Vestibular system

The Vestibular System Study Notes

Learning Objectives

  • A. Anatomy of the Vestibular Labyrinth

  • B. Transduction

  • C. Vestibulo-ocular Reflex

  • D. Caloric Testing

  • E. Other Vestibular Reflexes and Pathways

The Vestibular System Overview

Major Functions

  1. Sensation of Motion and Spatial Orientation

    • Linear Acceleration: Change in velocity without alteration of direction (movement in a straight line).

    • Angular Acceleration: Change in both velocity and direction simultaneously.

  2. Upright Posture: Maintains posture through input to axial muscle control areas.

  3. Eye Stabilization: Stabilizes visual focus during head movements by regulating eye motor control regions.

Anatomy of the Vestibular Labyrinth

  • Components:

    • Auditory Ossicles: Includes stapes, incus, malleus.

    • Petrous Portion of Temporal Bone: Contains the inner ear structures.

    • Auricle (Pinna), External Auditory Canal, Tympanic Cavity, Tympanic Membrane: Components of the auditory system.

  • Vestibular Branch of Cranial Nerve VIII (CN VIII): Key structure in vestibular function.

    • Cochlear Branch of CN VIII: Responsible for hearing.

  • The bony labyrinth contains cells that transduce information regarding body positioning in space, known as the vestibular sense.

Labyrinth Structure

  • Bony Labyrinth: Situated next to the cochlea.

  • Membranous Labyrinth: Enclosed within the bony labyrinth; floats in perilymph and is filled with endolymph.

Receptor Organs in the Labyrinth

  • Types of Receptor Organs:

    • There are five receptor organs:

      • Three Semicircular Canals: Positioned in different planes to detect angular accelerations.

      • Two Otolithic Organs: C………..,;,,,,….//»><><?
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Hair Cells and Transduction

  • Function of Hair Cells:

    • They serve as transducing cells in the semicircular canals and otolithic organs.

    • Structure includes mechanically-gated channels.

    • The specific proteins and molecules associated with hair cells include Myosin, USH1B, and various Cadherins.

Otolithic Organs

Structure and Function

  • The kinocilia of hair cells do not degenerate in the vestibular system and are oriented on the same side of a cell in a given region.

  • Striola:

    • The dividing line in both the utricle and saccule that separates hair cells with different orientations.

    • The ampulla has no striola, as all stereocilia are oriented similarly.

  • Otolithic Hair Cell Transduction:

    • Stereocilia: Embedded in a gelatinous substance called the otolithic membrane.

    • Otoconia: Calcium carbonate crystals located on the otolithic membrane.

    • Functional Mechanism: The otoconia add weight, allowing the membrane to respond to movements after inertia is surpassed.

    • Orientation:

      • Utricles are arranged for horizontal stimuli, while saccules respond to vertical stimuli.

      • Tilting of the head produces similar effects as acceleration, impacting activation and inhibition of hair cells.

Semicircular Canals

Structure and Function

  • Configuration:

    • Position of canals:

      • 1. Anterior

      • 2. Posterior

      • 3. Horizontal

    • They detect angular rotation.

  • Canal Anatomy: Each canal has an ampulla containing crista (with hair cells) and stereocilia immersed in a gelatinous cupula.

  • Transduction Mechanism:

    • No head rotation causes:

      • No movement of endolymph, leading to no cupula displacement and no change in hair cell activity, maintaining baseline.

    • During angular rotation, endolymph moves, causing cupula displacement and subsequently activated stereocilia.

Bilateral Symmetry in Canals

  • The vestibular system operates in pairs, providing redundant information for accuracy and stability.

  • Rotational movements lead to an increase in action potentials when hair cells are excited, with hyperpolarization resulting in a decrease once movement ceases.

Vestibular Pathways

  • Key Pathways in the Brain:

    • Connections from the vestibular nuclei (located in the brainstem) to various regions including:

      • Cerebellum

      • Oculomotor nerves

      • Neck, trunk, and limb motor control

  • Reflexes:

    • Vestibulo-ocular Reflex: Maintains eye position relative to head movements.

    • Vestibulo-collic Reflex: Stabilizes head during body movements.

    • Vestibulo-spinal Reflex: Adjusts posture during body position changes.

Vestibulo-ocular Reflex

Mechanism of Action

  • Hair cells on opposing sides exhibit opposite responses during head movement.

  • Components of the Reflex:

    • Slow phase maintains gaze focus while the fast phase resets eyes to a neutral position after head motion.

    • Afferent fibers from nerve VIII increase firing on one side while decreasing on the other, coordinating eye movements effectively.

Caloric Testing

Principal Methodology

  • This test assesses the function of brainstem in unconscious patients by introducing temperature changes in the ear:

    • Warm or cool water stimulates endolymph movement simulating head motions.

    • Produces physiological nystagmus:

      • Slow movement counter to head movement followed by fast reset.

    • Pathological Nystagmus:

      • Rhythmic eye movements with no head motion indicate potential canal or nerve damage.

    • Activity levels typically steady in nerves at rest; damage manifests as imbalanced firing.

    • Caloric Responses:

      • Warm water increases activity on the stimulated side (interpreted as head turn towards that side).

      • Cold water decreases activity (interpreted as head turn to the opposite side).

Reflexes Associated with Vestibular Pathways

Vestibulo-collic and Vestibulo-spinal Reflexes

  • Vestibulo-spinal Reflex:

    • Maintains upright posture involving cerebellar processing and activating the lateral vestibulospinal tract to enhance extensor tension ipsilaterally.

  • Vestibulo-collic Reflex:

    • Stabilizes the head; encapsulated by medial vestibulospinal tract pathways.

Thalamocortical Pathways

Overview

  • There is no specifically defined primary vestibular cortex, yet a vestibular cortical “system” exists within:

    • Parietal and Insular Regions

    • Parieto-insular Vestibular Cortex: Integrates multimodal proprioceptive signals and generates a spatial map for body orientation and motor control.