Anatomy and Physiology of Hearing
Anatomy
External Ear
• Pinna (auricle)
• External auditory canal
• Tympanic Membrane
External Ear
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Auricle
funnel-shaped elastic cartilage
Except lobule
Anterior : Zygomatic process
Superior : bony external meatus
Posterior : mastoid process
http://users.bergen.org/dondew/bio/AnP/AnP1/AnP1Tri2/FIGS/EARS/03hearing.html
External Ear
• ectodermal and mesodermal of 1st and 2nd branchial arches
• 1st branchial arch : tragus and most of the helix
• 2nd branchial arch : antihelix, antitragus, lobule, and inferior helix
External Ear
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External auditory canal :
Posterosuperior wall 2.5 cm long
Anteroinferior wall 3.1 cm long
lateral 1/3 : cartilage ,cerumenproducing glands and hair
follicles
• medial 2/3 : bone
External Ear
• Fissures of Santorini
• Inconstant fissure filled with
connective tissue occur in
cartilaginous part of EAC
• meatus to the superficial mastoid
, parotid gland
http://www.sciencedirect.com/science/article/pii/S0009926006000444
External Ear
• Foramen of Huschke
• Bony part of EAC is deficient
anteroinferior
• produces an opening into the
infratemporal region
• meatus to the deep lobe of the
parotid gland
http://www.scielo.br/scielo.php?pid=S0100-39842006000400009&script=sci_arttext&tlng=en
External Ear
• dorsal portion of 1st branchial
cleft, which extends toward and
eventually makes contact with
endoderm of the expanding
tubotympanic recess.
External Ear
• Blood supply
• Auricle : posterior auricular and small auricular rami of superficial
temporal vessels. (external carotid artery)
External Ear
• Blood supply
• EAC : posterior auricular ,
auricular rami of superficial
temporal vessels , deep
auricular artery (1st part of
maxillary artery)
External Ear
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Lymphatic drainage
Parotid node
Superficial cervical node
Retroauricular node
External Ear
• Innervation
• Auriculotemporal branch of the
trigeminal nerve
• Cutaneous branch of cervical plexus
- great auricular nerve (C2,C3)
- lesser occipital nerve (C2,C3 )
• Auricular branch of vagus nerve
(nerve of Arnold)
• CN 7,9
Tympanic Membrane
• medial wall of the EAC
• lateral wall of the middle ear
• Protects middle ear space from
foreign material
• Maintains air cushion that prevents
insufflation of foreign material from
the nasopharynx.
8-9 mm.
9-10mm.
Tympanic Membrane
• 4 layer, concave-shaped
1. Squamous epithelium
2. Radiating fibrous layer
3. Circular fibrous layer
4. Mucosa layer
• Pars flaccida or Shrapnell's membrane
• Pars tensa
Tympanic Membrane
• Blood supply
• Small peripheral vascular ring
formed by deep auricular branch
of maxillary artery
• Anterior tympanic branch of
maxillary artery
• Stylomastoid branch of posterior
auricular artery
http://otoscopy.hawkelibrary.com/album06/Anat_10_D
Tympanic Membrane
• Sensory innervation
• Outer surface
- Auriculotemporal branch of
trigeminal nerve
- Auricular branch of vagus nerve
• Inner surface : Tympanic plexus
Middle Ear
• transmits acoustic energy from the
air-filled EAC to the fluid-filled
cochlea
• Tympanic cavity
Middle Ear
• Tympanic cavity
• Cavum tympani, Tympanum proper,
middle ear cavity
• Temporal bone
• Bones of Middle ear
• Eustachian tube
Middle Ear
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Bones of Middle ear
Malleus
Incus
Stapes
Middle Ear
• 2 striated muscles : smallest ,
high innervation ratio
- tensor tympani :attaches to
malleus , trigeminal nerve
- stapedius : attaches to stapes
, stapedial branch of facial
nerve.
