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M01.02.029 The Inner Ear
The inner ear plays a critical role in hearing and balance. This guide presents its anatomy, structure, neurovascular supply, and clinical relevance in a structured format.
Overview of the Inner Ear
The ear consists of three main parts:
- Outer Ear
Captures and directs sound waves toward the eardrum. - Middle Ear
Transmits sound vibrations via ossicles to the inner ear. - Inner Ear
Converts mechanical signals to electrical impulses and detects head motion for balance.
Anatomy of the Inner Ear
Location
The inner ear is situated within the petrous part of the temporal bone, between:
- Middle ear (laterally)
- Internal acoustic meatus (medially)
Structure
The inner ear has two main components:
| Component | Description |
|---|---|
| Bony Labyrinth | Series of bony cavities containing perilymph. Includes cochlea, vestibule, and semi-circular canals. |
| Membranous Labyrinth | Continuous ducts filled with endolymph, housed within the bony labyrinth. Includes cochlear duct, semi-circular ducts, utricle, and saccule. |
Key Openings
- Oval window: Between middle ear and vestibule.
- Round window: Between middle ear and scala tympani.
Bony Labyrinth
| Component | Features |
|---|---|
| Cochlea | Twists around the modiolus, forming perilymph-filled chambers: scala vestibuli and scala tympani. |
| Vestibule | The central part housing the saccule and utricle, connected to the cochlea and semi-circular canals. |
| Semi-circular Canals | Three canals (anterior, lateral, posterior) positioned at right angles, containing ampullae for balance detection. |
Membranous Labyrinth
| Component | Function |
|---|---|
| Cochlear duct | Organ of hearing; houses the Organ of Corti (epithelial cells for sound detection). |
| Saccule and Utricle | Detect vertical (saccule) and horizontal (utricle) acceleration. |
| Semi-circular ducts | Detect angular motion via endolymph flow in the ampullae. |
Neurovascular Supply
Arterial Supply
| Structure | Arteries |
|---|---|
| Bony labyrinth | Anterior tympanic, petrosal, stylomastoid branches. |
| Membranous labyrinth | Labyrinthine artery (branch of inferior cerebellar or basilar artery). |
Venous Drainage
Labyrinthine veins drain into sigmoid sinus or inferior petrosal sinus.
Innervation
| Nerve | Role |
|---|---|
| Vestibulocochlear (CN VIII) | Splits into: |
- Vestibular nerve: Balance.
- Cochlear nerve: Hearing. | | Facial nerve (CN VII) | Passes through but does not innervate the inner ear structures. |
Clinical Relevance
Meniere’s Disease
- Symptoms: Vertigo, low-pitched tinnitus, hearing loss.
- Cause: Excess endolymph accumulation, distending ducts, and damaging sensory membranes.
Points to Remember
- The inner ear is divided into bony and membranous labyrinths, each with unique structures and functions.
- Endolymph (inside membranous labyrinth) and perilymph (inside bony labyrinth) are critical for signal transmission.
- The vestibulocochlear nerve (CN VIII) provides sensory innervation, with distinct branches for hearing and balance.
- Clinical conditions like Meniere’s disease are linked to disturbances in fluid balance within the labyrinth.
Figures
- Fig 1: Overview of the ear.
- Fig 2: Bony labyrinth components.
- Fig 3: Membranous labyrinth components.
- Fig 4: Structure of the cochlea.
- Fig 5: Arterial supply of the labyrinth.
Bibliography
- TeachMeSeries Ltd. (2024). Anatomy of the Inner Ear.