Smell and Taste- Heinbockel Flashcards
What are the two chemical senses?
- olfaction
- gustation
Both taste and smell chemoreceptors respond to chemicals in what?
aqueous solutions
Taste – to substances dissolved in saliva
Smell – to substances dissolved in fluids of the nasal membranes
The olfactory system deals with volatile or nonvolatile substances? What about the gustatory system?
olfactory system: general odors, VOLATILES, pheromones (food, predators, mating and aggression cues, individual identity)
Gustatory system – NONVOLATILES (sweet,
bitter, salty, sour, umami tasting substances)
What do the chemical senses need to do?
- Detect all relevant chemicals in the environment (including novel ones)
- Encode both QUALITY and QUANTITY of individual chemicals in the context of complex chemical mixtures; when you smell smoke it is not just one type of chemical but a mixture of different chemicals
- Decode and interpret this sensory information in the context of experience and other sensory input so that the nervous system can generate an appropriate behavioral response
What factors do the chemical senses encode in the chemicals?
both quality and quantity
From the Nose to the
Olfactory Bulb and on to
Higher Olfactory Centers
Olfactory receptor neurons (ORNs), typical bipolar neurons, are found in an epithelial sheet in the nasal epithelium
ORNs project axons (CN I) to the ipsilateral olfactory bulb
Synaptic interactions occur within glomeruli in the bulb.
Olfactory bulb neurons interact with central neurons (mitral and tufted cells) which send axons to olfactory cortex and other structures in the limbic system
axons travel through nasal epithelium to cribriform plate to olfactory bulb
Where is the olfactory epithelium?
on the superior concha in the nasal cavity
Describe the olfactory pathway.
Air and odorant molecules come in and go to the back of the nose where they activate olfactory receptor neurons through cilia (has membrane proteins) binding the molecules
the axons all together form CN I and go through the cribriform plate to the olfactory bulb where they synapse on central neurons like the mitral cells that will go to higher olfactory centers (olfactory cortex or limbic system)
ORNs project to the olfactory bulbs and synapse with mitral cells
Glomerular mitral cells process odor signals
Mitral cells project axons and send impulses to the olfactory cortex (anterior olfactory nucleus, piriform cortex, olfactory tubercle), amygdala, and entorhinal cortex
Secondary projections go to the thalamus, hypothalamus, and hippocampus
Where does olfactory transduction takes place where?
on the cilium at the end of dendrites of the olfactory receptor neurons (has specific membrane proteins and is bathed in serous fluid)
What are the two types of cells in the nasal epithelium?
- supporting cells (sustentacular cells)
- stem cell population of basal cells; need them due to the rapid turnover rate of olfactory receptor neurons (live only for months or two)
While olfactory receptor neurons have cilia, what do the supporting cells in the nasal epithelium have?
microvilli
Describe olfactory receptor neurons.
-odors interact with olfactory receptor neurons (ORNs) at the dendritic cilia
- these cilia contain the molecular components of the sensory transduction cascade
- ORNs are bipolar neurons with cilia
- ORNs send axonal projections to the olfactory bulb-supporting cells have microvilli
Why do olfactory receptor neurons have a short lifespan of 30-60 days?
constantly bombarded with air and odor molecules
How are ORN regenerated?
new cells generated by the stem cell population (basal cells) in the nasal epithelium
basal cells contain progenitor and stem cell populations–migrate, differentiate and replace ORNs and other OE cell types
What is interesting about the new ORN generated by the basal cells in the nasal epithelium?
the “new” ORN forms new connections in the olfactory bulb
they produce axons that project to the olfactory bulb; they know exactly where to go
Where does the watery fluid that bathes the cilia of ORNs come from?
secreted by olfactory (Bowman’s gland)
What is the arrangement of cilia in the respiratory system vs those in the olfactory system? Why is this arrangement important?
few nonmotile cilia in the olfactory system (just there to bind to air and odor molecules)
9 plus 2 arrangement (motile) cilia in the lower respiratory system (trachea and bronchi) important for moving material out of the airway
We have a Large Family of
Odorant Receptors that are of what type?
g protein coupled receptors
Each receptor consists of a protein chain that traverses the cell membrane seven times. When an odorant substance attaches to an olfactory receptor, the shape of the receptor protein is altered, leading to a G protein activation. An electric signal is triggered in the olfactory receptor neuron and sent to the brain via nerve processes.
What is the unexpected finding in terms of the expression of the olfactory receptors?
Every single olfactory receptor cell expresses one and only one gene of all the genes that code for olfactory receptor molecules.
Each olfactory receptor cell expresses one type of odorant receptor, and each receptor can detect a limited number of odorant substances.
Explain why the olfactory receptors are triggered by different odorant molecules.
