HUF 2-56 Sensations of smell and taste

Question 1

Olfactory epithelium

Answer 1

• Olfactory sensory neurons
• Dorsal and posterior nasal cavity
• Quite breathing: only small portion of air reaches it
• Active sniffing: ↑ fraction
• Surface of nasal cavity inflated by presence of conchae (highly vascular; erectile tissue)
• Cold: lose sensation of smell
  ∵ Conchae are engorged with blood
  => ↓ air reaching olfactory epithelium

Question 2

Olfactory sensory neurons

Answer 2

• Cilia: odorant receptor molecules
• Short-lived; constantly replaced by new cells from basal stem cells
• Bowman’s gland
  => Lipid-rich mucus
  ∴ Lipid-soluble => odor

Question 3

Odorant receptor superfamily

Answer 3

• GPCR; 7 transmembrane domains
• Diversity in aa sequence
  => Recognise diff. molecular features of odorants
• ~400 functional members in human (largest family in human genome)
• Each olfactory sensory neurons express ONE and ONLY ONE of these receptors
  ∵ aa changes in 1 single odorant receptor
  => Diff. in odor perception among individuals

Question 4

Chemoelectrical transduction by odorant receptors

Answer 4

- GPCR
=> cAMP
=> Open cyclic nucleotide-gated channels
=> Na+ and Ca2+ influx
=> Deplorisation
=> Firing of olfactory sensory receptors

• Desensitisation of odorant receptors
  => Cease to notice odor after cont. exposure

Question 5

How do odorant receptors code diff. odors?

Answer 5

Odorant molecule coded by combination of odorant receptors that the molecule activates

Question 6

Organisation of olfactory bulb

Answer 6

• Olfactory neurons bearing same olfactory receptors spread across olfactory epithelium
• Axons converge to glomerulus (specialised synaptic regions in olfactory epithelium)
• Diff. odorants produce diff. patterns of glomerulus activities
• Mitral cells: convey signal collected in glomeruli into CNS
  => Recognition

Question 7

Central processing of olfactory info.

Answer 7

1° olfactory cortical areas

• Ant. olfactory nu.
• Olfactory tubercle
• Piriform cortex
• Amygdala
• Entorhinal cortex
• Emotive aspects of olfactory sensation

2° olfacotry cortical areas

• Orbitofrontal cortex (from thalamus)
• Frontal cortex
• Conscious discrimination of odors

Question 8

Common disorders of smell sensation

Answer 8

Anosmia: absence of sense of smell
Hyposmia: diminished olfactory sensitivity
Dysosmia: distorted sense of smell

Causes:

• Absence / disrupted functions of odorant receptors
• Hypogonadism (Kallmann’s syndrome)
• Ageing: olfactory threshold ↑ with age

Question 9

Taste papilla

Answer 9

3 types:

1. Circumvallate
2. Foliate
3. Fungiform

Innervation:

• Ant. 2/3: chorda tympani
• Pos. 1/3: CN9

Tongue, oral mucosa of palate and epiglottis

Question 10

Taste buds and taste cells

Answer 10

• 50 - 150 taste cells in each taste bud => Gustatory aff. n.
• Microvilli in taste pore (in contact with dissolved tastants)
• Saliva: food molecule delivery
• Taste cells are short-lived (9-10 days)
  ∴ Constantly replaced by basal cells (stem cells)

Question 11

Types and meaning of taste

Answer 11

1. Sweet: source of metabolic energy
2. Bitter: poison
3. Sour: acidity e.g. fruit not ripe yet; bacterial activity
4. Salty: NaCl (electrolyte)
5. Umami: GLU (source of protein)

Specific region of tongue is more sensitive to specific taste

Question 12

Chemoelectric transduction of taste

Answer 12

Sweet: Molecules bind to GPCR
Bitter: Molecules block K+ channels directly OR bind to GPCR (intracelluar signalling => membrane excitability)
Sour: Direct entry of H+ through Na-channels OR blockade of K+ channels
Salty: Na+ pass through Na-selective ion channels directly

Question 13

Central projections of gustatory signals

Answer 13

1. Chorda tympani => Geniculate ganglion
   CN9 => Petrosal ganglion
   Pharynx => Nodose ganglion (CN10)
2. Gustatory area of nu. of solitary tract
3. VP / VM nu. of thalamus
4. Gustatory cortex (ant. insula-frontal operculum)
5. 2° gustatory cortical area in orbitofrontal cortex

Question 14

Common disorders of taste sensation

Answer 14

Ageusia: absence of sense of taste
Hypogeusia: diminished taste sensitivity
Dysgeusia: distorted sense of taste
Bell’s palsy: impair taste sensation

‘Taste’: combination of sensation of taste, olfaction, temp., somatosensation (pain, texture)

Gustatory cortex: also receives thermal, mechanical and nociceptive stimuli
Orbitofrontal cortex: integrate olfactory and gustatory signal
=> final taste sensation

• Diminished sensation of smell in common cold or flu
  => ↓ sensation of taste