Lecture 9 - Olfaction and Taste Flashcards
The receptor cells that transduce odorant stimulu are
bona fide neurons.
The cell bodies are located in
the olfactory epithelium (OE)
In humans the OE lines what?
the posterodorsal part of the nasal cavity below the cribriform plate on the nasal septum and lateral nasal wall.
The OE neurons relay information via
axonal projections through the cribriform plate to the olfactory bulb (OB).
The OB is
the first relay in the sensory chain.
What drains through the cribiform plate and what is the clinical relevance?
CSF drains through and is a potential opening for infection
What is the connection between emotion and smell?
The orbitofrontal cortex
What are the OB targets?
1) pyriform cortex 2) olfactory tubercle 3) amygdale 4) entorhinal cortex
Senory neuron in the OE
1) true sensory neuron 2) death of the sensory neurons leaves the tracks that the new axons can fill
Why might a patient smell an apple that smells like something else?
Death of a lot of sensory neurons in the OE result in the incoming axons forming new connections which changes the perception
Bulbar neurons
project via the lateral olfactory tract to the piriform (olfactory) cortex located at the tip of the temporal lobe.
What happens to the information after it gets to the olfactory cortex?
From here information flows in many directions: to parts of the neocortex via the mediodorsal nucleus of the thalamus and to the lateral hypothalamus
The OE contains:
basal cells, neurons and supporting cells.
The neurons of the OE are?
bipolar with an apical dendrite ending in a knob with immotile cilia. Each neuron also has a thin unmyelinated axon (C fiber) that projects to the bulb.
Neurogenesis in the olfactory epithelium
Basal cells continually undergo mitotic division in a stem cell mode. Sensory neurons have a life span of approx. 30 days. Therefore, epithelial projections are continually being broken down and reforming
What is the sequence of stimulus transduction?
(1)odorant delivered to the epithelium along the airstream (2)odorant diffuses through the mucus to reach the cilia (3)odorant interacts with receptor protein (4)binding to the receptor results in a depolarizing receptor potential (5)the biochemistry of t
What is the key ion in the sequence of stimulus transduction?
Ca
What is the largest GPCR family?
Olfactory receptors
What is the effect of a zonal distribution pattern?
Spatial pattern code where there is integration of the receptor type with differing binding affinity
What underlies odor coding?
Receptor expression patterns
Describe the olfactory receptor within a zone?
Can be either homogenously distributed or have a clustered distribution pattern
Are olfactory receptors specific?
Receptors are promiscuous with what they respond to
Single unit recordings of individual olfactory sensory neurons show what?
That neurons are broadly tuned.
The diversity of physiologically defined types of olfactory neurons parallels what?
the number of ORs.
What determines the physiological responsiveness of an olfactory neuron?
the particular OR a neuron expresses where any one OR is broadly tuned to respond to a lot of different odorants, which share a common molecular feature.
The encoding of odorants at the level of the olfactory epithelium is
spatial
Compare the areas of the epithelium in their response to odorants.
There are inherent or intrinsic differences
The inherent differences in response to odorants reflect what?
the distribution of ORs
What is the result of inherent differences in response to odorants?
Each odorant has its own fingerprint
What is the most rostral part of the CNS?
The olfactory bulb
What is the shape of the olfactory bulb and its cells?
It is tubular in shape and the cells and their neuropil are arranged in concentric layers like an onion.
What is the order for the layers of the olfactory bulb?
there is the glomerular layer, external plexiform layer, mitral cell layer and granule cell layer.
What are the mitral and tufted cells?
they are the relay neurons that project to olfactory cortex.
What are Periglomerular cells and granule cell?
they are inhibitory interneurons that modulate activity of the mitral tufted cells.
What do the periglomerular cells form?
the outmost shell of neurons. They cluster around neuropils called glomeruli.
What is the fundamental unit of anatomical and physiological organization in the bulb?
The glomerulus
How does the olfactory sensory system compare other sensory systems?
1) the projection of the sensory surface onto its central structures is not point-to-point. (2) First, it is quadrant-to-quadrant. These quadrants are defined by the boundaries of the OR expression zones.
What defines the quadrants of the olfactory sensory systems?
The boundaries of the OR expression zones
What is meant by a differential distribution pattern of specific OR expressing neurons?
1) any particular neuron expresses but one OR type 2) within a zone, neurons expressing a particular OR can be either homogenously distributed or have a clustered distribution pattern
What is the convergence of axons axons from specific OR expressing neurons?
