53^ 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



FR. S. 



SPH. S. 



FIG. I. Diagrammatic representation of the lateral wall of 

 the nasal cavity, indicating the general extent of the olfactory 

 epithelium (dotted). The main stream of inspired air passes 

 below the olfactory region. Secondary eddying of air currents 

 carries odors to the receptor region. Abbreviations: CR.PL., 

 cribriform plate, perforated by the olfactory nerves, FR.S., 

 frontal air sinus; I.T., .\I.T., S.T., inferior, middle and su- 

 perior turbinate bones projecting as ledges from the lateral 

 wall of the nasal cavity; SPH.S., sphenoidal air sinus. 



and oval. When seen by light microscopy, they pos- 

 sess distally small terminal swellings from each of 

 which five to six olfactory hairs commonly arise. In 

 the rabbit, these cells are estimated to number 150,000 

 per sq. mm (35). Electronmicroscopy indicates that 

 the hairs are considerably more numerous, with up 

 to 1000 hairs per cell (23). Each hair is i to 2 /:i long 

 and o.i /a in diameter. In this way the surface area ex- 

 posed by the receptor cell is greatly increased (fig. 2). 

 Clark (32) points out, however, that near the pe- 

 riphery of the olfactory epithelium there is some 

 intermingling of olfactory receptors and ciliated 

 epithelial cells of the 'respiratory' region, and that 

 this may lead to misinterpretations in electronmicros- 

 copy of the olfactory hairs, which he considers to 

 be coarser and to remain untapered at their free ex- 

 tremities. 



Removal of the olfactory bulb in the raljljit pro- 

 duces a very striking degeneration in 48 hr., with 

 almost complete removal of the debris of the mucosal 

 receptors within three days (33). However, only 

 about half the receptors degenerate after complete 

 removal of the bulb, the remainder persisting un- 

 altered up to six months after operation. The findings 

 do not support the concepts that the axons of the 

 residual elements may proceed to adjacent areas of 

 the olfactory epithelium, rather than to the ijulb, or 



that they may give off collaterals sufficient to main- 

 tain the cells. The possibility is considered of a cen- 

 trifugal system of fiijers arising in the bulb and 

 proceeding peripherally to the mucosa. 



The secretions of the numerous serous and mucous 

 glands in both the respiratory and olfactory regions 

 of the nose bathe the entire cavity in a liquid sheath 

 which is in a constant state of motion towards the 

 nasopharynx. This sheath may be of basic importance 

 in conveying odorous substances to the receptor cell, 

 since varying degrees of water and fat solubility in 

 the tissues of the mucosa may be related to the odorous 

 properties of a particular substance (see below). 



The electrical responses of the olfactory mucosa of 

 the frog have been successfully recorded by Otto.son 

 (76). Odorous air blown into the nasal cavity evokes 

 a slow negative monophasic potential in the olfactory 

 mucosa (fig. 3). The response is obtained only from 

 the olfactory area of the mucosa and is not abolished 

 by cocaine in concentrations sufficient to paralyze 

 olfactory nerve fibers. It is abolished i^y small amounts 

 of ether or chloroform vapor, and it is inferred that 

 the response arises in the olfactory hairs. The ampli- 

 tude of the respon.se is, within certain limits, propor- 

 tional to the logarithm of the stimulus intensity. 

 Equal amounts of odorous material distributed in 

 different volumes of air evoke responses of equal 

 amplitudes. The shape and time course of the re- 

 spon.se is related to the strength of the stimulus. With 

 an increase of odor intensits' in the stimidating air. 



HG. 2. Electronmicrograph of the surface of a human ol- 

 factory cell, showing a great number of thin finger -like processes 

 1.5 to 2.0 11 long. Magnification X 23,430. [From Bloom & 

 Engstrom (23).] 



