264 



BIOPHYSICAL STUDIES ON NERVE AND MUSCLE 



string 



Figure 10-1. Nerve Cell and Axons. The length of the axon is sometimes as 

 much as 1 00,000 times the length of the cell. Insertion of Micropipets and Micro- 

 electrodes. Stimulating (or detecting) electrodes touching myelin sheath. 



optical and electron-microscopic methods made it all the more attractive as 

 a subject for study. 



In the next section some of the pertinent information which has been ob- 

 tained from the lowly squid is summarized. This information has formed 

 the basis of a better understanding of the biophysics of nerve conduction. 

 Nerve is similar enough from one species to another that some generalities 

 can be assumed on the basis of information gained from the squid axon. 



The Era of the Squid 



Curtis and Cole by 1936 had placed metal electrodes inside and out- 

 side the squid's tube-shaped axon; and with a conventional Wheatstone 

 bridge, had made measurements of the electrical resistance (20,000 

 ohms/cm 2 ) and electrical capacitance (1 microfarad (^f) per cm 2 ) of the 

 membrane. Further, they showed that the resistance is much lower when 

 the nerve is actively transmitting impulses. 



With the development of electronic dc amplifiers and oscilloscopes, it 

 became possible to display the passage of the nerve impulse as detected by 

 thin platinum-wire contacts (electrodes) touching the nerve (see bottom of 

 Figure 10-1, for example). The impulse turned out to be a band of negative 

 charge passing down the outside surface of the axon, from the point of 

 stimulation to the far end. The insert in Figure 10-2 shows the electrical 