Second, within a quadrant the axonal projections bring together the axon terminals of all “like-neurons”. In other words, all of the neurons expressing a particular OR converge onto the same set of glomeruli.
all of the neurons expressing a particular OR
converge onto the same set of glomeruli
What is responsible for establishing the spatial code for olfactory stimuli across the bulb?
The pattern of the projections from the epithelium onto the bulb. The circuitry then acts on this information.
Describe the olfactory neurons and the olfactory receptor.
Recall that olfactory neurons are broadly tuned and that all the neurons that express a single OR synapse together in a single glomerulus. Thus, the rule – one OR, one glomerulus applies
The glomerulus is
both an anatomical unit and a functional unit.
The glomerulus as both an anatomical and functional unit is implied by what?
the connectivity, the unique responsiveness of individual ORs and, like the epithelium, the pattern of activation of the bulb in response to odorants.
Lateral inhibition
filters out the incidental information from the true stimulus
Glomeruli are activated by what?
odorants and different glomeruli are activated by different odorants.
In the working model for the olfactory system, what is an odotype?
there are many different ORs and each would recognize a single chemical moiety (e.g., a phenyl group) that would be considered an “odotope”. A single chemical would be composed of many such odotopes.
In the working model for the olfactory system, how are odors identified and discriminated?
by the overall activation of glomeruli across a the whole bulb based on the selective activation of glomeruli, one glomerulus is the unit for recognizing a particular odotope that has been presented
The five primary taste qualities are:
salt, sour, bitter, sweet and umami
Taste buds are innervated by
cranial nerves (VII via the chorda tympani nerve to the tongue and the greater superficial petrosal to the palate), and the IXth and Xth). Each axon innvervates from 2-10 taste cells
Gustatory axons innervate what?
the NTS (nucleous of the solitary tract).
The NTS has projections to
1) VPM of the thalamus 2) hypothalamus 3) amygda.
VPM, the hypothalamus and the amygdala project to what?
the insula and frontal cortex.
Taste buds on the anterior 2/3s of the tongue
CN VII
Taste buds on the posterior 1/3 of the tongue
CN IX
Taste buds for the epiglottis
CN X
What are the four primary taste fields?
1) circumvallate 2) foliate 3) fungiform papillae 4) buds on the soft palate.
What taste fields respond to what tastants?
All of the taste fields respond to all tastants, each taste field is most sensitive to a particular taste quality.
What taste buds are most sensitive to sweet?
the soft palate
Taste buds are limited to
specialized protrusions called papillae
Describe a taste bud.
pear-shaped collection of cells surrounded by a basal lamina that are embedded in a stratified squamous epithelium.Each bud has 50-100 cells that include receptor cells and basal cells (stem cells).
Microvilli of a taste cell extends through what?
a taste pore.
What synapses with the receptor (taste) cells?
Gustatory afferent axons
Stimuli (of the taste cell) causes what?
a deplorization which produce synaptic transmission across the receptor cell – axon synapse.
Salt and sour stimuli are transduced through
an ionic transduction mechanism which results in an intracellular increase in calcium and transmitter release.
Salt ion transduction mechanism
amiloride sensitive Na channel
Acid ion transduction mechanism
H+ sensitive cation channel
Receptor mediated transduction via second messenger in taste
Sweet, bitter and umami stimuli are transduced through a G-protein coupled mechanism. Increases in cAMP stimulate a TRPM5 calcium channel via IP3. Take home point = increase Ca influx
In the gustatory system, what do single axons respond to?
many different primary stimulus qualities although they will have a preferential maximal response to only one.
Recoding activity patterns for the gustatory system from the chorda tympani nerve?
one can demonstrate a differential firing pattern across a population of axons.
What was seen as the evidence for an across fiber “pattern code”?
a differential firing pattern across a population of axons
How does the same group of axons in the gustatory system differentially encode the stimulus quality?
It does this based on the differential firing pattern.
What does the evidence for labeled line taste coding suggest?
that specific gene knockout and rescue experiments specifically rescue or delete the perception of one taste quality without impacting the others.
Deletion in mice of a downstream transduction component (i.e., PLC beta 2) of all three tastants results in what?
an altered behavioral response to the stimulus qualities.
When PLC beta 2 is specifically rescued in cells that express the taste receptors for bitter (T2Rs) what is seen?
there is a recovery of function for quinine but not the other tastants.
Pungency vs odor
1) Pungency = irritation ex capsacin transmitted through lingual nerve through Trigeminal 2) Odor is through the ORs of CN I
