Physiology of balance, smell and taste

Question 1

Orientation of the semicircular canals and otolith organs

Answer 1

6 semicircular canals orientated at right angles to one another to detect head rotation in all direction

Left and right lateral are functionally paired

Left anterior and right posterior functionally paired and same opposite

Fourth otolith organs not exactly at right angles

Question 2

Structure of semicircular canals and otolith organs

Answer 2

Sensory cells in ampullae of semicircular canals embedded in cupula

Sensory cells in otolith organs embedded in gelatinous sheet covered with ‘heavy’ crystals of calcium carbonate

Question 3

Vestibular hair cells

Answer 3

Type II vestibular receive both afferent and efferent innervation

Type I vestibular are surrounded by afferent nerve calyx and are not directly contacted by efferent nerve fibres

Type II appear to be more sensitive

Question 4

Nystagmus

Answer 4

Slow eye movements followed by fast ones during continuous head rotation

In normal individuals, rotating the head elicits physiological nystagmus

Spontaneous nystagmus where eyes move rhythmically from side to side in absence of head . movements

Question 5

Causes of spontaneous nystagmus

Answer 5

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 6

Caloric testing to test the function of the brainstem in an unconscious patient

Answer 6

Slow eye movements resulting from cold water irrigation in one ear for three different conditions:

1. with the brainstem intact
2. with a lesion of the medial longitudinal fasciulus
3. with a low brainstem lesion

Question 7

Why does caloric testing work?

Answer 7

Irrigating an ear with water slightly warmer or colder than body temperature generated 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 increases rate on the warmed side and a decreased rate on the chilled side

Question 8

Causes of vestibular disorders

Answer 8

Ear infection

Head injury

Whiplash

Ageing

Certain drugs e.g. aminoglycoside antibiotics

Question 9

Disorders of the vestibular system

Answer 9

Patient complains of ‘dizziness’

Trauma

Benign paroxysmal positional vertigo

Meniere disease

Question 10

Dizziness

Answer 10

Light headed -> check CV

Vertigo -> check vestibular

Question 11

Trauma

Answer 11

Esp CN VIII e.g. motorcycle accident

Question 12

Bening paroxysmal positional vertigo

Answer 12

Vertigo caused by changes in head position

Question 13

Meniere disease

Answer 13

Progressive disease

Episodes of vertigo, tinnitus and progressive hearing loss, usually in one ear

Excess fluid in inner ear

Question 14

Sense of smell important for

Answer 14

Social interactions

Avoidance of poisons/ noxious gases

Smell plays a major role in the enjoyment of food

Question 15

Where is the olflactory epithelium?

Answer 15

Area of 2-3cm on each side of the nose

Question 16

Organisation of the olfactory epithelium

Answer 16

Ciliated receptor cells send their own afferent axons to the brain

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

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

Question 17

Mechanism of olfactory transduction

Answer 17

Depends on second messenger process, with cAMP being activated in response to odorant molecule

Leads to opening of cAMP dependent ligand gated ion channels

Permeable to Na+ and Ca2+

Influx depolarises receptor, signalling binding of odorant molecule leading to action potential

Ca2+ influx directly opens Cl- channels which contributes to depolarisation

Question 18

Clinical issues with olfaction

Answer 18

Hyposmia and anosmia

Very common, 5-10% of population

Question 19

Causes of olfaction issues

Answer 19

Upper respiratory tract infection

High age

Nasal polyps

Diabetes mellitus

Head trauma

High dose radiation at nasal epithelium

Some drugs

Question 20

Sweet

Answer 20

Identification of energy rich nutrients

Question 21

Umami

Answer 21

Recognition of amino acids

Question 22

Salty

Answer 22

Ensures proper dietary electrolyte balance

Question 23

Sour and bitter

Answer 23

Warn against the intake of potentially noxious and/ or poisonous chemicals

Question 24

Organisation of the gustatory system

Answer 24

The tongue is the principle organ of taste

Five different modalities of taste

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 25

Salt sensation

Answer 25

Depends on equilibrium potential for Na+ ions across the taste receptors

Question 26

Sour sensation

Answer 26

Depends on pH, with H+ ion closing K+ channels either directly or indirectly via cAMP as a second messenger

Leads to depolarisation of taste receptors

Question 27

Sweet sensation

Answer 27

Comes about via a second messenger system that closes K+ channels leading to depolarisation of the taste receptors

Question 28

Bitter and umami sensations

Answer 28

Due to second messenger induced increase in intracellular Ca2+ in the receptors

Ca2+ increase leads to neurotransmitter release

Question 29

Central pathways of the gustatory system

Answer 29

Taste signalled by CN VII, IX and X to the nucleus of solitary tract in the brainstem

Fibres from second order taste neurons project ipsilaterally to the central posterior nucleus of the thalamus

THalamic efferent projects to insula, defining primary gustatory cortex which projects to orbitofrontal cortex

Parabrachial nuclei of pons are relay for taste afferent in rodents

Question 30

Clinical issues in gustation

Answer 30

80% taste disorders are really smell disorders

Causes
- prior upper respiratory tract infection, head injury, poor oral hygiene