THE NUTRITION OF NERVOUS TISSUE 851 



Metabolism of the Central Nervous System 



The synapses are regions in which conductivity is modified by a va- 

 riety of conditions. It is to be expected consequently that nutritional 

 defects will influence the ease with whi<?h impulses may pass through 

 these regions. The cell body is the nutritive center for the neuron as a 

 whole. In the reflex arc, of which the synapses and nerve cell bodies 

 form a part it is logical to expect that deficiencies in nutrition would 

 show themselves more promptly than in the isolated nerve trunk. This 

 is indeed the case. While a nerve trunk will retain its conductivity 

 in an atmosphere of nitrogen for several hours, the reflexes of the frog, 

 deprived of oxygen supply, disappear in thirty minute. In the warm- 

 blooded mammal all reflexes disappear within a few minutes after fail- 

 ure of the blood supply, and in man unconsciousness may be the im- 

 mediate result of disturbance in the cranial circulation or of asphyxia. 



We have already seen how dependent the activity of bulbar centers, 

 which control the heart beat, the blood pressure, and the movements of 

 respiration, are on certain conditions, such as the hydrogen-ion concen- 

 tration of the blood, which must be profoundly altered by disturbances 

 in the circulation of the brain. In the same way the excitability of 

 the spinal centers concerned with reflex conduction is modified by asphyx- 

 ial conditions. Spasmodic contractions of skeletal muscles, or convul- 

 sions, commonly precede death from asphyxia. These occur in animals 

 in which the brain has been removed and are consequently due to the 

 activity of the spinal cord. In spinal animals in which reflexes may be 

 elicited with difficulty a mild degree of asphyxia frequently causes 

 a reflex to appear, which could not be produced previously. Deeper as- 

 phyxia will cause certain reflexes, i. e., the scratch reflex, to take place 

 spontaneously. When death is threatened, general convulsions of the 

 skeletal muscles of the trunk and limbs are invariably set up. These 

 effects show that asphyxia may increase the excitability of the spinal 

 centers of skeletal muscle and may result in the spontaneous contraction 

 of the muscles which they supply. The heightened excitability is of short 

 duration, however, if the asphyxia is complete, and passes into a 

 condition of depression, followed by the complete failure of the re- 

 flexes, which may be restored, however, if the respiration of air or oxygen 

 is established in time (Mathison 7 ). 



The responses of the spinal centers for movement of skeletal muscle 

 do not differ fundamentally from those of the medulla, which are con- 

 cerned with the regulation of the circulation and respiration. The dif- 

 ference in their behavior consists in the fact that the bulbar centers 

 react to smaller changes in the condition of circulation. Thus the vaso- 

 motor center reacts to thirty seconds of oxygen lack or to breathing 5 per 

 cent of C0 2 whereas the spinal centers require two minutes of oxygen 



