i88 4 



II VNIH'.i ii IK i H elfish il i >c\ 



NEURi H'H\Miil <><;Y hi 



ihc rate of perfusion is increased to three times normal. 

 Under these conditions, no metabolic substrate what- 

 soever is available from the vascular compartment, 

 ami the intrinsic glucose of the brain is exhausted 

 within 15 inin. Since the amount of endogenous glyco- 

 gen does not diminish during this period, and since 

 the respirators quotient decreases from its normal 

 value of approximately 1.00 to between 0.84 and 

 0.56, it is apparent that noncarbohydrate 'structural' 

 components of the central nervous system are being 

 catabolized. Studies with C u labeled glucose (49) 

 have shown that only 30 to 35 per cent of the glucose 

 taken up b\ the brain is directly oxidized to carbon 

 dioxide and water under "resting' conditions, and an 

 even smaller percentage during activity. Another 20 

 to 50 per cent of the glucose taken up is rapidly trans- 

 formed into acid-soluble components (largely amino 

 acids), and substantial amounts are built into lipids, 

 proteins and other acid insoluble components. 

 Further studies confirmed the fact that proteins, 

 lipids and other nonsoluble substances are being 

 constantly metabolized by the brain. Therefore, it is 

 proposed by Geiger that the ability to maintain a 

 functional central nervous system in the absence of 

 blood sugar is dependent upon the maintenance of a 

 blood flow rate rapid enough to eliminate waste 

 products originating from the noncarbohydrate 

 metabolism. (Further comments on Geiger's studies 

 will be found in the preceding chapter in this volume. I 

 A similar interpretation was proposed b\ Gellhorn & 

 Ressler ( 32 ) lo explain their observation that under 

 certain conditions in rats it is possible lor the electrical 

 activity of the cortex to be normal with the blood 

 sugar at coma level. They Inst removed the adrenal 

 medulla bilaterally in adult male rats. One or more 

 weeks later when coma had been produced by insulin 

 injection, the brain was subjected to an electric shock. 

 This was followed immediately by disappearance of 

 the com. 1, bv normal behavior and bv return of the 

 1.1.(1 to the original pattern although the blood sugar 

 was still at coma level. These effects were explained 

 as resulting from the increased blood supply to the 

 brain through excitation ol the sympathetic nervous 

 system. I hese observations suggest the possibility that 

 the blood-brain barrier may be as importantly a 

 process ol eliminating substances from the neuronal 

 milieu as in regulating solute entry from the plasma. 



inge and Brain Fun* tion 



rhere is no evidence to indicate thai a hairier to 



oxygen exchange exists between the blood and the 



central nervous system exlrav aseular fluids, and 

 neural function rapidly deteriorates following oxygen 

 deprivation. In general, the symptomatic progression 

 due to hypoglycemia 1 see above 1 is duplicated in acute 

 anoxia, except that in the latter case the events are 

 compressed into a few minutes instead of hours. 

 Sugar & Gerard (145) produced sudden and com- 

 plete cerebral anemia in cats by temporary occlusion 

 of all vascular channels to the brain, and observed 

 that the electrical activity of the cortex and caudate 

 nucleus disappeared earlier than that of the thalamus 

 and medulla. Provided that the anemia was not 

 maintained to the point of irreversible damage, re- 

 covery occurred in the reverse order. The survival 

 times of the electrical activitv in various cerebral 

 structures, following acute anemia, as found bv 

 Sugar & Gerard (143) were as follows: 



Cerebral cortex 14 15 sec. 



Caudate nucleus 2 5 _2 7 sec - 



Ventrolateral thalamic 28-33 sec. 



nucleus 



Medullary reticular 30-40 sec. 



formation 



The ability of the central nervous svstem to con- 

 tinue to function in the absence of oxygen decreases 

 with age in many species and irrespective of the man- 

 ner of producing anoxia (76, chap. 7). Newborn rats 

 are able to survive at 320 mm Hg atmospheric pres- 

 sure lour limes as long as adults. The tolerance de- 

 clines rapidly after birth until the third week when 

 the mature level is first attained. There is An associated 

 shift from anaerobic towards areobic supply of cere- 

 bral energy, so that the difference between the central 

 nervous svstem calorie requirement and the anaerobic 

 supply becomes greater in the adult. 



Transbarriei Potential Differenci 

 and Hydrogen l<>n Exchange 



The role of electric charge in the homeostasis ill 

 the neuronal milieu has received considerable atten- 

 tion since the early observation that acidic dvrs 

 behaved differently from basic dves in staining the 



brain /// vivo I he significance of solute charge was 

 intensively explored and discussed bv I'riedinann (44) 

 using aniline dves, toxins, viruses and drugs, and 

 Becker S: Aird (in emphasized the possibility that a 

 charged membrane might be involved in explaining 



the effect of acidic dissociation constant on the per- 

 meation of certain sulfonamides into the brain 1 j6 I. 

 Ischugi & Taylor (150) described an electrical 



