14.2 Neurobiology of smell & taste

Question 1

What is smell & taste known as?

Answer 1

• Smell = olfaction
• Taste = gustation

They are examples of visceral senses due to their close association with GI function

Question 2

What type of receptors are smell & taste and how are they stimulated?

Answer 2

• They are chemoreceptors
• Stimulated by chemical molecules in solution in mucus in the nose (odorants) and saliva in the mouth (tastants) and the sensations of smell and taste likely evolved as protective mechanisms to avoid the intake of potentially harmful substances

Question 3

Briefly describe the olfactory epithelium

Answer 3

• Yellowish pigmented
• Is a specialized portion of the nasal mucosa that covers an area of 10 cm2 in the roof of the nasal cavity near the septum in humans
• Is in an area where the nervous system is closest to the external world

Question 4

What are the 3 types of cells in the olfactory epithelium?

Label the diagram

Answer 4

There are three cell types:

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

Question 5

How is smell picked up?

Answer 5

• Each olfactory sensory neuron (bipolar olfactory sensory neurons - do olfactory transduction) has a dendrite (short & thick dendrites) 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 initiate a cascade of events leading to generation of action potentials in the sensory axon
• Each olfactory sensory neuron has a single axon that projects to the olfactory bulb, a small ovoid structure that rests on the cribriform plate of the ethmoid bone

Question 6

Label this diagram (to do with smell)

Answer 6

Question 7

What is the function of supporting cells?

Answer 7

Secrete the mucus that provides the appropriate molecular and ionic environment for odor detection in the olfactory epithelium

Question 8

How do odor-producing molecules (odorants) get picked up?

Answer 8

• Odor-producing molecules (odorants) dissolve in the mucus and bind to odorant receptors on the cilia of the olfactory sensory neurons
• Odorant- binding proteins in the mucus may facilitate the diffusion of odorants to and from the odorant receptor

Question 9

Label the diagram of the nose

Answer 9

Question 10

How do olfactory sensory neurons get replaced?

Answer 10

• By basal stem cells undergoing mitosis (into olfactory sensory neurons) e.g. from damage from environmental exposure
• Olfactory sensory neurons ~ survive 1-2 months

Question 11

Explain the olfactory mucosa from a histological view

Answer 11

• 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 cilia which initiate a nervous impulse, interpreted in the brain as an odor
• Supporting cells surround the olfactory cells
• Mucus-producing Bowman’s glands are embedded in the lamina propria
• This connective tissue is richly vascularized

Question 12

How can the olfactory system discriminate between different odors?

Answer 12

It can discriminate between different smells thanks to the existance of many different functional odorant receptors

Question 13

Explain the genetics behind olfaction

Answer 13

• 1000 olfactory genes
  
  • 400 of these are for olfactory receptors
• Amino acid sequence of odorant receptors = DIVERSE
  
  • BUT are ALL G protein-coupled receptors (GPCRs)

Question 14

On a microscopic level what happens when an odorant molecule binds to its receptor? (steps)

Answer 14

1. G-protein subunits (α, β, γ) dissociate
2. The α-subunit activates adenylyl cyclase to catalyze 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 an inward-directed Ca2+ current which produces the GRADED receptor potential
4. This then opens Ca2+ activated Cl− channels, further depolarizing 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 nerve) is triggered

6.

Question 15

What happens in olfaction in the olfactory bulb?

Answer 15

1. Axons of the olfactory sensory neurons synapse on the primary dendrites of the mitral cells and tufted cells to form olfactory glomeruli
   
   • Each olfactory sensory neuron projects to only one or two glomeruli
2. The mitral cells with their glomeruli project to different parts of the olfactory cortex
   
   • ​The central olfactory system is able to decode the pattern of receptor-cell activity that signals the identity of the odorant

Question 16

What are the inhibitory & excitatory neurotransmitters in the olfactory system?

Answer 16

• INHIBITION = GABA
• EXCITATORY = GLUTAMATE

Question 17

Label the diagram in the olfactory system

Answer 17

Question 18

Explain briefly about odorants and the odor detection threshold

Answer 18

• Odorants are generally small, containing 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
  
  • ​Is NOT the same in ALL individuals
    
    • Usually woman have a BETTER sense of smell than men & especially during OVULATION
• Some toxic substances are essentially odorless; they have odor detection thresholds higher than lethal concentrations
  
  • E.g. carbon dioxide (will die before detected)
• To detect a change in concentration of odor producing substance must be about 30% difference before detected

Question 19

What is anosmia & hyposmia/hypersthesia & congenital anosmia?

Answer 19

• Anosmia = inability too smell
• Hyposmia/hypersthesia = diminished olfactory sensitivity

These can be due to simple nasal congestion, nasal polyps, OR prolonged use of nasal decongestants

Can also be due to 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

Olfactory dysfunction = often early clinical symptom of Alzeihmer’s disease

Congenital anosmia = rare disorder where someone is BORN without the ability to smell

Question 20

What is hyperosmia & dysosmia? (& common causes)

Answer 20

• Hyperosmia = ENHANCED olfactory sensitivity
  
  • Causes:
    
    • In pregnant women
• Dysosmia = DISTORTED sense of smell
  
  • Causes:
    
    • Sinus infections
    • Partial damage to the oflactory nerves
    • Poor dental hygiene

Question 21

What is the organ of taste known as and where is it located?

Answer 21

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 22

What are the different types of papillae?

