CHEMICAL ARCHITECTURE OF THE CENTRAL NERVOUS SYSTEM 



l80 5 



for components of the acetylcholine system (36, 59, 

 148, 225), for other neurohumors and related com- 

 pounds (10, 167, 187, 240), and to a lesser extent for 

 oxygen uptake and glycolysis (54). The meaning of 

 these differences from a structural standpoint and a 

 more complete definition of differences among 

 various gray matter areas remain to be elucidated. 

 The application of the microchemical techniques 

 discussed above to such areas as the thalamus, 

 caudate nucleus and hypothalamus, for example, 

 would be most interesting. 



neurons vs. glia. The evidence reviewed so far has 

 not indicated what proportions of components and 

 enzyme activities can be ascribed to neurons or to 

 glia, or what differences may occur between them. 

 The fact that enzyme activities drop rather abruptly 

 in white matter beneath the cortex while total cell 

 density does not decrease in parallel (fig. 8, 9), 

 together with the fact that the glial populations in 

 cortical gray and subcortical white are roughly 

 comparable (table 1), would suggest that the 

 metabolic activity of glial cells is substantially less 

 than that of the neurons, particularly since the 

 neurons make up only about 20 per cent of the cortical 

 population. Whether glial and neuronal compositions 

 are different cannot be deduced from such data, and 

 there have been no studies on glia comparable to 

 those for neurons to be discussed in the next section. 

 By electron microscopy, glia and neurons appear to 

 contain the same cellular elements and differ only in 

 relative amounts visible (172). 



Substantiation of lesser metabolic activity for glia 

 has been provided by several studies (3, 100, 135). 

 Results of the study by Heller & Elliott (100) have 

 been summarized in table 5. Per unit weight, cortical 

 tissue respires and glycolyzes more rapidly than white 

 matter (from the corpus callosum), but when the 

 data are calculated in terms of cell numbers, the 

 cerebral cortex exhibits the highest activity, cerebellar 

 cortex the lowest, and corpus callosum intermediate 

 between the two. These results suggest that glial 

 cells may respire more rapidly than some neurons, 

 but in cerebral cortex neurons would appear to 

 respire more rapidly than either cerebellar neurons 

 or glia, possibly due to their larger size and more 

 extensive processes. Korey & Orchen (135) have 

 recently confirmed some of these findings by the use 

 of a somewhat more detailed method of tissue fraction- 

 ation. On the basis of these data, it is estimated that 

 for both cat and man about 75 per cent of the respi- 

 ration (oxygen consumption) of the cerebral cortex is 



table 5. Respiration and Gylcolysis of 

 Various Brain Areas* 



* From data of Heller & Elliott (100). 



i Assuming nuclear counts made on other areas apply. 



| Oj uptake in bicarbonate-buffered medium. All others 

 in phosphate-buffered medium. With normal tissues, values 

 in bicarbonate about 711' , of those in phosphate. 



attributable to neurons, although they comprise only 

 about 20 per cent of the total cortical cell population. 

 Korey & Orchen (135) have calculated that gray 

 matter accounts for 73 per cent of total brain respi- 

 ration and that neurons (including axoplasm) utilize 

 some 65 per cent of the total oxygen consumed by 

 the brain. These investigators also point out that if 

 the respiratory rate of neurons is compared with that 

 of hepatic parenchymal cells under the same experi- 

 mental conditions, the neuronal rate is four times 

 higher on a per cell basis. It is probable that it would 

 be more correct from a functional standpoint to 

 refer oxygen consumption to some parameter of cell 

 size (volume, surface area) rather than to cell number 

 or tissue weight (135). A preliminary estimate for cat 

 cerebral cortex, in terms of unit volumes of neurons 

 and of glia, suggests that the respiratory rate of 

 cortical glia in such terms is only one half that of the 

 neurons and that the "extra' neuronal oxygen con- 

 sumption may in part be attributable to special 

 metabolic systems unique to neurons (232). 



If the data on tumors can be applied to normal 

 tissues it would appear that the oligodendroglia 



