1804 



HANDBOOK OP PHYSIOLOGY 



NEUROPHYSIOLOGY III 



80 



60 



MOTOR CORTEX 



FUMARASE 

 (111 



fig. 10. Layer by layer plot of various constituents of motor 

 (agranular) cortex from the monkey. Cholesterol is expressed 

 in mmole per kg dry weight, aldolase and fumarase are ex- 

 pressed in mole per hr. per kg dry weight — all by the factors 

 indicated. Cell density is expressed as io 1 cells per cu mm. 

 [Adapted from Robins el al. (191-193).] 



80 



VISUAL CORTE X 



40 



t 



in;. 1 I. Plot, identical to that of Bg, 10, for visual (striate) 

 1 oi ten from the monkey. The arrows at layers fo and 5 indicate 

 the position of the outer and inner lines of Uaillargii . respec- 

 tively. Note the lack of significant change in cholesterol content 

 ni these two layers, [Adapted from Robins et id. (191-193).] 



■ uli.irljacciii white matter, as an indication of in- 

 . n asing tnyelination. These changes are mirrored by 

 I' 1 reases in protein and enz) me acth ities in the same 

 regions, Actually, until the deepest layers are readied, 

 mosl components are surprisingly constant from 

 layer to layer, even on subdivision of each layer 

 (192), although Pope (177) found a somewhat more 

 irregular distribution for eertain enzymes. It is also 



of interest that the levels of components and enzyme 

 activities are of the same order in various cortical 

 areas studied, although they differ not only in cellular 

 density but in histological and functional charac- 

 teristics. This situation is also apparent for other 

 components such as the acetylcholine system (225, 

 233), glutamic acid and glutamine (230, 231), and 

 sodium and potassium (unpublished observations). 



In such studies, estimation of enzyme activities 

 are done with a view to ascertaining distribution in 

 relation to histological organization. While such 

 correlations may in general be correct, most tissues 

 provide an excess of enzymes, so that under normal 

 conditions their concentration is rarely, if ever, the 

 limiting factor in rates of reactions. This fact is 

 illustrated nicely by several examples derived from 

 deficiency and toxicity studies (44, 161, 162, 228). 

 When acetylcholinesterase is irreversibly inactivated 

 by alkylfluorophosphates, clinical signs of toxicity 

 do not appear until enzyme inactivation has reached 

 90 per cent or more (161). A generous supply of 

 enzymes is provided in part at least because renewal 

 of depicted enzyme requires time. In the example 

 cited, the rate of regeneration was 4 per cent per day 

 during the initial two weeks and 0.4 per cent per day 

 thereafter, so that some 140 clays were needed to 

 achieve control levels of activity (161 ). 



While most of the constituents of the central 

 nervous system are fairly consistent in amounl from 

 species to species, there are certain cortical constit- 

 uents and activities which depart from this rule. 

 These are represented by activity of the acetylcholine 

 system (233) and oxygen uptake or respiration and 

 possibly anaerobic glycolysis (34, 56). The first two 

 decrease in going from small to large species ol 

 animals, acetylcholine activity correlating with 

 increasing brain weight (233) and oxygen uptake 

 correlating with increasing bod\ weight (56) of the 

 respective species. Anaerobic glycolysis shows an 

 increase with increasing size of species, but its 

 correlation with body or brain weight has not been 

 assessed. The significance of these latter two obser- 

 vations in terms of functional organization is not .it 

 present apparent. The decrease 111 acetylcholine 

 aelivilv appears to parallel the decrease in cortical 

 neuron density which is also correlated with increasing 

 species brain weight ( -'07, 225, 233). Some nt the 



possible implications "I this parallelism are discussed 

 elsewhere ( 22 ", I . 



In addition to the foregoing, there are differences 

 of distribution for certain constituents among various 



areas of gray in, liter in a given species. This is true 



