ODOR SPECIFICITIES OF THE FROG'S OLFACTORY RECEPTORS 23 



examine the shape of the spikes. Even if two units had approximately the 

 same ampHtude, their shape was almost always characteristic, and while 

 watching the expanded spikes, the electrode was moved slightly until 

 different units had clearly distinguishable amplitudes. Thus it was possible 

 to watch simultaneously the response of two or three cells to each stimulus. 

 Figure 2 shows the resting discharge and an expanded sweep and a normal 



Ffg. 2. Resting discharge and two responses to pyridine. Center sweep shows 

 expanded spikes. Each expansion has a 2.5 msec duration. All sweep lengths, 



10 sec. 



sweep of the response of two units to pyridine. Note the different shapes 

 of the action potentials of the two active units. The gate also was used to 

 provide relative brightening of the spikes compared with the base line, in 

 order to maintain more nearly uniform photographic exposure. 



ODOR RESPONSES AND SPECTFTCTTY 



A responsive unit will generally produce a burst of spikes from I to 4 sec 

 in duration. This is usually followed by a quiet period about as long. For 

 a much longer time after this there is a refractory period during which the 

 threshold of the unit is increased. The amount of increase and the dura- 

 tion depend upon the strength of the preceding stimulus and its constitu- 

 tion. For instance, Fig. 3 shows a unit stimulated with butyric acid twice 

 within 1 min. The second stimulus causes fewer action potentials than the 

 first. The stimulus strengths were about equal as is shown by the Ottoson 

 potentials. The time between the arrival of the stimulus at the mucosa 

 and the response also depend on the particular stimulus, but not on the 



