OLFACTION AND GUSTATION

What is Gustation? Taste , synonyms: smatch or gustation It is one of the five senses. It refers to the ability to detect the flavor of either beneficial (food, nutrition); or detrimental (poisons) chemical substances Sweetness helps to identify energy-rich foods, while bitterness serves as a warning sign of poisons.

Taste Buds We perceive tastes through taste cells embedded in taste buds which are embedded in papillae on tongue.

Where are these buds? There are approximately 100,000 taste buds They are located on the back and front of the tongue on the roof, sides and back of the mouth in the throat.

Tongue Papilla contains hundreds of taste buds Taste buds contain taste 50 – 150 taste cells (they’re not neurons) that have microvilli The microvilli project out of the pores in the papillae Chemicals binds to the receptors on the microvilli of the taste cells. This information is relayed to the axons of gustatory neurons with which the taste cells synape

How does taste get into the taste cells? Taste chemicals in food can either pass directly into the taste cells via different channels or bind to surface receptors on the microvilli The taste cells release neurotransmitters into the synapses between themselves and the axons of the gustatory neurons This triggers action potentials in the axons and the info travels to the brain via cranial nerves VII, IX, X

Information is relayed to the thalamus Information then goes to the primary gustatory cortex (parietal lobe)

TONGUE MAP MYTH the 'tongue map' concept which says we have different areas of tongue that senses different tastes, is utterly a false idea This century old misconception comes from the research paper of a German scientist D. P. Hanig published in 1901. in 1974, Virginia Collins found that there are variations in sensitivity to the four basic tastes, but it is too small to have any practical significance. Collins found that all tastes can be detected anywhere there are taste receptors—around the tongue, on the soft palate at back roof of the mouth, and even in the epiglottis, the flap that blocks food from the windpipe. 

How many tastes? The sensation of taste can be categorized into five basic tastes: sweet, bitter, sour, salty, and umami.

Basic Tastes Taste distribution Most of the tongue is receptive to all basic tastes But these regions are most sensitive to a given taste a. Bitter across the back b. Sour on side closest to the back c. Salty on side more rostral than sour d. Sweet across front

Umami? “Umami” is the Japanese word for “meaty” or “savory”. Therefore, it is the taste found in many oriental dishes. The amino acid glutamate produces a strong umami taste. MSG (mono sodium glutamate) has the umami taste

What’s smell got to do with it? Nasal passages are located in a way that odors can enter through the nose or pharynx and contribute to the perception of flavor

SMELLY STUFF There are as many as 100,000 unique odors that we can discriminate 80% of which are noxious Odors perceived to be noxious are often deleterious (rotting meat, etc.).

Organ of smell The nose is NOT an organ of smell The Olfactory epithelium, located on a thin sheet of cells high up in our nasal cavity is the actual “organ” of smell

The Neuroepithelium Made up of ciliated pseudostratified epithelial cells Goblet cells and mucous glands Olfactory receptors, which are neurons

How good is your sense of smell? The size of the olfactory epithelium is proportionate to olfactory acuity Man has 10 cm2 of olfactory epithelium Dog has 170 cm2 of olfactory epithelium Dogs also have 100x as many receptors per cm2 of olfactory epithelium

Unlike photoreceptors, cochlear hair cells and taste cells, olfactory receptors are actually neurons! They fire action potentials They are the only neurons in the nervous system that are replaced regularly throughout life (Every 4-8 weeks)

Path of Olfaction Odor receptors detect chemicals in air Their axons carry signals towards a fan-like structure, the glomerulus Collectively, these glomeruli compose the olfactory bulb, a brain structure responsible for transmitting scents from odor receptors in the nose to the processing regions of the brain. The olfactory bulb transfers incoming signals from odor receptors into outgoing signals through neurons called mitral cells, that carry odor information up to the cortex via cranial nerve I. In addition to the mitral cells, the olfactory bulb is packed with interneurons and modulatory neurons such as tufted cells, granule cells. There are also glial cells present.

I ThinkSnot! There are mucus secreting cells at the ends of the olfactory receptors This mucus contains cells of the immune system and is shed every ten minutes Individuals with an infection (cold, flu, etc.) one symptom is a runny nose Mucus is shed more frequently to protect the olfactory receptors from infection

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