CHAPTER LXXVIII 



Chemical environment of the 

 central nervous system 



nnDrnT „ -rcr- ii i r> /- t Departments of Physiology and Anatomy, University of California 



K.UBEK1 D. ISIHIKUI , , ,. ,. . 



School of Medicine, Los Angeles, California 



CHAPTER CONTENTS 



The Microenvironment 



Relation of Cerebrospinal Fluid to the Microenvironment 



Functional Anatomy of Fluid Compartments 



Composition of Interstitial Fluid 

 The Blood-Brain Barrier 



Anatomy of the Blood-Brain Barrier 

 Dynamics of the Chemical Microenvironment 



Electrolyte Exchange- 

 Potassium to Calcium Ion Ratio and Brain Function 



Water Exchange 



Exchange of Metabolic Intermediates 



Glucose Exchange 



Glucose and Brain Function 



Oxygen Exchange and Brain Function 



Transbarrier Potential Difference and Hydrogen Ion I \ 

 change 



Transbarrier Metabolic Pump 



environment will introduce noise, in the communica- 

 tions sense, and it is not surprising, therefore, to 

 discover unique homeostatic mechanisms designed to 

 buffer the central nervous s> stem against such changes 

 and thereby achieve a maximum signal to noise 

 ratio. On the other hand, controlled alterations of the 

 central environment may also represent meaningful 

 signals as, for example, the response of the respirator) 

 center to changes in [H + ], the temperature regulating 

 tenuis to changes in local temperature, and the less 

 well-defined influence of hormonal and metabolite 

 concentrations on integrated central nervous activity. 

 For this reason, it is important to consider the internal 

 microenvironment oi the central nervous systems not 

 as a primordial sea ol elaborately guarded eonstanc\ 

 hut as a dynamic mechanism within the machinery 

 of nervous activity whose changing pattern of in- 

 homogeneities modifies and regulates behavior. 



the functional capacity of every neuron within the 

 nervous system is dependent upon the nature of the 

 milieu which invests it. Minute changes in chemical 

 or physical parameters of this microenvironment will 

 alter the threshold of excitability in adjacent cell 

 membranes and thereby influence functional activity. 

 Not only is the maintenance of proper metabolite 

 concentrations of critical importance but the "atmos- 

 phere' ol" organic and inorganic ions must be 

 regulated within very narrow limits, for the phe- 

 nomenon of excitability displays exquisite sensitivity 

 to changes in electrolyte milieu. Since the commodity 

 of the nervous system is information, meaningless 

 fluctuations in significant components of the neuronal 



THE MICROENVIRONMENT 



Relation of Cerebrospinal Fluid to the Microenvironment 



Until recently, the basic anatomy of the central 

 nervous system microenvironment did not seem to 

 propose any considerable difficulties. As with other 

 tissues, a continuous fluid phase, the interstitial 

 compartment, was believed to exist outside of all cell 

 membranes and to be separated from the plasma by 

 the endothelial wall of the capillaries. A shell of this 

 fluid enveloping a cellular element comprised the 

 microenvironment of that element, and all exchange 



1865 



