Physiology of taste, balance and smell

Question 1

The endolymph in the scala media in the cochlea is continuous with

Answer 1

the endolymph on the apical surfaces of the vestibular hair cells

Question 2

Describe the organisation and structure of the semicircular canal

Answer 2

The six semicircular canals are oriented at right angles to one another to detect head rotation in all directions

Question 3

Where are the sensory cells in the ampullae of the semicircular canals embedded?

Answer 3

cupula

Question 4

Where are the sensory cells in the otolith organs (sacculus and utriculus) embedded

Answer 4

in a gelatinous sheet covered with ‘heavy’ cristals of calcium carbonate: the otoliths

Question 5

Describe the 2 types of vestibular hair cells

Answer 5

most are type II vestibular hair cells, which receive both afferent and efferent innervation
the type I vestibular hair cells are surrounded by an afferent nerve calyx and the hair cells are not directly contacted by efferent nerve fibres

Question 6

Which vestibular hair cells are more sensitive?

Answer 6

Type II

Question 7

What do the semicircular canal receptors detect?

Answer 7

Rotation of the head

Question 8

What is nystagmus?

Answer 8

slow eye movements followed by fast ones during continuous head rotation

Question 9

What is spontaneous nystagmus?

Answer 9

where the eyes move rhythmically from side to side in the absence of any head movements.

Question 10

When does spontaneous nystagmus occur?

Answer 10

• occurs when one of the canals is damaged
• net differences in vestibular nerve firing rates exist even when the head is stationary because the vestibular nerve innervating the intact canal fires steadily when at rest, in contrast to a lack of activity on the damaged side.

Question 11

What is caloric testing used for?

Answer 11

used to test the function of the brainstem in an unconscious patient

Question 12

Why does caloric testing work?

Answer 12

Irrigating an ear with water slightly warmer or colder than body temperature generates convection currents in the canal that mimic the endolymph movement induced by turning the head to the irrigated side or away from it, respectively.
These currents result in changes in the firing rate of the associated vestibular nerve, with an increased rate on the warmed side and a decreased rate on the chilled side. As in head rotation and spontaneous nystagmus, net differences in firing rates generate eye movements.

Question 13

What do the receptors in the otolith organs detect?

Answer 13

linear acceleration and tilting of the head

Question 14

List the causes if vestibular disorders

Answer 14

Ear infection
Head injury 
Whiplash
Ageing
Certain drugs, e.g. aminoglycoside antibiotics (gentamicin) – also affect hearing

Question 15

What causes dizziness?

Answer 15

Light-headed - check cardiovascular

Vertigo (spinning) - check vestibular

Question 16

What is Benign paroxysmal positional vertigo (BPPV)?

Answer 16

vertigo caused by changes in head position

Question 17

What is Ménière’s disease?

Answer 17

Progressive disease
episodes of vertigo, tinnitus and progressive hearing loss, usually in one ear
Excess fluid in inner ear

Question 18

What is the function of smell?

Answer 18

social interactions (perfumes, deodorants)
avoidance of poisons / noxious gases
smell plays a major role in the enjoyment of food

Question 19

Describe the olfactory epithelium

Answer 19

The human olfactory epithelium has an area of 2-3 cm2 on each side of the nose

Question 20

Describe the action of the ciliated receptor cells

Answer 20

The ciliated receptor cells send their own afferent axons to the brain

Question 21

Describe the odorant receptors

Answer 21

There are more than 1000 different odorant receptor proteins, with each receptor cell expressing just one of these.
Each receptor cell responds to a number of different odours with action-potential firing

Question 22

How is olfactory information coded?

Answer 22

Olfactory information is coded not by individual receptor types but in the pattern of stimulation that the brain learns to interpret

Question 23

Describe the mechanism of olfactory transduction

Answer 23

Olfactory transduction depends on a second messenger process, with cAMP being activated in response to a odorant molecule
This leads to opening of cAMP-dependent ligand-gated ion channels
non-selective cation channels, permeable to Na+ and Ca2+
Na+ and Ca2+ influx (inward current in the figure) depolarizes the olfactory receptor cells, signalling the binding of an odorant molecule, and leading to action potentials
The Ca2+ influx indirectly opens Cl- channels which, due to the unusual high intracellular Cl- concentration of the olfactory receptors, contributes to the depolarization

Question 24

Describe the central pathways of olfactory system

Answer 24

Olfactory receptor
Olfactory bulb
Olfactory bulb targets
Orbitofrontal cortex, thalamus, hypothalamus and hippocampal formation

Question 25

Describe some clinical issues with olfaction

Answer 25

Hyposmia & anosmia
Very common, 5-10 % of population
Reduced quality of life

Question 26

What causes hyposmia and anosmia?

Answer 26

upper respiratory tract infection (inflammation), high age, nasal polyps, diabetes mellitus, head trauma, high dose radiation at nasal epithelium, some drugs

Question 27

What is the purpose of gustation?

Answer 27

Evaluating the nutritious content of food and preventing the ingestion of toxic substances.

Sweet: identification of energy-rich nutrients.
Umami (‘meaty’): recognition of amino acids.
Salty: ensures proper dietary electrolyte balance.
Sour and bitter: warn against the intake of potentially noxious and/or poisonous chemicals.

Additional value: contributes to the overall pleasure and enjoyment of a meal.

Question 28

List the 5 different tastes?

Answer 28

salty, sour, sweet, bitter and umami

Question 29

Describe the variation of the different tastes

Answer 29

There is regional variation in the sensitivity to different tastes, but there is considerable overlap so that most parts of the tongue can detect all five modalities

Question 30

What does salt sensation depend on?

Answer 30

The equilibrium potential for Na+ ions across the taste receptors:

Question 31

What does sour sensation depend on?

Answer 31

pH (acidity), with H+ ions (protons) closing K+ channels either directly or indirectly via a cAMP as a second messenger. This leads to depolarization of the taste receptors

Question 32

What does sweet sensation depend on?

Answer 32

a second messenger system that closes K+ channels, leading to depolarization of the taste receptors

Question 33

What is a bitter or urami sensation depend on?

Answer 33

second-messenger induced increase in intracellular Ca2+ in the receptors. The Ca2+ increase leads to neurotransmitter release

Question 34

What evidence is available to suggest single tastes receptors respond to a single taste receptor?

Answer 34

second-messenger induced increase in intracellular Ca2+ in the receptors. The Ca2+ increase leads to neurotransmitter release

Question 35

Describe the central pathways of gustation

Answer 35

Taste is signalled by cranial nerves VII, IX and X to the nucleus of the solitary tract in the brainstem. In primates, fibres (red lines) from second-order taste neurons project ipsilaterally to the ventral posterior nucleus of the thalamus. Thalamic efferents (green lines) then project to the insula, defining the primary gustatory cortex which, in turn, projects (black lines) to the orbitofrontal cortex, sometimes defined as a secondary cortical taste area. The parabrachial nuclei of the pons are shown in orange. In rodents these are a relay for taste afferents The parabrachial nuclei have a dorsal thalamocortical projection and also a ventral projection that terminates in amygdalar and hypothalamic nuclei, among others.

Question 36

List some clinical issues in gustation

Answer 36

80% of taste disorders are really smell disorders
Causes of true taste disorders:
prior upper respiratory tract infection, head injury, poor oral hygiene
Diagnosis is less obvious
Compared to smell disorders