Physiology of Balance, Smell and Taste Flashcards
1
Q
Describe the organization of the peripheral vestibular system
A
- the endolymph in the chochlear duct is continuous with he endolymph on the apical surface of the vestibular hair cells
2
Q
What are the otolith organs? Explain their orientation and their structure
A
- they are sensory cells in the otolith organs (sacculus and utriculus cells) are embedded in a gelatinous sheet covered with crystals of calcium carbonate
- they sense linear and gravitational accelerations of the head
- the four otolith organs are not exactly perpendicular to enable them to resolve and tilt
- the six semicircular canals are oriented at right angles to one another to detect head rotation in all directions
3
Q
What are the two types of vestibular hair cells?
A
- Type II vestibular hair cells
- receive afferent and efferent information
- appear to be more sensitive
- Type I vestibular hair cells
- surrounded by an afferent nerve calyx and are not directly contacted by efferent nerve fibres
4
Q
What is the role of the semicircular canal receptors?
A
- they detect rotation of the head
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5
Q
What is Nystagmus?
A
- Slow eye movements followed by fast ones during continued head rotation - the fast phase defines the direction of the nystagmus
- physiological nystagmus occurs in the head rotation due to the vestibulo-ocular reflex
-
Spontaneous nystagmus is when the eyes move rhythmically from side to side in the absence of any head movements
- this occurs when one of the canals is damage
- no firing takes place instead of the reduced firing that would happen in physiological nystagmus
6
Q
What is the use of Caloric testing?
A
- water is used to test the function of the brainstem in an unconscious patient - uses ocular reflexes
- using slightly warmer or colder warmer than body temp generates currents in the canal that mimics endolymph movement induced by turning the head
- affects the firing rates of the associated vestibular nerve
- warmer water = higher frequency
- cooler water = decreased rate
- the net difference generates an eye movement
- affects the firing rates of the associated vestibular nerve
7
Q
The Central vestibular pathway
A
8
Q
What are causes of vestibular disorders?
A
- Ear infection
- Head injury
- Whiplash
- Ageing
- certain drugs: AMinoglycoside antibiotics (gentamicin)
9
Q
What are disorders of the vestibular system?
A
- Patient complains of “dizziness”
- Light-headed à check cardiovascular
- Vertigo (spinning) à check vestibular
- Trauma
- Esp. CN VIII, e.g. motorcycle accident
- Benign paroxysmal positional vertigo (BPPV)
- vertigo caused by changes in head position
- Ménière’s disease
- Progressive disease
- episodes of vertigo, tinnitus and progressive hearing loss, usually in one ear
- Excess fluid in inner ear
10
Q
Explain the location and organization of the olfactory epithelium
A
- ciliated receptor cells send their own afferent axons to the brain
- olfactory information is coded by the pattern of stimulation that the brain learns to interpret
11
Q
Explain the mechanism of olfactory transduction
A
- Olfactory transduction depends on a second messenger process, with cAMP being activated in response to an 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 unusually high intracellular Cl- concentration of the olfactory receptors, contributes to the depolarization
12
Q
What are the central pathways of the olfactory system?
A
13
Q
What is Hyposmia and Anosmia?
A
- reduced or increased olfaction- very common
- Causes:
- upper resp. tract infection
- older age
- nasal polyps
- diabetes mellitus
- head trauma, high dose radiation at nasal epithelium, some drugs
14
Q
Organization of the gustatory system
A
15
Q
Explain the mechanism of taste transduction
A
- Salt sensation depends on the equilibrium potential for Na+ ions across the taste receptors
-
Sour sensation depends on pH (acidity), with H+ ions (protons) closing K+ channels either directly or indirectly via a cAMP as a second messenger.
- his leads to depolarization of the taste receptors
- Sweet sensation comes about via a second messenger system that closes K+ channels, leading to depolarization of the taste receptors
- Bitter and umami sensation are due to a second-messenger induced increase in intracellular Ca2+ in the receptors. The Ca2+ increase leads to neurotransmitter release
16
Q
The central pathways of the gustatory system?
A
- Taste is signalled by cranial nerves VII (front 2/3 of the tongue), IX and X (both rear 1/3 of the tongue) to the nucleus of the solitary tract in the brainstem.
- 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. The parabrachial nuclei have a dorsal thalamocortical projection and also a ventral projection that terminates in amygdalar and hypothalamic nuclei, among others.
