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Special senses

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Olfactory epithelium
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Smells may contain 100s of different odorants
Humans have ~400 "smell" genes active in nose
Each encodes a unique receptor protein
Protein responds to one or more odors
Each odor binds to several different receptors
Each receptor has one type of receptor protein
Pain and temperature receptors are also in nasal cavities
Respond to irritants, such as ammonia, or can "smell" hot or cold (chili peppers, menthol)
In order to smell substance, it must be volatile
Must be in gaseous state
Odorant must also be able to dissolve in olfactory epithelium fluid
Activation of olfactory sensory neurons
Dissolved odorants bind to receptor proteins in olfactory cilium membranes
Open cation channels, generating receptor potential
At threshold, AP is conducted to first relay station in olfactory bulb
Anosmias: olfactory disorders; most result from:
Head injuries that tear olfactory nerves
Aftereffects of nasal cavity inflammation
Neurological disorders, such as Parkinson's disease
Olfactory hallucinations
Usually caused by temporal lobe epilepsy that involves olfactory cortex
Some people have olfactory auras prior to epileptic seizures
Taste buds: sensory organs for taste

Most of 10,000 taste buds are located on tongue in papillae, peglike projections of tongue mucosa

Fungiform papillae: tops of these mushroom-shaped structures house most taste buds; scattered across tongue

Foliate papillae: on side walls of tongue

Vallate papillae: largest taste buds with 8-12 forming "V" at back of tongue
Few on soft palate, cheeks, pharynx, epiglottis
Image: Location and Structure of Taste Buds
Each taste bud consists of 50-100 flask-shaped epithelial cells of two types:

Gustatory epithelial cells: taste receptor cells have microvilli called gustatory hairs that project into taste pores, bathed in saliva

Sensory dendrites coiled around gustatory epithelial cells send taste signals to brain

Three types of gustatory epithelial cells

Basal epithelial cells: dynamic stem cells that divide every 7-10 days
Sweet—sugars, saccharin, alcohol, some amino acids, some lead salts

Sour—hydrogen ions in solution

Salty—metal ions (inorganic salts); sodium chloride tastes saltiest
Perception of salty flavor can be adjusted over time - persons on low Na diet have an increased sensitivity to salty foods

Bitter—alkaloids such as quinine and nicotine, caffeine, and nonalkaloids such as aspirin
Unable to distinguish between many different bitter flavors

Umami—amino acids glutamate and aspartate; example: beef (meat) or cheese taste, and monosodium glutamate
Binding of food chemical (tastant) depolarizes cell membrane of gustatory epithelial cell membrane, causing release of neurotransmitter

Neurotransmitter binds to dendrite of sensory neuron and initiates a generator potential that lead to action potentials

Different gustatory cells have different thresholds for activation

Bitter receptors are most sensitive
All adapt in 3-5 seconds, with complete adaptation in 1-5 minutes