by Dr. M. Hutchins
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| This sensory system permits humans to recognize some 9,000 different odors. Humans are surrounded by air containing many diverse odorant molecules, yet our perception selects only one which attracts or distracts us. In the United States, we tend to disguise or modify environmental odors with a deodorant, i.e. replace an unpleasant odor with a stronger, acceptable one. | |
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Figure 14
Air warmed and transported to the olfactory mucosa. |
The olfactory epithelia are located within each nasal cavity. This tissue is a yellowish patch about 2 sq. cm and is located on the superior portion of the nasal cavity and on the sides of the nasal septum. It is surrounded by the reddish nasal mucosa which moistens and warms the inspired air carrying odorant molecules. An odorant binding protein is produced by the nasal mucosa, each protein molecule binds reversibly to several odorant molecules and transports these molecules to the receptor site in the epithelium. Sniffing the air rapidly increases the transport of the odorant chemicals to the olfactory system.
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Figure 15
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The olfactory epithelium consists of a group of columnar type cells overlying a basement membrane containing nerve axons and blood vessels. This tissue contains specialized bipolar neurons, the olfactory receptor cells. At the coronal end each receptor cell has about 10 motile cilia that project onto the surface of the olfactory mucosa. Each cilia contains numerous receptor proteins which bind to the odorant molecules transported to this site by inspired air. The basal portion of the olfactory neurons are axons which project into the basement membrane through the cribriform plate to synapse in the olfactory bulb. The olfactory epithelium also contains supporting cells which support the receptor cells and provide mucus around the cilia. Basal epithelial type cells are located near the basement membrane and their function is not known, although they may differentiate into new neurons to replace aging olfactory neurons. These specialized neurons are unusual because they are replaced about 70 days, unlike neurons of the central nervous system that are not replaced during our lifetime.
SENSORY TRANSDUCTION OF ODORS
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The olfactory stimuli, odorant molecules, bind reversibly to the receptor sites on the cilia of the receptor neurons. This receptor is coupled to a GTP-binding protein called GOLF. A second messenger, cyclic AMP, is activated which opens ion-gated channel membrane proteins to permit the influx of Ca++ and efflux of K+. In response to ion conductance, a receptor potential is generated at the coronal end of the olfactory neuron In response to this receptor potential, an action potential is generated and propagated toward the basal portion of the sensory neuron. |
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Figure 16
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Transduction of odorant chemicals to nerve impulses is similar to the transduction of many types of stimuli in all sensory receptors. It begins with stimulation of the receptor, an alteration in membrane potential, and if the receptor potential is strong enough, action potentials are propagated as nerve impulse down the sensory afferent nerve fiber. The nerve impulses are propagated at frequencies directly related to the strength of the stimuli.
TRANSMISSION OF NERVE IMPULSES THROUGH THE OLFACTORY LOBE
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Action potentials, i.e. nerve impulses, produced by stimulated olfactory neurons are propagated along the axon through the cribriform plate into the olfactory bulb. Neural convergence is the rule in the olfactory bulb where in humans it is estimated that as many as 10 million olfactory neurons send axons into the olfactory bulb to 65,000 glomeruli to synapse with about 100,000 Mitral cells. Numerous interneurons modify this neural input in the olfactory lobe, but the olfactory system like the gustatory or taste systems sends a neural code to the olfactory tract and into the central nervous sytem for each specific odorant molecule. |
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Figure 17
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This code consist of many action potentials propagated at different frequencies along the nerve fibers within the olfactory tract, The neural code is conducted to various areas of the paleocortex, such as the olfactory cortex and amygdala. Each specific generated code permits us to distinguish an odor in our home or in a dental office and to characterize this odor, i.e. floral, fruit, burnt, resin, spicy, or putrid.
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Figure 18
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DISORDERS OF THE OLFACTORY SYSTEM
The most common cause of a loss in the sense of smell is a temporary reduction in the transport of odorant molecules to the olfactory mucosa, which occurs with head colds, sinus, flu infection or allergies. Another cause is inhalation of chemical solvents which can injure the nasal and olfactory mucosa. Usually, recovery occurs unless there is significant, prolonged exposure with chronic damage to the olfactory mucosa. For example, many long term smokers, state that they have a loss in the sense of smell.
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Causes of Smell Disorders
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Figure 19
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Trauma to the head such as fractures, especially in the basal region of the brain can cause damage to the cribriform plate as well as to the olfactory tract. The olfactory cortex could be injured by an aneuryism of the Circle of Willis. Studies have shown that around 60 years of age, there is a decline in our ability to detect odors and by age 80 there may be a several fold decrease in sensitivity to certain odors. Men do not have the sensitivity to many odors that woman have and men have a greater loss in the sense of smell as humans age.
CONCLUSION
In conclusion, the flavor of foods is dependent upon taste, salivary flow, smell, masticatory efficiency and contact with the oral mucosa and the stimulation of touch, temperature and even pain. However, smell is the most important and neurally the most elusive because it is more directly involved with emotion and memory than the other senses.
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Content questions should be directed to: Dr.
Max Hutchins
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The University of Texas Health Science Center at Houston (UT-Houston)
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