Anatomy and Physiology of the Temporal Bone and Ear

Temporal Bone and Ear Anatomy

Temporal Bone: 4 Parts

The temporal bone consists of four parts: Squamous, Tympanic, Mastoid, and Petrous.

Squamous

  • Major part of the lateral surface of the temporal bone.

  • Parts:

    • Vertical: Lateral part of the mastoid process; Lateral wall of the middle cranial fossa

    • Horizontal: Zygomatic process; Glenoid fossa; Supramastoid crest; Parts of anterior and posterior EAC; Spine of Henle
      Temporal line
      EAC
      Glenoid cavity
      Petrotympanic suture
      Mastoid process
      Tympanic bone
      Styloid process

Tympanic

  • Gutter-shaped.

  • Forms the inferior wall and major parts of the anterior and posterior walls of the bony EAC.

  • Tympanosquamous and tympanomastoid suture lines.

Mastoid

  • Forms the posterior and inferior borders of the temporal bone.

  • Attachment for the sternocleidomastoid muscle.

  • Parts:

    • Digastric groove = posterior belly of digastric

    • Stylomastoid foramen = exit of the facial nerve

    • Jugular spine and fossa

    • Carotid foramen

    • Canal for Jacobson’s (IX) and Arnold’s (X) nerves

Petrous

  • Hardest bone of the human skull.

  • Medial aspect of the temporal bone.

  • Pyramid shaped:

    • Base = semicircular canals, vestibule, cochlea

    • Apex = internal carotid artery, eustachian tube

  • Superior surface = middle cranial fossa

  • Posterior surface = posterior cranial fossa

    • Endolympathic duct and sac

    • Internal auditory meatus

Temporal Bone Aspects

  • Lateral Aspect:

    1. External auditory canal

    2. Fossa mastoidea

    3. Squama

    4. Mastoid tip

    5. Tympanomastoid suture

    6. Zygoma

    7. Glenoid fossa

    8. Temporal line

    9. Petrotympanic fissure

    10. Styloid process

  • Medial Aspect:

    1. Squama

    2. Middle meningeal artery sulcus

    3. Petrosa

    4. Internal auditory canal

    5. Sigmoid sinus

    6. Superior petrosal sulcus

    7. Inferior petrosal sulcus

    8. Petrous apex

    9. Arcuate eminence

    10. Internal carotid artery foramen

  • Superior Aspect:

    1. Zygoma

    2. Petrous Apex

    3. Arcuate eminence

    4. Tegmen

    5. Facial Hiatus

  • Posterior Aspect:

    1. Styloid process

    2. Internal Auditory Canal

    3. Endolymphatic fossa

  • Inferior Aspect:

    1. Mastoid process

    2. Zygoma

    3. Glenoid fossa

    4. Styloid Process

    5. Stylomastoid foramen

    6. Jugular fossa

    7. External opening of cochlear aqueduct

External Ear - Auricle/Pinna

Key Features

  • Helix

  • Crus of Helix

  • Crus of Anti-helix

  • Anti-Helix

  • Tragus

  • Concha

  • Anti-tragus.

  • Lobule

Innervation and Blood Supply

  • Auricular branch of vagus

  • Auriculotemporal nerve

  • Lesser occipital nerve (C2 and C3)

  • Greater auricular nerve (C3)

  • Superficial temporal artery

  • Posterior auricular nerve (VII motor)

External Auditory Canal:

Innervation provided by C-3, VII, X, V

  • External auditory meatus/canal ~ 2.5 cm long

    • Lateral 1/3 fibrocartilage

      • Fissures of Santorini à vestigial lymph channels that drain into parotid gland

      • hair follicles, sebaceous glands, sweat glands

    • Medial 2/3 bony

      • No subcutaneous tissue

      • Devoid of hair follicles, ceruminous glands, and sebaceous glands

Middle Ear

Tympanic Membrane

  • Conical shaped, tilted antero-inferiorly

  • Antero-inferior wall is longer than the postero-superior wall

  • Anterior angle is often obstructed by a bony protrusion of anterior wall

  • Two parts, three layers

Parts of Tympanic Membrane

  • Pars tensa

    • Outer epithelial

    • Middle fibrous (radiating and circular)

