i8o6 



II WIUn h JK ( '1 I'll 1 ! si. i -i 



m ik' H'Iiysioi.ogy in 



respire much more rapidly than astrocytes (or other 

 nonneuronal elements). Other studies lend support 

 to this suggestion (3, 237). Since oligodcndroglia are 

 relatively more numerous in white matter (table 1 |, 

 they may account for the bulk of the nonneuronal 

 element respiratory activity. There are other clear 

 differentiations between cortical neurons and glia. 

 Using differential histochemical techniques, Koelle 

 (133) has found that acetylcholinesterase is restricted 

 largely to neuronal elements, whereas the serum 

 type ('non-specific' or 'pseudo 1 cholinesterase) resides 

 in the glial cells and blood vessels. Studies of the 

 distribution of enzymes responsible for the synthesis 

 and for the further metabolism of 7-aminobutyric 

 acid demonstrate that they are restricted to gray 

 matter areas of the central nervous system (7, 142, 

 198). The distribution of 7-aminobutyric acid content 

 in various parts of the brain conforms to that for the 

 enzymes and indicates that the 7-aminobutyric acid 

 system is probably exclusively a neuronal system (230, 

 231). further comparative studies along these lines 

 should be most informative. 



ORGANIZATION OF Mil. NEURON 



General Composition 



The developments of cytochcmical and cytological 

 methods, using ultraviolet and x-ray microspectro- 

 graphic analysis, microdissection with microchemical 

 analyses, cell fractionation by high-speed centri- 

 fugation, and electron microscopy, have made it 

 possible to determine something of the composition, 

 structure and organization of the neuron.- I he 

 principal advantages and disadvantages of these 

 methods in their application to neural tissues have 

 been discussed and summarized by Robins & Smith 

 (190, pp. 315-321). D.ii.i from a number of sources 

 have provided .1 fairly good idea of the general 



'Staining or 'slide' histochemistry lias been largely omitted 

 from tins consideration because >>t the lack of quantitation 

 inherent in current methods, ol difficulties in localization due 

 i" diffusion and nonspecific adsorption ol artefacts introduced 



In eml - dding and fixal in sues and ol the lai k ol methods 



foi -i number hi Important constituents. I Ins is nol to imply 

 thai the other methods have no analogous ilis.nh. mi, mis nor 



to detract 1 the many valuable studies employing these 



techniques which have provided information on thi pn enci 

 and distribution ol enzymes and othei constituents, such .is 

 the cholinesterases (81, 133, 224) and neurosecretory activity 

 ni hypothalamic neurons 200 Foi furthei discussion and 

 no ill. papi 1 bj Robins 8 Smith 190 should be 1 on 

 suited. 



composition of certain representative neurons (30-32, 



5'. 5-> 57> 5 8 > 8 7. "3. '43. '57. >59>- A summary 

 for the anterior horn cell of spinal cord derived from 

 these sources is shown on the left in figure 12. 



With the exception of the nucleolus, all areas of 

 the neuron consist primarily of water, and there is a 

 fairly regular, progressive increase in its percentage 

 from nucleolus to axoplasm. Similar findings for 

 nuclear and cytoplasmic areas of spinal ganglion 

 cells, supraoptic nucleus neurons and Purkinje cells 

 have been reported (31, 32, 159); but neurons from 

 Deiter's nucleus apparently contain a greater pro- 

 portion of solids (31, 32). The general picture, shown 

 in figure 12, appears to apply to all species studied 

 (rabbit, cat and rat) and does not differ significantly 

 from that for liver cells (159). Most of the nuclear 

 and perikaryal solids are protein in nature, but each 

 area contains about 30 per cent lipids. The nucleolus 

 does not seem to contain any lipid and the axoplasm 

 contains a relatively small amount (percentage not 

 known). The composition of the myelin sheath of the 

 axon is essentially similar to that inferred from 

 studies of Amnion's horn (fig. 8), except for a some- 

 what greater water content. By comparing the data 

 in figure 12 with those given in figures 1, 8, 10 and 

 11, it is apparent that the general constitution of 

 cortical gray matter is very similar to that of the 

 neuron. It is probable that the glial cells would, on 

 this basis, have a similar composition, although that 

 remains to lie determined. 



The nucleolus stands out as a very different 

 structure. Its hisih percentage "I solids confers upon 

 it a much higher density than any other cellular 

 element. Under the effects of gravity, nucleoli of 

 cells will fall through the nuclear fluid and, under 

 centrifugal force, nucleoli have been observed to 

 pass out through both nuclear and cell membranes 

 (2381. Nucleolar solids appear to be almost entirely 

 protein with .1 sin. ill percentage of associated ribo- 

 nucleic acids. Vincenl (238) has concluded that the 

 nucleoli of cells must be solid or nearly solid bodies 

 containing proteins in a state of considerable de- 

 hxdi.iiinn Ihden Ml ;' has pointed out that the 

 large nucleolus of neurons, together with the large 

 amounts of perikaryal ribonucleoprotein (Nissl 

 bodies), are features characteristic either of growing 

 cells or those in which intense production of protein 

 occurs. He has suggested, on the basis of ultraviolet 

 microspectrographic studies, that dming axonal 

 regeneration (alter experimental section 1, there is 

 nucleolar production ol ribonucleic a< ids and protein 

 which migrate to the exterior of the nuclear membrane 





