Neurobiology of smell and taste

Question 1

WHat is olfaction and gustation and how are they related?

Answer 1

• The related senses of taste and smell help us interpret the chemical world
• Smell (olfaction) and taste (gustation) are examples of visceral senses because of their close association with gI function
• Physiologically, they are related to each other as the flavor of food is a combination of its taste and smell.
• This is why food may taste “different” if one has a cold that depresses the sense of smell.

Question 2

How are smell and taste receptors stimulated?

Answer 2

Smell and taste receptors are chemoreceptors that are stimulated by chemical molecules in solution in mucus in the nose (odorants) and saliva in the mouth (tastants) and the sensation of smell and taste likely envolved as protective mechanisms to avoid the intake of potentially harmful subsstances.

Question 3

What is the olfactory Epithelium in SMELL

Answer 3

• The yellowish pigmented olfactory epithelium is a specialised portion of the nasal mucosa that covers an area of 10cm2 in the roof of the nasal cavity near the septum in humans.
• The oplfactory epithelium is the place in the body where the nervous system is closest to the external world

Question 4

3 cell types in olfactory epithelium

Answer 4

1. Olfactory sensory neurons (receptor)
2. Supporting Cells (sustentacular)
3. Basal stem cells - at base of epithelium

Question 5

WHo odorants lead to actionpotentials in sensory axon

Answer 5

• Each olfactory sensory neuron has a dendrite that projects to the epithelial surface
• Numerous cilia protrude into the mucus layer lining the nasal lumen
• Odorants bind to specific odorant receptors on the cilia and intiiate a cascade of events leading to the generation of action potentials in sensory axons
• Each olfactor sensory neuron has a single axon that projects to the olfactory bulb, a small ovid structure that rests on the cribiform plate on the ethmoid bone

Question 6

Describe Olfactory snesory neurons

Answer 6

• Bipolar
• Also called olfactory receptor cells
• Responsible for olfactory transduction
• Olfactory sensory neurons have a short, thick dendrite that projects into the nasal cavity
• Dendrite it terminates in a knob containing 6-12 cilia that protrude into the thin layer of mucus overlying the peithelium.
• The axons of the olfactory sensory neurons (ie olfactory nerve) pass through the cribiform plate of the ethmoid bone ot enter olfactory bulb

Question 8

Olfactory bulb and synapse

Answer 8

Question 9

Function of supporting cells in odor detection in olfactor epithelium

Answer 9

• The supporting cells secrete the mucus that provides the appropriate molecular and ionic environment for door detection in the olfacotry epithelium
• Odor-producing molecules (odorants) dissolve in the mucucs and bind to odorant receptors on cilia of olfactory sneosry neurons

Question 10

What is the role of Odorant binding proteins

Answer 10

Odorant binding proteins in the mucus may facilitate the diffusion of odorants to and from the odorant receptor.

Question 11

Function of Basal stem cells in smell

Answer 11

• Basal stem cells undergo mitosis to generate new olfactory snesory neurons as needed to replace those damaged by exposure to the environment; olfactory sensory neurons generally survive for only 1-2 months.
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Question 12

Describe the Olfactory Mucosa

Answer 12

• The olfactory mucosa is a pseudostratified ciliated columnar epithelium located in the superior most region of the nasal cavity, and contains bipolar olfactory cells whose cilia are embedded in mucus.
• Chemicals which dissolve in the mucus trigger responses in these ciliar which initiate a nervous impulse, intepreted in the brain as odor
• Supporting cells surround the olfactory cells
• Mucus producing Bowmans glands are embedded in the lamina propria
• This connective tissue is richly vascularised

Question 13

Olfactory membrane on microscope

Answer 13

Question 14

Olfactory membrane labelled hsiotlogy

Answer 14

Question 15

Can the olfactory system discriminate odors?

Answer 15

Yes the olfactory system can discriminate perhaps more than 1 million distinct odors due in part to the existence of many different functional odorant receptors.

Question 16

How many olfactory genes are there?

