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The main roles of the turbinates are:
- To direct and maintain laminar airflow, respiratory rate and velocity
- Air Conditioning (Humidification + Heating)
- Filtration
- Olfaction
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The middle and superior turbinates’ mucosa harbor olfactory nerves, extended into the nasal cavity, through the cribriform, from the olfactory bulb. But the main role of the turbinates (including the inferior ones) role in olfaction is – their direction over airflow. They deflect the inspired air not just to stream through the inferior, middle and superior meatuses, but also to have enough resistance and elevation to make the airflow reach the roof of the nose – the cribriform plate – where the olfactory bulb is located.
The air that reaches there is humidified, and saturated with moisturized scent molecules, that get trapped in the olfactory cilia.
Only a small part of the nose and nasal cavity is taken up by the organs of smell; the rest of it is mainly concerned with processing the airflow on its way through to the lungs. The sense organs are made up of two yellowish-gray patches of tissue, called the olfactory membranes, each about the size of a postage stamp. They are located in a pair of clefts just under the bridge of the nose and at the top of the nasal cavity. The reasons for the coloration are not completely clear, but it seems to be necessary for the membrane to work. During normal breathing, most of the air flows through the nose, with only a small part reaching the olfactory clefts, but this is enough to get a response to a new smell. When a person “sniffs the air” to detect smells, the air moves through the nose much faster, increasing the flow that makes its way to the olfactory clefts and so carrying more odor to those sensors. Olfactory receptor cells are bipolar neurons whose cell bodies lie within the olfactory mucosa that lines the cribriform plate. There is a constant turnover of olfactory receptor cells, as there is of gustatory receptor cells; their life cycle is approximately 60 days. The cells send a process toward the surface of the mucosa, which divides into several cilia that penetrate the layer of mucus. Odorous molecules must dissolve in the mucus and stimulate receptor molecules on the olfactory cilia. The axons of the olfactory receptor cells enter the skull through small holes in the cribriform (“perforated”) plate at the base of the skull. |
Fig 39: Olfactory epithelium and mucosa
Fig 40: Olactory mucosa
There are many elongated supporting cells or sustentacular cells surrounding the receptors. A thin layer of watery mucus made by the supporting cells and the olfactory glands covers the receptor cilia and microvilli. During inhalation, odorants are drawn into this fluid layer, where they dissolve and then bind to the cilia receptors.
Binding of the odorants causes the olfactory receptors to generate electro-chemical impulses (= action potentials). Receptor axons carry the impulses through the holes in the cribriform plate to the olfactory bulbs at the base of the brain.
The olfactory bulbs lie at the base of the brain on the end of the stalk like olfactory tracts. The axons of the olfactory receptors terminate in the olfactory bulbs, where they synapse with dendrites of mitral cells (named for their resemblance to a bishop’s miter). These synapses take place in the comlex axonal and dendritic arborizations called olfactory glomeruli (from glomous, “ball”). There are approximately ten thousand glomeruli, each of which receives input from a bundle of approximately 1,000 axons. The axons of the mitral cells travel to the rest of the brain through the olfactory tracts. Some of these axons synapse in the brain, whereas others cross the brain, enter the other olfactory nerve, and synapse in the contralateral olfactory bulb.
Fig 41: The olfactory system. The olfactory neural pathways in the brain
The pyriform cortex projects to the hypothalamus and to the dorsomedial thalamus, which projects to the orbitofrontal cortex (as shown in Fig – 41). The orbitofrontal cortex also receives a considerable amount of olfactory information, which is probably important for the acceptance or rejection of food and for the olfactory control of reproductive processes seen in many species of mammals. Most mammals (and perhaps humans too) have another organ that responds to olfactory stimuli: the vomeronasal organ. This organ plays an important role in animals’ responses to odors that affect reproductive physiology and behavior.
Efferent fibers from several locations in the brain enter the olfactory bulbs. The synapses of these fibers appear to be inhibitory, but their role in the processing of olfactory information is a mystery.
The importance of the human sense of smell has been largely underestimated. Many people believe that human olfactory acuity and specificity have deteriorated. Other mammals are believed to be macrosmatic (i.e. better smellers) because they have more olfactory receptor cells in their nasal mucosa than humans. For example, dogs have about 230 million olfactory receptor cells, while humans have about 10 million. Accordingly, humans and other primates typically are believed to be microsmatic (i.e. worse smellers) equipped with highly developed powers of vision that supposedly make humans “visual creatures.” This concept needs reconsideration since many recent studies have shown that olfaction plays a very important role in human reproductive biology and because human reproductive biology affects human behavior.
The sense of smell has also been found to be a strong associative registering agent of memories, and losing it (becoming anosmic) can cause clinical depression, and serious eating disorders that result in considerable gain of weight or loss weight.
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