1830 



HANDBOOK or l'HYSIOI.OGY 



NElkolMIYSIOIOClY III 



systems. Probably other organized enzyme sequences 

 will prove to have requirements as defined as those 

 for the well-investigated energy-yielding reactions, 

 but data are as yet incomplete 



Metabolic Response to Electrical Excitation 



Metabolic changes in response to applied electrical 

 pulses have been observed in vitro in the whole brain 

 from lower vertebrates and in cell-containing prepara- 

 tions from all parts of the mammalian central nervous 

 system which have been so examined (17, 19, 134, 

 136). There have been described suitable sources of 

 electrical pulses (10, 125), electrodes for their applica- 

 tion (10, 126), and precautions and control experi- 

 ments employed in interpreting results (125, 135, 

 [58). 



Techniques fall into two main categories. First are 

 those in which response is judged by measurement of 

 changes in the tissue environment, the consumption of 

 substrates or formation of products. The second 

 comprises those in which the tissue itself is analyzed. 

 In the first category, much use has been made of 

 orthodox manometric techniques for measuring oxy- 

 gen consumption (125, 136) or, in bicarbonate 

 salines, for measuring acid formation (137). With 

 cerebral cortex almost all the acid formed from tjlucose 

 is lactic acid (137, 143 1. These procedures have the 

 virtue of enabling, with a single sample of tissue, 

 measurement during successive periods in which the 

 1 issue is first unstimulated, then receives electrical 

 pulses and then is again unstimulated. Pulses may be 

 passed for some 2 hr. without adversely affecting the 

 tissue; in ordinary apparatus, 15 to 40 min. suffice 

 for accurate manometric measurements. More rapid 

 potentiometric methods for determing the oxygen 

 consumption of stimulated tissues have been devised 



and applied to ganglia (106). In the second Category, 

 the means often used for handling tissues for subse- 

 quent analysis are little different from those described 

 above I 1 jo); it isonl) necessary to note that for meas- 

 urement (ii labile substances, fixation must follow 

 prompt!) after cessation oi pulses For this purpose 



special apparatus has been devised and is described 



in the follow ing section. 



H\ whatever means response is measured, it is found 



that 1 dei 10 obtain maximal response from .1 tissue 



such .is the mammalian cerebral cortex, almost the 

 whole oi the tissue 5 pie must be within .m appreci- 

 able potential gradient Two main types of electrodes 

 have been employed to give the necessary gradient. 

 I :, the electrodes are at the cento and periphei \ 



of the annular space of a conical manometric vessel 

 (10, 126). They thus encompass almost the whole of a 

 shallow layer of fluid in which fragments of tissue of 

 0.1 to 10 nit; are floating freely. This is the arrange- 

 ment most appropriate when small and irregularly 

 shaped tissue fragments are to be examined. It is also 

 applicable to the cell-containing, chopped-tissue 

 suspensions described in a previous section, but is not 

 suitable if the tissue is to be collected rapidly for 

 analysis. A fixing agent may however be tipped from 

 a side arm during the passage of pulses. In the second 

 arrangement, the tissue as a sheet some 1x1.5 cm m 

 size lies between oriels of wires which alternately are 

 of opposite polarity when pulses are applied. This is 

 the arrangement most appropriate when the tissue is 

 to be analyzed at the end of an experiment, or when 

 the spread of excitation from the electrodes is under 

 investigation. 



Characteristics of electrical pulses required for 

 excitation are similar in the two experimental arrange- 

 ments when the potentials applied are expressed as 

 peak potential gradients (133, 134). Neither type of 

 electrode gives a uniform potential gradient; gradients 

 with the present types have been recorded (10, 77). 

 Maximal metabolic responses are typically obtained 

 when pulses of exponential time-voltage relationship 

 and peak potential gradient of 1 v. per mm with a 

 time-constant of 0.3 msec, are applied at 100 per sec. 

 by use of which response-voltage curves and other 

 relationships have been worked out (67, 133). Meta- 

 bolic response is given also 10 sine waves but requires 

 expenditure in the fluid of much more electrical 

 energy, and to bursts of pulses with relatively long 

 ( 1 sec. 1 inten als (114, 1 33, 227). 



Metaboli Studies i>* Brie) Experimental Periods 



The rapid action of the central nervous system /" 

 rim is paralleled by the rapidity oi change in its 

 metabolic activity. Stuck' of comparable phenomena 

 in vitro involves three operations: </> preparim: tissues 

 and re-establishing defined levels in the chemical or 

 metabolic characteristics under investigation which 

 max require, according; to the substances or processes 

 concerned, a few minutes, or an hour or more, />) 

 r.ipidU inducing a changed level of activity; and c) 

 rapid analysis 01 fixation. 



Of analytical methods, potentiometric means of 



tsuring respiration have already been noted. The 



methods of fixation applied to detecting change in 



peripheral nerve in periods of Some milliseconds (2 1 () 



do not set appear to have been applied to the central 



