PHYSIOLOGY OF CHEMORECEPTION 231 



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Figure 1 Types of electrical potentials re- 

 corded by various techniques: (A) generator 

 potential, (B) action potentials, (C) electro- 

 encephalogram. Lower line indicates duration 

 of applied stimulation, with the time sequence 

 reading from left to right. Note that some 

 "spontaneous" impulses and EEG potentials 

 preceed and follow the period of stimulation 

 in these examples. 



Electro-Olfactograms (EOG) and Related Potentials 



In the 1930s it was discovered that when odors were applied to olfactory 

 epithelia of various mammals, a slow electronegative potential appeared. 

 Ottoson, during extensive studies on this phenomenon in frog olfactory 

 epithelia, proposed the name "electro-olfactogram" (EOG), and a simi- 

 lar potential in olfactory organs (antennae) of insects led to the term 

 "electro-antennogram" (Ottoson 1963, Schneider 1963). 



The shapes and durations of these slow potentials from the olfactory 

 organs differ according to the type of stimulus used; consequently, they 

 can be convenient tools for analysis. EOGs or EAGs obviously resemble 

 generator potentials. Because of their complexities and their origins from 

 the summed potentials of many cells, however, they do not meet the more 

 rigorous criteria by which generator potentials are characterized. Although 

 attempts have been made to do so (Hodgson and Mathewson, unpublished), 

 these potentials have yet to be recorded in any consistent fashion from 

 chemosensory organs of elasmobranchs. 



Action Potentials (Impulses) 



Excitation of chemoreceptors is eventually communicated to the brain as 

 a change in afferent nerve impulses, action potentials, spike potentials, or 

 "impulses" (Figure IB). Despite the implications of the name, these poten- 

 tials may not simply appear during chemical stimulation and disappear when 

 stimulation ceases. In many cases a "spontaneous" afferent flow of action 

 potentials is decreased during chemical stimulation. Consequently, the in- 

 vestigator must be prepared to decode any type of change in afferent action 



