Physiology of Taste and Smell

Question 1

What are the receptors for taste and smell?

Answer 1

Chemoreceptors (stimulated by binding of particular chemicals)

Question 2

Purpose of ability to taste and smell?

Answer 2

Pleasurable/objectionable sensation

Provides a checkpoint for quality control, e.g: expired food has a bad smell

Influence flow of digestive juices

Question 3

How is taste perception influenced?

Answer 3

By info from smell receptors

Question 4

What is taste also known as?

Answer 4

Gustation

Question 5

Location of the sensory receptor cells of taste?

Answer 5

Mainly packaged in taste buds (sense of organs of taste)

Question 6

Constituents of taste buds?

Answer 6

Sensory receptor cells and support cells

These are both arranged alternately (like slices of an orange)

Additionally, there are also basal cells

Question 7

Life span of taste receptor cells?

Answer 7

10 days

Question 8

How are taste receptor cells replaced?

Answer 8

By basal cells that are also within the taste buds

Question 9

Locations of taste buds?

Answer 9

Mainly on the:
• Tongue
• Palate
• Epiglottis
• Pharynx

Question 10

How are taste buds arranged on the tongue?

Answer 10

In the papillae on the tongue

Question 11

What are papillae?

Answer 11

Finger-like, raised bumps on the dorsum of the tongue that produce the rough appearance

Question 12

4 types of papillae on the tongue?

Answer 12

1. Filiform (most common but are the only ones that do not contain taste buds)
2. Fingiform
3. Vallate
4. Foliate

Question 13

How do taste buds communicate with the NS?

Answer 13

Taste receptor cells, within the buds, synapse with afferent nerve fibres

Question 14

Steps in the transmission of info from the taste buds to the brain?

Answer 14

1. Binding of taste-provoking chemical (AKA tastant) to the receptor cells stimulates a depolarisaion
2. Receptor potential initiates an action potential in the afferent nerve fibres, which synapse with receptor cells
3. Signals are conveyed by the CNs, via the brainstem and the thalamus, to the cortical gustatory areas

Question 15

How do afferent taste fibres reach the brainstem?

Answer 15

Facial nerve (chorda tympani branch) - anterior 2/3rd region of the tongue

Glossopharyngeal nerve - posterior 1/3rd of the tongue

vagus nerve - areas other than the tongue, e.g: pharynx and epiglottis

Question 16

5 primary tastes?

Answer 16

Salty - stimulated by chemical salts, esp. NaCl

Sour - stimulated by acids which contain free H+ ions

Sweet - stimulated by configuration of glucose

Bitter - stimulated by a diverse group of tastants, inc. e.g: alkaloids, poisonous substances and toxic plant derivatives

Umami (meaty/savory): triggered by amino acids, esp. glutamate

Question 17

What is ageusia and potential causes?

Answer 17

Loss of taste function that may be caused by:
• Nerve damage
• Local inflammation, e.g: glossitis, radiation, tobacco
• Endocrine disorders

Question 18

What is hypogeusia?

Answer 18

Reduced taste function:
• Chemotherapy
• Drugs

Question 19

What is dysgeusia?

Answer 19

Distortion of taste function:
• Glossitis
• Gun infections
• Tooth decay
• Reflux
• URT infections
• Drugs and chemotherapy
• Neoplasms
• Zinc deficiency

Question 20

What is the olfactory mucosa and where is it located?

Answer 20

Patch of mucosa in the dorsal roof of the nasal cavity

Question 21

3 constituent cell types in the olfactory mucosa?

Answer 21

1. Olfactory receptor cells - specialised ending of renewable afferent neurons
2. Supporting cells -
3. Basal cells - secrete mucous

Question 22

Structure of the olfactory receptor cells?

Answer 22

Each neuron has a thick, short dendrite and an expanded end, called an OLFACTORY ROD; from here, cilia project from the surface of the olfactory mucosa

Question 23

How many cilia project from each olfactory receptor neuron?

Answer 23

10-12

Question 24

What are odorants?

Answer 24

Molecules that can be smelled; they bind to the cilia

Question 25

Lifespan of olfactory receptors and how are they replaced?

Answer 25

2 months

Basal cells act as precursors for new olfactory receptor cells

Question 26

How is the olfactory nerve formed?

Answer 26

Axons of olfactory receptors collectively form the afferent fibres of the olfactory nerve

Question 27

Describe the olfactory pathway

Answer 27

Olfactory nerve pierces the cribriform plate of the thmoid bone and enters the olfactory bulbs (in the inferior surface of the brain)

From here, neurones pass along the olfactory tract to reach the temporal love and olfactory areas

Question 28

Function of the olfactory bulbs?

Answer 28

Transmit smell info to the brain

Question 29

How do odorants reach the smell receptors?

Answer 29

During quiet breathing, by diffusion; sniffing enhances smelling by drawing air currents upwards within the nasal cavity

Question 30

2 necessities for a substance to be smelled?

Answer 30

1. Sufficiency volatile - some of its molecules can dissolve in the mucous coating of the olfactory mucosa
2. Sufficiency water soluble - dissolve in the mucous coating of the olfactory mucosa

Question 31

Sensitivity of olfactory receptors?

Answer 31

Different sensitivity to different substances

Question 32

What is anosmia and porential causes?

Answer 32

Inability to smell:
• Viral infection
• Allergy
• Nasal polyps
• Head injury

Question 33

What is hyposmia?

Answer 33

Reduced ability to smell:
• As for anosmia

May be an EARLY SIGN of Parkinson’s disease

Question 34

What is dysosmia?

Answer 34

Altered sense of smell:
• Differently interpreting some odours
• Hallucinations of smell