Answer 22

• Fungiform papillae: rounded structures most numerous near the tip of the tongue
  
  • Up to 5 taste buds (on top)
• Foliate papillae: are on the posterior edge of the tongue
  
  • Contain up to 100 taste buds (sides)
• Circumvallate papillae: are prominent structures arranged in a V on the back of the tongue
  
  • Contain up to 100 taste buds (sides)

Question 23

Explain the innervation of the tongue

Answer 23

Taste buds on the anterior two-thirds of the tongue are innervated by the chorda tympani branch of the facial nerve; those on the posterior one-third of the tongue are innervated by the lingual branch of the glossopharyngeal nerve

Question 24

What are taste buds composed of?

Answer 24

• Taste buds are composed of basal stem cells and three types of taste cells:
  
  1. Dark
  2. Light
  3. 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 25

What are **taste receptors** and how do they work?

Answer 25

* The 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 surface of the tongue where *tastes* cells are exposed to the oral contents

Question 26

How does **saliva** function in taste?

Answer 26

* Acts as a ***_solvent_*** for tastants; after *dissolving*, 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 fibers_*, and conversely, each nerve fiber receives input from an average of five taste buds

Question 27

How are **taste cells** produced and how long do they live for?

Answer 27

* **Basal cells** (from epithelial cells aroud taste bud) into **taste cells** * Survive ~**10 days**

Question 28

What happens to taste buds when **sensory fibres** are cut?

Answer 28

Taste buds that it innervates **degenerate** and then suddenly disappear.

Question 29

Explain the **gustation** area with the 3 nerves

Answer 29

On each side, the myelinated but relatively slowly conducting taste fibers in these three nerves unite in the *_gustatory portion_* of the ***_nucleus of the tractus solitarius (NTS)_*** in the *_medulla oblongata_*

Question 30

Explain the nerve pathway from the tongue to brain

Answer 30

1. On each side, the myelinated but relatively slowly conducting taste fibers in these three nerves unite in the gustatory portion of the ***_nucleus of the tractus solitarius (NTS)_*** in the medulla oblongata 2. From there, axons of second-order neurons ascend in the ***_ipsilateral medial lemniscus_*** and project directly to the ***_ventral posteromedial nucleus_*** of the thalamus 3. From the thalamus, the axons of the third-order neurons pass to neurons in the ***_anterior insula_*** and the ***_frontal operculum_*** in the ipsilateral cerebral cortex 4. This region is rostral to the face area of the *postcentral gyrus,* which is probably the area that mediates conscious perception of taste and taste discrimination

Question 31

Explain where the **gustation cortex** is?

Answer 31

Question 32

Explain the **TRPV1 receptors** of taste buds

Answer 32

* 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_*** (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 33

What are the basic taste modalities?

Answer 33

* 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) * These have **2** major types of receptors: 1. **_Ligand-gated channels_** (ionotropic receptors) * *Salt & sour* 2. **_GPCRs_** (metabotropic receptors) * *Sour, bitter & umami*

Question 34

What is **salt-sensitive taste** mediated by?

Answer 34

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 35

Explain the mechanism of **sour tasting**?

Answer 35

* The sour taste is triggered by ***_protons_*** (H+ ions) as ***_ENaCs_***(epithelial sodium channel) permit the entry of *protons* and may contribute to the sensation of sour taste * The H+ ions can also bind to and *_block_* a ***_K+ sensitive channel_*** * The fall in K+ permeability can *depolarize* the membrane * Also, a *hyperpolarization*-activated cyclic nucleotide-gated cation channel (HCN) and other mechanisms may contribute to sour transduction

Question 36

What picks up **sweet** taste?

Answer 36

**2** *GPCRs*: * **T1R2** * **T1R3**

Question 37

Process of **bitter taste** explain

Answer 37

* Bitter taste is produced by a variety of unrelated compounds * Many of these are *_poisons_*, and bitterness serves as a warning to avoid them * Some bitter compounds (eg, quinine) are membrane permeable and bind to and *_block K+-selective channels_* * Many bitter tastants (eg, strychnine) bind to *_GPCRs_* (T2R family) that couple to the heterotrimeric G-protein, gustducin * Gustducin *lowers* *cAMP* and increases the formation of inositol phosphates (IP3), which *_releases Ca2+ to trigger depolarization_*

Question 38

Process of **umami** taste explain

Answer 38

* Umami tastants activate a receptor comprised of **_T1R1_** and **_T1R3_** * Umami taste may also involve the activation of a ***_truncated metabotropic glutamate receptor, mGluR4_***, in the taste buds

Question 39

What ***_concentration change_*** of tastant is nercessary to detect a difference?

Answer 39

**30%**

Question 40

What is a **taste threshold**?

Answer 40

Minimum concentration at which a substance can be perceived (*bitter* taste usually lower threshold)

Question 41

What can damage to **linguinal**/**glossopharyngeal nerve** cause (taste)?

Answer 41

* ***_Ageusia_*** (absence of the sense of taste) * ***_Hypogeusia_*** (diminished taste sensitivity)

Question 42

What can problems with ***taste sensitivity*** be due to?

Answer 42

* Neurological disorders such as vestibular schwannoma * Bell palsy * Familial dysautonomia * Multiple sclerosis * Certain infections (eg, primary ameboidmeningoencephalopathy) * Poor oral hygiene * When *serotonin &/or norepinephrine* levels are altered (conditions - *_taste disturbances_*)

Question 43

What can **diminished** taste be due to?

Answer 43

* Aging * Tabacco use

Question 44

What is **dysgeusia/parageusia**?

Answer 44

* *_Unpleasant perception_* of taste causes a *metallic*, *salty*, *foul*, or rancid taste and in many cases, dysgeusia is a temporary problem

Question 45

Where are the taste types found in a taste bud?

Answer 45