THE SENSE OF SMELL 



541 



FIG. 6. Intrinsic waves' recorded in the olfactory bulb in light thiopental anesthesia. A: No ol- 

 factory stimulus. Frequency, 100 /sec. B: Abolition of intrinsic rhythm by weak olfactory stimulation 

 with amyl acetate. C: In another preparation, strong stimulation with amyl acetate abolishes the 

 intrinsic rhythm with substitution of a slower induced rhythm. [From .\drian (5).] 



SPONT.ANEnus WAVES. Thcsc are usually smaller and 

 less regular than waves evoked by olfactory stimula- 

 tion and are associated with persistent activity in the 

 cells of the bulb. They were described in the isolated 

 olfactory bulb of the frog (47). Their frequencies are 

 as high as 70 to 100 per sec (fig. 6). They are sup- 

 pressed in the mammal by deep anesthesia and they 

 accompany a persistent irregular discharge of axon 

 spikes in the deeper layers. In medium anesthesia 

 this discharge may be so large as to conceal any 

 change induced by a weak olfactory stimulus. This 

 continuous activity persists after complete destruction 

 of the olfactory epithelium and after isolation of the 

 bulb from the forebrain but ceases after interference 

 with the blood supply of the bulb. 



Adrian (5) regards this activity as largely spon- 

 taneous or intrinsic, expressing the continuous break- 

 down and repair of cells not stabilized by deep anes- 

 thesia. Although mitral cells certainly take part in 

 this activity, there is some reason to suppose that it 

 may originate in cells with short axons (granule 

 cells) arranged in layers deep to the mitral cells. This 

 is suggested by the fact that the intrinsic waves can 

 exhibit considerably higher frequencies than the in- 

 duced waves. If the induced waves indicate the 

 maximum frequency of discharge of the direct olfac- 

 tory pathwa\', a higher rhythm probably has a dif- 

 ferent origin. 



In the phase of recovery from deep anesthesia, the 

 bulb is quiescent unless stimulated, but a breath of 

 odorous air will produce a few waves accompanied 

 bv .scattered discharges in the mitral cells. \Vith 



lightening anesthesia such a stimulus may start a 

 longer train of waves of gradually decreasing fre- 

 quency. Ultimately a stage is reached at which the 

 bulb reacts with a train of waves which may con- 

 tinue indefinitely. This phenomenon has been named 

 by Adrian the ' awakening reaction' of the bulb, and 

 it is suggested that in medium or light anesthesia the 

 granule cells have become capable of maintaining 

 themselves in continued activity and that their ac- 

 tivity leads to a continued discharge in the mitral 

 cell axons. 



WAVES AND UNIT ACTIVITY ACCOMPANYING OLF.^CTORY 



sTiMUL.-iiTiON. In such animals as the cat and rabbit, 

 strong olfactory stimuli elicit large sinusoidal oscil- 

 lations in the bulb, usually at a fixed frequency and 

 occurring only with each period of stimulation (fig. 7). 

 These large regular waves are produced only by 

 olfactory stimuli given at three or four times the 

 threshold concentration, and may appear at fre- 

 quencies of 50 to 60 per sec. against a silent back- 

 ground in moderately deep urethane anesthesia, and 

 at 10 to 15 per sec. under allobarbital or pentobarbi- 

 tal. Their frequency does not bear any relationship 

 to the quality or intensity of the stimulus. 



In moderately deep allobarbital or pentobarbital 

 anesthesia, with the bulb exhibiting regular intrinsic 

 waves at a low frequency, an olfactory stimulus usu- 

 ally abolishes the waves at each inspiration. If the 

 stimulus is strong the gap in intrinsic activity may be 

 filled with induced waves. As anesthesia lightens, the 

 rhvthm becomes more firmlv established and the 



