22 



R. C. GESTELAND AND OTHERS 



current flow set by the electrode being the determining factor. The dura- 

 tion of the three phases of the action potential of olfactory receptors, 

 extracellularly recorded, is from 3 to 5 msec if the electrode is deep, that 

 is, near the axon hillock or axon. Figure 1 shows three sweeps of resting 



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Fig. 1. Resting discharge of olfactory receptor cell axons. The lengths of the 

 sweeps are 0.5 sec and the largest spikes are 0.4 mV peak-to-peak amplitude. 



activity of units recorded in this position. The spike duration is longer, 

 between 5 and 7 msec, when the electrode tip is on the mucosa side of the 

 basement membrane near the cell bodies, or very near the surface of the 

 mucosa. Action potentials recorded from fifth-nerve axons and fifth- 

 nerve endings in the mucosa are much shorter, approximately 1-1.5 msec, 

 typical of myelinated axon activity. For olfactory action potentials, 

 optimal amplifier frequency response is roughly from 8 c/s to 1 kc/s. The 

 maximum repetition rate of olfactory spikes is 20 per sec, and this is seen 

 only rarely when a particularly appropriate stimulus is presented to a cell. 

 A common rate for a responding cell is from 1 to 5 spikes per sec. The 

 resting rate (which may be low-level activity of the receptors caused by 

 room odors) is usually a few spikes in 10 sec. 



Our display system operated in the following way. The vertical signal 

 output of a monitoring oscilloscope was used to open a gate that passed 

 the spike on to a second oscilloscope. The gate could be set to open only 

 for spikes exceeding a preset amplitude. The Ottoson potential was added 

 on the second oscilloscope, deflecting the base line to show the time of 

 arrival of the stimulus at the mucosa and its relative strength. Since two 

 or more units are often picked up by the microelectrode, we arranged a 

 sweep expansion saw tooth to be triggered by the gate so that we could 



