CHAPTER LXXIV 



Chemical architecture of the central nervous system 



DONALD B. TOWER National Institute of Neurological Diseases and Blindness, Bethesda, Maryland 



CHAPTER CONTENTS 



Composition of the Central Nervous System 

 Macrochemical Data 



Principal divisions and constituents 

 Fluid spaces and solutes 

 Solids 

 Microchemical Data 

 Amnion's horn 

 Other cortical areas 

 Neurons vs. glia 

 Organization of the Neuron 

 General Composition 

 Fractional Composition 



THE GENERAL CHEMICAL MAKE-UP of the Central 



nervous system has been known for well over a 

 century, but the details of its composition and par- 

 ticularly of its organization are still far from complete. 

 The difficulty has been in part due to the nature of 

 the components of neural tissues for which suitable 

 analytical approaches have only recently been 

 developed. There is a more fundamental difficulty 

 related to the nonhomogeneous nature of the nervous 

 system both in its organization and in its function. 



The division of the brain and spinal cord into 

 gray and white matter is convenient both structurally 

 and functionally, and it has been estimated that the 

 two portions are approximately equal in volume 

 (gray 45 per cent, white 55 per cent of the total) 

 (223). Yet myelinated fibers are prominent in certain 

 gray areas, notably in the cerebral cortex and 

 thalamus; and in certain species, like the whale, 

 neurons are present deep in white matter (225). 

 Even if this division is accepted as a general approxi- 

 mation, each portion is not homogeneous within 

 itself. While this is strictly true for most organs of the 



body, compared to the liver, for example, the brain 

 is much less so. This is partly due to the variety of 

 cell types, an example of which is summarized in 

 table 1 based on studies of the rat (158, 177); similar 

 data have been reported for other species (2, 88, 

 99, 191). Thus, in gray areas, neurons comprise only 

 about 20 per cent of the total cell population, the 

 remainder including four other cell types which 

 contribute to both structure and function. A similar 

 situation exists in white matter and it is noteworthy 

 that the total cell population here approaches that 

 for cerebral cortex. The cerebellar cortex on the 

 other hand has many more cells than either of the 

 foregoing. Looking at another aspect of this situation, 

 the cell bodies of neurons in cerebral cortex occupy 

 only about 5 per cent of the total volume (146, 207), 

 yet their dendrites and axons ramify throughout 

 many layers and areas, representing a vastly greater 

 cytoplasmic volume (some 25 per cent of the cortical 

 total) than that of the perikaryon. Pope (179) has 

 discussed some of the problems which these features 

 pose for neurochemical studies. 



Although the gross composition and, in some 

 instances, the finer composition of gray and white 

 areas, respectively, are similar, they are very different 

 from many physiological and biochemical stand- 

 points. It would seem that such differences should be 

 reflected in the structural chemical organization of 

 the individual areas. This may be so, but the eluci- 

 dation of such differences has not yet been achieved. 



The foregoing sets forth some of the basic problems 

 which must be borne in mind during consideration 

 of the material to follow. This chapter is not intended 

 to be a detailed review of the subject since these are 

 available elsewhere (153, 168, 194). Detailed con- 

 sideration of species differences cannot be presented 



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