The Olfactory System: Anatomy and Physiology


The Nasal Cavity

Did you know that approximately 95% of the nasal cavity has nothing to do with smell? Most of the area of the nasal cavity serves the function of cleaning the air we breathe before it reaches the lungs. It does this with the help of the respiratory mucosa, which lines the walls of the nasal cavity. Within this mucosa, small, hair-like cilia move in a wave-like motion, moving mucus to the back of the throat. Dust, bacteria, and other chemicals get trapped in the mucus and are denatured by stomach acid once the mucus is swallowed (Stoddart and Whitfield, 1984)

Airflow through the nose is actually fast (250mL/sec) and fairly turbulent. Because of this, only a small fraction of the air we breathe is able to enter to the olfactory cleft. When we breathe deeply, of sniff, the speed of the air in the nostrils increases, and more of the air comes in contact with the olfactory epithelium When we have a cold, the production of mucus increases and this may obstruct the narrow openings to the olfactory tissue, making it more difficult to smell. (Stoddart & Whitfield, 1984)

Much is still unknown concerning the nasal cavity. For instance, the shape of the nasal cavity varies from person to person. For some unknown reason, the exit space from the back of the nasal cavity is larger in women than in men. Also, the olfactory membrane is pigmented. No one knows why, but we do know that the pigmentation does play a role in olfaction, because albino animals lack a sense of smell. (Stoddart &Whitfield, 1984)

 

Olfactory Membrane

The olfactory membrane, or epithelium, is a layer of cells on the roof of the nasal cavity. It is made up of three layers of cells: the supporting cells, the olfactory receptor cells, and the basal cells. Supporting cells are similar to glial cells and help produce mucus. Basal cells are the source of new receptors. Olfactory receptor cells, actually neurons, are the site of transduction. Olfactory receptor cells are the only neurons in the nervous system that are regularly replaced (they last about 4-8 weeks).

The size of the epithelium is a good indicator of the acuity of an animal's sense of smell. For example, the surface area of a human epithelium is 10 cm, whereas the surface area of a dog's epithelium is 170cm^2. Dogs also have 100 times more receptors per square centimeter than humans, so, needless to say, dogs have a much better sense of smell! (Bear, Connors, and Paradiso, 1996)

When we inhale, most of the chemical stimuli in the air are dissolved in the layer of mucus coating the epithelium. This mucus contains water, mucopolysaccharides, antibodies, enzymes, salts, and odorant-binding proteins. Olfactory receptor cells are bipolar, meaning that they have one dendrite and one axon. The dendrite ends in a knob, from which several small cilia dangle. So, when the chemical stimuli, or odorants, dissolve in the mucus on the epithelium, they bind specific receptor proteins on these cilia. This begins the transduction process. (Bear et al., 1996)

The process begins when a G-protein is stimulated. This triggers the activation of Adenylyl cyclase, the enzyme that speeds up the conversion of ATP to cAMP, a second messenger. Cyclic adenosine monophosphate, or cAMP, then binds to action channels in the membrane of the cilia. This causes the channels to open and Ca++ to enter the cilia. The influx of Ca++ activates chloride channels to open and Cl- leaves through these channels. This causes membrane charge to become more positive, or depolarize, and an action potential is created. The action potential travels down the axon of the olfactory receptor cell, eventually meets with the axons of the other olfactory receptor cells to form the olfactory nerve (Cranial Nerve 1). (Bear et al., 1996)

(Bear et al., 1996)

In order for the axons of the receptor cell to form the olfactory nerve, they must pass through the cribiform plate, a thin sheet of bone. The olfactory bulb lies on the cribiform plate, and the once the axons have reached it, they make their first synapse in the glomeruli, within the bulb. There are many more receptor cells than glomeruli, so about 10 million receptor cells converge on 2,000 glomeruli. From each glomerulus, there are two sets of output cells. The first set is a group of twenty-four "tufted" cells that cross to the other olfactory lobe. The second set, also of twenty-four, travel on to the brain. Each cell in a group of twenty-four can either be "on" or "off" and the number of combinations of "ons" and "offs" is approximately 16 million. Humans have the ability to smell approximately 16 million different smells!! (Stoddart & Whitfield, 1984)

Within the olfactory bulb, the axons of the olfactory receptor cells branch and synapse on second order neurons with spherical glomeruli, or cluster of neurons. These secondary neurons then merge to form the olfactory tract and travel further into the brain. (Bear et al., 1996) The olfactory tract travels first to the "primitive regions" of the cortex, and then moves onto the thalamus and neocortex, where the information is interpreted. What is interesting about this is that sight and hearing are processed by a relay center in the cerebral hemisphere before reaching most areas of the brain. Smell, on the other hand, has a direct route to many parts of the brain. This is because the sense of smell is evolutionarily older than sight or hearing. Sight and hearing are closely connected with higher functioning, whereas smell is associated with emotion and sexual behavior, as is discussed in more detail on this web site. (Stoddart & Whitfield, 1984)

 

 

(Bear et al., 1996)


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