Neurobiology of smell and taste

Question 1

LOs:

• Describe the structure and function of the neural elements in the olfactory epithelium and olfactory bulb.
• Identify the significance of the family of olfactory receptor genes.
• Explain how odorant receptors are activated and the mechanism by which signal transduction occurs in these receptors.
• Label the components of the pathway by which impulses generated in the olfactory epithelium reach five regions of the olfactory cortex.
• Describe the location and cellular composition of taste buds.
• Name the five major taste modalities and compare the signal transduction mechanisms in the receptors mediating these different taste modalities.
• Label the components of the pathways by which impulses generated in taste receptors reach the gustatory region of the insular cortex.
• Name and discuss abnormalities in odor and taste sensations.

Question 2

Relationship between smell and taste

Answer 2

Both help us interpret the chemical world.

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

Question 3

Olfactory epithelium

what kind if tissue is it

what is it a part of?

where is it?

which 3 cells make it up?

Answer 3

pseudostratified epithelium

yellowish pigmented olfactory epithelium

specialised part of he nasal mucosa in the roof of the nasal cavity near the septum in humans

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

Question 4

Olfactory sensory/ receptor neurone

Answer 4

Responsible for olfactory transduction

Each has a dendrite that projects to the epithelial surface, at the end of each one is a knob from it numerous cilia protrude into the mucous 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 sensory neurone has a single axon that projects to the olfactory bulb thorugh the cribiform plate of the ethmoid bone to enter the olfactory bulb.

Question 5

olfactory bulb

Answer 5

small ovoid structure that rests on the cribiform plate of the ethmoid bone

sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OFC) and the hippocampus

Question 6

supporting cells in the olfactory epithelium

Answer 6

they secrete the mucous that provides the appropraite molecular nad ionic environment for odor detection in the olfcatory epithelium

Question 7

odor producing molecules and odorant binding proteins

Answer 7

odor producing molecules (ordorants)- dissolve in the mucous and bind to odorant receptors on the cilia of the olfactory sensory neurones

odorant binding proteins- in the mucous may facilitate the diffusion of odorants to and from the odorant receptor

Question 8

basal stem cells

Answer 8

undergo mitosis to undergo new olfactory sensory neurones as needed to replace those damaged by exposure to the environment

Question 9

odorant receptors and signal transduction

Answer 9

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

1. odorant binds to its receptor
2. the G-protein subunits dissociate
3. the a subunit activates adenylyl cyclase
4. adenylyl cyclase catalyzes the production of cAMP
5. cAMP acts a second messenger to open cation channels
6. membrane permeabilty to Na+, K+ and Ca2+ increases
7. net effect: inward directed Ca2+ current creating a graded potential
8. Ca2+ activated Cl- cahnnels open further depolarising cell as Cl- kleaves neurone

if the stimulus is great enough and the threshold potential is overcome then an action potential in CNI is generated

Question 11

olfactory sensory pathway

Question 12

Odor detection threshold

Answer 12

odorants are generally small (3-> 20 Carbons) molecules with the same number of carbons but different structural configurations have different odours

relaticely high water and lipid solubility is characteristic of substances with strong odours

odour detection threshold- lowest concentration of a chemical that can be detected (this is not the same in everyone)

takes a big change to perceive a change in smell intensity

Question 13

abnormalities in odour detection

Asomnia-

hyposmia

Dyosmia-

serious head trauma to the cribiform plate or head, tumours and resp. tract infections can casue serious damage to what

Answer 13

Asomnia- inability to smell

hyposmia and hypesthesia- diminished olfactory sensitivity

Hyperosmia- enhanced olfactory sensitivity (pregnant women)

Dyosmia- distorted sense of smell

olfactory nerve

Question 14

what is the primary organ of taste (gustation)

where are they found

what are the types

Answer 14

5,000 taste buds located on papillae on the dorsal aspect of the tongue

• fungiform pupillae- rounded structurees near tip of tongue
• foliate papillae- posterior edge of the tongue
• circumvallate papillae- prominent structrues arranged in a V in the back of the tongue

taste buds can also be found in the soft palate, epiglottis and pharynx

Question 15

taste buds on the anterior 2/3 are innervated by

posterior a

/3 innervated by

Answer 15

chorda tympani branch of the facial nerve

lingual branch of the glossopharyngeal nerve

Question 16

taste buds are composed of 2 groups of cells

Answer 16

stem cells

taste cells

Question 17

taste cells

Answer 17

3 kinds:

dark, light. intermediate

extend from the base of the taste bud to the taste pore, where microvilli contact tastants dissolved in mucous and saliva

Each taste bud contains 50–100 taste receptor cells and numerous basal cells and support cells

Question 18

taste receptors

Answer 18

modified epithelial cells that respond to chemical stimuli or tastants

Apical ends have microvilli that project into the taste pore, a small opening on the dorsal surface of the tongue where taste cells are exposed to the oral contents.

