Physiology of balance, taste and smell Flashcards
Outline the orientation and structure of the semicircle canals
How is head rotation detected?
6 semicircle canals orientated at right angles to one another to allow detection of head rotations in all directions (3 in each ear)
- Left and right lateral semicircular canal are functionally paired
- Left anterior (superior) and right posterior canals are functionally paired
- Left posterior and right anterior are functionally paired
Sensory cells in the ampullae of the semicircular canals are embedded in a cupula (thick gel cap)
- Hair cells are stimulated by movement towards the tallest hair (Endolymph movement causes this)
- Results in increased firing of CN8 on side of head turning
Outline the orientation and structure of the otolith organs
What is the stimulus for otolith organs?
There are 4 otolith organs, positioned just off right angles to allow them to resolve head tilt and linear acceleration in all directions(changes to the position of head in respect to gravity)
-Sensory cells in the otolith organs (sacculus and utriculus) are embedded in a gelatinous sheet covered with heavy calcium carbonate crystals.
Gravity and linear acceleration provide stimulus to otolith organs according to F=MxA
Whats the difference between the two types of vestibular hair cells?
BOTH FOUND IN BOTH THE SEMI CIRCULAR CANALS AND OTOLITH ORGANS
- TYPE1: majority, receive both afferent and efferent innervation, most sensitive
- TYPE2: surrounded by an afferent nerve calyx and the hair cells are not directly contacted by efferent nerves
What is nystagmus?
Slow eye movements followed by fast ones during continuous head rotation
-Rotation of the head elicits physiological nystagmus
Spontaneous nystagmus: eyes move rhythmically from side to side in the absence of head movements.
- Occurs when 1 of the semicircular canals are damaged
- Net difference in CN8 firing rates exist even when the head is stationary because the CN8 innevating the intact canal fires steadily when at rest, in contrast to a lack of actions on damaged side
What is calonic testing
Used to test the function of the brainstem in an unconscious patient
- Slow eye movements resulting from cold water irrigation in one ear
- Intact ear: both eyes deviate to side of irrigation
- Lesion of medial/longitudinal fasciculus: lateral eye deviates to side of irrigation, medial eye fixed
- Low brainstem lesion: no deviation
Why does calonic testing work?
Warm/cool water generates convection currents in canal that mimic the endolymph movement induced by head turning
These currents result in changes in firing rates of the associated vestibular nerve, with increased rate on warm side and decreased rate on cool side
As in head rotation and spontaneous nystagmus, net differences in firing rates generate eye movements
Outline the central vestibular pathway
Vestibular organs (semicircular canals, utricle, saccule)
Vestibular nuclei
Oculomotor nuclei
Extraocular muscles
From vestibular nuclei can go to the cerebellum (or other direction) or spinal cord, reticular formation (which provides an alternate pathway to oculomotor nuclei) or cerebral cortex.
Can also go straight from vestibular organs to cerebellum
States 6 causes of vestibular disorders
Drugs - gentamycin Ear infection Head injury Whiplash Ageing Other
State 4 types of vestibular disorder and. how they present
- “Dizziness”- if light headed check CVS, if vertigo check vestible
- Trauma - especially in CN8 following motorcycle accident
- Benign paroxysmal positional vertigo
- spinning caused by change in head position - Meniere’s disease- progressive, episodes of vertigo, tinnitis, and hearing loss in 1 ear, excess fluid in inner ear
What is the function of smell and where is it detected?
Smell is important for social interaction, avoidance of poisons and enjoyment of food
2-3cm of olfactory epithelium on each side of nose
Ciliated receptor cells send their own afferent axons –> brain
>1000 different odorant receptor proteins, each receptor cell expressing just one
-Each receptor cell responds to a number of different odours with AP firing
Olfactory information is coded not by individual receptor types but in the pattern of stimulation that the brain learns to interpret
Outline the mechanism of olfactory transduction
Occurs via 2nd messenger transduction
Odorant –> cAMP activation –> opening of cAMP-dependent ligand-gated ion channels –> Na+ and Ca2+ influx depolarises the olfactory receptor, signalling the odorant molecule –> AP
**Ca2+ influx indirectly opens Cl- channels due to unusual increase in intracellular Cl- concentration of the olfactory receptors, contributes to depolarisation
Outline the central pathways to the olfactory system
Olfactory receptors –> olfactory bulb (via olfactory nerve) –> Olfactory bulb targets (via olfactory tract)
Olfactory targets (all to hippocampal formation):
Pyriform cortex–> orbitofrontal cortex
Olfactory tubercle–> orbitofrontal cortex, hypothalamus, thalamus,
Amygdala
Entorhinal cortex
Give 2 examples of olfactory disorders
How common are they?
Causes?
Effects?
Hyposmia- difficulty smelling
Anosmia- complete or partial lack of smell
Common- 5-10%
Causes: head trauma, URTI (inflammation), ageing, nasal polyps, DM, high dose radiation at nasal epithelium, drugs
Reduces quality of life
What is the importance of gustation?
How are the taste organs organised?
Important in detecting nutritious content of food and preventing ingestion of toxic substances.
Overall contributes to overall pleasure and enjoyment of meal
Tongue is principle organ with 5 modalities of taste to be distinguished: salty, umami, sweet, sour, bitter
-Regional variation in sensitivity but generally there is overlap such that most parts of the tongue detect all 5
Individual taste receptors respond to single taste modality
What is the function of detecting sweet substances?
How does this occur?
Sweet: identification of energy rich foods
Via 2nd messenger system that closes K+ channels –> depolarisation