    • Inner mucosal

  • Pars flaccida

    • Outer epithelial

    • Loose connective tissue

    • Inner mucosal

Landmarks of Tympanic Membrane

  • Tympanic annulus

    • Edge of the TM that is thickened and attached to the bony annulus

  • Notch of Rivinus

    • Small defect in the posterior edge of the bony annulus, located superior to the tympanomastoid suture line

  • Prussak’s space

    • Small recess bordered laterally the pars flaccida, bounded superiorly by scutum, inferiorly by lateral process of malleus, and medially by neck of malleus

Ossicular Chain

  • Malleus

    • Head

    • Neck

    • Anterior Process

    • Manubrium

  • Incus

    • Body

    • Short Process

    • Long Process

    • Lenticular Process

  • Stapes

    • Head

    • Anterior Crus

    • Posterior Crus

    • Footplate

Muscles of Middle Ear

  • Both contract to dampen damage of loud sound

    1. Tensor Tympani

      • Innervation: Trigeminal nerve (CN V3)

      • Blood supply: Superior tympanic artery

      • Attachment: Cochleariform process

    2. Stapedius

      • Innervation: Facial nerve (CN VII)

      • Blood supply: Stapedial artery

      • Attachment: Pyramidal eminence

Tympanic Cavity Divisions

  • Epitympanum

  • Mesotympanum

  • Hypotympanum

Medial Wall

  • Promontory

    • Prominent eminence located anteroinferior to the oval window and anterior to round window

    • Corresponds to the basal turn of the cochlea

    • Axis of cochlea is directed anteriorly and laterally

  • Oval window

    • Where the stapes footplate is lodged

    • Transmission of mechanical energy to the cochlea

    • Inferior to the tympanic segment of the facial nerve

  • Round window

    • The other opening of the labyrinth to the middle ear

    • Inferior to the oval window

    • Round window niche

Posterior Wall

  • Aditus ad antrum

  • Pyramidal eminence

  • Chordal eminence

  • Styloid eminence

  • Facial recess

  • Tympanic sinus

  • Attic

Details

  • Facial recess

    • Bony annulus laterally and facial canal medially

    • Divided by the chordal crest to form:

      • Chordal eminence

      • Pyramidal eminence

  • Tympanic sinus

    • Medial to the facial nerve

    • Visualization of its base is difficult

    • Ponticulus (superior)

      • Connects the pyramidal eminence and promontory

    • Subiculum (inferior)

      • Between the posterior wall and round window niche

  • Attic (Epitympanum)

    • Cog

      • Bony spur that extends vertically from the tegmen to point anterior to head of malleus

      • Divides attic into posterior and anterior divisions (supratubal recess)

      • Superior to facial nerve

    • Floor contains post-geniculate portion of facial nerve

    • Aditus ad antrum

      • Opening of the antrum located posterior to the attic

  • Antrum

    • Connects the mastoid air cells with the attic

    • Posterior to attic, inferior to middle fossa plate, lateral to labyrinth

    • One of the most consistent and important landmarks in mastoidectomy

    • Prominence of horizontal semicircular canal

Eustachian Tube

  • 17 to 18 mm in children à 10° from horizontal

  • 35-37 mm in adults à 45° from horizontal

  • Supero-lateral 1/3 bony

  • Antero-medial 2/3 cartilaginous

  • Flat, closed at rest

  • Opens by contraction of tensor veli palatini and levator veli palatini, the tensor veli palatini being more important in adults.

Mastoid Air Cells Development

Stages of Development

  • Conception to Birth: Infantile type air cells appear.

  • 22-24 weeks: Mastoid antrum develops.

