Lecture 7: Chemical Senses Flashcards
Outline the neural pathway of olfaction.
Odorant –> Olfactory epithelium
—> olfactory receptor neurons (ORNs) which project, via olfactory nerve, to…
Olfactory bulb (glomeruli) where ORN synapse with mitral cells
mitral cell axons form the lateral olfactory tract that projects to Pyriform cortex
Describe the pyriform cortex
Pyriform cortex
Primary cortical target of olfactory bulb/olfactory tract (in the temporal lobe)
“Archicortex”, meaning evolutionarily older than other parts of the cortex (3 layers rather than 6)
Pyriform cortex outputs (pyramidal cells) travel to the thalamus, which then projects to association cortex regions
How is odorant perception in humans different than rats and dogs?
We have the least ORCs and therefore the worst sense of smell.
What is the structure of the olfactory epithelium?
Olfactory Receptor Neurons (ORNs)
Receptor cells with olfactory cilia (not true cilia)
Contain chemoreceptors on cilia.
ORNs project through the cribriform plate to the olfactory bulb.
ORNs die after several days are continually replenished by basal cells, which are stem cells similar to those observed in the developing nervous systems.
What is the function of the olfactory epithelium?
Cilia of ORNs detect odorants.
Effectively increase the surface area of the odorant-sensing region of the neuron, increasing sensitivity.
The olfactory cilia possess the chemoreceptors that produce the odorant receptor potential (this is a depolarizing potential, the figure above is showing the inward cation current).
What is the structure and function of the odorant receptor of the ORN?
Actual odorant receptors are 7-transmembrane G protein-coupled receptors (Part of family that includes β-adrenergic receptors, muscarinic receptors & rhodopsin/opsin).
Although there are a large number of odorant receptor genes, but only a fraction are actually expressed.
Each receptor responds to a specific (& probably limited) set of odorants.
There appears to be a bilateral spatial arrangement of receptor expression.
What is currently known about the neural distribution of distinct ORNs that pick up certain odorants?
In (A) and (B), green label is for olfactory marker protein (OMP) that labels all olfactory neurons. The red label in (A) is for adenylate cyclase III & is restricted to the olfactory cilia.
In (C) and (D) the green marker is for specific olfactory receptor types; note the distinct distribution.
Describe the molecular mechanism for odorant transduction.
Odorant binds to receptor which activates a G-protein only found in olfactory receptor cells.
This activates adenylate cyclase III, which then activates a cyclic nucletide-gated channel.
The resulting Ca2+/Na+ influx depolarizes the cilia. This depolarization is boosted by a Cl- efflux mediated by a Ca2+-gated Cl-
channel.
The Na+/Ca2+ exchanger pumps Ca2+ out so that intracellular Ca2+ levels are returned to a level that does not activate the cyclic nucleotide gated channels.
The depolarization in the cilia propagates passively to the axon (see slide 4) where it initiates a Na +-mediated action potential the travels to the olfactory bulb.
How do different ORNs respond to different odorants? (e.g. do they all respond the same?)
Even though we don’t know *why* the ORNs do what they do, we can clearly see a sort of labeled-line encoding.
Some neurons respond to one smell, others to a few smells, and then some respond to lots of smells.
How is the olfactory bulb organized?
into glomeruli, which are balls of synapses between ORNs and mitral cells of the Olfactory Bulb
Note: 25,000 ORNs provide input in a glomeruli, while 25 mitral cells receive it.
Describe the glomeruli in the olfactory bulb.
In the olfactory bulb are glomeruli, spherical structures where ORN axons form excitatory synaptic connections with dendrites from mitral cells (principal projection neurons).
Each glomeruli contains dendrites from ≈ 25 mitral cells and receives input from ≈ 25,000 ORNs.
Each glomeruli receives input from ORNs expressing the same single olfactory receptor.
Maximizes sensitivity, signal fidelity & signal/ noise ratio.
Glomeruli also contain dendrites from tufted cells and periglomerular cells.
What is the role of granule cells in the olfactory bulb?
