Physiology of Taste and Smell

Question 1

“Receptors for taste and smell are chemoreceptors”

T/F?

Answer 1

TRUE

Receptors for taste and smell are chemoreceptors which are stimulated by binding to particular chemicals

Question 2

What are the sense organs of taste?

Answer 2

Taste buds

Question 3

Where are sensory receptor cells of taste found?

Answer 3

Sensory receptor cells of taste are mainly packahed in taste buds

Question 4

Discuss briefly the histology of taste buds

Answer 4

Taste buds consist of :

• sensory receptor cells

and

• support cells

arranfed like slices of orange

Question 5

Discuss briefly the histology of taste receptor cells

Answer 5

• Taste receptor cells are replaced from basal cells within the taste buds
• Taste receptor cells, within taste buds, synapse with afferent nerve fibres

Question 6

What is the life span of a taste receptor cell?

Answer 6

about 10 days

Question 7

Outline the structure and location of taste buds in the oral cavity and throat

Answer 7

The majority of taste buds sit in the papillae in the tongue

• papillae are finger-like structures which gives rise to the rough appearance of the dorsum of the tongue
• the human tongue contains 4 types of papillae; these are:
  
  • filliform: most numerous- do not contain taste buds
  • fungiform- contains taste buds
  • vallate- contains taste buds
  • foliate- contains taste buds

BUT taste buds are present in the

• tongue
• palate
• epiglottis
• pharynx

Question 8

Outline how the impulses from taste receptors are transmitted to the brain

Answer 8

1. Binding of tastant (taste provoking chemical) to receptor cells
2. Alters cell ionic channels and produces depolarising receptor potential
3. Receptor potential initiates action potentials in afferent nerve fibres
4. Signals conveyed by cranial nerves via brainstem adn thalamus to corticol gustatory areas

Question 9

Which cranial nerves convey signals from taste receptor cells to the brainstem?

Answer 9

Afferent taste fibres reach the brainstem via:

• VIIth cranial nerve (chorda tympani branch of facial nerve)
  
  • anterior 2/3 of the tongue
• IXth cranial (glossopharyngeal) nerve
  
  • posterior 1/3 of the tongue
• Xth cranial (vagus) nerve
  
  • areas oether than tongue including e.g. epiglottis and pharynx

Question 10

Outline the primary five modalities of taste

Answer 10

Five primary tastes

• salty
  
  • stimualated by chemical salts esp. NaCl
• sour
  
  • stimulated by acids which contain free hydrogen ions (H⁺)
• sweet
  
  • stimulated by configuration of glucose
• bitter
  
  • stimulated by diverse group of tastants incl. alkaloids, poisonous substances and toxic plant derivatives
• Umami (meaty or savory)
  
  • triggered by amino acids esp. glutamate

Question 11

Outline abnormalities of taste and give examples of causes

Answer 11

(may be temporal or permanent)

Abnormalities of taste include:

• Ageusia (loss of taste function)
  
  • can be caused by e.g. nerve damage, local inflammation (including e.g. glossitis, radiation, tobacco), some endocrine disorders
• Hypogeusia (reduced taste function)
  
  • can be caused by e.g. chemotherapy, medications
• Dysgeusia (distortion of taste function)
  
  • Many causes including: glossitis, gum infections, tooth decay, reflux, URTI, medicaitons, neoplasmas, chemotherapy, zinc deficiency

Question 12

Where is the olfactory mucosa found and what cell types can be found there?

Answer 12

Olfactory mucosa is a patch of mucosa in the ceiling (dorsal roof) of the nasal cavity

Contains 3 cell types:

• olfactory receptor cells
• supporting cells
• basal cells (secrete mucous)

Question 13

What is interesting about olfactory receptors?

Answer 13

Olfactory receptors are specialised endings of renewable afferent neurons

Each neuron has a thick short dendrite and an expanded end called an olfactory rod

From the olfactory rods, cilia project to the surface of olfactory mucosa

about 10-12 cilia per olfactory receptor neuron

odorants (molecules that can be smelled) bind to the cilia

Question 14

What is the life span of olfactory receptors?

Answer 14

about 2 months

Question 15

Discuss the afferent fibres of olfactory nerves

Answer 15

• Axons of olfactory receptors collectively form afferent fibres of olfactory nerve
• These pierce the cribiform plate of the ethmoid bone and enter the olfactory bulbs in the inferior surface of the brain
• Olfactory bulbs transmit smell information to the brain
• from the olfactory bulb neurons pass along the olfactory tract to reach the temporal lobe and

Question 16

How do odorants reach olfactory receptors?

Answer 16

During quiet breathing odorants only reach smell receptors by diffusion

The act of sniffing enhances smelling by drawing air currents upwards within the nasal cavity

Question 17

What is required for a substance to be an odorant?

Answer 17

To be smelled a substance must be:

1. Sufficiently volatile
   
   • some of its molecules can enter the nose with inspired air
2. Sufficiently water soluble
   
   • i.e. can dissolve in the mucus coating of olfactory mucosa
     3.

Question 18

How are humans able to differentiate between thousands of differnet odours?

Answer 18

Olfactory receptors have different sensitivity to different substances

Question 19

What is the name for the inability to smell?

What can cause this?

Answer 19

• Anosmia
  
  • inability to smell
  • can be temporary/permanent
  • causes include:
    
    • viral infections
    • allergy
    • nasal polyps
    • head injury

Question 20

What is the name for the reduced ability to smell?

What are some of the potential causes?

Answer 20

Hyposmia

Causes include:

1. viral infections
2. allergy
3. nasal polyps
4. head inury
5. may be an early sign of Parkinson’s disease

Question 21

What is the name for an altered sense of smell?

What are some examples of this?

Answer 21

Dysosmia

Examples include

• differently interpreting some odours
• hallucinations of smell