36 D. OTTOSON 



are separated from the odorous particles by a thin membrane. We may 

 therefore safely assume that in order to excite, the odorous particles must 

 be brought into contact with the receptors. 



The events that take place in the sensory membrane after the particles 

 have reached the receptors have been the subject of a great deal of specula- 

 tion. It seems most likely that the first step in the excitatory process in- 

 volves the adsorption of the molecules on the membrane. It has been 

 suggested by Beets (1962) that this stage also includes an orientation of the 

 molecules so that they fit into certain sites of the membrane. It is generally 

 recognized that the steric configuration of the molecule is of essential 



Fig. 1. The electro-olfactogram. Response obtained from the frog's nasal 



mucosa to stimulation with butanol vapour. Vertical line 1 mV. Time bar 



2 sec. (From Ottoson, 1956.) 



importance. At the present state of knowledge no definite conclusion can be 

 drawn as to the interaction and relative significance of the various factors 

 which may influence the action of the substance on the end organs. In dis- 

 cussing the processes that may precede excitation, the possibility has also 

 to be taken into consideration that the action of the stimulating agent may 

 be mediated by the release of some intermediary substance. It has been 

 suggested by Nachmansohn (1959) that ACh might participate in the 

 normal process of excitation of sensory endings. Experimental tests on the 

 effect of ACh on various types of sensory end organs have shown that ACh 

 initiates a discharge in some types of endings, whereas others are unaffected. 

 The effect of ACh on the olfactory receptors has been studied by Skouby 

 et ai (1954) who found that topical application of ACh produced an in- 

 creased olfactory sensitivity. However, the observation that anticholine- 

 sterases do not block the activity of the olfactory receptors (Ottoson, 1962) 

 indicates that ACh does not participate in the excitatory process. What- 

 ever intermediary process might be involved, the ultimate effect of the 

 odorous particles on the receptors is a reduction of their membrane poten- 

 tial. This change is the first step in the excitatory process that is amenable 

 to direct examination. 



