Physiology of Balance, Smell and Taste

Question 1

Describe the organization of the peripheral vestibular system

Answer 1

• the endolymph in the chochlear duct is continuous with he endolymph on the apical surface of the vestibular hair cells

Question 2

What are the otolith organs? Explain their orientation and their structure

Answer 2

• 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

Question 3

What are the two types of vestibular hair cells?

Answer 3

• 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

Question 4

What is the role of the semicircular canal receptors?

Answer 4

• they detect rotation of the head
  *

Question 5

What is Nystagmus?

Answer 5

• 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

Question 6

What is the use of Caloric testing?

Answer 6

• 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

Question 7

The Central vestibular pathway

Answer 7

Question 8

What are causes of vestibular disorders?

Answer 8

• Ear infection
• Head injury
• Whiplash
• Ageing
• certain drugs: AMinoglycoside antibiotics (gentamicin)

Question 9

What are disorders of the vestibular system?

Answer 9

• 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

Question 10

Explain the location and organization of the olfactory epithelium

Answer 10

• 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

Question 11

Explain the mechanism of olfactory transduction

Answer 11

• 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

Question 12

What are the central pathways of the olfactory system?

Answer 12

Question 13

What is Hyposmia and Anosmia?

Answer 13

• 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

Question 14

Organization of the gustatory system

Answer 14

Question 15

Explain the mechanism of taste transduction

Answer 15

• 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

Question 16

The central pathways of the gustatory system?

Answer 16

• 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.

Question 17

Clinical issues in gustation

Answer 17

• 80% of taste disorders are smell disorders

True taste disorders

• prior upper respiratory tract infection
• head injury
• poor oral hygiene

less obvious daignosis