40 D. OTTOSON 



perties of the fibres, such as their conduction velocity and excitability cycle 

 may, however, be examined by recording the activity in response to elec- 

 trical stimulation. Amphibians and certain fishes are particularly useful 

 preparations for such studies since the olfactory fibres in these species are 

 grouped together into one single nerve trunk. Gasser (1956) has demon- 

 strated that the: spike potential of the pike's olfactory nerve consists of one 

 single wave. The response of the frog's olfactory nerve (Fig. 5) has the 



Fig. 5. Action potential of the frog's olfactory nerve. A, superimposed records 

 of responses to electrical stimulation of increasing strength. Vertical line 

 0.5 mV. Time mark 50 msec. B, superimposed records showing the relative 

 refractory period of the olfactory nerve. Vertical line 200 /<V. Time bar 100 msec 

 (From Ottoson, 1959c.) 



same characteristic appearance (Ottoson, 1959c). The simple configuration 

 of these responses can be attributed to the fact that all the fibres have 

 approximately the same diameter (Gasser, 1956 ; de Lorenzo, 1960). 

 Owing to their small dimensions conduction velocity is very low. In the 

 pike, Gasser (1956) found the velocity to be about 0.2 msec. In the 

 frog, the olfactory fibres conduct at a velocity of 0.14 msec. These find- 

 ings indicate that time is of relatively little importance in the transmission of 

 the olfactory message to the brain. In providing the olfactory cortex with 

 information about the chemical environment, the absolute sensitivity of the 

 olfactory receptors and their ability to function as peripheral analysers are 

 most certainly far more important. 



The transmission of the olfactory signals from the afferent fibres to the 

 secondary neurons takes place in the glomeruli in the bulb. For the under- 

 standing of the function of the olfactory system it is important to note that 

 there are no synaptic connections between the sensory cells in the mucosa. 

 An interactiort between the primary cells, resulting for instance in an 

 inhibition of the type seen in the Limulus eye (Hartline et aJ., 1956) is 

 therefore less likely to occur. Each receptor cell with its axon functions as 

 an independent input channel to the bulb. 



As shown in Fig. 6, natural stimulation of the olfactory mucosa gives 

 rise to a slow potential change in the bulb. Superimposed upon this 

 potential there are regular oscillations which in the frog have a frequency 

 of 8-12 per sec. The experimental evidence suggests that the slow potential 

 comes from the dendritic network in the glomeruli and that it is of the same 

 nature as the enduring potentials recorded from sensory cortical areas 



