133. Vestibular Apparatus – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• The vestibular apparatus is the sensory system responsible for maintaining balance, posture, equilibrium, and stable vision during head movement. It consists of the otolith organs (utricle and saccule) and the three semicircular canals. Dysfunction commonly causes vertigo and imbalance.
• The membranous labyrinth contains endolymph (potassium-rich), while the space between the membranous and bony labyrinth contains perilymph. Vestibular receptors are hair cells, each having stereocilia and one kinocilium. Bending of stereocilia toward the kinocilium causes depolarization and increased vestibular nerve firing, whereas bending away causes hyperpolarization and decreased firing.
• The otolith organs detect linear acceleration and head position relative to gravity.
  • Utricle: Detects horizontal linear acceleration.
  • Saccule: Detects vertical linear acceleration.
  • Their hair cells are embedded in the otolithic membrane, which contains otoconia (calcium carbonate crystals). Displacement of this membrane during head tilt or linear acceleration bends the hair cells and alters vestibular nerve activity.
• The semicircular canals detect angular (rotational) acceleration.
  • Hair cells are located in the crista ampullaris of the ampulla.
  • Their stereocilia project into the cupula, which does not contain otoconia.
  • During head rotation, endolymph initially lags behind because of inertia, deflecting the cupula and bending the hair cells.
  • The semicircular canals respond mainly to the onset and cessation of rotation, not to constant rotation. Paired canals on both sides help determine the direction of head movement.
• Vestibular impulses travel through the vestibular division of the eighth cranial nerve to the vestibular nuclei (lateral, medial, superior, and inferior) and the cerebellum.
  • Lateral vestibular nucleus: Gives rise to the lateral vestibulospinal tract, facilitating extensor muscles for posture.
  • Medial vestibular nucleus: Forms the medial vestibulospinal tract, controlling head and neck muscles.
  • Vestibular pathways also project to the ocular motor nuclei, coordinating head and eye movements.
• Vestibular reflexes include:
  • Postural reflexes: Maintain body stability through tonic labyrinthine, labyrinthine righting, and vestibulocollic reflexes.
  • Vestibulo-ocular reflex (VOR): Stabilizes images on the retina by moving the eyes opposite to head movement. Large or sustained head rotation produces nystagmus, in which the slow phase is vestibular and the fast phase is generated by brainstem mechanisms.
• Motion sickness results from conflict between vestibular, visual, and proprioceptive inputs. It presents with nausea, dizziness, sweating, pallor, and vomiting. Symptoms improve by minimizing head movement and using vestibular suppressants when required.
• Vertigo is the false sensation of movement due to abnormal vestibular activity.
  • Peripheral vertigo: Caused by vestibular apparatus or vestibular nerve disorders (e.g., Ménière disease, benign paroxysmal positional vertigo).
  • Central vertigo: Caused by lesions of the brainstem or cerebellum.
• Vestibular function tests include:
  • Romberg test.
  • Rotational test.
  • Caloric test—remember COWS: Cold Opposite, Warm Same (fast phase of nystagmus).