542 



HANDBOOK OF FHVSIOLOGY 



NEUROPHYSIOLOGY I 



FIG. 7- Induced waves develop in the olfactory bulb after commencement of the olfactory dis- 

 charge. Rabbit under deep urethane anesthesia breathing air containing amyl acetate. The upper 

 oscillograph tracing shows the waves from the surface of the bulb; the lower shows the axon spikes 

 from the white matter. Inspiration indicated by white line above. Time marker, o. i sec. [From 

 Adrian (5).] 



waves may be merely reduced or scarcely altered 

 during stimulation. When the iaulb is quiet in very 

 deep anesthesia, a moderate olfactory stimulus sets 

 up a mitral cell di.scharge with each inspiration, but 

 no discharges are visible between inspirations. With 

 lightening anesthesia, evoked discharges appear 

 against a background of continuous irregular activity 

 which ultimately becomes so prominent as to obscure 

 entirely any change evoked by the stimulus. In very 

 light anesthesia the olfactory stimuli regain some 

 control over the mitral pathway, and both weak and 

 strong stimuli evoke an obvious increase in discharge 

 during each inspiration, with suppression of the 

 mitral discharges in the periods between each in- 

 spiration. 



The complete suppression of intrinsic activity in 

 the bulb of the rabbit is seldom long maintained. The 

 return of activity takes place more slowly when the 

 smell is strong and the anesthesia light. Adrian (5) 

 suggests that the phenomenon offers an explanation 

 of olfactory adaptation as seen in man, although, as 

 mentioned above, records from the mucosa during 

 continuous stimulation indicate that at least some 

 adaptation occurs at the receptor level (76). 



Records from the olfactory bulb in man show a 

 series of rhythmic waves at each inspiration while 

 breathing tincture of valerian and benzene, whereas 

 room air yields no response. No spontaneous waves 

 of the type seen in the rabbit have been noted in 

 man. Thiopental anesthesia abolishes all responses 

 (80). 



Unit activity recorded with microelectrodes in the 

 olfactory bulb of a variety of animals favors the mitral 

 cells as the site of origin of the axon spikes, with 

 tufted cells and glomeruli possibly also contributing. 

 Where it is possible to record both wave and spike 

 components of the response, it is found that the fast 

 spikes are evoked first, followed by the waves, with 

 the spikes becoming synchronous with the waves as 

 the wave response develops (5, 72). 



Differential Excitation of Receptors 



DIFFERENTIATION OF RESPONSE IN AREA. Substances 



soluble in water (e.g. amyl acetate, ethyl acetate, 

 ether, acetone) have a lower threshold for spike dis- 

 charges in the anterior part of the bulb, where mitral 

 cells synapse with fibers from the anterior and dorsal 

 parts of the mucosa (fig. 8). Conversely, substances 

 soluble in lipoids (e.g. pentane, coal gas and ben- 

 zene) have a lower threshold for spike discharges in 

 the posterior part of the bulb which receives fibers 

 from the posterior and ventral parts of the olfactory 

 epithelium (9, 10). This difference does not neces- 

 sarily imply a differential excitability of the receptors 

 at the front and back of the organ and may well re- 

 sult from structural factors, difference in the velocity 

 of the air current and in the composition of the sur- 

 face film through which molecules of odorous sub- 

 stance pass to reach the receptor surface. 



In records from the middle part of the bulb (9) 

 there may be a single series of large spikes or a mix- 

 ture of large and small spikes (fig. 9). The single 

 series presumably represents a discharge from one 

 cell, whereas small spikes come from neighboring 

 units. Adrian found that at any one recording point 

 one substance in low concentration would give a 

 single series of large spikes. Each large spike thus 

 appears to have a special relation to a particular 

 stimulus. Units have been observed displaying this 

 specific sensitivity to such diverse substances as 

 trimethylamine, acetone, ethyl acetate, amyl acetate, 

 pentane, octane, xylol, petrol, clove oil, oil of euca- 

 lyptus and thick machine oil. Despite the improb- 

 abilitv of finding a few primary smells out of which 

 all others can be compounded, .Adrian (lo) has de- 

 fined four groups of substances with one substance in 

 each group most frequently evoking a single unit 

 discharge (see above). Strong concentrations of 

 odorants will bring in other units, but critical regions 

 will always exist where the concentration is only 



