572 SIGNIFICANCE OF THE NERVOUS SYSTEM 



preceding period of activity. This interval during which the irrita- 

 bility is at low ebb, constitutes the refractory period. 



In the case of nerve fibers, the period of refraction is extremely 

 brief, in spite of the fact that they undergo metabolic changes. It 

 amounts to only 0.002-0.006 sec. Their extremely rapid power of 

 recuperation is dependent upon their great affinity for oxygen. It is 

 possible, however, to render this period more evident by lessening the 

 amount of the available oxygen which can be done most easily by sur- 

 rounding the *nerve fiber with some inert gas or narcotizing agent. 1 

 Cell-bodies behave very similarly, but as their metabolic requirements 

 are much greater than those of the nerve fibers, their refractory period 

 is also more clearly marked. Thus, it has been found that a refrac- 

 tory period of 0.006 sec. for the nerve fibers of the frog corresponds to 

 a refractory period of 0.1 sec. for the ganglion cells of the same animal. 

 This time may be varied by altering the irritability of the cell, either by 

 lessening the amount of the available oxygen or by narcosis. 



As the cell-bodies of different groups of neurons are destined to 

 perform different functions, it may be conjectured that their anabolic 

 requirements are subject to considerable variations. Hence, although 

 their refractory period is much longer than that of the nerve fiber, the 

 value of 0.1 sec. must vary somewhat in accordance with the type 

 of cell under consideration. It has also been suggested that the 

 refractory period acts as a check upon those impulses which ganglion 

 cells discharge automatically. It is a well-known fact that the dif- 

 ferent motor organs, such as muscle tissue and glandular tissue, are 

 constantly kept in a condition of tonus in consequence of an outpouring 

 of subminimal impulses by their respective centers. These impulses 

 are said to be generated at the rate of about 10 in a second. Obvi- 

 ously, as the refractory period amounts to 0.1 sec., they could not 

 be repeated at shorter intervals. Nor could they recur at longer 

 intervals, because the excessive rise in irritability would eventually 

 cause them to be discharged irrespective of any stimulation. It is 

 believed that some ganglion cells discharge their impulses even more 

 rapidly than 10 in a second, namely, 40-100 in a second, but a mere 

 difference in rate does not destroy the principle involved in this 

 process of self-regulation, because the refractory period must neces- 

 sarily become the shorter, the greater the rate of discharge. At no 

 tune, however, could it equal the refractory period of nerve fibers. 



Summation of Stimuli in Nerve Cells. The phenomenon of sum- 

 mation is well illustrated by the summation of the contractions of 

 skeletal muscle. If a number of stimuli of the same intensity are 

 passed into muscle tissue at brief intervals, the resulting contractions 

 are added to one another until the total reaction displays a very much 

 greater amplitude than that of the single contractions. A strength of 

 stimulus may also be employed which does not give rise to a reaction, 

 while two or three stimuli of this intensity applied in rapid succession, 



1 Frohlich, Zeitschr. fur allg. Physiol., iii, 1904, 148. 