Middle Ear
• 1st Arch (Mandibular Arch)
- Malleus head and neck
- Incus body and short process
- Tensor tympani
• 2nd Arch (Hyoid Arch)
- Stapes
- Malleus manubrium
- Incus long process
- Facial expression
- stapedius
Middle Ear
• Artery
• Anterior tympanic artery (the maxillary artery)
• Inferior tympanic artery (derived from ascending
pharyngeal artery)
• Stylomastoid artery (posterior auricular artery)
• Posterior tympanic artery
• Middle meningeal artery (Maxillary artery)
• Caroticotympanic artery (Internal carotid artery)
Middle Ear
• Vein
• Ptergoid plexus
• Superior petrosal sinus
• Lymphatic
• Retropharyngeal , Parotid
lymph node
Middle Ear
• Innervation
• Tympanic plexus
Middle Ear
• Eustachian tube is the conduit
through which air is exchanged
between middle ear space and
upper aerodigestive tract.
• 45 degrees , 17-18mm. at birth
grows to 35mm. in adult
• proximal 1/3 (11mm.)
• distal 2/3 (24mm.)
Middle Ear
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Blood supply
Maxillary artery
Ascending pharyngeal artery
Ascending palatine branch of facial artery
Superior tympanic branch of middle meningeal artery
Artery of pterygoid canal
Middle Ear
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Innervation
Tympanic plexus
Chorda tympani
Mandibular nerve
Inner Ear
• Osseous labyrinth
• Membranous labyrinth
Inner Ear
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Osseous labyrinth
Bony
Vestibule
Semicircular canal
Cochlea
Inner Ear
• Cochlea : 35 mm long
• Snail-shaped cavity within mastoid bone
• 2 ½ turns, 3 fluid-filled chambers
Inner Ear
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Membranous labyrinth
Semicircular ducts
Utricle
Saccule
Utriculosaccular duct
Endolymphatic duct , sac
Cochlear duct
Inner Ear
• Cochlear duct
Inner Ear
• Cochlear duct
• Organ of Corti
Inner Ear
• Outer hair cell
• Inner hair cell
• Outer and inner hair cells :
mechanical (acoustic)
energy -> electrical
(neural) energy
Inner Ear
• Artery
• Internal auditory artery (anterior inferior cerebellar artery 83% , basillar
artery 17%)
Inner Ear
• Vein
• Vestibular vein
• Spiral vein
• Lymphatic
• Not usually described
Inner Ear
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Innervation
Vestibulocochlear nerve (CN 8)
Vestibular nerve
Cochlear nerve
Physiology
Physiology
External Ear
• External ear : 15 dB gain (3kHz), 10 dB gain (2-5kHz)
• Concha : resonant frequency 5 kHz
• EAC : resonant frequency 3.5 kHz
External Ear
• NIHL occur first and most prominently
at the 4kHz (boilermaker notch).
External Ear
• >2 kHz : head shadow effect , interaural intensity differences of 5-15 dB
• <2 kHz : Interaural time differences ~0.6 ms
Middle Ear
• low impedance of air ->high impedance of the fluid-filled cochlea
• Impedance match : 25-30 dB
1. area ratio = 17-20 times , 26 dB
2. lever ratio = 1:1.3 times , 2.3 dB
3. shape of TM
Middle Ear
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One function of the middle ear muscles is to protect the cochlea from
loud sounds (>80 dB SPL ,low-frequency (<2 kHz)) :CN V and VII
Inner Ear
• Oval window -> cochlea -> scala vestibuli -> Helicotrema -> scala
tympani -> round window
Inner Ear
• Organ of Corti
• cochlear amplifier : outer hair cells , stereocilia ,tectorial membrane
Inner Ear
• Organ of Corti
• Na-K-ATPase pumps
Inner Ear
• Organ of Corti
Inner Ear
• Cochlea
• Transduction: displacement of the
basilar membrane in response to
displacement of the stapes due to
acoustic energy
• basilar membrane (base ,apex)
Auditory cortex
• Primary auditory cortex : area A1 , Brodmann’s area 41
• Associated auditory cortex : area A2 , Broadmann’s area 22,42