Ear Anatomy

ANATOMY OF THE EAR

Camille Q. Tolentino, MD

Fellow - PSOHNS

FUNCTIONS OF THE EAR

• HEARING

• BALANCE

TEMPORAL BONE DEVELOPMENT

• Temporal Bone

  • Sphenoidal fontanelle

  • Frontal bone

  • Squamous part

  • Supraorbital notch (foramen)

  • Ethmoid bone

  • Anterior ethmoidal foramen

  • Orbital plate

FONTANELLES AND SUTURES

• Features:

  • Anterior fontanelle

  • Posterior fontanelle

  • Lambdoid suture

  • Coronal suture

  • Squamous suture

BONES OF THE TEMPORAL BONE

• Parietal bone

• Sphenoid bone

  • Greater wing

• Occipital bone

• Zygomatic bone

• Lacrimal bone

  • Infraorbital foramen

• Palatine bone

• Maxilla

  • Alveolar process

ANATOMY OF THE TEMPORAL BONE

• Temporal Bone:

  • Articulates with sphenoid, parietal, occipital, and zygomatic bones

  • Shape: Pyramidal

  • Contains squamous, petrous, mastoid, and tympanic parts

SQUAMOUS PORTION

• Zygomatic process

• Tympanosquamous suture (fissure)

• External auditory meatus

• Origin of posterior belly of digastric muscle

• Mastoid process

• Tympanomastoid suture (fissure)

• Styloid process

PHARYNGEAL APPARATUS

• Development:

  • Does not form or regress

  • Derived from primitive pharynx

  • Arch from mesoderm

  • Occurs during weeks 4-5 of gestation

  • Components:

  • Cleft: ectoderm (head)

  • Cranio-pouch: endoderm (tail)

EARLY DEVELOPMENTAL STAGES

• 20XX:

  • Structures Forming:

  • Developing endolymphatic duct

  • Early otocyst

  • Ascending 1st pharyngeal pouch (future Eustachian tube)

  • Early ossicular condensations

  • Inverting 1st pharyngeal cleft (future external auditory canal)

  • Developing semicircular canals

  • Stapes

  • Facial nerve

  • Incus

  • Malleus

  • Formation of the tympanic membrane and Eustachian tube

AURICLE ANATOMY

• Features:

  • Crux of helix

  • External auditory meatus

  • Tragus

ANATOMICAL LANDMARKS

• Intertragic notch

• Antitragus

• Triangular fossa

• Helix

• Scaphoid fossa

• Crura of antihelix

• Auricular tubercle (of Darwin)

• Antihelix

• Concha of auricle

• Lobule of auricle

AURICULAR STRUCTURE

• Composition:

  • Fibroelastic cartilage

  • Covered by perichondrium and skin

  • Formed by 6 Hillocks of His

SENSORY INNERVATION OF AURICLE

• Innervation details

  • Great auricular nerve (C-3)

  • Auricular branch of vagus nerve (CN X)

  • Arnold nerve (CN VII)

  • Dorsal rami of cervical spinal nerves

  • Trigeminal nerve branches

  • Ophthalmic nerve (CN V)

  • Maxillary nerve (V2)

  • Mandibular nerve (V3)

BLOOD SUPPLY OF AURICLE

• External Carotid Artery: Major blood supply to auricle and external auditory canal (EAC)

  • Sources:

  • Posterior auricular artery: mastoid and auricle

  • Superficial temporal artery: EAC and auricle

EXTERNAL AUDITORY CANAL

• Length: ~2.5 cm long

• Composition:

  • Lateral 1/3: fibrocartilage (contains hair follicles, sebaceous and sweat glands)

  • Medial 2/3: bony (devoid of hair follicles, ceruminous glands, and sebaceous glands)

• Orientation:

  • Adult: upward, backward, outwards

  • Infants: downward and backward

EXTERNAL AUDITORY CANAL PATHOLOGIES

• Narrowest Portion:

  • Tumor spread

  • Osteomyelitis/MOE (Malignant Otitis Externa)

• Fissures of Santorini: Vestigial lymph channels that drain into superficial parotid gland

• Foramen of Huschke: Incomplete ossification, anterior EAC, deep lobe of parotid gland

MIDDLE EAR

• Structure:

  • Air-filled space lined with mucosa

  • Functions as a sound-pressure transformer

  • Contains tympanic segment of CN VII (facial nerve) and vessels/nerves

  • Includes osseous Eustachian tube

TYMPANIC MEMBRANE

• Location:

  • Medial wall of external auditory canal, lateral wall of middle ear

• Structure:

  • 3-layered concave membrane

  • Connected centrally to the manubrium and tympanic sulcus peripherally

  • Parts:

  • Pars flaccida (Shrapnell membrane)

  • Pars tensa

OSSICULAR CHAIN

• Components:

  • Malleus, incus, stapes

• Articulations:

