ENT (Otorhinolaryngology) · MU Sofia

5. Physiology of the vestibular apparatus.

Covers how the vestibular apparatus informs the brain about head position and movement, starting with the otolith organs, the utricle and saccule, and how their hair cells respond to gravity and linear acceleration. It then moves to the semicircular canals and their cupula-based response to rotation, followed by the central vestibular pathway through the vestibular nuclei to the spinal cord, cerebellum, oculomotor nuclei and cortex. It closes with the vestibulo-ocular and vestibulospinal reflexes, nystagmus, and vestibular compensation, and includes two diagrams.

Inside: 3 sections · 2 labelled pictures · 15 flashcards · 6 min

Topic 5 of 52 in the MU Sofia syllabus for ENT (Otorhinolaryngology) (Medicine, Year 4). The free AceMed note covers it in 3 sections, about 913 words (6 min of reading), with 15 flashcards.

In this note

  1. Physiology of the Vestibular Apparatus
  2. II. The Semicircular Canals (SCCs)
  3. III. The Central Vestibular Pathway

Physiology of the Vestibular Apparatus

The vestibular apparatus is responsible for providing the brain with information about the head's position and movement in space, which is critical for maintaining balance, posture, and gaze stability. it's housed within the inner ear and consists of the Otolith Organs (detect linear acceleration and gravity) and the Semicircular Canals (detect angular (rotational) acceleration).

I. The Otolith Organs (Utricle and Saccule)

These organs detect the head's position relative to gravity and linear (straight-line) acceleration/deceleration.

A. Anatomy and Sensory Unit

  • Sensory Receptor: The Macula, a patch of hair cells embedded in a gelatinous membrane.
  • Otolithic Membrane: A thick, gelatinous layer overlying the macula, containing dense, crystalline calcium carbonate particles called Otoliths or Otolithia (literally "ear stones").
  • Hair Cells: Hold stereocilia and one large kinocilium. The hair cell bundle is responsible for transduction.
  • Striola: A central curved line on the macula that divides the hair cells into two populations with opposing polarities (their kinocilia face opposite directions). This polarity is key to coding directional movement.

B. Function and Transduction

  1. 1. Detection of Head Position (Gravity): When the head is tilted, gravity pulls on the heavy otoliths, causing the otolithic membrane to shift. This shear force bends the hair cell bundles.
  • Bending towards the kinocilium depolarises the hair cell, increasing the firing rate of the vestibular nerve.
  • Bending away from the kinocilium hyperpolarizes the hair cell, decreasing the firing rate.
  • The brain interprets the pattern of excitation and inhibition across the maculae of both ears to find the exact angle of head tilt.
  1. 2. Detection of Linear Acceleration: During linear acceleration, the inertia of the heavy otoliths causes them to lag behind, again creating a shear force on the hair cells, which the brain interprets as movement.

C. Orientation and Specificity

  • Utricle: The macula is primarily oriented in the horizontal plane, horizontal linear acceleration (forward/backward) and horizontal head tilts.
  • Saccule: The macula is primarily oriented in the vertical plane, vertical linear acceleration (up/down).

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