Anatomy of ear + development of the Ear + vestibule cochlear nerve

📚 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)

1. External Ear:
2. • Auricle (pinna)
   • External Auditory Meatus (EAM)
2. Middle Ear (Tympanic Cavity):
3. • Separated from EAM by Tympanic Membrane (Eardrum)
3. Internal Ear (Labyrinth):
4. • 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:

1. Trigeminal (CN V)
2. Vagus (CN X)
3. 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:

1. Outer Layer: Cutaneous (Skin/Ectoderm)
2. Middle Layer: Fibrous (Mesoderm)
3. 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)

1. Mesotympanum: area directly behind eardrum
2. Epitympanum (Attic): upper part, above TM
3. 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:

1. Greater Superficial Petrosal Nerve:
2. • Branch of CN VII (Facial)
   • Goes to Pterygopalatine Ganglion
2. Lesser Superficial Petrosal Nerve:
3. • From Tympanic Plexus (CN IX origin)
   • Goes to Otic Ganglion
3. Deep Petrosal Nerve:
4. • 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:

1. Sensory innervation to medial TM surface
2. 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:

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

🟣 D. POSTERIOR WALL (MASTOID WALL)

📌 Key Structures:

1. Aditus to Mastoid Antrum:
2. • Opening leading to Mastoid Antrum
   • Connected to Mastoid Air Cells
2. Pyramid:
3. • Conical bony projection
   • Encloses tiny Stapedius Muscle
   • Only tendon emerges from pyramid → attaches to stapes
3. Facial Nerve Course:
4. • 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)

1. Fenestra Ovalis (Oval Window):
2. • Located superiorly
2. Fenestra Cochleae (Round Window):
3. • 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):

1. Malleus (largest)
2. Incus
3. 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:

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

🟠 MIDDLE EAR MUSCLES (ACOUSTIC REFLEX)

📌 Two Muscles:

1. Tensor Tympani (anterior)
2. 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)

1. Tensor Tympani:
2. • Supplied by Mandibular Nerve (V3)
2. Stapedius Muscle:
3. • 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