i8->j 



lUMllmiiK (iF PHYSIOLOGY 



NEUROPHYSIOLOGY III 



inactivate the fatty acid oxidase system. The problem 

 of fattv acid metabolism in neural tissue is unquestion- 

 ably an important one and offers a challenge for 

 physiologists and biochemists alike. 



The problem with regard to phospholipid me- 

 tabolism is more promising but considerably more 

 I >lex. When brain homogenates are actively 

 incorporating; orthophosphate-P- into lipids, phos- 

 phatidylethanolamine, phosphatidylserine and sphin- 

 lyelin show only a slight incorporation of P 32 , 

 whereas diphosphoinositide has a specific activity 

 comparable to ATP and other acid-soluble phosphates 

 (23). Lecithin showed a low, yet significant turnover. 

 Peripheral nerve respiring in glucose will incorporate 

 orthophosphate-P' 2 into phospholipids, and at a 

 considerably greater rate during nerve degeneration 

 (81). The metabolically active lipids are presumably 

 in the Schwann cell, the myelin lipids being relatively 

 inert. Myelin formation and, therefore, a significant 

 fraction of phospholipid synthesis in the central 

 nervous system have been attributed to certain 

 neuroglia (73). In the peripheral nervous system the 

 Schwann cells, which are embryologically similar to 

 oligodendroglia, seem definitely to be involved in 

 myelin formation (47) 



Phospholipid synthesis involves a specific coenzyme, 

 cvtidine diphosphate, which in the presence of 

 phosphotransferases will react with choline or 

 ethanolamine t< form CDP-choline or CDP-ethanol- 

 ai 1 line (66, 67). The next enzymatic step involves the 

 conversion of the cytidyl compounds to lecithin and 

 phosphatidylethanolarnine. This enzymatic system is 

 widely distributed in nature and is present in cat 

 brain (Kennedy, E. P., personal communication). 



that the RXA and protein content showed the normal 

 increase only if the animal received normal light 

 stimulation (16). Under a variety of conditions 

 resulting in a loss of function, the RXA content of 

 neurons is rapidly depleted. [This topic has been 

 reviewed by Myden (62).] 



A marked depletion of the cytoplasmic RXA in 

 the supraoptic nuclei of the hypothalamus occurred 

 as a result of fasting for J4 hr., while the cytoplasmic 

 protein content appeared to increase (89). If rats are 

 subjected to a brief period of cold, the cytoplasmic 

 RXA of the supraoptic nuclei increases significantly; 

 but after exposure to cold for 1 hr., a precipitous 

 decrease in the RXA occurs. 



Since the role of RXA and DXA in the cell is 

 closely linked with protein synthesis, changes in their 

 cellular content would be expected to correlate with 

 the appearance of enzymes. In the embryonic stage 

 of the nervous system when the neuron begins to 

 mature and numerous respiratory enzymes appear, 

 the content of RXA and DXA is extremely high. As 

 the respiratory enzymes reach a nearly maximal rate, 

 just before birth, the RXA has decreased one third, 

 while the DXA has fallen to less than one half its 

 initial value (90). After birth the nucleic acid content 

 of the central nervous system remains fairly constant. 

 Both the activity of ribonuclease and desoxyribo- 

 nuclease, as well as the phosphate turnover of the 

 nucleic acids, are correlated with the rate of dis- 

 appearance of the nucleic acids (bo). There appears 

 to be a relationship between the density of RXA and 

 the degree of neuronal differentiation in the em- 

 bryonic nervous system (bi ). 



NUCLEI) VOIDS 



H\ means of (ley. mi biophysical and cytochemical 

 techniques, Hyd£n and collaborators have made 

 extensive studies on the nuclcoprotcin content and 

 distribution of various nerve cells under conditions of 



rest and acti\ iiv After intense muscular exercise both 

 the RXA and protein content of the anterior horn 

 cells decrease to less than one third the original 

 amount, while the ganglion cells associated with the 



1I1 nerve show .1 significant increase in RXA 

 during the 1-1 hr, oi electrical stimulation and a 

 considerable decrease with the onset of fatigue during 

 prolonged stimulation < (.9). I he necessity of nonn.il 

 stimulation for adequate development of retinal 



■lion cells is made apparenl from the observation 



RELATIONSHIP OF CHEMISTRY TO FUNCTION 



Even a brief survey of the historical development 

 of the chemistry of neural tissue reveals that the 

 problem has been consistently dealt with in terms of 

 its function; and although such attempts have been 

 largely inconclusive, owing to the difficulty of tech- 

 niques, important groundwork has been laid for 

 future developments. 



Ever since the hypothesis was proposed that muscle 



Contraction involved the splitting of ATP and w.is 



consequently an endergonic process, the tendency 

 to regard nerve conduction in the same way has been 

 prevalent, and almost categorical. The evidence that 

 respiratory metabolism increased during activity in 

 neural tissue is overwhelming (25, 29), and the 

 increased production of lie.it (luring nerve conduction 





