📚 EXTERNAL & MIDDLE EAR ANATOMY

🖇 I. INTRODUCTION & LECTURE SCOPE

📌 Coverage Strategy

Content is too vast for 2 hours → summarized/focused approach
Main focus: Anatomy of the Ear (especially Middle Ear and External Ear) (Exam Point)
Minimal coverage: Development (5 min or by Dr. Hala), Inner Ear (rapid overview only)
Inner Ear details → more relevant for postgraduate level, not undergraduate emphasis
High-yield information: frequently questioned exam points + clinically relevant material (Exam Point)

🖇 II. FUNCTIONS AND DIVISIONS OF THE EAR

📌 Primary Functions

Hearing (primary function)
Equilibrium/Balance (secondary function, specific to Inner Ear)

🚨 Functional Separation (Exam Point)

Hearing and equilibrium are anatomically adjacent but functionally distinct
Absolutely no relation to each other
Same nerve carries both but:
- Fibers are separate
- Nuclei are separate
- Pathways are separate
Enter/exit inner ear together as one nerve → immediately separate inside inner ear → travel to different locations

📌 Nerve Nomenclature (Exam Point)

8th cranial nerve = Vestibulocochlear Nerve (correct term)
❌ Avoid "Auditory Nerve" → ignores significant vestibular (balance) component

🟠 Anatomical Divisions (Coronal Section)

External Ear:
- Auricle (pinna)
- External Auditory Meatus (EAM)
Middle Ear (Tympanic Cavity):
- Separated from EAM by Tympanic Membrane (Eardrum)
Internal Ear (Labyrinth):
- Cochlea (anterior)
- Vestibule (middle)
- Semicircular Canals (posterior/upper)

🖇 III. THE EXTERNAL EAR

📌 Components:

Auricle
External Auditory Meatus (EAM)
Tympanic Membrane (TM) - acts as common boundary:
- Lateral wall of Middle Ear
- Medial wall of EAM

🟠 A. THE AURICLE (PINNA)

📌 Structure

Projection of elastic cartilage
Covered by skin
Attached to side of head

📌 Anatomical Parts

Tragus (important anterior prominence)
Lobule (lower fleshy part):
- Composed of fat (not cartilage)
Helix (outer rim)
Antihelix

🚨 Function of Convolutions (Exam Point)

Folds and ridges ≠ aesthetic appearance
Function: directing sound waves from any source into the External Auditory Meatus

🟣 AURICULAR MUSCLES

📌 Three Rudimentary Muscles:

Anterior Auricular Muscle
Superior Auricular Muscle
Posterior Auricular Muscle

🚨 Characteristics (Exam Point)

Rudimentary in humans (non-functional)
Function of moving pinna is lost in humans
Functional in animals (dogs, horses)

📌 Motor Innervation

Facial Nerve (CN VII)
Specifically: Temporal and Buccal branches

🟣 SENSORY INNERVATION (Exam Point - Highly Emphasized)

🚨 LATERAL (OUTER) SURFACE:

Upper 2/3:
- Auriculotemporal Nerve (branch of Mandibular division of CN V = V3)
Lower 1/3:
- Great Auricular Nerve (Cervical Plexus, C1 and C2 ventral rami)

🚨 MEDIAL (POSTERIOR) SURFACE: (distribution is reversed)

Upper 2/3:
- Lesser Occipital Nerve (from C2)
Lower 1/3 (including Lobule):
- Great Auricular Nerve (C2)

🚨 ROOT OF AURICLE (Exam Point)

Small cartilaginous area at entrance of EAM (immediately posterior to tragus)
Supplied by: Auricular Branch of Vagus Nerve (CN X)

📌 Summary: Three major nerve groups supply auricle:

Trigeminal (CN V)
Vagus (CN X)
Cervical nerves (C1, C2)

🟣 ARTERIAL SUPPLY

📌 Two Main Arteries (both from External Carotid Artery):

Superficial Temporal Artery → supplies lateral surface
Posterior Auricular Artery → supplies medial surface

🟣 LYMPHATIC DRAINAGE (Exam Point)

📌 Drainage Pattern:

Lateral Surface → Pre-auricular Nodes
Medial/Posterior Surface → Post-auricular (Mastoid) Nodes
Both nodal groups → Deep Cervical Nodes

