58 METABOLISM IN CHANGED CEREBRAL ACTIVITY 



hypoxic levels. Nevertheless, phosphocreatine still decreased and 

 inorganic phosphate increased. The change in oxygen tension v^^as 

 not detectable for some 20-30 sec after giving metrazole (Gurdjian 

 et al, 1947). 



It seems certain that the low^ered tension is due to an increased 

 oxygen consumption by the tissue. Thus, Davies and Remond 

 (1947) found that the oxygen tension of the blood in the smallest 

 pial arterioles remained constant throughout seizure activity 

 induced by metrazole, but that the tension was lowered in venous 

 blood. Measurements of changes in regions removed from visible 

 blood vessels revealed that the tension fell to about 30-35% of the 

 initial value during the convulsive discharge. The drop began after 

 the initial spike discharge had ended and took about 5 sec for 

 completion. Oxygen consumption was found not to increase until 

 the electrical seizure activity was well developed and did not reach 

 its maximum until the end of the convulsive discharge. The im- 

 portance of a definite minimum oxygen tension for the maintenance 

 of convulsive activity was shown by Gurdjian et al. (1947). 

 Seizure activity, induced during hypoxia with 4-1% oxygen 

 rapidly ceased though the quantities of adenosine triphosphate and 

 phosphocreatine were not altered further. Breathing air restored 

 the electrical activity and restarted the seizures which subse- 

 quently disappeared and reappeared as hypoxia was alternately 

 induced and released. 



The above experiments were of critical importance in establish- 

 ing that the changes in constituents, such as phosphates, during 

 seizure activity were not due to a local hypoxia and that they 

 preceded measurable changes in oxygen tension. Increased 

 oxygen consumption is thus an after effect of increased nerve cell 

 activity. It also seems clear that the actual quantities of energy- 

 rich phosphates in the brain vary little when compared with the 

 factors of energy supply and expenditure. In convulsive activity 

 a new steady state is established in which the levels of phospho- 

 creatine, though lower than in the normal brain, are maintained by 

 the oxidative metabolism of carbohydrate. 



The balances which exist between the oxidative synthesis of 

 phosphates and their metabolism during convulsions is well 

 illustrated by the effects of including carbon dioxide in the inspired 

 gas mixtures. F'ollowing the observation (Gibbs et al., 1938) that 

 petit mal seizures could be interrupted by increasing the carbon 



