ODOR SPECIFICITIES OF THE FROG'S OLFACTORY RECEPTORS 25 



Stimuli, the threshold is below the level at which an Ottoson potential can 

 be distinguished. Overstimulation causes extended high thresholds. If the 

 unit is overstimulated by several different stimuli, it will often discharge 

 at a very high rate for many seconds, and then go into a state with 

 prolonged high threshold. It is not a dead receptor, however, and will res- 

 pond with a few spikes to an appropriate stimulus. After a long period of 

 stimulation, even when the stimuli strengths have been kept low enough 

 to avoid these effects, many of the units that we record are generally 

 irritable, responding strongly to all successive stimuli. The mucosa 

 does not return to its initial or normal state after this effect has set in. 

 An interesting phenomenon is seen if a puff of cigarette smoke is blown 

 at the exposed mucosa with the decorporate preparation. There is a big in- 

 crease in background activity (activity of cells too far from the electrode tip 

 to be distinguished from noise) which suddenly becomes an oscillation of 

 5-10 c/s. This oscillation lasts for a few seconds and is apparently phasic 

 activity of many receptors. The receptors do not show much activity or 

 selectivity after such an oscillation has occurred. However, it can be 

 obtained repeatedly, and the frequency of the oscillations changes some- 

 what with composition of the smoke. We do not get the oscillations when 

 circulation is intact. The dc potential of the mucosa becomes very erratic 

 sometime after single units show the effects of massive stimulation. 



When we section the olfactory nerve and let degeneration take place for a 

 week or more, the effects on the Ottoson potentials and single-unit res- 

 ponses are very apparent. The Ottoson potential is reduced to approxi- 

 mately 50 per cent of the amplitude recorded from the mucosa with an intact 

 nerve, and the frequency with which one can find a unit with an electrode 

 is markedly reduced. With careful exploring, spikes can be found and they 

 are odor-specific in their responses. This agrees with Le Gros Clark's histo- 

 logical studies on degeneration following first-nerve section in the rabbit 

 where he found at least half of the olfactory receptor cells to have dege- 

 nerated (Le Gros Clark, 1957). Section of the fifth nerve, on the other 

 hand, has no obvious effect on the Ottoson potential, amplitude, and the 

 number of active single units or their response properties. On a few 

 occasions with a preparation with intact fifth nerve we have encountered a 

 single unit in the mucosa with an action potential that is short compared 

 with olfactory units, approximately 1-1.5 msec. Figure 5 shows such a 

 unit. The large spike has a short duration, and the small one is more than 

 twice as long. The top trace shows the resting rate, the middle trace shows 

 the large unit responding to butyric acid, and the bottom trace, a weak 

 response of the small unit to musk xylene. The large unit showed some 

 response to camphor and mercaptoacetic acid, but to nothing else, even 

 if very strong puffs were used. The small unit responded to a larger group 

 of odors. It seems most likely that the large unit is a fifth-nerve ending. 



