Lecture 26: Vestibular System

Lecture 26: Vestibular System

• The vestibular system is responsible for sensing motion, posture, and image stabilization during head movement.
• Vestibular receptors are located within the inner ear labyrinth.
• The system is composed of otolith organs (saccule and utricle) and semicircular canals.

Topics

• Vestibular hair cells
• Semicircular canals
• Otolith organs

Overview

• The vestibular system senses motion, postural stability, and image stabilization during head movement.
• Receptors in the inner ear labyrinth provide information about head motion and orientation relative to gravity.
• The system assists in coordinating body position with other sensory information.

Vestibular Hair Cells

• Motion is detected by hair cells (kinocilium and stereocilia) in the vestibular organs.
• Deflection of these hairs causes hyperpolarization or depolarization depending on the direction of deflection.
• Depolarization is caused by stereocilia movement towards the kinocilium.
• Hyperpolarization is caused by stereocilia movement away from the kinocilium.
• Different polarization affects the primary vestibular afferent firing rate to the brainstem.
• Spontaneous activity at rest is due to open Ca²⁺ channels, causing a constant neurotransmitter release.
• Motion in one direction stimulates firing, while the opposite direction inhibits firing of the cranial 8th nerve.

Semicircular Canals

• Each canal is designed to detect angular acceleration during head rotation.
• The membranous canal is within a bony canal, filled with endolymph (high K⁺, low Na⁺).
• The space between the membranous and bony canals is filled with perilymph (low K⁺, high Na⁺).
• Hair cells inside the ampulla are located within a gelatinous structure called the cupula.
• Endolymph motion caused by angular head motion results in cupula deflection, which causes stereocilia movement towards or away from the kinocilium.
• Motion generates differences in transmitter release and discharge rates in the afferent nerves, proportional to the angular velocity.
• Semicircular canals are functionally paired, with opposite canals acting as push-pull pairs, one exciting and the other inhibiting.

Otolith Organs (Utricle and Saccule)

• These organs provide information about linear acceleration and changes in head position relative to gravity.
• The utricle lies horizontally, and the saccule is oriented vertically.
• Hair cells (macula) within the utricle and saccule have cilia embedded in a gelatinous mass containing calcium carbonate crystals (otoliths).
• Otoliths move due to gravity or acceleration, and cause a shear force on the stereocilia.
• This shear force leads to depolarization (and excitation) or hyperpolarization (inhibition) in the hair cells.
• The striola is the central stripe dividing the macula into regions of opposing hair cell polarity (towards or away).
• Otolith organs are sensitive to linear motion due to the striola's curvature.
• In contrast, semicircular canal hair cells sense motion in the same direction.

Similarities between the auditory and vestibular systems

• Both systems use a common fluid system (membranous labyrinth).
• Both involve hair cell motion detectors with similar principles.
• Both are innervated by branches of the same cranial nerve (8th).

Summary

• Specialized receptors (otolith organs and semicircular canals) within the inner ear detect forces related to head movement and gravity.
• Motion is detected by vestibular sensory organs through the deflection of hair cells.
• Motion causes changes in the electrical potential, transmitter release, and firing rates.
• Hair cells in semicircular canals are oriented in the same direction.
• Hair cells within otolith organs demonstrate opposing orientations to detect opposing directions of motion and acceleration.
• Vestibular and auditory systems share a common fluid system, hair cell detectors, and innervation.