Intro | Olfactory Nerve Axons | Olfactory Bulb | Olfactory Cilia | Olfactory Nerve
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
Olfactory Cilia are located along the upper surface of the inside of the nasal passages. These hair-like receptor cells respond to chemical stimuli that have dissolved in the nasal mucus. Olfactory cilia are constantly replaced, an ability not characteristic of the oth er sensory receptors.
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Any volatile substance capable of stimulating olfactory cilia will have a molecular weight somewhere in the range of 15 to 300. Most odorous substances are lipid soluble and organic in origin. Greater than 50 million olfactory cilia are found in one of two patches of tissue, each approximately 1 square inch in area. Researchers estimate that there are over 100 different receptors (in the form of proteins) represented in the olfactory cilia that lead to the recognition of up to ten thousand different odors. These receptive proteins transverse the cell membrane seven times, similar to the receptive proteins responding to neurotransmitters. As mentioned previously, olfactory receptor cells are replaced after a life cycle of approximately 60 days. Less than 10 % of the air that enters the nasal passage will reach these olfactory receptors. An active sniff is usually necessary to inhale air high enough to reach them. Approximately 300 milliseconds are needed for any aromatic compound to dissolve in the mucus of the receptive patches. Thus, the response time to olfactory stimulation is slow relative to the other sensory systems.
Olfactory receptor cells are bipolar cells whose cell bodies lie deep within the mucus lining (or epithelium) of the nose. One appendage, reaches from the cell body to the surface of the mucus where is branches into an array of 10-20 cilia. The cilia must reach the mucus environment. This is where odorous molecules must dissolve before they are capable of stimulating the receptors. The second receptor cell appendage, the olfactory nerve axon, carries information away from the cell body deep in the nasal mucus lining to the mitral cells (named for the bishop's miter, which they resemble) located in the olfactory bulbs. Axons leaving the mitral cells in the olfactory bulbs collect to form the olfactory nerve or tract.
On a cellular level, the aromatic molecules produce potentials on the cilia by interacting with the receptor proteins. They couple to a special G protein, called Golf. This protein induces the synthesis of cyclic AMP. The cyclic AMP in turn opens the sodium channels of the membrane and causes depolarization.