T2 L13 Physiology of balance, taste & stimuli Flashcards
What is the endolymph in the scala media continuous with?
Apical surface of vestibular hair cells
Describe the 6 semi-circular canals
At right angles to one another to detect head rotation in all directions
Left & right lateral semi-circular canals are functionally pairs
Describe the 4 otolith organs
Aren’t at right angles to resolve head tilt & linear acceleration in all directions
What are the sensory cels in the ampullae of the semi-circular canals embedded in?
Cupula
What are the sensory cells in the otolith organs embedded in?
Gelatinous sheet covered with heavy crystals of calcium carbonate
What does Newton’s second law show?
Gravity & linear acceleration provide same stimulus to otolith organs
What is Newton’s second law?
F = m x a
Describe type II vestibular hair cells
Most common
Receive afferent & efferent innervation
Describe type I vestibular hair cells
Surrounded by afferent nerve calyx
Not direct contact with efferent nerve fibres
Which type of vestibular hair cell appears to be more sensitive?
Type II
What is nystagmus?
Slow eye movements followed by fast ones during a continuous head rotation
When does physiological nystagmus occur?
When rotating the head
What is spontaneous nystagmus?
Eyes move rhythmically side to side in absence of head movements
When does spontaneous nystagmus occur?
When one of the canals is damage
There is difference in the vestibular nerve firing rates when the head is stationary as the vestibular nerve innervating the intact canal is still firing
What is caloric testing used for?
Test brainstem function in unconscious patient
What happens during caloric testing if the brainstem is intact?
Both eyes move toward the side of the cold water
What happens during caloric testing if there is a lesion of the medial longitudinal fasciculus?
Movement of eye only on the irritated side
What is the diagnosis if neither eyes move during caloric testing?
Low brainstem lesion
Why does cold water irrigation cause eye movements?
Increases density of water in semi-circular canals –> convection currents in canal that mimic endolymph movement induced by turning head
What is the result of warm water irrigation?
Endolymph rises
Increased firing
What is the result of cold water irrigation?
Endolymph falls
Decreased firing
What are some causes of vestibular disorders?
Ear infection Head injury Whiplash Ageing Certain drugs
What are some causes if the patient complains of dizziness?
Light headed - check cardiovascular
Vertigo - check vestibular
What is benign paroxysmal positional vertigo?
Vertigo caused by changes in head position
What is Meniere’s disease?
Progressive disease
Episodes of vertigo, tinnitus & progressive hearing loss
Why is smell important?
Social interactions - perfumes, deodorants
Avoidance of poisons / noxious gases
Enjoyment of food
How many odourant receptor proteins are there?
More than 1000
Each receptor cells expresses just 1 of these proteins
What is olfactory information coded by?
Pattern of stimulation that the brain learns to interpret
Not coded by individual receptor types
What does olfactory transduction depend on?
Second messenger process with cAMP being activated in response to an odourant molecule
What happens after the opening of cAMP-dependent ligand gated ion channels?
Non-selective cation channels permeable to Na+ & Ca2+
Influx of Na+ & Ca2+ –> depolarisation of olfactory receptor cells which signals binding of odourant molecule –> action potentials
Ca2+ influx indirectly opens Cl- channels –> high intracellular Cl- concentration of olfactory receptors –> contributes to depolarisation
What can cause issues with olfaction?
Upper respiratory tract infection High age Nasal polyps Diabetes mellitus Head trauma High dose radiation at nasal epithelium Some drugs
What is the function of taste?
Evaluate nutritious content of food & prevent ingestion of toxic substances
What are the 5 different modalities of taste that can be distinguished?
Salty Sour Sweet Bitter Umami
What is the function of a sweet taste receptor?
Identify energy-rich nutrients
What is the function of a umami receptor?
Recognition of amino acids
What is the function of a salty receptor?
Ensures proper electrolyte balance
What is the function of the sour & bitter receptors?
Warn against intake of potentially noxious and/or poisonous chemicals
What is the mechanism for salt taste transduction?
Depends on equilibrium potential for Na+ across taste receptors
What is the mechanism for sour taste transduction?
Depends on pH with H+ closing K+ channels either directly or indirectly via cAMP as a second messenger
What is the mechanism for sweet taste transduction?
Via second messenger system which closes K+ channels –> depolarisation of taste receptors
What is the mechanism for bitter & umami taste transduction?
Second-messenger induced increase in intracellular Ca2+ receptors –> neurotransmitter release
What cranial nerves signal taste to the nucleus of solitary tract in the brainstem?
VII
IX
X
Where do the fibres from 2nd order taste neurons project to in primates?
Ipsilaterally to ventral posterior nucleus of thalamus
Where do thalamic efferents project to?
Insula, defining primary gustatory cortex which projects to orbitofrontal cortex
Describe the parabrachial nuclei in pons in rodents
Dorsal thalamic projection & ventral projection that terminates in amygdala & hypothalamic nuclei
What are the main causes of issues with olfaction?
Prior upper respiratory tract infection
Head injury
Poor oral hygiene