  • 0-2 years: Air cells develop, forming a tip.

  • 5 years: Diploic type mastoid development occurs, with the SCM pulling the mastoid.

  • Transitional type: mastoid enlarges with migration of air cells toward periphery

  • Mature system: Pneumatization ceases.

Mastoid Air Cell Regions

  • Mastoid Perilabyrinthine Region

  • Petrous Region

  • Apex Region

  • Tegmental Cells

  • Subarcuate Tract

  • Posterosuperior Tract

  • Squamous

  • Accessory Cells

  • Supralabyrinthine Area

  • Sinodural Cells

  • Antrum

  • Posteromedial Tract

  • Central Mastoid Tract

  • Occipital Accessory Cells

  • Sinal Cells

  • Retrofacial Cells

  • Zygomatic

  • Peritubal Area

  • Medial Tip Cells

  • Intralabyrinthine Area

  • Lateral Tip Cells

  • Carotid A.

  • Styloid Cells

  • Jugular V.

Inner Ear

The inner ear comprises the Bony Labyrinth and the Membranous Labyrinth.

Bony Labyrinth

  • Posterior: 5 openings with semicircular canals

  • Lateral: opening of oval and round windows

  • Bony semicircular canals: 3 canals at right angles to one another, 2/3 of circle

  • Bony Cochlea:

    • 2.5 turns

    • Core --> Modiolus

    • Osseous Spiral Lamina à spiral crest of bone diving lumen into two tubes

    • Helecotrema à Apex of modiolus

Membranous Labyrinth

  • Membranous semicircular canals

    • Superior and posterior share one crus cummunis

  • Membranous vestibule

    • Utricle

    • Saccule

    • Saccus endolymphaticus

    • Scala media

      • Contains Organ of Corti

Semicircular Canals

  • Lateral

    • Located at the medial wall of antrum sloping at 30∘30^{\circ}

    • Most vulnerable part of the labyrinth to pathologies of mastoid and middle ear

    • Ampulla is at its anterior end

  • Posterior

    • Posterior to the lateral SCC

    • Parallel to the posterior fossa dura

    • Ampulla is at its inferior end

    • Superior end joins the superior SCC to form common crus

  • Superior

    • Located inferior to the middle cranial fossa plate

    • Ampulla is at its anterior end, superomedial to ampulla of lateral SCC

    • Perpendicular to long axis of the petrous pyramid

Vestibule

  • Hollow space within the petrous bone that contains otolithic organs

    • Utricle

      • Detects head movements on a horizontal plane

    • Saccule

      • Detects head movements on a vertical plane

Cochlea

  • Anterior to the vestibule

  • 2.5 turns

  • Wide base, narrow apex

  • Promontory = projection of the basal turn of cochlea into the middle ear

  • Core à Modiolus

  • Osseous Spiral Lamina à spiral crest of bone diving lumen into two tubes

  • Helicotrema à Apex of modiolus

Organ of Corti

Key Components:

  • 16,000 hair cells with 30-100 stereocilia (microvilli).

  • Microvilli make contact with tectorial membrane (gelatinous membrane).

  • Basal sides of inner hair cells synapse with 1st order sensory neurons in spiral ganglion.

  • Scala vestibuli (perilymph): [Na] > [K]

  • Scala tympani (perilymph): [Na] > [K]

Facial Nerve

Key Segments:

  1. Intracranial – 23 to 24 mm long

  2. Meatal – 13 to 15 mm long

  3. Labyrinthine – 3 to 4 mm

    • shortest and narrowest segment

    • fundus to geniculate ganglion

    • susceptible to compression by means of edema, embolic phenomena, low-flow states, and vascular compression

  4. Tympanic – 8 to 11 mm

    • cog and cochleariform process

    • inferior to the lateral SCC

    • posterior and inferior to the oval window

    • higher incidence of dehiscence (25 – 50%)

  5. Mastoid – 10 to 14 mm

    • medial to short process of incus

    • anterior edge of digastric ridge

    • exits at stylomastoid foramen

    • longest intratemporal branch of the facial nerve

Internal Auditory Canal

Mnemonic

  • 7up, coke down is a mnemonic to remember the relative position of nerves inside the internal auditory canal (IAC).

