Taste and Smell

How many things can we taste, how many things do

we smell?

4 tastes

Research has demonstrated that there are only four basic taste sensations: salty, sour, bitter, and sweet. Further, each of these sensations is associated with a different area of the tongue.

To demonstrate some or all of the four basic taste sensations• Take some toothpicks, and dip them in sugar water,

lemon juice, Angostura bitters and salt water • Then touch each toothpick to the various parts of

your tongue identified in the diagram – sugar water will taste the sweetest on the tip of

your tongue– lemon juice should taste the sourest on the sides

of your tongue– salt water should taste the saltiest just in back of

the tip of your tongue – Angostura bitters you can test the area at the back

of your tongue for its ability to detect bitter tastes

What is “Bitter?”

• Some people are much more able to taste bitter sensations. Those who dislike diet soft drinks usually say they do not like them because they leave a bitter aftertaste.

• Some people are not able to taste that bitter sensation.

• Also, most poisons are bitter. So, it may make evolutionary sense for people to dislike bitter tastes.

• In fact, growers have developed strains of celery that are not bitter for that reason.

Senses of Smell and Taste

• taste and smell represent phylogenetically old sensory system

• they are the sensory systems that let us detect and discriminate the molecules in our environment

• these senses help to link the external environment with internal needs, e.g. hunger, thirst, etc.

• are vivid emotionally and perceptually   • important nutritionally for regulating food

intake  

Taste (gustatory) -- transduction of soluble chemicals

Taste is sensed by gustatory receptor cells (taste buds) on the tongue, palate, and pharynx, with some on the epiglottis and upper esophagus

Different regions of the tongue are more sensitive to either bitter, sour, salty, or sweet.

Where’s the taste?

• sweet - associated with organic molecules sweetest compounds are the proteins, thaumatin and monellin can be detected at concentration of 10-8 M

• bitter - often associated with harmful stimuli not know which chemical structures elicit the taste

• sourness - acids elicit sour taste • saltiness - a molecule that breaks into

its ions, the ions pass through a channel in the cell membrane

drawing of taste bud

Microscopic view of taste bud

• each taste bud contains 50-150 receptors (S and N), basal cells (B), and support cells

• the taste bud contacts substances through the taste pore (arrow at top of image)

taste buds (TB) are found on papillae

How are the different tastes encoded?

• different receptors respond preferentially to 1 of the 4 basic tastes

• taste receptors are differentially distributed across the tongue • specific pathway theory -- nerve fiber responds to only one taste

(labeled-line) • across-fiber pattern coding -- central neuron compares pattern

of input from a group of fibers, each fiber is more sensitive to a particular taste but also responds to other taste stimuli

• probably both theories are correct • taste coding involves comparison of activity across groups of

fibers responding preferentially to different tastes (contrast)

Smell

Smell

•some at concentrations of only a few parts per trillion      •odors enter the nasal cavity •olfactory binding protein attaches to the odorant molecule •odorants cause olfactory neurons to depolarize •Chemically, in order for a molecule to possess a scent, it must have at least the following two qualities:

•It must be somewhat volatile so that it can reach the nose •It must interact with the protein receptors located in the nerve endings.

Smell

The process of smellWhen an animal inhales odorous molecules, these

molecules bind to specialized proteins, known as receptor proteins, that are found in the nerve endings in the olfactory epithelium.

Binding of odors to these receptors initiates an electrical signal that travels along the axons to the olfactory bulb, which is located in the front of the brain, right behind the nose.

The olfactory bulb serves as the first relay station for processing olfactory information in the brain; the bulb connects the nose with the olfactory cortex, which then projects to higher sensory centers in the cerebral cortex, the area of the brain the controls thoughts and behaviors.

Smelly facts…• fruity, floral, and herbaceous most potent

scents • putrid stimuli are much weaker scents • individual olfactory neurons respond to

multiple odorants • seems likely that individual neurons

respond to only a few distinct odorant molecules

• the sensation of smell rapidly adapts• repeated presentation of an odorant

results in reduced sensitivity • sensitivity to other odors can be effected• smell is more sensitive than taste

• Humans can discriminate 5% change in the intensity of an odorant

• Taste Preferences -- Learned and Innate– Chocolate Lover or Broccoli Hater? Both genes

and experience effect which tastes we prefer. – intake of food greatly effected by the smell and

taste of the food

• We have developed extensive nomenclature to describe foods (e.g. coffee tasting terminology or wine tasting terminology) that derives from the smells, flavors, and other sensations associated with the food

• When deprived of salt we will seek out foods high in salt to compensate

Some of what we have learned…

• obvious evolutionary advantage to not eat a particular food that makes you sick

• taste and smell allow us to perceive chemicals in our environment

• taste and smell sensations seem to be coded following the same general rules used by other sensory systems

Straight shot to the cerebral cortex

Note that the olfactory system is the only sense system that does not synapse through the thalamus. Rather it is connected directly to the cortex.

Your House Stinks!

• When you return to your house after a long absence, it always seems to smell worse than usual. Well, it always smells that way.

• It is just the phenomenon of sensory fatigue. As you live in your house, you no longer can perceive the smells around you. But, while on a trip, your nose can recover. Then when you return, you smell your house like everyone else smells it.