All odorant receptors are related proteins and differ only in some amino acid residues. The subtle differences in the protein chains explain why the receptors are triggered by different odorant molecules.
How are the different olfactory receptors arranged in the nasal epithelium?
There are separate zones of the olfactory epithelium. A different set of odorant receptor genes is expressed in each zone.
How many different odorants can be distinguished?
350
What allows us to detect many odorants that outnumber the amount of olfactory receptors we have?
a given olfactory receptor is activated by several odorants and a given odorant can activate different types of olfactory receptors
Describe the olfactory transduction.
Odors are transduced by a G Protein-coupled cascade
the odorant binds to the odorant receptor in the cilia of the dendrites of the olfactory receptor neurons which activates the G protein.
the alpha subunit (Galphaolf) activates the adenylate cyclase which makes cAMP from ATP
the cAMP activates the cyclic nucleotide gated channel that allows sodium and calcium to rush into the cell which depolarizes the cell leading to an AP
-the incoming calcium also opens up the chloride anion channel making the inside of the cell more positive (making chloride leave the cell) allowing for further depolarization
another pathway is the (inositol triphosphate pathway) IP3 pathway
What are the major molecular components of the olfactory transduction cascade?
-Olfactory receptor (OR)–member of the
G-protein coupled receptor family (7TMs), there are ~900 functional OR genes in mouse, ~400 in humans
- G protein–couples OR to ACIII, includes Golf a-subunit
- Adenylyl cyclase type III–synthesizes the second messenger cAMP
-CNG channel–gated by cAMP, contributes to depolarization of the ORN and
Ca2+ influx (underlies both additional activation of Ca2+-sensitive Cl- conductance and odor adaptation through negative feedback)
-Ca2+-sensitive Cl- channel–has not been cloned, contributes majority of depolarizing current
Not all the genes are expressed throughout the entire olfactory epithelium. Certain genes are expressed only in cells in a given zone or region. How many zones are there and what is represented in each?
4 zones
Zone I
Zone II
Zone III
Zone IV
Describe the intrinsic organization of the olfactory bulb.
-Convergent Projection Pattern of Olfactory
Sensory Neurons to the Main Olfactory Bulb
- 2,000 glomeruli
- Each glomerulus also contains branched dendrites of mitral and tufted cells
- 1 ORN for 1 olfactory receptor gene for 1 glomerulus
ORNs expressing a given receptor protein converge onto the same glomerulus AKA onto only a few individual glomeruli in the bulb
What is the first relay station in brain
for the processing of olfactory information?
olfactory bulb
The ORN axons enter the olfactory bulb and end in structures called what?
glomeruli
What are the output neurons of the olfactory bulb?
mitral and tufted cells
State the relative amount of projection from the olfactory bulb to higher olfactory centers and vice versa.
There are more axons going down to the olfactory bulb from the higher olfactory centers than those going from the olfactory bulb to higher olfactory centers
Modulation of bulb activity is done by descending (centrifugal) circuits.
What is the purpose of inhibitory interneurons?
mediate lateral inhibition by connecting mitral and tufted cells of other input channels (banana smell more enhanced than the apple smell)
Compare the input into the olfactory bulb to that of the olfactory cortex.
convergence to the olfactory bulb and divergence to the olfactory cortex
Tracing the olfactory Pathway
- 5 million olfactory sensory neurons
- 1000 different odorant receptor genes
- 2000 glomeruli in the olfactory bulb
- 1 olfactory receptor neuron - 1 odorant receptor- 1 glomerulus
What does the olfactory cortex consist of?
- anterior olfactory nucleus
- piriform cortex
- olfactory tubercle
- amygdala
- entorhinal cortex
Where do secondary projections of the olfactory pathway go to?
thalamus , hypothalamus, hippocampus
Define the following olfactory disorder:
- anosmia
- hyposmia
- dysosmia
- phantosmia
Anosmia (loss of smell)
hyposmia (diminished smell)
-both result from a number of etiologies
Dysosmia (distorted smell)
phantosmia (phantom smells = smelling something that is not really there)
-may accompany conditions such as prior URI, nasal/sinus diseases, head trauma, aging
How does head trauma result in anosmia?
hard hit to the nose:
-all the axons reaching through the cribriform plate will be torn as the nose is displaced resulting in no sense of smell
the axons no longer have connections to the olfactory
Taste primarily occurs where?
on taste buds on the tongue
Taste buds are found in papillae of the tongue mucosa
What are the different types of taste buds (papillae) on the tongue?
circumvallate: really big
foliate: leaf
fungiform: mushroom
filiform: really thin
How is the sense of taste organized on the tongue?
in the taste buds are receptor cells which can detect the different taste qualities
so you can taste anything anywhere on the tongue
there is not region of the tongue set out for a particular taste like sweet for example
What are the different type of cells found in each gourd-shaped taste bud and what are their functions?
gustatory cells:
supporting cells: insulate the receptor
basal cells: dynamic stem cells
Are the gustatory receptors neuronal in origin?
are not neuronal cells but epithelial in origin
thus they have no axons
but they do have cranial fibers that go right up the taste buds
form chemical synapse on sensory fibers and voltage gated ion channels to generate action potentials
Where does taste transduction take place?
on gustatory hairs, microvilli, exposed to oral cavity
Where are taste buds located?
in papillae on the tongue (also in epithelium of soft palate, nasopharynx, and epiglottis)
The taste bud contains what different kinds of taste receptor cells?
