Vestibule of the Inner Ear

The vestibule is the central part of the bony labyrinth of the inner ear and plays a crucial role in the sense of balance. It connects the semicircular canals and the cochlea and houses important sensory organs responsible for detecting linear acceleration and head position relative to gravity.

Location

The vestibule is located within the petrous part of the temporal bone, between the cochlea (anteriorly) and the semicircular canals (posteriorly). It forms the medial wall of the middle ear cavity and is separated from it by the oval window (fenestra vestibuli), into which the base of the stapes fits.

Structure

The vestibule is a small, oval-shaped cavity that is part of the bony labyrinth. It contains two membranous structures:

• Utricle: Located in the superior part, detects horizontal linear acceleration
• Saccule: Located in the inferior part, detects vertical linear acceleration

Both the utricle and saccule contain specialized sensory regions called **maculae**, which are responsible for detecting changes in head position.

Function

The vestibule is a key component of the **vestibular system**, which contributes to balance and spatial orientation. Its functions include:

• Detecting linear acceleration of the head
• Sensing static head position (upright, tilted, etc.)
• Sending sensory input to the brainstem for postural adjustments and eye movements (via vestibulo-ocular reflex)

Membranous Labyrinth Components

The vestibule contains:

• Utricle: Connected to all three semicircular ducts and oriented in the horizontal plane
• Saccule: Communicates with the cochlear duct and oriented in the vertical plane
• Both utricle and saccule contain **otolith organs**, composed of hair cells embedded in a gelatinous layer topped with otoliths (calcium carbonate crystals)

Innervation

The vestibule is innervated by the **vestibular division of the vestibulocochlear nerve (cranial nerve VIII)**. Specifically:

• Superior vestibular nerve: Supplies the utricle and the anterior and lateral semicircular ducts
• Inferior vestibular nerve: Supplies the saccule and posterior semicircular duct

Blood Supply

The vestibule receives arterial blood from the **labyrinthine artery**, a branch of the anterior inferior cerebellar artery (AICA). Venous drainage is through the labyrinthine vein.

Clinical Significance

Vestibular Dysfunction

Damage to the vestibule or its associated structures can cause:

• Vertigo: Sensation of spinning or imbalance
• Dizziness and unsteadiness: Especially during head movements
• Oculomotor disturbances: Such as nystagmus (involuntary eye movements)

Benign Paroxysmal Positional Vertigo (BPPV)

BPPV is caused by displaced otoliths from the utricle that migrate into a semicircular canal, leading to abnormal stimulation of the vestibular system during head movements.

Labyrinthitis and Vestibular Neuritis

Inflammation of the vestibule and its associated nerve can lead to acute vertigo, nausea, and imbalance. Vestibular neuritis typically affects only the nerve, while labyrinthitis may also impair hearing.

Meniere’s Disease

A disorder involving endolymphatic hydrops (fluid buildup) that can affect the vestibule, leading to episodic vertigo, tinnitus, and fluctuating hearing loss.

Development

The vestibule arises from the **otic vesicle**, an ectodermal structure that develops from the otic placode. During embryogenesis, the vesicle differentiates into the utricle and saccule, which give rise to the membranous labyrinth of the vestibule.

Imaging

The vestibule can be visualized using high-resolution CT (for bony structures) and MRI (for soft tissues and membranous labyrinth). Imaging is particularly useful in assessing vestibular schwannomas, congenital anomalies, or trauma to the inner ear.