Olfaction or getting information about the smell of any substance is the main deed of nose. As the substance must be in solution form for getting information of the taste similarly the thing must be in gaseous or vapor form for getting information of the smell or olfaction. Gas or vapor gets dissolved in local secretion after reaching into the nose and stimulate olfactory cells. The impulses of stimulus reach into the olfactory bulb and then into olfactory region of brain by going through the olfactory path where they are analyzed. In this way, information of smell becomes clear.
Structure of nose and sense of smell:
The mucus of nasal cavity gets divided into different sections by three nasal conchae which start from the external wall of the nose. Three small protuberances like three small mounds take birth on this place because of three nasal conchae. A nasal mucus membrane is spread on the total region which is made of columnar ciliated epithelium and there are goblet cells too in it. The mucus membrane of nose remains sticky and greasy because of the secretion of these cells.
Vertical, middle and lower horizontal Vallecula take birth in the nose because of nasal conchae. All the three bone made vallecula divides each half portion of the nose into four sections incompletely which go from front side to back side and situated one above another. The breathing air goes through the lower three sections but the air does not go in the upper most section normally. The organs of small are situated in the olfactory mucous membrane of the upper most section. The end organs of olfactory cells are spread in this membrane. The section of upper most sections is like the dark canal or pocket which terminates on the same place. The olfactory epithelium is sensitive for smell which is near the roof of the nose. In it, there are three types of cells supporting cells, basal cells and bipolar cells. Besides these, there are some bowman’s glands in the lamina propria. The excretion of these glands keeps moist to the whole surface and does the work to dissolve odorous vapors. The base of supporting cells is upwardly and makes a membrane of surface and cilia like hair of olfactory cells come out from the vacant place of the middle portion.
Generally, bipolar nerve cells are the receptor cells of smell. The number of these cells is 25 million in human being and each cell is surrounded by supporting cells. There is a round nucleolus in each cell and there is protoplasm in the form of thread. These cells are present in the mucus membrane. Two processes of each cell are present on both points. Cells are close by to each other in a long axis and stand in perpendicular from the surface. The dendrites of these cells are afferent nerve fibers in small size which keep on coming out from the middle portion of epithelium. On the above side, these fibers make olfactory rods by expanding a little and come to an end into bulbous olfactory vesicle. Six to twenty cilia are coming out from the vesicles which are covering to the surface epithelium. These are called smell hairy cells. Vesicle and its cilia are its olfactory end organs. Axons of the receptor cells which make olfactory nerve raise high by coming out from the second edge and go through the Cribriform plate of Ethmoid bone and reach into olfactory bulbs. Olfactory bulbs are special structures in brown color which are the enlarged form of pole of the olfactory region of brain. There are terminal axons of receptors cells, tufted cells, granule cells and synapse of mitral cells of dendrites inside the olfactory bulbs. Because of this reason a special round oragan named olfactory glomeruli takes birth. Each glomerulus gets about 26,000 impulses of receptor from the axons. Axons of mitral and tufted cells make olfactory tract which reaches into the olfactory or smell center in temporal lobe of cerebral cortex where impulses are analyzed and we feel a special smell.