  • BB: Bill's bar

  • FC: Falciform crest

  • SVN: Superior vestibular nerve

  • IVN: Inferior vestibular nerve

  • 7: Facial (VII) nerve

  • C: Cochlear nerve

Embryology

External Ear Development

  • 6 HILLOCKS OF HIS (1st and 2nd branchial arches)

Middle Ear Development
Inner Ear Development

Physiology of Hearing and Balance

Nature of Sound

§ Sound is any audible vibration of molecules
§ Vibrating object pushes air molecules into zones of compression separated by zones of rarefaction

Properties of Sound

§ Frequency – the number of waves that pass a given point in a given time
§ Pitch – perception of different frequencies (we hear from 20–20,000 Hz)
§ Intensity – The power transmitted by a wave through an unit area
§ Loudness – The perception of intensity

Main Components of the Hearing Mechanism Divided into 4 parts (by function):

§ Outer Ear
§ Middle Ear
§ Inner Ear
§ Central Auditory Nervous System

Functions of the Outer Ear
• Gathers sound waves
• Increases pressure in a frequency- sensitive mechanism
• Aids in localization

Functions of the Middle Ear

• Couple sound energy to the cochlea
• Impedance matching
• Protects the cochlea
• Preferential application of sound to one window

Impedance Transformer

• Large area of TM in comparison to small area of foot plate (pressure increases inversely to the ratio of these areas)
• Ossicular lever ratio (Malleus is 1.3 times longer than incus)
• Buckling action of TM
• Ligaments suspending ossicles

Impedance Efficiency

• Middle ear converts low pressure high displacement movements of the eardrum into high pressure low displacement movements needed for the cochlear fluid movement.
• 50% of sound energy from TM gets transmitted and absorbed in the cochlea.
• Without middle ear, only 1% of sound energy will be absorbed by the cochlea

Role of Middle Ear Muscles

• Tensor tympani attaches to the neck of malleus. It pulls the drum medially.
• Stapedius muscle attaches to the posterior aspect of neck of stapes.
• Contraction of these muscles increase the stiffness of ossicular chain thus blunting low frequencies
• Stapedius contracts in response to loud sounds and acts as an in built ear plug

Bone Conduction

• Bone vibration conducted through ext canal
• Skull vibration – ossicles lag behind
• Differential distortion of bony cochlea
• Direct vibration of osseous spiral lamina
• Skull vibration via CSF to endolymph

Structures of the Inner Ear

• Bony Labyrinth
• Bony cochlea
• Vestibule
• Semicircular canals
• Membranous Labyrinth
• Cochlear duct
• Utricle & Saccule
• Semicircular Canals

Organ of Corti

• 16,000 hair cells have 30-100 stereocilia (microvilli )
• Microvilli make contact with tectorial membrane (gelatinous membrane that overlaps the spiral organ of Corti)
• Basal sides of inner hair cells synapse with 1st order sensory neurons whose cell body is in spiral ganglion

Potassium Gates of Cochlear Hair Cells

 Stereocilia bathed in high K+ concentration creating electrochemical gradient from tip to base
 Stereocilia of OHCs have tip embedded in tectorial membrane which is anchored
 Movement of basilar membrane bends stereocilia
 Bending pulls on tip links and opens ion channels
 K+ flows in -- depolarizing it & causing release of neurotransmitter stimulating sensory dendrites at its base

Theories Of Hearing

• Place theory of Helmholtz
• Telephone theory of Rutherford
• Volley theory of Wever
• Traveling wave theory of von Bekesy