🟠 B. EXTERNAL AUDITORY MEATUS (EAM)

📌 Dimensions

Length: approximately 1 inch (2.5 cm or 24 mm)

📌 Structure

Lateral 1/3: cartilaginous
Medial 2/3: bony
Junction: narrow area = isthmus

🚨 Shape and Direction (Exam Point/Clinical)

S-shaped
Direction:
1. Initially: backward and upward
2. Then: inward and downward
3. Finally: downward and forward

🚨 CLINICAL: Examination Technique (Exam Point)

To examine eardrum → EAM must be straightened:
- Adults: Pull auricle Upward and Backward
- Children/Newborns: Pull pinna Backward and Downward
- - (cartilaginous part direction is reversed in children)

📌 Glands

Skin of outer 1/3 contains: Ceruminous Glands
- Modified sweat glands
- Produce wax (cerumen) instead of sebum

🟠 C. TYMPANIC MEMBRANE (TM) / EARDRUM

📌 Physical Characteristics

Thickness: 0.1 mm (very thin)
Diameter: approximately 10 mm
Appearance: semitransparent membrane

🚨 Angulation (Exam Point)

Slanted/inclined
Forms angle of approximately 55 degrees with meatus floor
This inclination helps capture sounds from below

🟣 PARTS OF TM (Classification)

📌 1. Pars Tensa:

Large portion
Highly stretched
Pulled medially (inward) by handle of malleus
Makes TM concave laterally

📌 2. Pars Flaccida:

Small portion
Located posteriorly
Relaxed (not pulled by malleus)

📌 Boundary of Pars Tensa:

Periphery fixed to surrounding bone via fibrous Annulus Fibrosus
Fits into bony groove (sulcus)

📌 Otoscopic Appearance:

Inward concavity creates visible light reflection = Cone of Light

🟣 EMBRYOLOGICAL LAYERS (Exam Point)

🚨 TM derived from all 3 embryonic layers:

Outer Layer: Cutaneous (Skin/Ectoderm)
Middle Layer: Fibrous (Mesoderm)
Inner Layer: Mucosal (Endoderm)

🟣 INNERVATION (Exam Point)

📌 Sensory only (NO motor innervation)

🚨 INNER (MEDIAL) SURFACE:

Tympanic Branch of Glossopharyngeal Nerve (CN IX)
Via Tympanic Plexus

🚨 OUTER (LATERAL) SURFACE:

Extension of EAM innervation:
- Upper half: Auriculotemporal Nerve (V3)
- Lower half: Auricular Branch of Vagus Nerve (CN X)

🖇 IV. THE MIDDLE EAR (TYMPANIC CAVITY)

📌 "The Big Story" - most detailed topic of lecture

📌 Definition

Air-filled cavity
Situated between External Ear and Inner Ear

📌 Shape

Six-sided cube compressed from side-to-side
Horizontal diameter:
- Widest: superiorly and inferiorly
- Narrowest: in the middle

🟠 DIVISIONS (Classification by TM relation)

Mesotympanum: area directly behind eardrum
Epitympanum (Attic): upper part, above TM
Hypotympanum: lower part, below TM

🟠 SIX BOUNDARIES (WALLS) (Exam Point)

🟣 A. ROOF (SUPERIOR WALL)

📌 Structure

Formed by: Tegmen Tympani
Very thin plate of bone
Separates middle ear from middle cranial fossa

🚨 Vascular/Neural Structures (Exam Point) Roof pierced by 2 openings transmitting nerves in grooves:

Greater Superficial Petrosal Nerve:
- Branch of CN VII (Facial)
- Goes to Pterygopalatine Ganglion
Lesser Superficial Petrosal Nerve:
- From Tympanic Plexus (CN IX origin)
- Goes to Otic Ganglion
Deep Petrosal Nerve:
- Sympathetic fibers from internal carotid plexus
- Joins greater petrosal nerve

📌 Facial Nerve Relationship:

Runs in bony canal coursing over roof
Gives off Greater Superficial Petrosal Nerve near roof

🟣 B. FLOOR (INFERIOR WALL)

📌 Structure

Contains bony ridge between two small fossae
Large nearby fossa = Jugular Fossa

🚨 Neural Structures (Exam Point)

