224 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



refractory period. The excitability during this period of time is lower 

 than normal; it is possible to set up an impulse, but the stimulus to 

 accomplish it must be more intense than that which was needed to excite 

 the normal pulse. 



Frequency of Impulses 



The relatively refractory period spaces the impulses so that the normal 

 impulses are a minimum distance apart, making overlapping or continu- 

 ous activity impossible. For instance, in the phrenic nerve of the cat, at 

 37° C under electric shock stimulation, it was found impossible to excite 

 more than 1000 spike potentials per second. 



The frequency of innervation of the motor units of muscle is ordinarily 

 only 5 to 50 per second, depending on the intensity of muscular contrac- 

 tion (Adrian and Bronk [1929]). Even in extreme conditions it does not 

 rise above 100 per second. In sensory fibers the frequencies of inner- 

 vation are of about the same order of magnitude. Thus, for example, 

 Adrian [1932] found that, when pressure was applied to the pad of a cat's 

 foot, the nerve impulses propagated in single fibers varied from 9 to 100 

 per second, according to the magnitude of the pressure applied. 

 * In general, it has been found that the rate of recovery of excitability in 

 the most rapidly conducting and most excitable mammalian fibers is 

 such that it would prevent conduction of as many as 1000 impulses per 

 second. 



Speed of Propagation 



It has been known since 1913, from the work of Lapique and Legendre, 

 that the speed of propagation of nerve impulses increases with increase in 

 fiber size. 



Direct measurements on nerve fibers, ranging in diameter from 1 to 

 16 microns, have shown the speed of propagation of the nerve impulses 

 to be proportional to the diameter of the fiber. Hursh [1939] found that 

 in the nerves of four-day-old kittens the maximal velocity of propagation 

 is 11 meters per second, as compared with 80 to 90 meters per second in 

 the saphenous nerve of full-grown cats. As the kitten grows to maturity, 

 the maximal velocity increases at a rate directly proportional to the 

 diameter of the largest fibers. Gasser and Grundfest [1939], using myeli- 

 nated fibers in the phrenic nerve of the cat, which vary in diameter from 

 1 to 14 microns, also found that the speed of propagation was propor- 

 tional to the diameters of these fibers. 



From the evidence on the relation between velocity of propagation 

 and amplitudes of the recorded axon potentials, they concluded that the 