Granule cells make dendro-dendritic connections with mitral cells & form a lateral inhibitory network; contribute to both odorant processing and synaptic plasticity.
remember mitral cells are what synapse with the ORNs
What is observed when we attempt to map responses of glomeruli to chemically-distinct odorants?
Odorants elicit a response in 1 or more characteristic glomeruli.
Increasing odorant concentration leads to increased level of glomeruli activation and increased number of activated glomeruli.
However, the response profile of glomeruli is not necessarily unique for an individual odorant.
Also, complex odors that have many separate components only activate a relatively small number of glomeruli that seem to represent key elements of a given odor.
So it’s unclear exactly how odorant identity is encoded
Part of the explanation may be that coding is sparse; little or no background activity and actual stimuli elicit only a few action potentials
There may be a temporal coding component on top of the spatial component.
What three cranial nerves are involved with taste? What area do they innervate?
VII- Facial- anterior portion of the tongue
IX - posterior portion of the tongue
X- innervates epiglottis, gets some taste info
All 3 of these send their information to the medulla
Taste cells synapse onto _____________ that project to the nucleus of the _________in the medulla.
primary afferents
solitary tract (NST)
How is the NST involved in satiation?
(first, NST is getting information about taste)
The NST also receives visceral afferent input via the autonomic nervous system.
This represents a region in which chemosensory information is integrated with visceral input/output.
Involved in satiation/apetite response, insulin release.
What reciprocal connections does the NST make?
There are also reciprocal
connections between NST with the
hypothalamus & amygdala (and
cortico-amygdala connections too).
Mediate subjective aspects of taste,
appetite, satiation, food-seeking
behavior.
Fungiform papillae
highest density
contain 25% of the taste buds
only 2-3 taste buds per papilla
Foliate papillae
only two bilateral pairs (posteriolateral surface)
contain 600 taste buds each (25%)
circumvallate papillae
9, each with about 250 taste buds (50%)
Taste buds are also present on what surfaces other than the tongue?
Soft palate, larynx, esophagus
total of 4000
Name the 5 distinct categories of taste perception
salt
sweet
sour
bitter
umami
Describe salt taste.
detection of dietary salts (e.g. NaCl)
and essential amino acids (needed for
protein synthesis)
Describe sweet taste.
detection of sugars and other
carbohydrates (energy source)
Describe sour taste
detection of acidity which can be
indicative of food palatability
Describe bitter taste. (two examples, know)
detection of compounds that may
indicate a food item is poisonous
(e.g. quinine, strychnine)
Describe umami taste.
savory (think Doritos); detection of
amino acids present in cooked
meats and other high-protein foods
Describe the spatial distribution of taste buds on the tongue.
There is a differential spatial distribution of tastes bids sensitive to the 5 categories of
tastants, with sweet/salty/umami concentrated in the rostral tongue and bitter/sour in more
caudal areas.
Priority to assess nutrient content first and than palatability.
Sweet/salty/umami elicit salivation, mouth movements & swallowing behaviors, insulin release
plus pleasurable sensations.
Sour elicits larger salivary response to dilute out the offending taste and motor behaviors
(grimace, puckering) that discourage continual intake of the food item.
Bitter elicits protrusion of the tongue, gagging, spitting to prevent ingestion
Name the thresholds for bitter, sour, salt, sweet, and umami tastants.
bitter tastants is quite low (8 μM quinine and 1 μM strychnine)
sour is much higher (2 mM for citric acid)
salt and sweet is even higher (10 mM NaCl and 20 mM sucrose)
umami is quite low (7 μM for glutamate
Describe taste bud anatomy.
Taste buds consists of the taste pore, taste cells,
basal cells and axons from primary taste afferents.
The basal cells are stem cells that replace the taste cells when they degenerate (2 weeks).
The apical region of the taste cells have microvilli in
which the taste receptors are localized.
Tastant binding to the receptors produces a
depolarizing receptor potential that spreads to the
basal region of the cell where it elicits
neurotransmitter release onto primary sensory
afferents.
