GENERATION AND TRANSMISSION OF SIGNALS 



39 



plateau phase is produced when a continuous stream of air is passed over 

 the olfactory mucosa. The level of the plateau in relation to the initial 

 peak of the response is a direct measure of the adaptation of the receptors. 

 With a stimulus of low or medium intensity this level amounts to 50-60 

 per cent of the height of the initial peak. The fact that this potential level 

 is maintained with very little decline brings direct evidence that the olfac- 

 tory receptors have to be classified as comparatively slowly adapting end 

 organs. This is also evidenced by the slow decline of the response during 



Fig. 4. Adaptation of the olfactory receptors. Responses obtained from the 

 rabbit's nasal mucosa to stimulation with a constant stream of odorized air. 

 Duration of stimulation : A, 0.5 sec. B, 1 sec. C, 3 sec. Vertical line ImV. 

 Time bar 0.5 sec. (From Ottoson, 1959a.) 



repeated stimulation of the nasal mucosa. How are we then to explain the 

 fact that an odour that at first appears strong rapidly weakens and soon 

 becomes imperceptible? Adrian (1950) has suggested that this phenomenon 

 may be explained by the fact that the incoming signals are suppressed by 

 the intrinsic activity of the bulb. It is also likely that the efferent system 

 (Kerr and Hagbarth, 1955) participates in this inhibitory action. Even if 

 we accept this explanation, we are still faced with the difficulty of explaining 

 the regularity of the adaptive process. 



It has been demonstrated that the potential recorded from the olfactory 

 membrane is not affected by antidromic stimulation of the olfactory nerves. 

 This fact shows that the membrane producing this potential does not par- 

 ticipate in the impulse activity. The potential generated by the receptors 

 spreads electrotonically in the nerves and by reducing their membrane 

 potentials initiates the afferent discharge. It has been demonstrated in 

 studies on the impulse initiation in the lobster's stretch receptors (Edwards 

 and Ottoson, 1958), in spinal motorneurons (Coombs et a/., 1957), as well 

 as in the giant cells of Aplysia (Tauc, 1962), that the cell body does not 

 participate in the production of impulses. These findings may also apply 

 to the olfactory primary neurons. If this view is correct the impulse 

 initiation in the olfactory fibres would occur through the electrotonic 

 spread of the potential generated by the hairs to the olfactory rod and cell 

 body and further out in the axon where the impulse would be set up at the 

 portion with the lowest threshold. 



The extreme fineness of the olfactory fibres has been the major obstacle 

 to the study of the characteristics of the afferent inflow and particularly to 

 the successful analysis of the activity in single units. Some of the pro- 



