220 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



potential is smooth, and its crest is reached in 0.135 millisecond. The 

 falling phase is much slower, and its contour is usually not smooth. 



The velocity of propagation of such a pulse, if taken from the saphen- 

 ous nerve of the cat (Hursh [1939]), was found to be about 70 meters per 

 second at 37° C, so that the crest is 0.95 cm behind the start. 



It will be noticed that the rise and fall of the potential pulse is com- 

 pleted in about 0.5 millisecond. Erlanger and Gasser [1937] refer to it 

 as a spike-like variation having the form of a skewed probability curve 

 followed by after-potentials that may last for as much as 1 second (see 

 Table VI-1). 



Composite Action Potential of a Nerve Trunk 



After the successful adoption of the variable-frequency stimulator 

 and the cathode-ray oscillograph with the pre-amplification of the electri- 

 cal potential impulses up to a millionfold, the study of nerve reactions to 

 artificial electrical stimulation was given a new impetus. 



The sciatic nerve of the bullfrog has served physiologists as a sort of 

 standard in investigations of this type because it has the advantage of 

 length and multiplicity of fibers. It consists of many hundreds of fibers 

 of every size and variety. The diameters of the fibers range from 12 

 microns, which are myelinated, down to the smallest, which are 

 unmyelinated. 



Figure VI-9 shows some semi-diagrammatic typical results compiled 

 from data obtained by Erlanger and Gasser [1937] from a peroneal nerve 

 of a bullfrog, stimulated with an electric shock strong enough to excite all 

 the fibers. Such an action potential record is divisible into three charac- 

 teristic groups of elevations associated with 04) fibers of highest velocity, 

 (B) those of intermediate velocity, and (C) the slowest pulse-propagating 

 fibers. In group A are included the large myelinated fibers; in group C 

 are included all unmyelinated fibers. 



Experimentally, it was found that the threshold of stimulation for fast 

 fibers is lower than that for slow ones. When the shock intensity was 

 gradually raised, the a spike was developed first and with increasing 

 intensity of shock the 6 and y, and finally the B and C negative potential 

 waves were developed. 



This initial qualitative classification of nerve fibers in terms of their 

 velocity of nerve pulse propagation has been justified by the recent 

 experimental work of Gasser and Grundfest [1939] and by Grundfest 

 [1939]. The view that nerve fibers of mammals may be divided into 

 three groups, each of which possesses a characteristic action potential, 

 seems to be well established. 



