Physiology of balance, taste and smell Flashcards
What are the semi-circular canals
- The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear.
- The three canals are the horizontal, superior and posterior semicircular canals
What is the cochlear duct/scala media
- Is an endolymph filled cavity inside the cochlea, located in between the tympanic duct and the vestibular duct, separated by the basilar membrane and Reissner’s membrane
What is the endolymph in the scala media continuous with
- Endolymph in scala media in the cochlea is continuous with the endolymph on the apical surfaces of the vestibular hair cells
How many semicircular canals are there
- The six semicircular canals
How are the six semicircular canals orientated
- Orientated at right angles to one another to detect head rotation in all directions
Which semicircular canals are functionally paired
- Left and right lateral semicircular canals
- Left anterior(= superior) and right posterior canals
- Left posterior and right anterior canals
What are otolith organs
- In vertebrates, the saccule and utricle make the otolith organs
Orientation of the otolith organs
- The four otolith organs are not exactly at right angles, to enable them to resolve head tilt and linear acceleration in all directions
Where are the sensory cells in the ampullae and the semicircular canals embedded
- In a cupula
Where are the sensory cells in the otolith organs embedded
- Embedded in a gelatinous sheet covered with ‘heavy’ crystals of calcium carbonate(the otoliths)
What is the membranous labyrinth
- The membranous labyrinth is a collection of fluid filled tubes and chambers which contain receptors for the senses of equilibrium and hearing
What separates the bony and membranous labyrinth
- Partly separated by the perilymph
What does the membranous labyrinth contain
- Endolymph
What are the two types of vestibular hair cells in semicircular canals and the otolith organs
- Type I
- Type II
Innervation of type II vestibular hair cells
- Receive both afferent and efferent innervation
Innervation of type I vestibular hair cells
- Surrounded by an afferent nerve calyx and the hair cells are not directly contacted by efferent nerve fibres
Functional difference between type 1 and type 2 vestibular hair cells
- Unclear but type II cells appear to be more sensitive
What is the cupula
- A cupula is a small, inverted cup or dome-shaped cap over a structure
Names of cupula in ear
- Ampullary cupula
- Cochlear cupula
Role of ampullary cupula
- A structure in the vestibular system, providing the sense of spatial orientation
How do semicircular canals detect rotation of the head
- Receptors
What elicits physiological nystagmus
- In normal individuals, rotating the head elicits physiological nystagmus
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
How can brainstem be tested in an unconscious patient
- Caloric testing
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 receptors in otolith organs detect
- Linear acceleration and tilting of the head
Causes of vestibular disorders
- Ear infection
- Head injury
- Whiplash
- Ageing
- Certain drugs, e.g. aminoglycoside antibiotics (gentamicin) – also affect hearing etc
Disorders of the vestibular system
- Patient complains of ‘dizziness’
- Trauma
- Benign paroxysmal positional vertigo(BPPV)
- Meniere’s disease
“Dizziness” - Vestibular system
- Light-headed à check cardiovascular
* Vertigo (spinning) à check vestibular
Nerve likely to be affected in trauma to vestibular system
- CN VIII
- eg. motorcycle accident
What is benign paroxysmal positional vertigo caused by
- Vertigo caused by changes in head position
What is meniere’s disease
- Progressive disease
- Episodes of vertigo, tinnitus and progressive hearing loss, usually in one ear
- Excess fluid in inner ear
Signal transmission between ciliated receptors cells and brain
- The ciliated receptor cells send their own afferent axons to the brain
Interpretation of olfactory info
- Olfactory information is coded not by individual receptor types, but in the pattern of stimulation that the brain learns to interpret
- Each receptor cell responds to a number of different odours with action-potential firing
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
Central pathways of the olfactory system
Olfactory receptors –> Olfactory bulb –> Olfactory bulb targets –> Orbitofrontal cortex + Thalamus + Hypothalamus + Hippocampal formation
Olfactory bulb targets
- Pyriform cortex
- Olfactory tubercle
- Amygdala
- Entorhinal cortex
Clinical issues with olfaction
- Hyposmia and anosmia
- Very common, 5-10% of population
Causes of clinical issues with olfaction
- Causes: upper respiratory tract infection (inflammation), high age, nasal polyps, diabetes mellitus, head trauma, high dose radiation at nasal epithelium, some drugs
- Reduced quality of life
- E.g. during eating and drinking
Modalities of taste
• 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.
What does salt sensation depend on
- Equilibrium potential for Na+ ions across the taste receptors
What does sour sensation depend on
- Depends 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
Sweet sensation comes about via a second messenger system that closes K+ channels, leading to depolarization of the taste receptors
What does bitter and umami sensation depend on
Sweet sensation comes about via a second messenger system that closes K+ channels, leading to depolarization of the taste receptors
Nerves involved in signalling taste (1st order)
Taste is signalled by cranial nerves VII, IX and X to the nucleus of the solitary tract in the brainstem
Path of neurons after the nucleus of the solitary tract in signalling taste
In primates, fibres from second-order taste neurons project ipsilaterally to the ventral posterior nucleus of the thalamus.
Path of neurons after the thalamus in signalling taste
Thalamic efferents 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
What are the parabrachial nuclei of the pons
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
What percentage of taste disorders are really smell disorders
- 80%
Causes of true taste disorders
- Prior upper respiratory tract infection, head injury, poor oral hygiene
