88 MARTIN G. LARRABEE AND PAUL HOROWICZ 



tion which also pertains to experiments on brain in situ, where the spontaneous 

 activity of the neurons presumably provides a background of activity metab- 

 olism. Therefore depression of oxygen consumption by anesthetics is perhaps a 

 secondary effect of depression of the spontaneous activity. A more definite 

 conclusion might be reached if enough data were available to permit applica- 

 tion of a criterion employed by Lewis and Mcllwain (1954) in interpreting 

 effects of other agents on electrically stimulated cerebral cortex. According to 

 this criterion a blocking agent which acts non-metabolically would be expected 

 to alter the various aspects of metabolism in the same way as withdrawal of 

 excitation. A metabolic poison, on the other hand, should cause a disproportion- 

 ately large change in that portion of the metabolism most closely related to its 

 site of action. Unfortunately definitive experiments along these lines have not 

 yet been performed on ganglia. 



Selective Action of Anesthetics in Relation to Metabolic Hypotheses 



An example of selective block of synaptic transmission through an excised 

 rat ganglion is shown in Figure 2. The middle and lower records on the left 

 show how pentobarbital first reduced the height of the postganglionic response 

 (middle record) and then at high concentrations abolished it (lower record), 

 with little or no change in impulse conduction along the preganglionic nerve. 

 In other words, this agent selectively blocked synaptic transmission. On the 



JilUUL 



Fig. 2. Left: Effects of pentobarbital on transmission through a superior cervical 

 ganglion excised from a rat. Right: Effects of urethane. In each record eight supra- 

 maximal stimuli were applied in succession to the preganglionic nerve at a frequency 

 of 30 per second. Each stimulus evoked two upward deflections. Recording conditions 

 were such that in the control (upper) records the first (smaller) deflection of each pair 

 signalled the conduction of impulses towards the ganglion along the preganglionic 

 nerve and the second (larger) deflection signalled the discharge of impulses away from 

 the ganglion over the postganglionic nerve. Middle and lower records show effects of 

 0.3 and 1.0 mM pentobarbital (on left) and 56 and 112 mM urethane (on right). 

 (Modified from Larrabee and Posternak, 1952). 



