PHSL

• Vestibular system


  Provides the brain with information about head movement and location

  
• The vestibular system reports on linear and angular acceleration of the head (NOT velocity)
• The stimulus is change in head position, and is typically reliant on gravity
• Vestibular functions


  • Generates reflexes to compensate for head movement and the perception of motion in space
  • Provides info to help with maintenance of posture
  • Provides info to allow conscious awareness of position/movement/acceleration (linear and rotational) of the head and body

  
• Semicircular canals


  • Sensitive to angular acceleration during head rotation
  • Aligned in 3 axes
  • Each most vigorously stimulated by acceleration in its preferred plane

  
• Ampulla


  • Each of the 6 SC canals has an ampulla
  • In the ampulla is the hair cell membrane (crista ampullaris)
  • The hairs of the vestibular hair cells are embedded in a gelatinous matrix, the cupula
  • During acceleration, the relative movement of the enclosed endolymph pushes against the cupula, which displaces the hairs on the hair cells causing the opening of mechanically gated ion channels
• Discharge rate of vestibular hair cells

  • Vestibular hair cells have a resting rate of discharge
  • As a result of head movement, the rate of discharge can therefore increase or decrease from the resting rate
  • Individual hair cells can thereby signal movement in two directions

  
• Utricle and Saccule


  • Detect linear acceleration (as opposed to angular)
  • Utricle approximately horizontal when standing, provides information on horizontal linear acceleration
  • Saccule approximately vertical when standing, provides information on vertical acceleration (gravity)

  
• Central pathways


  • Axons from VS project to vestibular nuclei in brainstem
  • Information from there used to: 1) stabilise eyes (via oculomotor nuclei)
  • 2) stabilise the head (via input to neck muscle motoneurons)
  • 3) maintain balance (via pathways to cerebellum and spinal cord)

  
• Vestibular disorders


  • Vertigo - caused by diseases affecting the vestibule or its afferent fibres, illusion of movement, dizziness, nausea
  • Motion sickness - caused by mismatch between visual and vestibular information, improved if horizon visible
  • "Bedspins" - caused by alcohol, ethanol infiltrates cupula, lowers density and causes it to "float", creating perception of movement
• Over 600 medications are known to be harmful to hair cells, can result in temporary or permanent hearing loss and disorders of balance. Aminoglycoside antibiotics (eg gentamycin, streptomycin, kanamycin) are amongst the worst
• Normal hair cells can be damaged by ototoxic drugs like kanamycin