. Biophysics: concepts and mechanisms. Biophysics. TRANSIENT BIOELECTRICS IN NERVE 265 shape of the impulse. Further, the use of two pickup electrodes placed a few centimeters apart, each feeding an oscilloscope, permitted measurement of the time it takes the impulse to cover the distance between them. The speed of transmission was thus shown to be about 100 m/sec (about 200 miles/hr), less if the nerve were bathed in media of low electrical conductivity. Since an excised squid axon bathed in seawater would live and reliably transmit for about 1 hr, one can well imagine the exciting days for H
. Biophysics: concepts and mechanisms. Biophysics. TRANSIENT BIOELECTRICS IN NERVE 265 shape of the impulse. Further, the use of two pickup electrodes placed a few centimeters apart, each feeding an oscilloscope, permitted measurement of the time it takes the impulse to cover the distance between them. The speed of transmission was thus shown to be about 100 m/sec (about 200 miles/hr), less if the nerve were bathed in media of low electrical conductivity. Since an excised squid axon bathed in seawater would live and reliably transmit for about 1 hr, one can well imagine the exciting days for Hodgkin and Hux- ley of Cambridge University, working at Plymouth, England; and for Curtis and Cole at the famed Marine Biological Institute at Wood's Hole, Massa- chusetts—but hard times for the squid population in the waters close by. direction of propagation f (impulse) *g£ ?^. ^ wave of negativity' leading edge trailing edge Figure 10-2. Potential Changes as the Impulse Passes Detector Electrodes, One Inside and One Outside the Axon. Normally the axon is negative to the outside electrolyte, but as the impulse passes, the potential is momentarily reversed. By 1939 researchers had micropipets inside the axons (see Figure 10-1, bottom) to sample the fluids during stimulation (Table 10-1); and micro- electrodes, too, to record the change in voltage across the membrane. J. Bernstein's hypothesis (1902), that the potential difference across the rest- ing membrane is due to a difference in salt concentration, was fully con- firmed. However, as the impulse passed any point on the nerve, the nerve mem- brane's voltage-difference from inside to outside at that point was found not only to drop to zero, but actually to reverse—the inside to become positive some 40 mv (the so-called "spike"), before it started its recovery to the normal state! There the puzzle had to stand during World War II. Fig- ure 10-2 shows how the potential difference between inside and outside the
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