Answer 16

There are about 1000 olfactory genes in humans, accounting for 3% of the human genome; approximately 400 of these genes function as odorant receptors

The amino acid sequences of odorant receptors are very diverse, but all are Gprotein-coupled receptors (GPCRs)

Question 17

Odorant receptors and the process of signla transduction

Answer 17

1. When an odorant molecule binds to its receptor, the G-protein subunits (a,B,y) dissociate
2. The a-subunit acitvates adenylyl cyclase to catalyse the production of cAMP which acts as a second messenger to open cation channels, increasing the membrane permeability to na+, K- and Ca2+
3. The net effect is in inward-directed Ca2+ current which produces the graded receptor potential
4. This opens Ca2+ activated Cl- channels, further depolarising the cell due to the high intracellular Cl- levels in olfactory sensory neurons
5. If the stimulus is sufficient for the receptor potential to exceed its threshold, an action potential in the olfactory nerve (1st cranial nerve0 is triggered.

Question 18

The Olfactory Sensory Pathway

Answer 18

1. In the olfactory bulb, the axons of the olfactory sensory neurons synapse on the primary dendrites of the mitral cells and tufted cells to form olfactory glomeruli
2. Each olfactory sensory neuron expresses only one of the 400 functional olfactory genes, but each odorant can bind to a large pool of odorant receptors
3. Each olfactory sensory neuron projects to only one or two glomeruli.
4. This provides a distinct 2D map in the olfactory bulb that is unique to the odorant.
5. The mitral cells with their glomeruli project to different parts of olfactory cortex
6. The central olfactory system is able to decode the pattern of receptor-cell activity that signals the identity of the odorant.

Question 19

Basal Neural Circuits in the Olfactory Bulb

Answer 19

Olfactory receptor cells with one type of odorant receptor project to one olfactory glomerulus (OG) and olfactory receptor cells with another type of receptor project to a different OG

Solid black arrows signify inhibition via GABA release, and white arrows signify excitatory connections via glutamate release

• CP, cribriform plate
• Gr, granule cell
• M, mitral cell
• PG, periglomerular cell
• T, tufted cell

Question 20

diagram of the olfactory pathway

Answer 20

Question 21

Odor Detection threshold - odorants

Answer 21

• Odorants are generally small, contain between 3 and 20 carbon atoms molecules with the same number of carbon atoms but different structural configurations have different odors.
• Relatively high water and lipid solubility is characteristic of substances with strong odors
• The odor detection threshold is the lowest concentration of a chemical that can be detected
• Examples of substances detected at very low concentration include hydrogen sulfide
• (0.0005 parts per million, ppm), acetic acid (0.016 ppm), kerosene (0.1 ppm), and gasoline (0.3 ppm)
• Some toxic substances are essentially odorless;they have odor detection thresholds higher than lethal concentrations
• For example, carbon dioxide is detected at 74,000 ppm but is lethal at 50,000 ppm
• The odor detection threshold for a given odorant is not the same in all individuals
• The sense of smell is said to be more acute in women than in men, and in women it is most acute at the time of ovulation
• Although olfactory discrimination is remarkable, determination of differences in the intensity of any given odor is poor
• The concentration of an odor-producing substance must be changed by about 30% before a difference can be detected
• The comparable visual discrimination threshold is a 1% change in light intensity.

Question 22

Abnormalities in odour detection

Answer 22

• Anosmia (inability to smell) and hyposmia or hypesthesia (diminished olfactory sensitivity) can result from simple nasal congestion, nasal polyps, or prolonged use of nasal decongestants. May also be sign of more serious problem like damage to olfactory nerves due to fractures of cribriform plate or head trauma, tumours (eg, neuroblastomas or meningiomas and respiratory tract infections)
• Congenital anosmia - rare disorder where an individual born without ability to smell. Olfactory dysfunction is often one of the earliest clinical symptoms of Alzheimers disease.
• Hyperosmia = enhaned oflactory sensitivity. less common than loss of smell, but pregnant women often become oversensitive to smell
• Dysosmia = Distorted sense of smell. Can be cause by several disorders including sinus infections, partial damage to olfactory nerces and poor dental hygiene.
• An aura of a disagreeable odor (eg burning rubber) can occur when individual experience an uncinate seizure that originates in medial tmeporal lobe.

