpredictable. However, Charlie pointed out that I
was a good frog surgeon and I didn't seem to be doing any more damage
than before. He asked, "Could there be any difference in the nerve where
you extended the dissection?"
"Not likely," I said. "The sciatic nerve does split up into two
branches, but you only find them below the knee, when one goes to the
front of the calf and one to the back."
"How do you know it doesn't separate before it gets to the knee?" he asked.