Anatomy of the Ear

External Ear - Outer Sound Funnel

• Components: Auricle (Pinna) & External Acoustic Meatus (EAM).
• Auricle:
  • Elastic fibrocartilage (lobule: fat).
  • Function: Sound collection, localization.
• EAM:
  • Length: ~2.5 cm (24 mm); S-shaped.
  • Outer 1/3 (cartilaginous, 8 mm): Cerumen, hair.
  • Inner 2/3 (bony, 16 mm): Thin skin.
  • Isthmus: Narrowest part.
  • Nerves: V3, VII, X (Arnold's - cough reflex).

⭐ Arnold's nerve (Vagus branch) in EAM causes cough reflex (Arnold's reflex) on stimulation.

Middle Ear - Tiny Bones, Big Job

• Cavity: Air-filled tympanic cavity (temporal bone); TM to oval window.
• Ossicles (MIS): Malleus, Incus, Stapes. Function: Sound transmission & amplification.
  • Malleus: Attached to TM; head in epitympanum.
  • Incus: Connects malleus to stapes.
  • Stapes: Footplate on oval window. Smallest bone.
  • 📌 Mnemonic: Make It Sound.
• Muscles: Protective reflexes.
  • Tensor tympani (CN V3): To malleus; tenses TM.
  • Stapedius (CN VII): To stapes; dampens loud sounds. Smallest skeletal muscle.
• Function: Impedance matching (air to cochlear fluid). Amplifies sound pressure ~22x.
• Communications: Eustachian tube (pressure balance), Aditus ad antrum (to mastoid cells).

⭐ Stapedius muscle paralysis (CN VII lesion) causes hyperacusis (increased sensitivity to sounds).

Inner Ear - Maze of Hearing & Balance

• Two main parts: Bony & Membranous Labyrinths.
  • Bony Labyrinth: Contains perilymph (↑Na+, ↓K+).
    • Cochlea: Hearing. Snail-shaped, 2.5-2.75 turns. Scala vestibuli & tympani.
    • Vestibule: Balance. Houses utricle & saccule.
    • Semicircular Canals (SCCs): Balance. Superior, posterior, lateral.
  • Membranous Labyrinth: Contains endolymph (↑K+, ↓Na+; from stria vascularis).
    • Cochlear Duct (Scala Media): Within cochlea. Houses Organ of Corti (hearing sensory organ).
    • Utricle & Saccule: Within vestibule. Detect linear acceleration, gravity (maculae).
    • Semicircular Ducts: Within SCCs. Detect angular acceleration (cristae ampullares).
• Innervation: Vestibulocochlear nerve (CN VIII).

⭐ Endolymph has a high K+ concentration, crucial for hair cell depolarization.

Nerves & Vessels - Ear's Lifelines

• Nerve Supply:
  • Ext. Ear: Auriculotemporal n. (V3), Great auricular (C2,C3), Vagal (Arnold's - X), Facial (VII - sensory).
  • Middle Ear: Tympanic plexus (IX) - sensory; Chorda tympani (VII) - taste.
  • Inner Ear: Vestibulocochlear n. (VIII) - hearing & balance.
• Arterial Supply:
  • Ext. Ear: Posterior auricular a., Superficial temporal a. (from ECA).
  • Middle Ear: Branches from ECA (e.g., Maxillary, Post. auricular) & ICA.
  • Inner Ear: Labyrinthine artery (from AICA/Basilar a.).

⭐ Arnold's nerve (Vagal br. - CN X) stimulation causes cough (Arnold's reflex).

High‑Yield Points - ⚡ Biggest Takeaways

• External Auditory Canal (EAC): S-shaped; outer 1/3 cartilaginous, inner 2/3 bony. Isthmus narrowest.
• Tympanic Membrane (TM): 3 layers; Pars Tensa (vibrates), Pars Flaccida (retraction).
• Middle Ear Ossicles: Malleus, Incus, Stapes (smallest bone); transmit sound.
• Eustachian Tube: Links middle ear to nasopharynx; shorter, wider, more horizontal in infants.
• Inner Ear: Cochlea (hearing); Vestibular system (balance: semicircular canals, utricle, saccule).
• Facial Nerve (CN VII): Close relation to middle ear/mastoid; vulnerable in surgery.
• Round & Oval Windows: Connect middle to inner ear; Stapes at Oval Window.