Chapter VI 



THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



A sensation invoked in the brain has no resemblance to the physical 

 events which redirect the activity of the nervous system. The sensa- 

 tion of light may be aroused not only through absorption by the retinal 

 receptors of radiant energy of a limited group of frequencies, but also by 

 electrical or mechanical stimulation of the retina. The sensation called 

 sound may be experienced by placing a low-frequency tuning fork with 

 its base in contact with the head so that the vibrations are communicated 

 to the bones of the skull. The same fork pressed to the surface of the 

 skin produces a sensation of vibration, that is, a series of tactile impres- 

 sions repeated at rapid intervals. 



The experienced sensations are not the reproductions of the physical 

 stimuli; they are, however, symbols of the stimuli that can be used to 

 inform us of events occurring in our physical environment. Therefore 

 it is important to inquire how nerves transport quantitative information 

 about these stimuli to the central nervous system. 



The experienced sensations are aroused by messages which are trans- 

 mitted from the excited receptors by means of the nerves to the cere- 

 brum. The problems of the kind of messages or nerve impulses and of 

 the speed of their propagation have been under investigation since the 

 time of Galvani and Volta, but not until 1908 did the researches initi- 

 ated by Gotch and Keith Lucas make it possible to give a quantitative 

 interpretation to the problem of nerve conduction. 



The most general features of the so-called moving nerve impulses are 

 best studied by first examining those irritable tissues or cells which can 

 develop quantitatively interpretable responses to mechanical, thermal, 

 chemical, or electrical stimulation, such as excised nerves and muscles of 

 higher animals. 



The impulse set up in response to an electric stimulus applied to tissue 

 isolated from its normal environment is an artificial effect. The accumu- 

 lated evidence, however, shows that its counterpart must be the funda- 

 mental activity of the nerve fiber in the body. 



Relation of Stimulus to Response 



The word " stimulus " will be used to mean any local artificial change 

 in the environment of tissues which causes an excitatory process to be set 

 up in the tissues localized at the point of stimulation. 



211 



