112 FACTORS AFFECTING METABOLISM in Vttro 



saline, promoted a marked increase in both the rates of oxygen 

 uptake and in the production of lactic acid. These changes were 

 rapid, occurring within 30 sec of the application of impulses. This 

 period represents the bottom limits of measurement with the 

 apparatus used and it is likely that the changes in oxygen uptake 

 are even more prompt. Since the use of this technique has pro- 

 vided the majority of evidence relating an electrical event in 

 cerebral tissue in vitro to a metabolic event it is appropriate to 

 describe certain major features. A detailed account of the many 

 physical factors involved in the response of cerebral tissue to 

 electrical stimulus has been given by Mcllwain (1956). 



Tissues consist of thin slices of cerebral cortex or other parts of 

 the brain and are either held in the wire grids which act as the 

 electrodes, or float, as chopped fragments, between concentric ring 

 electrodes. Both types of electrodes are constructed to be used 

 with conventional Warburg manometers slightly modified by 

 fitting points of entry for the electrode contacts. The electrodes 

 used consist of pure silver wire, platinum wire electroplated with 

 gold, or molybdenum wire (Mcllwain, 1951 ; Ayres and Mcllwain, 

 1953). With such electrodes no difficulties are encountered due to 

 artifacts such as electrolysis of the salines. The majority of experi- 

 ments have been conducted with grid electrodes of pure silver 

 wire, both in the manometers and in an apparatus permitting the 

 removal and transfer of tissues from an incubation medium to a 

 fixing agent such as trichloracetic acid within 0-2 sec, with pulses 

 applied throughout this period or interrupted if necessary before 

 fixing (Heald and Mcllwain, 1956). Response to electrical stimulus 

 is confined to intact nervous tissue or to tissue with a well-developed 

 nervous supply, for kidney, liver and testis show no response 

 (Kratzing, 1951). It also is essential that the cellular structure 

 remains intact for the respiration of disintegrated preparations 

 prepared in isotonic sucrose is not affected by impulses of varying 

 types which produce a response with intact slices when incubated 

 in the same apparatus (Narayanaswami and Mcllwain, 1954; 

 Mcllwain, 1951) though alternative claims, discussed below, 

 have been made. However, it should be noted that disintegrated 

 systems are usually fortified to exhibit maximal respiratory 

 capacity and under such conditions additional response to electrical 

 stimulus is not likely to be detected. 



Application of electrical stimuli to cerebral slices in vitro 