Question 23

Anosmia

Answer 23

Anosmia (inability to smell) and hyposmia or hypesthesia (diminished olfactory sensitivity) can result from simple nasal congestion, nasal polyps, or prolonged use of nasal decongestants

It may also be a sign of a more serious problem such as damage to the olfactory nerves due to fractures of the cribriform plate or head trauma, tumors (eg, neuroblastomas or meningiomas), and respiratory tract infections

Question 24

Congenital anosmia

Answer 24

Congenital anosmia is a rare disorder in which an individual is born without the ability to smell

Olfactory dysfunction is often one of the earliest clinical symptoms of Alzheimer disease

According to the National Institutes of Health, 1-2% of the North American population under the age of 65 experiences a significant degree of loss of smell

However, 50% of individuals between the ages of 65 and 80 and >75% of those over the age of 80 have an impaired ability to identify smells

Question 25

Hyperosmia

Answer 25

Hyperosmia (enhanced olfactory sensitivity) is less common than loss of smell, but pregnant women commonly become oversensitive to smell

Question 26

Dysosmia

Answer 26

Dysosmia (distorted sense of smell) can be caused by several disorders including sinus infections, partial damage to the olfactory nerves, and poor dental hygiene

Question 27

Where are tastebuds and how many do we have

Answer 27

The organ for taste (gustation) consists of about 5000 taste buds located primarily on the papillae of the dorsal surface of the tongue in humans.

Question 28

What are the types of Papillae on the tongue?

Answer 28

• Fungiform papillae = rounded structures most numerous near the tip of the tongue
• Foliate papillae = are on the posterior edge of the tongue
• Circumvallate papillae = prominent structures arranged in a V on the back of the tongue

Each fungiform papilla has up to 5 taste buds, mostly located at the top of the papilla, while each circumvallate and folate papilla contains up to 200 taste buds, mostly located along the sides of the papillae. Taste buds are also located in the soft palate, epiglottis, and pharynx.

Question 30

How are taste buds innervated?

Answer 30

Question 31

WHat are taste buds composed of and taste cells what are these?

Answer 31

• Taste buds are composed of basal stem cells and 3 types of taste cells: Dark, light, intermediate
• Taste cells extend from the base of the taste bud to the taste pore, where microvilli contact tastants dissolved in saliva and mucus
• Each taste bud contains 50-100 taste receptor cells and numerous basal cells and support cells.

Question 32

How are the taste receptor cells modified?

Answer 32

• Taste receptor cells are modified epithelial cells that respond to chemical stimuli or tastants
• The apical ends of taste cells have microvilli that project into the taste pore, a small opening on the dorsal surgave of the tingue where atstes cells are exposed to the oral contents.

Question 33

What is the function of saliva?

Answer 33

• Acts as a solvent for tastants, after dissolve, the chemical diffuses to the taste receptor sites
• Function to cleanse the mouth to prepare the taste receptors for a new stimulant

Each taste bud is innervated by about 50 nerve fibres, and conversely, each nerve fibre receives input from an average of 5 taste buds

The basal cells arise from the epithelial cells surrounding the taste bud and differentiate into new taste cells as taste cells survive for only about 10 days

If the sensory nerve is cut, the taste buds it innervates degenerate and eventually disappear.

Question 34

Describe the Taste pathways

Answer 34

• The sensory nerve fibres from the taste buds on the anterior 2/3 of tongue travel in chorda tympani branch of the facial nerve and those from the posterior 1/4 of the tongue reach the brianstem via the glossopharyngeal nerve.
• The fibres from areas other than the tongue (eg, pharynx) reach the brian stem via the the vagus nerve
• On each side, the myelinated but relatively slowly conducting taste fibres in these 3 nerves unit in the gustatory portion of the nucleus of the tractus solitarius (NTS) in the medulla oblongata
• From there, axons of second-order neurons ascend in the ipsilateral medial lemniscus and project directly to the ventral posteromedial nucleus of thlamaus.
• From the thalamus, the axons of the 1/3 order neurons pass to neurons in the anterior insula and frontal operculum in the ipsilateral cerebral cortex.
• This region is rostral to the face area of the postercentral gyrus, which si probably the area that mediates conscious perception of taste and taste discrimination.