Type I
Type II
Type III taste receptor cells
-all with modified epithelium
What is the function of the basal cells in the taste buds? What is the lifespan of taste receptor cells?
- basal cells give rise to new receptor cells
- lifespan of TRC is 10 days (shorter than olfactory receptor cells) as they are constantly bombarded with food
What are the five basic taste sensations?
Sweet – sugars, saccharin, alcohol, and some amino acids
Salt – metal ions
Sour – hydrogen ions
Bitter – alkaloids such as quinine and nicotine
Umami – elicited by the amino acid glutamate
Explain the overall characteristics of taste transduction.
Taste stimuli are transduced by a G protein-coupled cascade
Have G-couple protein receptors (taste receptor) and membrane proteins that allow certain ions to get into the cell
signal transduction that involves calcium channels
there are no axons but there is neurotransmitter release onto the primary sensory neurons onto some cranial nerve fibers
Describe the process of taste transduction.
taste stimulus binds to taste receptors which activates g protein that activates phospholipase C which then generates diacylglycerol (DAG) and IP3
transient receptor potential (TRP): new class of ion channels involved in sensory functions
-sodium and calcium rushing into the cells and additional IP3 receptors that evoke calcium release
What are the different taste transduction mechanism?
some of the atse (bitter, sweet, umami) work through a g-protein coupled mechanism
some other (sour and salty) use other mechanisms
they all converge o a common signal in the center in that the cell get depolarized (either calcium rushing in or closing sodium channels) and release of NTs
Describe the taste transduction for salty and sour taste.
The stimulus energy of taste is converted into a nerve impulse by:
- Salty & sour involve permeation, or blockade, of ion channels
- Na+ influx in salty tastes
- H+ in sour tastes (by directly entering the cell, by opening cation channels, or by blockade of K+ channels)
- Sweet & bitter tastes mediated by G protein-coupled mechanisms or direct effects on ion channels
- Gustducin (G protein) in sweet and bitter tastes
- Umami: metabotropic glutamate receptor-mediated (very acting glutamate receptor)
In order for a chemical to activate the taste receptor AKA be tasted, what needs to occur?
taste (food) must be dissolved in saliva and contact the gustatory hair cells
Binding of the food chemical:
- Depolarizes the taste cell membrane, releasing neurotransmitter
- Initiates a generator potential that elicits an action potential in the cranial nerve fibers
Describe the gustatory pathway.
depending on where you are in the tongue you are activating different types of cranial nerves (either facial, glossopharyngeal, or vagus)
irrespective of which cranial nerve gets activated here they all project to the nucleus solitarius in the medulla oblongata and from there they project to the thalamus (in the olfactory system thalamus is INDIRECTLY involved) to go to the gustatory cortex
Cranial Nerves VII (branch of facial nerve, chorda tympani) and IX (lingual branch of glossopharyngeal nerve) carry impulses from taste buds to the solitary nucleus of the medulla
Which higher order brain centers receive fibers from the thalamus in the gustatory pathway?
- Cortex (insular or gustatory cortex) (taste)
- Amygdala (central nucleus) (appreciation of taste)
- Hypothalamus (lateral hypothalamic nucleus) to brainstem reticular formation structures
- -> control salivation, insulin release and relevant musculature (tongue, face muscles, swallowing muscles)
List the cranial nerve associated with each of the following:
- soft palate
- epiglottis
- vallate papillae
- foliate papillae
- fungiform papillae
- soft palate: VII
- epiglottis: X
- vallate papillae: IX
- foliate papillae: IX, VII
- fungiform papillae: VII
How is taste and olfaction related to one another?
there is another pathway that makes us think that smell is really involved
- Taste is 80% smell, retronasal activation (odors from the food you taste on your tongue go to the olfactory receptors so when you can’t smell you can taste your food)
- Thermoreceptors, mechanoreceptors, nociceptors also influence tastes
- Temperature and texture enhance or detract from taste
- Taste is in the mouth, not in the nose
- Coincidence of somatosensory stimulation of tongue and retronasal passage of odorants into nose –> odorants are perceived as flavors in mouth
