Vestibular Anatomy

Semicircular canals
1. Monitors head rotation and spatial positioning
2. Three semicircular canals with endolymph fluid filled ampullae
  1. Canals lie at right angles to one another
  2. One for each direction (e.g. X,Y and Z axis)
  3. Fluid filled canals contain crista ampullaris
    1. Crista ampullaris is a sensory organ detecting sudden head rotation, acceleration and deceleration
    2. Hair cells within the crista ampullaris detect fluid motion
    3. Hair cells generate an impulse that is transmitted centrally via Vestibular Nerve (CN 8)
Otolithic organs
1. Utricle
  1. Contains otoliths (small stones) that settle with position changes
  2. Detects horizontal linear movement and Head Tilting
2. Sacule
  1. Smaller, more globular organ without utricle communication
  2. Detects vertical linear movement (e.g. gravity)
3. Both Utricle and Sacule contain a Macula (sack)
  1. Macula contains thick, gelatinous material and otoliths
  2. Otoliths (statoconia) are Calcium Carbonate crystals that move with gravity and head movement
  3. Hair cells within the Macula detect static head position as well as head movement
Perilymphatic fluid
1. Surrounds vestibular apparatus (semicircular canals and otolithic organs)
2. Communicates with the perilymph of the Cochlea (from oval window to round window)
3. Communicates with perilymphatic duct (and the subarachnoid space)

Cranial Nerve 8 (two tracts)
1. Vestibular tract carries signals from the utricle and sacule
2. Cochlear tract carries signals from the Hearing apparatus in the Cochlea
Vestibular nucleii
1. Located in the pons and Medulla and receive input from the respective tracts of Cranial Nerve 8
2. Transmit signals to the rest of the Cerebrum, Cerebellum and spinal cord
  1. Combines with other inputs (e.g. occiptal lobe, dorsal columns)
Medial Longitudinal Fasciculus and Oculomotor Nucleus
1. Brainstem fiber tracts that connect with the Oculomotor Nucleus
  1. Carry information directing eye movement
2. Connects with CN 3, CN 4, and CN 6 for eye motor activity
  1. Also Synapses with CN 8 for head position input related to eye saccades