Retronasal olfactory sensation
The sensation of an odor that is perceived when chewing and swallowing force an odorant in the mouth up behind the palate into the nose
- Such odor sensations are perceived as originating from the mouth, even though the actual contact of odorant and receptor occurs at the olfactory mucosa
Flavor
The combination of true taste (sweet, salty, sour, bitter) and retronasal olfaction
Patient case
Damaged taste, but normal olfaction —could smell lasagna, but had no flavor
Similar effect created in lab: Chorda tympani anesthetized with lidocaine
Chorda tympani
The branch of cranial nerve VII (the facial nerve) that carries taste information from the anterior, mobile tongue (the part you can stick out)
Brain imaging studies
Brain processes odors differently, depending on whether they come from nose or mouth
Food industry: Adds sugar to intensify sensation of fruit juice § Increase in sweetness (a pure taste sensation) increases perceived olfactory sensation of fruit
Taste buds
- Create neural signals conveyed to brain by taste
nerves - Embedded in structures: Papillae (bumps on
tongue) - Each taste bud contains taste receptor cells
- Information is sent to brain via cranial nerves
Four kinds of papillae
- Filiform papillae: Small structures on the tongue
that = the bumpy appearance. Have
no taste function
- Filiform papillae: Small structures on the tongue
- Fungiform papillae: Mushroom-shaped
structures (maximum diameter 1 mm) that are
distributed most densely on the edges of the
tongue, especially the tip. An average of six taste
buds per papilla are buried in the surface
- Fungiform papillae: Mushroom-shaped
- Foliate papillae: Folds of tissue containing taste
buds. Located on the rear of the tongue lateral to
the circumvallate papillae, where the tongue
attaches to the mouth
- Foliate papillae: Folds of tissue containing taste
- Circumvallate papillae: Circular structures that
form an inverted V on the rear of the tongue (three
to five on each side). Moundlike structures
surrounded by a trench. Much larger than
fungiform papillae
- Circumvallate papillae: Circular structures that
Microvilli
Slender projections on the tips of some
taste bud cells that extend into the taste pore
Contain the sites that bind to taste substances
Not tiny hairs (as the name implies): We now
know they are extensions of the cell membrane
Tastant
Any stimulus that can be tasted
Tastants can be divided into two large categories:
Some are made up of small, charged particles that
taste salty or sour
Small ion channels in microvilli membranes
allow some types of charged particles to enter
but not others
Pathway
Taste buds to cranial nerves to medulla
and thalamus and then on to cortex
Insular cortex
Primary cortical processing area for
taste. The part of the cortex that first receives taste
information
Orbitofrontal cortex
The part of the frontal lobe of
the cortex that lies above the bone (orbit)
containing the eyes
Receives projections from insular cortex
Involved in processing of temperature, touch,
smell, and taste, suggesting it may be an
integration area
Inhibition
Plays an important role in processing taste
information in the brain.
Function
To protect our whole mouth perception of
taste when we have injuries to taste system.
Descending inhibition from taste cortex blocks pain
perception
* Has survival value because we need to eat even if
our mouth has been injured
Four basic tastes
- Salty
- Sour
- Bitter
- Sweet
Salty:
- Salt is made up of two charged particles: Cation
and anion. - Ability to perceive salt is not static
§ Low-sodium diets will increase in intensity of
salty foods over time - Liking for saltiness is not static
§ Early experiences can modify salt preference.
Chloride-deficiency in childhood leads to
increased preference for salty foods later
§ Gestational experiences may affect liking for
Saltiness
Sour:
- Comes from acidic substances
- At high concentrations, acids will damage both
external and internal body tissues
Bitter:
- Quinine: Prototypically bitter-tasting substance
- Cannot distinguish between tastes of different
bitter compounds - Many bitter substances are poisonous
Sweet:
- Evoked by sugars
- Many different sugars that taste sweet:
§ Glucose: Principle source of energy for most
animals
§ Fructose: Even sweeter than glucose
§ Sucrose: Common table sugar. Combination of
glucose and fructose
The special case of umami:
- Candidate for fifth basic taste
- Comes from monosodium glutamate (MSG)
- Glutamate: Important neurotransmitter
Survival value of taste
Taste is a system for detecting nutrients and
antinutrients
Bitter: Might signal poisons
Sour: Configured to detect acidic solutions that
might harm the body
Sweet and Salty: Our bodies need sodium and
sugar to survive
Different flavored foods placed on tips of infants’
tongues
- Sweet food evokes a “smilelike” expression
followed by sucking. - Sour produced pursing and protrusion of lips
- Bitter produced gaping, movements of spitting, and
sometimes vomiting movements
Specific hungers theory
The idea that deficiency of a
given nutrient produces craving (a specific hunger) for
that nutrient
only holds for sweet and salty foods
Labeled lines:
- Theory of taste coding in which each taste fiber
carries a particular taste quality
§ Major source of controversy in literature - Other possibility: Patterns of activity across many
different taste neurons
