GENERATION AND TRANSMISSION OF SIGNALS 



37 



The electrical response evoked when a small volume of odorous air is 

 passed over the mucosa consists of a purely monophasic negative potential 

 with a fast rising phase and an exponential fall (Fig. 1). This response is 

 the generator potential of the olfactory organ and is homologous to poten- 

 tials of the same type recorded from other sense organs. The configuration 

 of the potential is influenced by a number of factors such as the parameters 



lOOf 



0.001 0.01 



StiiTiulus strength ( molar i oiitenl rat ion) 



0. I 



Fig. 2. 



Relation between amplitude of the electro-olfactogram and the stimulus 

 strencth. (From Ottoson, 1956.) 



of the stimulus and the local conditions in the mucosa. Tn the frog, an air 

 puff of a duration of about 1 sec. and of low or medium stimulus 

 strength gives rise to a response that lasts for 4 to 6 sec. Compared with the 

 corresponding potentials in other types of sense organs, the response of the 

 olfactory membrane is relatively slow. This is to be expected, considering 

 the fact that the exciting odorous particles have to pass through a layer of 

 mucus before they reach the receptors. Excitation may therefore be 

 assumed to take place gradually, its time-course being a function of the 

 number of particles which reach the receptor per time unit. The temporal 

 course of the excitatory process also depends on what may be called the 

 "' lifetime " of the odorous particles, i.e. the time that passes until they 

 become inactivated. 



The potential recorded from the sensory epithelium in the nasal mucosa 

 is a mass response and as such it tells us very little about the function of 

 the individual receptors. Nonetheless the response may provide informa- 

 tion that might be useful for the evaluation of the function of the peri- 

 pheral olfactory apparatus. By using the potential as an index of the 

 activity of the olfactory membrane, the effect of different substances can 

 be quantitatively measured. This method has been particularly valuable 

 in studies on the relation between the physicochemical properties of 

 different odorous compounds and their stimulating effects (Ottoson, 1958). 

 By measuring the increase in amplitude of the response with increasing 



