Olfactory receptors Each odor molecule activates a particular combination of olfactory receptors. The activated receptor cells send impulses up through nerve axons to the olfactory bulb for processing.
Inside the brain Signals then travel along the olfactory tract to the olfactory cortex. The cortex is located in the limbic system, which is responsible for emotions and memory. Signals are also sent to the amygdala and orbitofrontal cortex. Smell enters the nose Odor molecules are drawn up through the nose and warmed to enhance the scent. The molecules dissolve in mucus produced by the olfactory epithelium and stimulate cilia that are connected to receptor cells.
1 2
3 Receptor cellSupporting cell
Nerve axon Mucus gland Odor molecule dissolving in mucus
Olfactory bulb Airborne odor molecules enter nostril
Olfactory bulb processes signals before passing to olfactory cortex Orbitofrontal cortex involved in decision- making and emotions as well as processing smells Cilia
SA NA AV L C ITY
Bone Mucus
Dura mater OLFACTORY BULBAMYGDALAOLFACTORY CORTEX
ORBITOFRONTAL CORTEX
FA OL OR CT EP Y H IT
MIUEL
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BRAIN FUNCTIONS AND THE SENSES
Smell 78 79
S me ll Id en ti fy in g a s m el l o ut o f t h e m an y o d or s i n t h e w or ld ar ou n d u s i nv ol ve s t h e o lf ac to ry s ys te m , w h ic h i so la te s d if fe re nt c h emi ca ls a n d t h en p as se s s ig n al s o n t o t h e b ra in t o d et er m in e w h et h er t h ey a re “ g oo d” o r “ b ad .” Dimethyl sulphide (DMS) is a very smelly compound. A whiff of the raw chemical can make you wonder whether something is rotting or if a pungent cheese is in the room. However, flavor chemists find it useful in creating all sorts of tastes. It is used in meat, seafood, milk, egg, wine, beer, vegetable, and fruit flavorings, usually at minuscule concentrations.
SM EL LY O R S W EE T?
W h at m a k es a s m el l? How we identify smells is still a matter of debate. Research suggests that most odors fall into 10 groups—or primary odors—each of which alerts us to something in the environment. Most smells are made up of a combination of these groups. Smell is a key part of survival, telling us whether something is safe or dangerous. Fragrant Light, natural scents such as flowers, grasses, and herbs, typically used in perfumery.
Sweet Warm, rich, sugary smells with a touch of creaminess, including chocolate, malt, and vanilla. Fruity Typically includes warm, ripe fruits and other fresh scents that have a sense of smoothness on the nose.
Minty Cool, fresh, and invigorating, epitomized by mint, eucalyptus, and camphor. Citrus Separate from other fruits, citrus has fresh, clean, acidic aromas with a touch of sweetness.
Toasted and nutty Slightly burned and caramelized with warm and fatty overtones, such as popcorn and peanut butter. Woody and resinous Earthy, natural smells, such as compost, fungi, spices, cedar, pine, and mold.
Pungent Often unpleasant smells such as manure or sour milk, also onions, garlic, and pickles. Chemical Includes synthetic, medicinal, solvent, and gasoline odors that are easily identifiable.
Decayed Beyond pungent are the odors of rotting food, sewage, household gas, and other “sickening” substances.
WHY DO SMELLS TRIGGER MEMORIES? U n lik e o ur o th er s en se s, s m el ls b yp as s t h e t h al am u s a nd g o st ra ig h t to t h e l im b ic s ys te m . Em o ti on s a nd m em or ie s a re p ro ce ss ed a nd s tor ed h er e, esp eci al ly in th e am yg da la .
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Taste
Fueling the body requires the intake of nourishing foods and liquids. Choosing what is safe to eat is largely influenced by our senses of taste and smell.
The five basic tastes
Taste is an evolutionary adaptation for survival.
Being able to determine whether something is nutritious or potentially poisonous before taking it into the body is enormously important.
So far, only five basic tastes have been discovered, although there may be others.
Picking up taste
Taste is actually a limited sense; there are only five basic tastes that can be detected (see right). Like smell, taste is a chemosense.
Chemical substances in food are picked up by the taste buds, which are mainly found on the tongue. Receptor cells, housed in structures called microvilli within the taste bud, detect these chemicals and send signals to the brain for processing.
Tongue
The tongue is a strong, flexible muscle. It is used to push food around the mouth and for speech. Its upper surface is covered in tiny projections called papillae. Most of the papillae are filiform, or threadlike, structures and contain no taste buds. They help grip and wear down food while it is being chewed.
1
Papillae
In addition to filiform papillae, the tongue has fungiform (mushroomlike), foliate (leaflike), and circumvallate (wall-like) papillae, which all contain taste buds. Most taste buds are found in the foliate papillae on the back and sides of the tongue.
2 Taste budsA taste bud is a collection of 50–100 cells that are clustered together like segments in an orange. They are located in the walls of papillae. One end of each cell protrudes out of the bud, where it gets washed with saliva containing food molecules.
3 Taste bud cells
When food molecules hit the cells, they interact with either receptor proteins or porelike proteins called ion channels. This causes electrical changes in the cell, which prompt neurons at the base of the cell to send signals to the brain.
4
Sweet
Signals the presence of carbohydrates, which are sources of vital sugars.
Salty
Detects chemical salts and minerals that are needed by the body.
Sour
Warns against foods that may be unripe or going bad.
Bitter
Poisons and other toxins are often bitter or unpalatable.
Umami
Detects glutamate salts and amino acids, which are found in meat, cheese, and other aged or fermented foods.
Microvilli contain receptor proteins, which bind with chemicals in food
Gustatory receptor cell Nerve
fiber Taste
pore Circumvallate papilla Surface of
tongue
Filiform papilla
Food molecule
Supporting cell Taste
bud Neuron
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Taste and smell
Detecting flavors depends as much on the nose as on the taste buds. The nose picks up external odors from food (see pp.78–79), but this is increased significantly by food- particle odors carried up into the nasal cavity by expired air from the lungs (retronasal olfaction). Some smell receptors have also been found in the taste buds. The brain combines the information from the nose and taste buds to perceive all the different flavors in the food. These are not the only sensations that contribute to the taste experience—the
somatosensory cortex detects the texture and temperature of food, adding context to the flavor.
KEY
Taste signals Retronasal smell Expired air
WHY DON’T BABIES LIKE BITTER FOODS?
Babies have many more taste buds than adults so they
taste bitter foods more intensely. They instinctively
refuse foods that aren’t as sweet or fatty
as breast milk.
The taste pathway
Information from the taste buds travels to the brain via cranial nerves in the jaw and throat. Impulses travel up the brain stem to the thalamus and are forwarded to the taste regions of the frontal cortex and insula, a fold of cortex deep in the brain.