What neurotransmitters are thought to be released by taste cells?
5HT and ATP
What two types of receptor mechanisms do taste receptors utilize?
Some of the taste receptors are ion channels.
Others are G-protein coupled receptors that activate
ion channels.
What is the receptor protein for salty things and how does it work?
amiloride-sensitive Na channel
Salt – simply Na+ influx through a voltageinsensitive
cation channel (amiloride
blocks ability to taste salt).
What is the receptor protein for sour things and how does it work?
“H+ sensitive TRP channel”
Sour – appears to be mediated by a protonpermeant
TRP channel.
What receptor subunit is shared by the sweet and umami taste cells?
T1R3
Both activate the same PLC–> IP3 –> TRPM channel pathway.
T1R2 is the distinct subunit of what tastant receptor?
sweet!
(then PLC IP3 to TRPM channel pathway
T1R1 is the distinct subunit of what tastant receptor?
umami (amino acids)
then PLC-IP3–> TRPM calcium channel opens.
TRPM channels are part of what chemoreceptor pathway? What do they do?
taste reception
they allow Ca ion influx, activated by IP3.
Describe the bitter tastant receptor protein.
Mediated by a single 7-transmembrane protein, T2R.
There are 30 T2R subtypes, each encoded by single gene, one of which mediates the ability to taste PTC.
Utilize same PLC–> IP3 –> TRPM channel pathway as sweet and umami
Briefly describe the salt, sour, sweet, umami, and bitter taste transduction via ion channels and G-protein coupled receptors (picture).
view slide 16/19 in lecture 16
How are different tastes distinguished?
If there is overlap in the sharing of signaling mechanisms, how are different tastes
distinguished?
Answer: Each taste cell expresses only one type of tastant receptor (see A on next slide).
This indicate a labeled-line coding for taste; a given taste elicits activity in a specific receptor
to CNS pathway. Segregation of specific taste pathways appears to be maintained all the
way to the insular cortex.
There is some degree of coding amount (concentration) and hedonic value by the number of action
potentials elicited in the primary afferents.
Describe an experiment that demonstrated the labeled line coding of the taste system.
That labeled-line coding mediated taste was shown in experiments in which the TRPM
channel or PLC was genetically knocked-out.
In the TRPM KO animals, the capacity to perceive bitter, sweet and umami was eliminated.
In the PLC KO animals, PLC expression was selectively reinstated only in cells that express the T2R
receptor (bitter).
In those animals, the perception of bitter returned, but not the perception of sweet or umami.
This indicates that even though sweet, bitter & umami all share the same intracellular signaling pathway, the neural signals encoding these tastants are segregated at the receptor level and remain segregated all the way to the olfactory cortex.
Graphically display how a TRPM knockout ould affect sweet, umami, and bitter taste. How would a T2R-rescue change this?
look up and study slide 18 of 19 in chemical senses lecture 16
seriously
Why is trigeminal chemoreception important?
Detection of chemical irritants to the soft and mucousal-lined tissues of the head (nasal
cavity, scalp, cornea, oral cavity).
What is the neuron type involved in the trigeminal chemoreceptive pathway ?
Is mediated by polymodal nociceptive neurons (similar to those that innervate the skin)
that project axons via the trigeminal nerve (CN V) to the trigeminal nucleus in the spinal
cord.
Where do the axons involved in the trigeminal chemoreception pathway go? What do they do there?
Axons from the trigeminal nucleus project to the VPM of the thalamus and then to the
somatosensory cortex.
They elicit salivation, tearing, vasodilation, sweating, bronchial constriction.
What potential irritants would trigger trigeminal chemoreception?
Potential irritants include sulfur dioxide, ammonia, methanol, acetic acid and capsaicin;
are detected at a higher concentration compared to the chemoreceptors involved in taste
and smell
Receptors mediating this chemoreception are not well known, but include the TRPV
receptors which probably detect acidic irritants. Other TRP channels are likely to be
receptors for other irritants.
