Physiology of taste, balance and smell Flashcards
The endolymph in the scala media in the cochlea is continuous with
the endolymph on the apical surfaces of the vestibular hair cells
Describe the organisation and structure of the semicircular canal
The six semicircular canals are oriented at right angles to one another to detect head rotation in all directions
Where are the sensory cells in the ampullae of the semicircular canals embedded?
cupula
Where are the sensory cells in the otolith organs (sacculus and utriculus) embedded
in a gelatinous sheet covered with ‘heavy’ cristals of calcium carbonate: the otoliths
Describe the 2 types of vestibular hair cells
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
Which vestibular hair cells are more sensitive?
Type II
What do the semicircular canal receptors detect?
Rotation of the head
What is nystagmus?
slow eye movements followed by fast ones during continuous head rotation
What is spontaneous nystagmus?
where the eyes move rhythmically from side to side in the absence of any head movements.
When does spontaneous nystagmus occur?
- 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.
What is caloric testing used for?
used to test the function of the brainstem in an unconscious patient
Why does caloric testing work?
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.
What do the receptors in the otolith organs detect?
linear acceleration and tilting of the head
List the causes if vestibular disorders
Ear infection Head injury Whiplash Ageing Certain drugs, e.g. aminoglycoside antibiotics (gentamicin) – also affect hearing
What causes dizziness?
Light-headed - check cardiovascular
Vertigo (spinning) - check vestibular
What is Benign paroxysmal positional vertigo (BPPV)?
vertigo caused by changes in head position
What is Ménière’s disease?
Progressive disease
episodes of vertigo, tinnitus and progressive hearing loss, usually in one ear
Excess fluid in inner ear
What is the function of smell?
social interactions (perfumes, deodorants) avoidance of poisons / noxious gases smell plays a major role in the enjoyment of food
Describe the olfactory epithelium
The human olfactory epithelium has an area of 2-3 cm2 on each side of the nose
Describe the action of the ciliated receptor cells
The ciliated receptor cells send their own afferent axons to the brain
Describe the odorant receptors
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
How is olfactory information coded?
Olfactory information is coded not by individual receptor types but in the pattern of stimulation that the brain learns to interpret
Describe the mechanism of olfactory transduction
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
Describe the central pathways of olfactory system
Olfactory receptor
Olfactory bulb
Olfactory bulb targets
Orbitofrontal cortex, thalamus, hypothalamus and hippocampal formation
Describe some clinical issues with olfaction
Hyposmia & anosmia
Very common, 5-10 % of population
Reduced quality of life
What causes hyposmia and anosmia?
upper respiratory tract infection (inflammation), high age, nasal polyps, diabetes mellitus, head trauma, high dose radiation at nasal epithelium, some drugs
What is the purpose of gustation?
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.
List the 5 different tastes?
salty, sour, sweet, bitter and umami
Describe the variation of the different tastes
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
What does salt sensation depend on?
The equilibrium potential for Na+ ions across the taste receptors:
What does sour sensation depend on?
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
What does sweet sensation depend on?
a second messenger system that closes K+ channels, leading to depolarization of the taste receptors
What is a bitter or urami sensation depend on?
second-messenger induced increase in intracellular Ca2+ in the receptors. The Ca2+ increase leads to neurotransmitter release
What evidence is available to suggest single tastes receptors respond to a single taste receptor?
second-messenger induced increase in intracellular Ca2+ in the receptors. The Ca2+ increase leads to neurotransmitter release
Describe the central pathways of gustation
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.
List some clinical issues in gustation
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