Question 35

Primary Gustatory cortex

Answer 35

Question 36

What other sensory fibres innervate the tongue besides facial and glosospharyngeal

Answer 36

Sensory fibers in the trigeminal (5th cranial) nerve also innervate the tongue and contribute to the burning sensation experienced when we eat foods containing capsaicin

Taste buds are surrounded

by TRPV1 receptors on

trigeminal nociceptive

fibers that are activated in

response to eating spicy foods

Question 37

What are taste buds surrounded by?

Answer 37

Taste buds are surrounded by TRPV1 receptors (is a protein that, in humans, is encoded by the TRPV1 gene) on trigeminal nociceptive fibers that are activated in response to eating spicy foods

Question 38

Taste modalities, receptors and transduction

Answer 38

Humans have five basic taste modalities are:

▪ Salt (common stimuli: sodium chloride)

▪ Sweet (common stimuli: sucrose)

▪ Sour (common stimuli: hydrochloric acid)

▪ Bitter (common stimuli: quinine)

▪ Umami (common stimuli: monosodium glutamate)

All tastants are sensed from all parts of the tongue and adjacent structures and afferent nerves to the NTS contain fibers from all types of taste receptors, without any clear localization of types

Question 39

How can an individual taste receptor respond to more than one type of tastant?

Answer 39

The central nervous system can distinguish the various tastes from one another because each type of taste receptor cell connects to a particular gustatory axon

The putative receptors for the five modalities of taste include the two major types of receptors:

▪ Ligand-gated channels (ionotropic receptors)

▪ GPCRs (metabotropic receptors)

Question 40

How are different tastes triggered?

Answer 40

Salt and sour tastes are triggered by activation of ionotropicreceptors; sour, bitter, and umami tastes are triggered by activation of metabotropic receptors. Many GPCRs in the human genome are taste receptors (T1Rs, T2Rs families)

Question 41

Mammalian taste receptors, cells and ligands

Answer 41

Question 42

What is salt sensitive taste mediated by?

Answer 42

Salt-sensitive taste is mediated by an epithelial sodium channel (ENaC) and the entry of Na+ into the salt receptor depolarizes the membrane, generating a receptor potential

Question 43

What is sour taste mediated by?

Answer 43

Question 44

What are sweet tastes detected by?

Answer 44

Question 45

WHta is bitter taste mediated by and produced by?

Answer 45

Question 46

Transduction of 5 basic taste modalities

Answer 46

Question 47

Taste Thresholds and intensity discrimination

Answer 47

• The ability of humans to discriminate differences in the intensity of tastes is relatively crude
• A 30% change in the concentration of the tastant is necessary before a difference can be detected
• Taste threshold refers to the minimum concentration at which a substance can be perceived
• Bitter substances tend to have the lowest threshold
• Some toxic substances such as strychnine have a
• bitter taste at very low concentrations, preventing
• accidental ingestion of this chemical, which causes
• atal convulsions
• Some common abnormalities in taste detection are described in Clinical

Question 48

ABnormalities in taste detection

Answer 48

Damage to lingual or glossopharyngeal nerve:

• Ageusia = absence of the sense of taste
• Hypogeusia = diminished taste sensitivity

Problems with taste sensitivity = Neurological disorders (eg vestibular schwannoma), bells palsy, familial dysautonomia, Multiple sclerosis, certain infections (eg primary ameboid meningoencephalopathy) and poor oral hygiene.

Ageusia = Adverse side effect of various drugs, including cisplatin and captopril or vitamin B3. Zinc deficiencies and also covid-19

DIminished taste = Aging, tobacco abuse.

• Dysgeusia or Parageusia = unpleasant perception of taste which causes a metallic, salty, foul or rancid taste and in many cases, dysgeusia is a temporary problem
• Factors contributing to ageusia or hypogeusia can also lead to abnormal taste sensitivity
• Taste disturbances can also occur under conditions in which serotonin (5-HT) and norepinephrine (NE) levels are altered (eg during anxiety or depression)

Question 49

New type taste cell researchers found

Answer 49

Now, a study from researchers at the University at Buffalo identifies a new type of taste cell capable of identifying a ‘broad range’ of flavors. The team states their data shines more light on how flavor is transmitted to the brain for processing. In addition, their trial also suggests taste buds are far more complex than previously thought. The opensource study is published in the journal PLOS Genetics.

Question 50