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 

 axon at a point on the surface changes as the wave of negativity passes. 



