Vestibular Apparatus Overview

Overview

This lecture covers the structure of the vestibular apparatus, its components, and how it detects motion to maintain body equilibrium and posture.

The vestibular apparatus is a sensory organ located in the inner ear.
It detects sensations of motion and is critical for maintaining equilibrium and posture.
The inner ear consists of the bony labyrinth (outer part) and membranous labyrinth (inner part).
Perilymph fluid is present between the bony and membranous labyrinths; endolymph fluid fills the inside of the membranous labyrinth.
The vestibular apparatus consists of three semicircular canals, the utricle, and the saccule.

Semicircular canals: anterior, lateral, and posterior canals detect angular (rotational) acceleration.
Ampulla: expanded end of each semicircular canal containing the crista ampullaris (crista ampullae).
Crista ampullaris: sensory region with hair cells that detect angular acceleration, helping maintain dynamic equilibrium.
Utricle and saccule (otolith organs): contain the macula, which senses linear acceleration.
Utricle detects horizontal linear acceleration; saccule detects vertical linear acceleration.
Static equilibrium is maintained by macula; dynamic equilibrium by crista ampullaris.

Hair cells are receptor cells found in the crista ampullaris and macula, with stereocilia and a single, longer kinocilium.
Stereocilia movement towards kinocilium opens mechanosensitive ion channels, allowing potassium influx and causing cell depolarization.
Depolarization triggers neurotransmitter (glutamate) release, increasing nerve fiber activity.
Movement away from kinocilium causes hyperpolarization and reduced nerve activity.
Hair cells connect to afferent nerve fibers forming the vestibular division of the eighth cranial nerve.

In semicircular canals, hair cell cilia are embedded in the gelatinous cupula, displaced by endolymph movement during angular acceleration.
Utricle and saccule hair cells are embedded in the otolith membrane, which contains calcium carbonate crystals (otoconia), making it denser than endolymph.
Otolith membrane movement stimulates hair cells during linear acceleration.

Vestibular apparatus — sensory organ in the inner ear for balance.
Bony labyrinth — outer bony structure of the inner ear.
Membranous labyrinth — inner tubule system within the bony labyrinth.
Perilymph — fluid between bony and membranous labyrinths.
Endolymph — potassium-rich fluid inside the membranous labyrinth.
Crista ampullaris — sensory organ in semicircular canals detecting angular acceleration.
Macula — sensory area in utricle and saccule detecting linear acceleration.
Otolith organs — utricle and saccule, responsible for linear acceleration detection.
Kinocilium — single large cilium on hair cells.
Cupula — gelatinous structure in semicircular canals embedding hair cell cilia.
Otolith membrane — membrane with calcium carbonate crystals over hair cells in utricle/saccule.

Review part two for the vestibular pathway and mechanism of semicircular canal stimulation.
Study diagrams of vestibular apparatus, hair cells, and otolith/cupula structures for better understanding.