ODOR SPECIFICITIES OF THE FROG'S OLFACTORY RECEPTORS 21 



positivity for certain stimuli. The magnitude of the slow potential depends 

 upon the nature and the strength of the stimulus. 



Action potentials were measured with a platinum-black plated, metal- 

 filled microelectrode, coupled capacitively to a cathode follower with 30 

 Mil input resistance. The indifferent electrode for both microelectrodes 

 was the chlorided silver plate. We found that some slight variations in the 

 procedure for preparing the metal microelectrode, which we have described 

 previously (Gesteland et a/.. 1959) greatly improved its ability to pick up 

 the extracellular olfactory action potentials. We break off the tip of a 

 glass micropipette so that it is from 2 to 5// in diameter. Next, we extrude 

 Cerrelow 136 alloy down the pipette to fill it to the end. If responses 

 from the region of the axon hillock are desired, the tip is next plated with 

 platinum black from a solution of chloroplatinic acid with a little added 

 agar. The platinum black is first deposited slowly, then rapidly enough 

 to cause bubbling, until a large, bushy glob is formed at the tip. The agar 

 in the plating bath is most important. It reduces the impedance of the 

 resulting electrode in tissue, as compared with an electrode plated from a 

 solution without agar. If responses from the cell body or distal process 

 are desired, it is best either to grind the tip of the electrode before plating 

 so that it is beveled like the tip of a hypodermic needle, or to break off the 

 tip so that the glass is jagged. Then a little alloy is dissolved out of the tip with 

 sulfuric acid and hydrogen peroxide. Platinum black is plated to fill the 

 hollow left by alloy dissolution. This results in a low-impedance metal 

 electrode with a glass cutting edge to lead the way into tissue. The big, 

 bushy ball-tip electrode was used for most of the experiments described 

 in this paper. It will easily record from receptors singly or a few at a time, 

 and on one occasion recorded for a few moments action potentials from a 

 single fiber in the first nerve far from the mucosa. 



A heavy micromanipulator was necessary in order to stay with units 

 for long periods of time (2 or 3 hr sometimes). 



We insert the metal electrode into the mucosa in such a way that its 

 path is very nearly tangent to the mucosa surface. The electrode will 

 also pick up units if it is normal to the surface, but the probability of record- 

 ing is greatly diminished, no doubt because of both a reduced likelihood 

 of contacting a unit in the optimal way and because the entering electrode 

 is much more likely to damage the receptor terminal structures and block 

 the sensitive area on the way in. Furthermore, the electrode irritates the 

 mucosa in the place where it penetrates, and there is movement of the cilia 

 and mucus as the animal tries to wash away the irritant. This may well 

 block activity of the receptors in that area. 



The metal microelectrode ses^ spike amplitudes ranging from the noise 

 level of 20 /lY up to 2 mV. The spikes may be monophasic, diphasic or 

 triphasic and of either initial polarity, the local boundary conditions for 



