TRANSIENT BIOELECTRICS IN NERVE 263 



common, however, throughout living tissue, and play a uniquely important 

 role in nerve conduction. Here an electrical transient — the change in voltage 

 across the nerve cell membrane — is propagated with great speed along the 

 surface of the cell and along the nerve fiber formed by many axons in paral- 

 lel. The voltage change is the unit of information. First we describe how 

 this transmission takes place. 



From Volta* to Hodgkin 



It was in the late 1770's in Bologna that the Italian physician, Luigi 

 Galvani, and his wife Lucia observed quite by accident that the leg of the 

 frog with which they were experimenting could be made to twitch if certain 

 parts of the animal were touched simultaneously with the ends of two differ- 

 ent pieces of metal (iron and zinc, for example) joined together. Actually 

 they had discovered two things: the electrical voltage of a Zn-Fe couple, and 

 the electrochemical exictation of living tissue. In the succeeding two hun- 

 dred years a great body of facts has accumulated; these have demonstrated 

 quite conclusively the electrochemical nature of nerve conduction and the 

 resulting stimulation of excitable tissue. The afferent and efferent nerve 

 systems have been well tracked and catalogued — the job of the former being 

 to conduct commands, despatched by the brain, out to muscles and other 

 effector tissues. The so-called "all-or-none law," which says simply that the 

 excitable tissue will not fire (act) unless the stimulus has some minimum 

 power, and that the impulse moves down the nerve with constant amplitude 

 and velocity,** is now an accepted working principle for the physiologist. 

 Various chemical and physical methods have been developed to modify the 

 sensitivity of the nerve to stimulating agents — chemical catalysts in the form 

 of drugs; electrical pacemakers, etc. 



However, even with all this great accumulation of useful knowledge on 

 how to modify the operation of the nervous system, it has only been since the 

 early 1930's that definitive examination could be made of several of the many 

 theories of operation of the nerve fiber. About that time it was realized 

 that the main nerve axon of the squid — in this respect unique among all 

 others — is a tube large enough (~1 mm od and several centimeters long 

 (see Figure 10-1)) to be examined both electrically and chemically, inside 

 and out. The fact that its physical structure could be examined by both 



*In Phil. Trans., 1800, Alcssandro Volta, Professor Natural Philosophy, University of 

 Pavia, published a paper in which he not only described his new "artificial electric organ" 

 (i.e., the first storage battery), but also discussed the effects which electric current from this 

 invention "exercises on the different parts of our body," effects "which will open a very wide 

 field for reflection, . . . particularly interesting to Medicine." 



**In certain unnatural media (sodium-deficient, for example) decremental propagation 

 occurs: both amplitude and velocity decrease as the impulse moves along the nerve. 