  • Head of malleus attached to body of incus

  • Base of stapes in contact with the oval window

MUSCLES OF THE MIDDLE EAR

• Tensor tympani muscle:

  • Origin: wall of auditory tube and own canal

  • Nerve supply: Mandibular division of trigeminal nerve (V3)

  • Action: dampens vibrations of tympanic membrane

• Stapedius muscle:

  • Origin: pyramid (bony projection on posterior wall of middle ear)

  • Nerve supply: facial nerve

  • Action: dampens vibrations of stapes

EUSTACHIAN TUBE

• Function: Conduit for air exchange between middle ear and upper aerodigestive tract

• Structure:

  • Proximal 1/3: petrous

  • Distal 2/3: fibrocartilaginous

  • Collapsed at rest, opens in an inverted J-shape during swallowing/yawning

MUSCLES OF THE EUSTACHIAN TUBE

• Tensor Veli Palatini (CN V3): Retracts to enlarge the tubal lumen during swallowing and yawning

• Levator Veli Palatini (CN X): Elevates the velum, closing the gap between oral and nasal cavities

SEMICIRCULAR CANALS AND TISSUES

• Canals:

  • Types:

  • Superior (anterior), posterior, lateral

  • Role: Detect angular accelerations or rotations

• Otoliths/Maculae: Detect linear accelerations

• VESTIBULAR APPARATUS:

  • Enclosed in bony labyrinth in petrous temporal bone

  • Utricle and saccule are lodged within the vestibule

COCHLEA ANATOMY

• Structure: Shaped like a snail, with a wide base that narrows at the apex

• Components:

  • 2 1/4 turns, helicotrema, spiral ganglion, modiolus, Reissner's membrane, organ of Corti, Scala vestibuli, Scala media, Scala tympani

ORGAN OF CORTI

• Composition:

  • Contains sensory hair cells and supporting cells

  • Critical for mechanical to electrochemical signal transduction

• Influence of Hair Cells:

  • Inner hair cells: ~3500, arranged in one row

    • Flasks shaped, passive transducers

  • Outer hair cells: ~12,000, arranged in three rows

  • Function: Act to amplify sounds

AUDITORY SYSTEM

• Function: Extracts specific information from complex waveforms of speech, music, and environmental sounds

VESTIBULAR SYSTEM

• Function: Detects head position and movement in space, enabling eye movement coordination, posture maintenance, and equilibrium

CENTRAL AUDITORY PATHWAYS

• Involves all ascending and descending neuronal projections interconnecting the auditory nerve, brainstem, midbrain, thalamus, and cerebral cortex

COMPONENTS

• Cochlear nucleus

• Superior olivary complex

• Inferior colliculus

• Medial geniculate body

• Auditory cortex

EIGHTH CRANIAL NERVE (COCHLEAR NERVE)

• Afferent fibers carry auditory information from inner and outer hair cells to the brainstem

  • Located anterior and inferior within lateral internal auditory canal (IAC)

COCHLEAR NUCLEUS

• Point of entry for peripheral auditory information into central auditory system

• First relay station for all ascending auditory information

• Located at the pontomedullary junction of the dorsolateral brainstem

SUPERIOR OLIVARY COMPLEX

• Found at the caudal aspect of the pons

• First central auditory center receiving binaural innervation, critical for sound localization and detection

LATERAL LEMNISCUS

• Main pathway for medullary and pontine auditory nerves reaching the inferior colliculus

• Important for sound localization, processing, and acoustic startle reflex

INFERIOR COLLICULUS

• A midbrain structure extensively innervated by higher and lower brain regions

• Responsible for sound localization, frequency determination, and integration of auditory and non-auditory systems

MEDIAL GENICULATE BODY

• Portal for all ascending auditory innervation to the telencephalon (cerebral hemispheres)

• Divided into ventral, dorsal, and medial divisions

AUDITORY CORTEX

• Located deep within the Sylvian fissure on the superior aspect of the temporal lobe

  • Brodmann areas:

  • Primary auditory cortex (AI): area 41

  • Secondary auditory cortex (AII): area 42

  • Wernicke area (dominant on left side): area 22

- Broca areas for language and musical syntax: areas 44 and 45

SEMICIRCULAR CANALS

• Structure:

  • Three orthogonal sensors: anterior, posterior, and horizontal

  • Sensitive to rotations in the same plane, specifying head rotation direction and amplitude

END ORGAN ANATOMY

• Ampulla:

  • Contains crista ampullaris, supporting cells, and cupula

• Crista: Saddle-shaped, maximizing specialized mechanoreceptor hair cell density

INNERVATION

• Cranial Nerves VII and VIII:

  • Form from brainstem laterally to CPA and IAC, carrying sensory information from vestibular and cochlear mechanisms

BLOOD SUPPLY TO THE EAR

• Anterior inferior cerebellar artery, labyrinthine artery, common cochlear artery, proper cochlear artery, and others

• They ensure vascular necrosis does not affect Sensitive structures such as cochlea