THE SENSE OF SMELL 



537 



FIG. 3. Rhytlimic waves superimposed upon the slow potential. Stimulus; amy] acetate. Volume 

 of stimulating air, 0.25 cc. Vertical line 5.0 mv. Time bar, i sec. [From Ottoson (76).] 



FIG. 4. Rhythmic waves evoked by continuous stimulation of the olfactory epithelium. Stimulus: 

 0.1 mole butanol. Velocity of stimulating air stream, i.o cc per sec. Vertical line i.o mv. Time bar 

 0.5 sec. [From Ottoson (76).] 



the potential rises at a faster rate, the crest of the 

 response broadens and the decay time lengthens. The 

 'wave form' of the stimulating air current is of great 

 importance in determining the shape and time course 

 of the response. The latency of the response to stimu- 

 lation with butanol of different stimulus strengths 

 varies from 0.2 to 0.4 sec. 



Ottoson has found that during continuous stimu- 

 lation the evoked response in the olfactory epithelium 

 declines from the initial peak to a lower level which 

 continues throughout stimulation (fig. 4). The ampli- 

 tude of this residual response is lower at higher stimu- 

 lus intensities. With repeated stimulation at short 

 intervals, the first three or four responses are pro- 

 gressively diminished, with greater reductions at 

 higher stimulus strengths. The sensitivity of the 

 epithelium to different substances can be selectively 

 reduced by repetitive stimulation, and rhythmic 

 oscillations are often seen on the peak of the slow 

 response evoked by strong stimuli. 



Olfactory Bulk and Its Conrnrllons with Olfactvry Mucosa 



The olfactory nerve fibers arising from the hair 

 cells penetrate the overlying cribriform plate of the 

 ethmoid bone and enter the olfactory bulb. Electron- 



microscopy of the olfactory nerves indicates a unique 

 appearance, with large numbers of very small nerve 

 fibers having a modal diameter of 0.2 n. They num- 

 ber six million from one side of the nasal septum in 

 the pig and are considerably more numerous from 

 the turbinates (45). There appears to be a one-to- 

 one relationship between receptor cells and axons. 

 Their conduction velocity in the pike is 0.2 m per 

 sec, thus resembling the last elevation in the C fiber 

 action potential in the frog's .sciatic nerv-e. 



Within the outer layers of the bulb the fibers of the 

 olfactory nerves enter into the formation of glomeruli 

 (fig. 5). Each glomerulus is formed jointly by enter- 

 ing olfactory nerve fibers and also from the dendrites 

 of more deeply situated mitral and tufted cells. These 

 cells forrn the succeeding second order neurons on 

 the olfactory pathway. The arrangement is an excel- 

 lent one for spatial summation, since each glomerulus 

 in the rabbit receives impulses from 26,000 receptors 

 and passes this information through 24 mitral cells 

 and 68 tufted cells (15, 16). The axons of the 60,000 

 mitral cells form the bulk of the lateral olfactory 

 stria passing to higher olfactory centers. 



Physiological and anatomical evidence confirms the 

 existence of a regional projection pattern from the 

 olfactory mucosa to the bulb. Impulses from the 