Central Auditory Pathways
Auditory Cortex
Applied Physiology

 EAC blockage = 30db HL
 TM perforation = 26db HL
 TM perforation with ossicular interruption 26.5 +7.3+ 5=38.3dbHL38.3\text{dbHL}
 Total TM loss with ossicular interruption - 26.5 +7.3+ 16.2=50dbHL50\text{dbHL}
 Ossicular interruption with an intact TM 38+15=54dbHL54\text{dbHL}
 Ossicular interruption with an intact TM and closed oval window = 60dbHL60\text{dbHL}

Vestibular Apparatus

Equilibrium

• Static equilibrium is perception of head orientation
• perceived by macula
• Dynamic equilibrium is perception of motion or acceleration
• linear acceleration perceived by macula
• angular acceleration perceived by crista

The Saccule and Utricle

Vertical Plane (up-down; forward-backward movement in a sagittal plane)
Utricle = Horizontal Plane (sideways head tilts, rapid lateral movements)

Macula of Saccule and Utricle

With the head erect, stimulation is minimal, but when the head is tilted, weight of membrane bends the stereocilia (static equilibrium)
Linear acceleration is detected since heavy otolith lags behind (one type of dynamic equilibrium)

Crista Ampullaris of Semicircular Ducts

Orientation of ducts causes different ducts to be stimulated by rotation in different planes

Equilibrium Projection Pathways
Vestibular Nuclei

Cristae of SCC & Cerebellum
Medial Longitudinal Fasciculus
Cerebellum & Utricular Macula
Vestibulo Spinal Tract, Reticulo Spinal Tract
Cristae Cerebellum
Medial Longitudinal Fasciculus
Utricular & Sacular Maculae

Ascending Vestibular Projections
Vestibular Reflexes

Vestibulo-spinal Helps maintain center of gravity
Vestibulo-ocular Helps maintain stability of the visual field
Vestibulo-collic Helps maintain stability of the head during movement of the torso

Vestibulo Ocular Reflexes
Clinical Relevance - Giddiness

  1. NON CORRECTABLE VISUAL IMPAIRMENT.

  2. NEUROPATHY.

  3. VESTIBULAR DYSFUNCTION.

  4. CERVICAL SPONDYLOSIS.

  5. ORTHOPAEDIC DISTURBANCES.

  6. CARDIAC DISORDERS.

  7. NEUROLOGICAL DEFICITS.

Assessment

IDENTIFICATION OF PRESENCE/ ABSENCE OF VESTIBULAR COMPONENT.

  1. VESTIBULO-SPINAL FUNCTION.

  2. VESTIBULO – OCULAR FUNCTION.

Vestibulo-Spinal Function

  • ROMBERGS TEST

  • UNTERBERGERS TEST

Vestibulo-Ocular Function
Nystagmus

INVOLUNTARY DEVIATION OF EYES AWAY FROM DIRECTION OF GAZE FOLLOWED BY A RETURN OF THE EYES TO THEIR ORIGINAL POSITION. 3 TYPES - CENTRAL, OCULAR, VESTIBULAR

Vestibular Nystagmus

  • RHYTHMIC FAST AND SLOW PHASES NAMED AFTER FAST PHASE.

  • 3 TYPES

    1. SPONTANEOUS

    2. POSITIONAL

    3. INDUCED.

Vestibular Nystagmus

  • SPONTANEOUS NYSTAGMUS

  • GRADE 1.

  • GRADE 2.

  • GRADE 3.

  • POSITIONAL NYSTAGMUS

  • HALLPIKE MANOEUVRE

Induced Nystagmus

ROTATIONAL TESTS

  • Nystagmus Induced by accelerating and decelerating rotating chair, tests both labyrinths simultaneously

CALORIC TESTS

  • COWS- cold water opposite side, warm water same side, direction of nystagmus

  • Extent of caloric response indicates function of labyrinth

Electronystagmograghy

  • Positive potential between the cornea and retina recorded as eyes move from straight ahead gaze

  • Test includes different head positions, eyes open, closed and caloric tests