The Senses

Fox, part of Chapter 10, figure from chapter 8


There are 5 "special" senses
touch (somesthesis) - already covered in previous outline
taste (gustation) - next (this outline)
smell (olfaction - after taste (this outline)
hearing (audition) - a big topic, next outline
sight (vision) - a huge topic, the outline after next

Taste (Gustation)

for chemicals dissolved in water
Gustation - chemicals - Many "flavors" are smell
Hanig (1901) - preferential localization:
sweet - tip of tongue
salt - front sides of tongue
sour - back sides of tongue
bitter - back middle of tongue

On tongue:
Papillae: Circumvallate (preference for quinine), foliate, fungiform (preference for sucrose)
also receptors in epiglottis
epiglottis via nerve X (vagus), circumvallate (9 of them) via IX (glossopharyngial), others via VII (facial)

Fig. 10.7
Several types of papilla including the circumvallate papillae on the back of the tongue, shown in this picture from our histology course
Within each papilla are numerous clusters of cells called taste buds shown in this histology picture. support cells and sensory cells

Note that there are taste "blindnesses" Ptc = phenylthiocarbamide, taster is dominant and nontaster
Use taste vs. non-taste to screen for G-protein coupled receptors (M. Barinaga, Family of bitter taste receptors found, Science 287, 2133-2135, 2000)

Fig. 10.8
generally, channel or G-protein linked receptor ultimately increasing calcium somehow for synapse
note receptor does not have axon

salt - Na+ channel opens (depolarization)

sour - pH sensitive channel closes (depolarization)

sweet - G-protein linked cAMP close K+ channel - depolarize

bitter -G-protein cascade

umami (glutamate) - and amino acids, channels as well as G-protein cascade

In each case, Ca2+ involved in vesicle release

Taste Projection
(much simpler than for olfaction)
Gustatory nucleus in medulla,
there to thalamus and then to sensory cortex, overlap to touch area - postcentral gyrus
also from medulla to hypothalamus

Smell- Olfaction

There are unusual primaries like aromatic and putrid , there may be many primaries, although mixtures give a single perception confounding the ability to define primaries
Relative to other senss, receptors difficult to stimulate
Even more than with the sense of touch, olfaction is related to motivational "affect"
The sense of smell is especially important in other animals (dogs)

Fig. 10.9
Anatomy of olfactory epithelium.
Note: the receptors are neurons with axons, unlike for taste
Receptors are ciliary, with cilia in mucus

Transduction - G protein coupled receptor via adenylyl cyclase
There is a specialized olfactory alpha subunit of the G protein (Golf)
Na+ - Ca2+ channel is like that of photoreceptor in that cAMP acts as a ligand to open the channel from inside the cell
Ca2+ opens Cl- channel
there is also a pathway involving PLC and IP3, but which is otherwise similar
in the background, there is a Na+/Ca++ exchanger

G-protein coupled receptor is very variable (there may be thousands, meaning that olfactory receptors contribute predominantly to diversity of G-protein-coupled receptors) and has specific variable regions
Axel and Buck won the 2004 Nobel prize (Physiology and medicine for this contribution)

Glomeruli - > Mitral cells -> lateral olfactory tract

Fig. 8.15
Olfaction is a complex sensory system in part because of the CNS projection to amygdala, hypothalamus, hippocampus) , areas called Limbic system


There are senses outside the 5 special senses

Vestibular sense

Fig. 10.13
note proximity with cochlea (for hearing)
utricle and sacculus linear motions
3 semicircular canals - rotations

Fig. 10.15
stones (otoconia) provide mass for bending in utricle and sacculus

Fig. 10.16
cupula displaced as semicircular canal fluid is displaced


hair cells (also for audition) [mechanoreceptor]
kinocilium (real cilium)
plus about 30 stereoocilia
mechanoreception assisted by tip links - depolarization if move toward kinocilium
hyperpolarize if in opposite direction

Test questions from 2004 - 2011 relating to this outline

Name (or give the number for) one of nerves that connect the taste receptors to the brain.

vagus 10, glossopharyngial 9, mostly facial 7

A gustatory receptor sensitive to quinine has a G protein-coupled receptor; the cascade results in calcium ions being released from an endoplasmic reticulum. Why would an increase in cytoplasmic calcium ions be useful in gustatory receptors?

for release of synaptic transmitter vesicles

Monosodium glutamate affects which taste receptor primary?


