GENERATION AND TRANSMISSION OF SIGNALS IN 

 THE OLFACTORY SYSTEM 



D. Ottoson 



Department of Physiology, Karolinska Institutet, Stockholm 



The olfactory receptor cell is a bipolar neuron with a peripheral extension, 

 from which a number of hairlike filaments protrude into the mucus cover- 

 ing the epithelium. The afferent nerve fibre emerges from the proximal 

 pole of the cell and runs together with fibres from adjacent cells in small 

 fascicles to the brain. This small neuron and its processes exhibit some 

 rather remarkable functional features. It is able to detect minute amounts 

 of certain substances and to transduce the stimulus into a coded message 

 that provides the brain with information about the nature and strength 

 of the stimulating agent. How the cell carries out this function has for 

 long been a challenging problem to physiologists and biochemists. It is 

 the aim of the present paper to discuss some of the problems concerning 

 the transformation of the events in the receptor membrane into the elec- 

 trical signals carried to the brain. 



One of the basic problems in the study of the function of the olfactory 

 sense organ concerns the question of which part of the primary neuron 

 serves as the sensory element. Various parts of the cell, such as the olfac- 

 tory rod, the vesicle or the hairs, have been suggested to re^present the 

 chemosensitive portion of the cell. The experimental evidence supports the 

 view that the primary reaction between the stimulating agent and the 

 neuron takes place in the membrane of the hairs. The morphological fea- 

 tures of these structures seem to vary greatly from one species to another. 

 In the frog the hairs have a length varying from 20 to 200// and form a felt- 

 work of densely interwoven filaments close to the surface of the mucus 

 layer. This feltwork may be regarded as the sensory membrane and chemo- 

 electrical transducer of the olfactory apparatus. 



In spite of all the efforts to evaluate the mechanisms underlying the 

 action of the odorous particles on the receptors we still don't know what is 

 the essential stimulus. A number of theories have been advanced which 

 assume that the olfactory membrane is excited by some sort of radiation 

 emitted from or absorbed by the particles. Experimental evidence speaks 

 strongly against these theories. It has, for instance, been demonstrated 

 (Ottoson, 1956) that the olfactory receptors cannot be excited when they 



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