Nervous Systems 



783 



TARl F 7^ 

 VELOCITY OF CONDUCTION IN NERVE NETS AND GANGLIONIC CHAINS 



The numerical relationship of velocity to fiber size differs according to 

 fiber type and sheath characteristics. In mammalian A fibers the velocity 

 in meters per second numerically equals the diameter in microns times a 

 constant (approximately 6 or 7^^^^' ^^^' ^^^ In giant fibers of the squid 

 velocity increases as the diameter raised to the 0.61 power.^''^'' Nerve fibers 

 are attenuated near their terminations, and here conduction is slow. 



Table 76 gives the velocities in moist air and, where possible, the total 

 fiber diameters in a variety of nerve fibers. The general relationship between 

 diameter and velocity holds. Velocities in situ are sHghtly greater. The 

 most striking instance of large fiber diameter and rapid conduction is in 

 the giant fibers of crustaceans, cephalopods, and annelids, which may have 

 diameters of tens of microns or even several hundred microns, and conduct 

 many times faster than the smaller fibers of these animals. Insects in gen- 

 eral have very small nerve fibers but conduction distances are short, hence 

 fast conduction is less important to the animal than a large number of fibers. 

 Mosquito larvae have fibers less than 1-2 fi in diameter, although cock- 

 roaches have some fibers as large as 10 ;u. in diameter.^^''^ 



Excitation times also vary inversely with fiber diameters, i.e., fast fibers 

 have short chronaxies although for a given fiber type chronaxies are rela- 

 tively constant over wide ranges in size.^^®- ^^"^ 



