ANATOMY AND PHYSIOLOGY OF THE EXTERNAL EAR, MIDDLE EAR AND INNER EAR

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ANATOMY AND PHYSIOLOGY OF THE EXTERNAL EAR,
MIDDLE EAR AND INNER EAR

• Prof. Alexander I. Yashan, MD, PhD

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EXTERNAL, MIDDLE AND THE INTERNAL EAR
1 - external auditory meatus 2 - Pinna 3
ossicles (malleus incus stapes) and tympanic
membrane 4 labyrinth (vestibule semicircular
canals cochlea) 5 tympanic cavity 6 -
auditory (acoustic) nerve 7 arteria carotis 8
- Eustachian tube
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PINNA
Is composed of a skin-covered cartilaginous
lamella whose posterior surface is evenly convex
and smooth, while its anterior surface is
concave, with semilunar folds and hollows between
them. The free anteroexternal margin of the pinna
is known as the helix (8) towards the bottom the
pinna gradually turns into the lobe (1) devoid of
cartilage and consisting of well-developed fat
and cellular tissue with a small number of
vessels and nerves. The small protuberance of
cartilage projecting over the external auditory
meatus is named the tragus (3). In front of the
helix and parallel to it is a ridge known as the
an thelix (10), with the antitragus (12) at its
posterior end. The skin on the anterior surface
of the pinna adheres directly to the
perichondrium on the posterior surface, however,
it may form folds owing to the presence of a
small layer of loose cellular tissue.
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External Ear

• Auricle - framework of elastic cartilage covered
  by skin
• Ear canal - about 3,5 cm long, consists of outer
  cartilaginous part and inner bony part
• The cartilaginous part is curved and lies at an
  angle to the bony part, it also narrows medially
  - ear drum is protected from trauma

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Removing of Round shaped Foreign body
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1 umbo2 - handle of malleus (3) (6) anterior
and posterior malleus folds4 - Pars flaccida
or Shrapnell's membrane 5 - short process of
malleus 7 pars tensa8 light reflex
Tympanic Membrane (eardrum)
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Interconnected Auditory Ossicles

• 1 - malleus handle
• 2 - head and neck of malleus
• 3- Malleus-incudal joint
• 4- body of incus
• 5 - short process of incus
• 6 - long process of incus
• 7 - head of stapes
• 8 base of stapes
• 9 - cruses of stapes

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Anatomy of Middle Ear
Lateral wall
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Middle Ear 3 compartments

• The tympanic cavity is a small chamber, about 1
  cm3 in size, lying in the depth of the temporal
  bone, between the tympanic membrane and the
  internal ear.
• In front, through the Eustachian tube, the
  tympanic cavity communicates with the
  nasopharynx
• behind, through the entrance into the mastoid
  antrum (aditus ad antrum mastoideum),
  communicates with the latter and the cells of the
  mastoid Process.

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TYMPANIC CAVITY

• It is customary to divide the into three parts
• the middle and biggest part, mesotympanum,
  corresponding to the pars tensa of the drum
• the upper part, epitympanum, lying above the
  former and also known as the epitympanic recess
  or attic
• the lower part, hypotympanum, lying below the
  drum level.

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Epitympanum

• Lies above the level of the short process of the
  malleus
• Contents
• Head of the malleus
• Body of the incus
• Associated ligaments and mucosal folds

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Mesotympanum

• Contents
• Stapes
• Long process of the incus
• Handle of the malleus
• Oval and round windows
• Eustachian tube exits from the anterior aspect
• Two recesses extend posteriorly that are often
  not visible directly
• Facial recess
• Lateral to facial nerve
• Bounded by the fossa incudis superiorly
• Bounded by the chorda tympani nerve laterally
• Sinus tympani
• Lies between the facial nerve and the medial wall
  of the mesotympanum

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Hypotympanum

• Lies inferior and medial to the floor of the bony
  ear canal
• Irregular bony groove that is seldom involved by
  cholesteatoma

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TYMPANIC CAVITY 6 WALLS

1. The roof of the tympanic cavity is a thin plate
   of bone separating the tympanic cavity from the
   middle cranial fossa where the temporal lobe is
   situated. This plate often has congenital
   fissures through which vessels pass from the
   middle cranial fossa. These anatomical features
   may account for the meningeal symptoms frequently
   observed in young children with acute otitis
   media.
2. The internal wall separates the tympanic cavity
   from the internal ear. It is marked by a gentle
   eminence, the promontory (promontorium),
   corresponding to the basal turn of the
   cochlea. Above and behind the promontory is
   an oval window or the fenestra vestibuli (oval
   window) which leads into the vestibule and is
   closed by the foot plate of the stapes. Upwards
   Facial Nerve is situated. Behind and below the
   promontory in a niche is a round window or the
   fenestra cochlea (round window) which leads into
   the cochlea, and is filled with a thin membrane,
   the secondary tympanic membrane.
3. External wall - tympanic membrane (Eardrum)

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• The inferior wall or floor of the tympanic cavity
  is separated from the jugular bulb by a fairly
  thick bony plate. Bone fissures in this wall are
  rarely found.
• The Eustachian tube begins with an opening in the
  anterior wall separating the tympanic cavity from
  the internal carotid canal.
• An opening in the upper part of the posterior
  wall leads to the mastoid antrum (aditus ad
  antrum mastoideum).

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TYMPANIC MUSCLES

• There are two muscles in the tympanic cavity
• (1) The tensor tympani muscle which stretches the
  tympanic membrane. It lies in the bony canal
  above the Eustachian tube, and is attached to the
  handle of the malleus.
• (2) The stapedius muscle which arises from the
  posterior wall of the tympanic cavity and is
  attached to the head of the stapes by a slender
  tendon. The tensor tympani is innervated by a
  branch of the trigeminal nerve, and the stapedius
  muscle by a branch of the facial nerve.