The diagram of axons from olfactory receptors does not show connection to the closest glomerulus. What characteristic of the olfactory receptor determines which glomerulus it connects to?

ones that respond to the same primary pool to the same glomerulus

Approximately how many G protein coupled receptors are there for human olfaction?

lots, 500-1000

Gustatory receptors connect to cranial nerves that project to the brain. Name one of the three places in this projection pathway.

medulla, thalamus, postcentral gyrus

What is the cause of the difference among the students in the physiology class as to whether they could taste PTC?

genetic, non-tasters are homozygous recessive

When a G protein coupled cascade in an olfactory receptor alters the cAMP level, what does this cAMP do?

gates a channel

Say something about what "hair" means with respect to hair cells in the vestibular or auditory systems.

real cilium=kinociliun and cilia-like stereocilia

Some taste cells depolarize in response to the appropriate chemical stimulation. What must happen, downstream of depolarization, for vesicles of neurotransmitter to be released?
Calcium ions must come in through calcium channels
Contrast how cAMP gates a cation channel in an olfactory receptor cell with the way acetylcholine gates the nicotinic receptor.
cAMP from inside the cell, Ach from outside, both channels are ligand gated

The receptor molecule for gustation is either (what?) or (what?).
a channel or a G protein coupled receptor
The chemoreceptive part of the olfactory cell is in the nasal cavity. Where is the synapse?
In the olfactory bulb (first cranial nerve) of the brain
In the vestibular system there are 3 fluid-filled (what) plus 2 organs with otoliths, the (what?) and the (what?). [Answer one of the above.]
semicircular canals, utricle & saccule

What type of molecule must come in many varieties to mediate the richness of olfaction you enjoy?

G protein linked receptor

Where is the first synapse in the olfactory system?

olfactory bulb

What is detected when endolymph bends the cupula?

head rotation

What does the term "umami" refer to?

a taste, glutamate

Which sensory system projects to the limbic system, including the amygdala?


After a bitter tastant causes Ca2+ to increase in the cell, what does that Ca2+ do?

cause transmitter release

Out of the 5 special senses, which one does not have a localized area of the cerebral cortex as its final projection?


About how many different G protein coupled receptors are involved in human olfaction?


In addition to the vestibular sense, which utilizes hair cells where "hair" refers to stereocilia?


All taste cell types, by one mechanism or another, have an influx of Ca2+. What process does this increased cytoplasmic calcium mediate?

synaptic vesicle release

A portion of the brain hypothesized to be involved in olfaction, emotion, and memory.

limbic system (or any part of it)

What is PTC (phenylthiocarbamide) and what did it reveal about sensory transduction?

a substance that tastes bitter and helped in the isolation of the G protein-coupled receptor

How is the richness of olfactory experience coded in the genes for olfactory receptor molecules?

a different gene for each G protein-coupled receptor, very variable

Upon stimulation, for each taste primary, cytoplasmic Ca2+ increases. What is it used for?

for release of transmitter vesicles

The ciliary receptor cells in the nasal epithelium have axons that terminate (where)?

olfactory bulb (first cranial "nerve")

What brain center involved in motivational aspects of hunger does the olfactory bulb project to?


There are three nerves that carry taste information to the brain. These three nerves are among a famous set of about a dozen that are collectively referred to as (what)?

cranial nerves

In olfactory transduction, what does cAMP do to affect the electrical properties of the receptor cell membrane?

cAMP is the ligand that gates the cation channel from inside the cell

Where are the sense organs that monitor your head position to keep your eyes upright for slight tilting of the head?

near the cochlea (for hearing)

In what way does the tastant for umami relate to a central nervous system excitatory neurotransmitter?


About how many olfactory receptor molecules does a human have?


In addition to G protein-coupled receptors, what is the other type of taste receptor molecule?


In taste receptors, Ca2+, mediating transmitter release, comes from either outside the cell or (what subcellular structure?)?

endoplasmic reticulum

Where do olfactory neurons make their synapses?

olfactory bulb

What are the "hairs" of hair cells, and what happens when they are bent?

cilia (stereocilia), open or close channel, depolarize or hyperpolarize

In what way are calcium ions essential for the signaling of the taste receptor cells to the cranial nerves?

mediate exocytosis of vesicles

In some cases, taste receptor molecules are channels. Alternatively they are (what?).

G protein coupled receptors

Why do olfactory axons seem to zig-zag rather than just connect to the closest glomerulus? (That pertained to yellow and green receptors, as colorized in your figure, connecting to yellow and green glomeruli.)

presumably, each glomerulus receives only from receptors expressing the same receptor

In the G protein-coupled receptor cascade in the olfactory cell, how did the cAMP affect the cell's response?

ligand close K+ channel

Describe the receptor cells of the vestibular system with respect to their (1) structure, or (2) the type of stimulus that excites them.

(1) "hair" cells with stereocilia, mechanical

On the back of the tongue, taste buds are found on what larger structure (hint, nine of them, and the nerve projects by the IX cranial nerve, the glossopharyngeal).

Circumvallate papillae

65. In terms of the respective molecular biology of transduction, why is olfaction a much richer sense than taste in humans?

There are maybe a thousand receptors for different primaries

Where is the first synapse in the sense of taste?

in the taste receptor cell

In olfactory transduction, what does cAMP do?

ligand for channel

By looking in the genome at G protein coupled receptors, what was isolated when responders vs. nonresponders to PTC (phenylthiocarbamide) were compared?

bitter taste receptor

For the fifth taste primary (other than sweet, sour, salt and bitter), what chemical(s) stimulate it?

glutamate, amino acids

Where is the first synapse in the sense of smell?

olfactory bulb

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