850 CENTRAL NERVOUS SYSTEM 



of animals and its limited power of regeneration is correlated with the 

 fact that in prolonged starvation its metabolism is maintained at the 

 expense of other organs of the body with the result that, excepting the 

 heart, it is the organ which least and last undergoes a diminution in 

 weight. 



The Metabolism of the Nerve Fiber 



The all-or-none nature of the nerve impulse has given reason to be- 

 lieve that the energy of the nerve impulse is derived from processes 

 going on in each part of the fiber which it traverses. The refractory 

 period in conduction is occupied with processes which restore the nerve 

 fiber to its original state of conductivity. These processes constitute 

 the metabolism of activity in the nerve fiber ; in addition to them we may 

 expect a certain amount of metabolism concerned with the maintenance 

 of the normal condition of the tissue. 



With regard to the chemical exchange in nervous tissue, practically 

 nothing is known except regarding the consumption of oxygen, the 

 output of carbon dioxide, and the effects of disturbances in these proc- 

 esses produced by abnormal conditions of circulation and respiration. 



Some uncertainty exists regarding the production of carbon dioxide 

 during the activity of the nerve fiber. It is maintained by Tashiro, 5 

 who has used a very sensitive method for its detection, that the carbon 

 dioxide production of nerve is distinctly increased while it is being 

 stimulated. On the other hand A. V. Hill 6 has failed to detect any heat 

 production occasioned by the passage of nerve impulses 'through a nerve, 

 although he used a method capable of detecting a change of one hundred- 

 millionth of a degree in the temperature of the tissue. If any oxidative 

 process had occurred in the nerve, such as might have resulted in the 

 liberation of carbon dioxide, it should have been accompanied by a 

 heat production readily detected by this method. The presence of dis- 

 agreement on this point at least emphasizes one point, that the carbon 

 dioxide production in nerve fibers, both at rest and in activity, is small and 

 must contribute a minor share to the metabolic requirements of the 

 body as a whole. 



The nerve fiber is dependent on a supply of oxygen to only a limited 

 degree, its requirements being apparently small. A nerve trunk of 

 the frog may retain its ability to conduct for three to five hours in an 

 atmosphere of pure nitrogen, but at the end of that time its ability 

 to transmit an impulse will come to an end. On supplying it with oxy- 

 gen its conductivity is restored again in a few minutes. Oxygen is 

 consequently necessary for the continued function of the nerve fiber, 

 which can, however, be deprived of its oxygen supply for long periods 

 without losing its ability to recover. 