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Medial Wall
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Eustachian or auditory tube

• About 3.5 cm in length connects the tympanic
  cavity with the naso-pharynx. The upper third of
  this tube, adjoining the tympanic cavity, has
  bony walls, while the remaining lower portion
  leading into the nasopharynx is made up of
  membrane and cartilage.
• The movement of the cilia of the ciliated
  epithelium lining the Eustachian tube is towards
  the nasopharynx.
• At rest, the Eustachian tube is in a collapsed
  state, but with each swallowing movement it opens
  by contraction of the soft palatal muscles
  attached to it, to let air into the tympanic
  cavity.

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MASTOID PROCESS

• Is located just behind the external auditory
  meatus is a bony structure protruding downwards
  with the sternocleidomastoid muscle attached to
  it. In young children, the mastoid process is not
  fully developed and represents a bony tubercle
  behind the osseous tympanic ring.
• The antrum communicates with the tympanic cavity
  and the air-filled cells of the mastoid process.
  The superior wall or roof of the antrum separates
  it from the middle cranial fossa.
• The following types of structure are to be found
  in the mastoid process the pneumatic or
  large-celled,
• diploic ". In the case of pneumatic structures,
  the cavity of the mastoid process is divided by
  thin bony partitions into a lattice of larger and
  smaller cells. The diploic structure has tiny
  cells resembling a diploetic bone
• the most frequent is the mixed form of mastoid
  structure where smaller cells are to be found
  alongside bigger ones. In compact structures the
  bone is indurated and the cells are very few
  this structure frequently occurs as a result of
  chronic suppurative otitis media.
• sclerotic

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Temporal bone Inner Ear (Labyrinth)
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Internal Ear or Labyrinth

• (1 frontal semicircular canal (4) ampulla of
  frontal semicircular canal (3) apex of cochlea
  (4) medial turn of cochlea (5) apical turn of
  cochlea (6) basal turn of cochlea (7) round
  window (8) oval window (9) ampulla of sagittal
  semicircular canal (10) sagittal semicircular
  canal (11), (12) crura (13) ampulla of
  horizontal semicircular canal crus commune of
  frontal and sagittal semicircular canals

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VESTIBULE (VESTIBULUM)

• The) lies in the centre of the bony labyrinth on
  whose external wall is the oval window
• on the opposite, internal wall, there are two
  recesses for the two membranous sacs of the
  vestibule.
• The front sac known as the saccule (sacculus)
  communicates with the membranous cochlea lying
  before the vestibule, while the rear sac or
  utricle (utriculus) is connected with the three
  membranous semicircular canals passing behind and
  above the vestibule.
• The intercommunicating sacs of the vestibule
  contain the statokinetic receptors or maculae
  acusticae,
• otolithic organs made up of a highly-differentiate
  d specific neuroepithelium covered with a
  membrane containing granules of carbonate and
  phosphorate of lime, i.e. the otoliths

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SEMICIRCULAR CANALS

• The are set at right angles to each other and
  represent the three planes of space.
• They are three in number the external or
  horizontal, the superior or frontal, and the
  posterior or sagittal. One end of each canal
  opens out into a larger space known as ampulla,
  the other end is even. The frontal and sagittal
  canals have a common even stem (crus commune).
• The ampulla of each membranous canal contains a
  ridge.
• the crista ampullaris, which is a receptor, i.e.
  a nerve ending consisting ot a highly-differentiat
  ed neuroepithelium or hair and supporting cells.
• The free surface of the hair cells is covered
  with hairs which respond to the slightest
  displacement or pressure of the endolymph.
• The receptors of the vestibule and semicircular
  canals are the peripheral nerve endings of the
  vestibular analysator.

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COCHLEA
Modiolus the external wall and also turning round
the former, divides the tube lumen into two
directions, the upper or scala vestibuli and the
lower or scala tympani which communicate at the
apex of the cochlea through a small opening known
as the helicotrema. Both channels are filled
with perilymph. The scala vestibuli communicates
with the vestibule, while the scala tympani
borders on the tympanic cavity through the round
window covered by the secondary tympanic
membrane. The scala vestibuli of the cochlea
contains the thin Reisner's membrane which
extends from the osseous spirn lamina to cut
off a small membranous canal of trianguli section
filled with endolymph and known as the cochles
duct or ductus cochlearis.
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ORGAN OF CORTI

• (1) basilar membrane (2) tectorial membrane
  (3), (4) hair cells (5) supporting cells (6)
  nerve fibres reaching hair cells

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Mechanism of Hearing

• External ear collects the sound
• Ossicles vibrate in the air-filled environment,
  magnifying the sound and aiming it to the oval
  window
• The cochlea converts a mechanical stimulus into
  an electrical one
• The message is transported to the brain by the
  VIII-th nerve

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Otoskopy
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Pure Tone Audiometry

• Audiometry is the precise method of hearing
  assessment. It is performed in
• a soundproof compartment by an audiologist, who
  uses an audiometer to
• introduce measured sound intensities of selected
  tones to the listener,
• usually through earphones. Pure tone thresholds
  are the minimal intensities
• of given tones (frequencies) that can be heard by
  the person being tested. These thresholds are
  usually recorded on a grid, to create an
  audiogram. This one has no responses recorded on
  it and is presented purely to give an idea of the
  frequencies and loudness of common sounds.
• On this graph, the vertical axis plots hearing
  level (HL) in decibels (dB),
• tiny units of loudness. These two abbreviations
  are usually used together
• (dB HL) to report a patients test results or to
  refer to the level of loudness
• of a given tone. Zero dB HL is near the top of
  the graph this level is barely
• audible to a human with perfect hearing.

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