Question 19

role of saliva in taste-

Answer 19

Acts as a solvent for tastants; after dissolving, the chemical diffuses to the taste receptor sites

cleanse the mouth to prepare the taste receptors for new stimulant

Question 20

basal cells

Answer 20

Basal cells arise from the epithelial cells surrounding the taste bud and differentiate into new taste cells

Question 21

If the sensory nerve to a taste bud is cut

Answer 21

the bud will degnerate and eventually disappear

Question 22

taste pathways

Answer 22

The sensory nerve fibres from the taste buds on the anterior 2/3 of the tongue travel in the chorda tympani branch of the facial nerve

those from the posterior 1/3 of the tongue reach the brainstem via the glossopharyngeal neve.

Fibres from areas other than the tongue (e.g., pharynx), reach the brainstem via the vagus nerve (CNX)

these 3 nerve fibres unite in the gustatory portion of the nucleus of the tractus solitarius (NTS) in the medulla oblongata

from there seconf order neurones ascend in the ipsilateral medial lemniscus and project to the ventral posteromedial nucleus of the thalamaus

third order neurones pass to neurones in the anterior insula and the frontal operculum in the postcentral gyrus

primary gustatory cortex- located in the parietal lobe within the insular and opercular cortex of the frontal lobe

Question 24

which receptors and nerve is involved in transmission of spicy food taste info

Answer 24

Sensory fibres in the trigeminal contribute to the burning sensation experienced when we eat foods containing capsaicin.

Taste buds are surrounded by TRPV1 receptors

Question 25

what are the 5 basic modalities of taste

give an example of a common stimuli for each

Answer 25

Salt (common stimuli- sodium chloride)

Sweet (-sucrose)

Sour (-hydrochloric acid)

Bitter (-quinine)

Umami (-monosodium glutamate)

Question 26

can a taste cell respond to more than one type of tastant?

Answer 26

yes

CNS can distinguish tastes from one another because each one connects to a particular gustatory axon

Question 27

what are the 2 major types of receptor involved in taste receptors

which flavours trigger each

Answer 27

Ligand gated (ionotropic)- salt and sour trigger them

GPCRs (metabolic)- sour, bitter and umami trigger

Question 28

salt sensitive taste which channels

Answer 28

mediated by ENaC’s (epithelial sodium channel)

entry of Na+ into the salt receptor depolarizes the membrane, generating a receptor potential

Question 29

sour taste

Answer 29

triggered by H+ protons

1. ENaCs permit entry of H+ so they may be involved
2. also bind to and block a K+ sensitive channel, the fall in K+ permeability can depolarise the membrane
3. HCN and other mechanisms also

Question 30

bitter

Answer 30

produced by a variety of unrelated compounds, many of which are poisonous

Some bitter compounds (e.g., quinine) are membrane permeable and bind to and block K+ selective channels

Many bitter tastants bind to GPCRs that couple to the G-protein, gustdicing.

Gustducin lowers cAMP and increases the formation of inositol phosphates, which releases Ca2+ to trigger depolarisation.

Question 31

umami

Answer 31

tastants activate a receptor comprised of T1R1 and T1R3

may involve the activation of a truncated metabotropic glutamate receptor, mGluR4, in the taste buds.

Question 32

intensity discrimination

Answer 32

30% change in tastant concentration is necessary before a difference can be detected

Question 33

taste threshold

Answer 33

minimum concentration at which a substance can be perceived

Question 34

Damage to the lingual or glossopharyngeal nerve may cause

Answer 34

Ageusia (absence of the sense of taste)

Hypogeusia (diminished taste sensitivity)

Question 35

causes for problems with taste sensitivity

Answer 35

Neurological disorders such as vestibular schwannoma

▪ Bell palsy

▪ Familial dysautonomia

▪ Multiple sclerosis

▪ Certain infections (eg, primary ameboid meningoencephalopathy)

▪ poor oral hygiene

Diminished taste:

▪ Aging

▪ Tobacco abuse

occur under conditions in which serotonin (5-HT) and norepinephrine (NE) levels are altered (eg, during anxiety or depression)

Question 36

Dysgeusia or Parageusia

Answer 36

unpleasant perception of taste) causes a metallic, salty, foul, or rancid taste and in many cases, dysgeusia is a temporary problem