Physiology of balance, smell and taste Flashcards
Orientation of the semicircular canals and otolith organs
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
Structure of semicircular canals and otolith organs
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
Vestibular hair cells
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
Nystagmus
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
Causes of spontaneous nystagmus
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
Caloric testing to test the function of the brainstem in an unconscious patient
Slow eye movements resulting from cold water irrigation in one ear for three different conditions:
- with the brainstem intact
- with a lesion of the medial longitudinal fasciulus
- with a low brainstem lesion
Why does caloric testing work?
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
Causes of vestibular disorders
Ear infection
Head injury
Whiplash
Ageing
Certain drugs e.g. aminoglycoside antibiotics
Disorders of the vestibular system
Patient complains of ‘dizziness’
Trauma
Benign paroxysmal positional vertigo
Meniere disease
Dizziness
Light headed -> check CV
Vertigo -> check vestibular
Trauma
Esp CN VIII e.g. motorcycle accident
Bening paroxysmal positional vertigo
Vertigo caused by changes in head position
Meniere disease
Progressive disease
Episodes of vertigo, tinnitus and progressive hearing loss, usually in one ear
Excess fluid in inner ear
Sense of smell important for
Social interactions
Avoidance of poisons/ noxious gases
Smell plays a major role in the enjoyment of food
Where is the olflactory epithelium?
Area of 2-3cm on each side of the nose
Organisation of the olfactory epithelium
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
Mechanism of olfactory transduction
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
Clinical issues with olfaction
Hyposmia and anosmia
Very common, 5-10% of population
Causes of olfaction issues
Upper respiratory tract infection
High age
Nasal polyps
Diabetes mellitus
Head trauma
High dose radiation at nasal epithelium
Some drugs
Sweet
Identification of energy rich nutrients
Umami
Recognition of amino acids
Salty
Ensures proper dietary electrolyte balance
Sour and bitter
Warn against the intake of potentially noxious and/ or poisonous chemicals
Organisation of the gustatory system
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
Salt sensation
Depends on equilibrium potential for Na+ ions across the taste receptors
Sour sensation
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
Sweet sensation
Comes about via a second messenger system that closes K+ channels leading to depolarisation of the taste receptors
Bitter and umami sensations
Due to second messenger induced increase in intracellular Ca2+ in the receptors
Ca2+ increase leads to neurotransmitter release
Central pathways of the gustatory system
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
Clinical issues in gustation
80% taste disorders are really smell disorders
Causes
- prior upper respiratory tract infection, head injury, poor oral hygiene