Floor perforated by: Tympanic Canaliculi (small openings)

📌 Tympanic Plexus Formation:

Tympanic Branch of Glossopharyngeal Nerve (CN IX) enters through canaliculi
Forms Tympanic Plexus with contributions from:
- Vagus Nerve
- Facial Nerve

📌 Plexus Functions:

Sensory innervation to medial TM surface
Gives rise to Lesser Superficial Petrosal Nerve

🟣 C. ANTERIOR WALL (CAROTID WALL)

📌 Structure

Defined by adjacent Internal Carotid Artery in its canal

📌 Two Superior Openings:

Opening for Eustachian Tube (Pharyngotympanic Tube)
Opening for canal housing Tensor Tympani Muscle

🟣 D. POSTERIOR WALL (MASTOID WALL)

📌 Key Structures:

Aditus to Mastoid Antrum:
- Opening leading to Mastoid Antrum
- Connected to Mastoid Air Cells
Pyramid:
- Conical bony projection
- Encloses tiny Stapedius Muscle
- Only tendon emerges from pyramid → attaches to stapes
Facial Nerve Course:
- Descends behind this wall
- Before exiting skull

🟣 E. MEDIAL WALL (LABYRINTHINE WALL)

📌 Promontory:

Central bulge
Caused by underlying Vestibule of Inner Ear
Tympanic Plexus lies over promontory

🚨 WINDOWS (FORAMINA) (Exam Point)

Fenestra Ovalis (Oval Window):
- Located superiorly
Fenestra Cochleae (Round Window):
- Located inferiorly

🟣 F. LATERAL WALL

📌 Structure

Formed by Eardrum (TM)

📌 Visible Contents (view from Middle Ear):

Handle of malleus (clearly visible)
Tendon of Tensor Tympani muscle (attaches to malleus)
Chorda Tympani Nerve (crossing middle ear cavity above eardrum)

🖇 V. AUDITORY OSSICLES & SOUND TRANSMISSION

📌 Three Ossicles (connected):

Malleus (largest)
Incus
Stapes (smallest)

📌 Malleus Anatomy:

Handle: attached to TM
Head: located in Epitympanum

📌 Stapes Anatomy:

Footplate: rests on Oval Window

🟠 MECHANISM OF HEARING (Exam Point)

🚨 Acoustic Impedance Matching Problem:

Middle ear designed to overcome impedance difference between:
- Air (in external/middle ear)
- Fluid/endolymph (in inner ear)
Fluid resistance = 5× greater than air

🚨 AMPLIFICATION (Exam Point)

Compensated by anatomical ratio:
- TM area : Oval Window area = 18:1

🟣 SOUND TRANSMISSION PATHWAY:

Sound vibration → pushes Eardrum
Eardrum movement → moves handle of Malleus inward
Malleus head → moves forward → pushes Incus
Incus → causes Stapes footplate to push inward on Oval Window
Fluid wave generation → wave in inner ear fluid (endolymph/perilymph)
Cochlea activation → wave travels through Membranous Labyrinth to Cochlea
Neural signal → sensitive organ (Organ of Corti) in cochlea registers vibrations
Transmission → information via Cochlear Nerve to brainstem nuclei

🟠 MIDDLE EAR MUSCLES (ACOUSTIC REFLEX)

📌 Two Muscles:

Tensor Tympani (anterior)
Stapedius Muscle (posterior)

🚨 Function (Exam Point)

NO ROLE in normal hearing mechanism
Function is solely protective

📌 Trigger:

Contract simultaneously ONLY in response to High-Pitch Sounds
Frequency: above 20,000 Hertz

📌 Protection Mechanism:

Contraction dampens movement
Prevents high-energy wave from damaging delicate cochlear fibers
One muscle stops Malleus
Other stops Stapes

🚨 INNERVATION (Exam Point)

Tensor Tympani:
- Supplied by Mandibular Nerve (V3)
Stapedius Muscle:
- Supplied by Facial Nerve (CN VII)

📌 END OF LECTURE NOTES

Key Focus Areas for Exams:

Sensory innervation of auricle (complex distribution)
TM layers and innervation
Six walls of middle ear and their contents
Mechanism of hearing and impedance matching
Middle ear muscle innervation and function