Lecture 2Ear and Hearing

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Chapter 12

• Lecture 2-Ear and Hearing

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Equilibrium and Hearing

• The ear is divided into 3 parts external,
  middle, and inner
• External ear-holds up your glasses on the sides
  of your head
• Auricle (pinna) is the funnel shaped part
• Functions to funnel sound waves into the ear
• External acoustic meatus is the bony tube that
  leads to the tympanic membrane (ear drum)

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Ear
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Middle ear

• Contains the air filled tympanic cavity
• This cavity has an opening to the auditory tube
  (eustachian tube) that leads to the nasopharynx
• The 3 smallest bones of the body are auditory
  ossicles and they are also found in the tympanic
  cavity
• Malleus (hammer), incus (anvil), stapes (stirrup)

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Ossicles

• Malleus attaches to the tympanic membrane
• Stapes attaches to the oval window (the opening
  to the inner ear)
• These ossicles amplify sound waves and transmit
  them to the inner ear via the oval window
• Stapedius muscle and tensor tympani muscle
  restrict movement when loud noises occur

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Inner Ear

• Begins at oval window
• Found in the petrous portion of the temporal
  bone, the bony canal is called the bony labyrinth
• Bony labyrinth contains membranous labyrinth
• Receptors for hearing and equilibrium and housed
  in the membranous labyrinth
• Space between bony and membranous labyrinths is
  filled with perilymph
• The membranous labyrinth contains endolymph

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Inner Ear
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Bony labyrinth

• Structurally and functionally divided into 3
  distinct regions
• 1. Vestibule-(entrance court)
• Contains a utricle and saccule
• 2. Semicircular canals
• Membranous labyrinth here is called semi circular
  ducts
• 3. Cochlea (snail shell)-organ of hearing
• Membranous labyrinth here is cochlear duct

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Structures for Hearing

• Cochlea-hearing organs are housed here in both
  ears
• The spongy bone axis is called the modiolus
• The membranous labyrinth in the modiolus contains
  the spiral organ (Organ of Corti) which is
  responsible for hearing
• The cochlear duct (scala media) is the membranous
  labyrinth that runs through the cochlea (filled
  with endolymph)

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More on the cochlea

• The roof is formed by the vestibular membrane
• The floor is formed by the basilar membrane
• These help divide the bony labyrinth into scala
  vestibuli and scala tympani (both are filled with
  perilymph)
• The scala tympani ends at the round window
  covered by the secondary tympanic membrane

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Spiral Organ

• Found in cochlear duct
• Contains thick sensory epithelium with hair cells
  and supporting cells that rest on the basilar
  membrane
• Hair cells (receptors) extend stereocilia into
  gelatinous structure called tectorial membrane
• Any movement of basilar membrane causes
  stereocilia to distort, and sensory neurons are
  stimulated
• Cell bodies of sensory nerves are in spiral
  ganglia of modiolus

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Organ of Corti and Tectorial Membrane
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Process of hearing

• 1. sound waves enter external acoustic meatus and
  tympanic membrane vibrates
• 2. auditory ossicles begin moving and sound
  waves are amplified
• 3. oval window vibrates and pressure waves
  travel through perilymph in scala vestibuli
• 4. High frequency and medium frequency pressure
  waves in scala vestibuli cause vestibular
  membrane to vibrate, causing endolymph in
  cochlear duct to move
• 5. Pressure waves in endolymph cause basilar
  membrane to move, hair cells distort, stimulus
  occurs in cochlear branch of VIII

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Frequencies

• Frequency is the number of sound waves that move
  past a point during a specific amount of time and
  is measured in Hz (hertz)
• different frequencies of vibrations stimulate
  different receptor cells the human ear can
  detect sound frequencies from about 20 to 20,000
  vibrations per second, but greatest sensitivity
  is 2,000 to 3,000
• a dB is decibels which is a measure of sound
  intensity or loudness
• whisper is 40, normal conversation is 60-70,
  traffic is 80, rock band 120, airplane 140.
  Frequent exposure to over 90 can cause damage and
  permanent hearing loss.

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Structures and Mechanisms of Equilibrium

• Receptors for equilibrium constitute the
  vestibular apparatus
• The vestibular apparatus includes the 3
  semicircular ducts (canals) and two chambers
  called the utricle and saccule
• Equilibrium is divided into
• static equilibrium or when perception of the
  orientation of the head when your body is still
  (gravity)
• Dynamic equilibrium or the perception of motion
  or acceleration
• Acceleration can be linear (car or elevator) and
  angular (change in rate of rotation)

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Utricle and Saccule

• Located inside the vestibule and participate in
  static equilibrium (key role in posture)
• Each contains patch of hair cells and supporting
  cells called a macula
• The macula sacculi lies vertically on the wall of
  the saccule (responds best to vertical movement
  like elevator)
• The macula utriculi lies horizontally on the
  floor of the utricle
• Each hair cell of the macula the 40 to 70
  stereocilia and one true cilium called a
  kinocilium
• The tips of these are embedded in a gelatinous
  otolithic membrane
• The membrane is weighted with calcium
  carbonate-protein granules called otoliths

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Detection of movement

• When head is erect, the otolithic membrane bears
  down directly on hair cells and stimulation is
  minimal
• When head tilts, the weight of the membrane bends
  the stereocilia and stimulates hair cells
• Brain interprets the message from both utricles
  and saccules in both ears and makes adjustments
• Important in linear acceleration-stoplight, no
  motion, proceed through light, OM of Utricle lags
  behind and bends stereocilia backward (same with
  running)
• When you stop at the next light, the opposite
  happens, and the OM keeps going after the macula
  stops and the stereocilia bend forward

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Macula responds to change
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Semicircular ducts and dynamic equilibrium

• Anterior and posterior ducts are vertical
• Lateral duct is nearly horizontal
• Different ducts are stimulated when you turn your
  head, nod, shake back and forth
• Each has endolymph and opens into the utricle
• A swelling at the end is an ampulla containing
  hair cells, supporting cells, and crista
  ampullaris

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Crista ampullaris
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Ducts continued

• Hair cells contain the stereocilia and kinocilium
  which are embedded into the cupula (gelatinous
  membrane)
• When head turns (like dancing), the ducts rotate,
  endolymph lags behind, pushing cupula, which
  bends stereocilia and stimulates hair cells
• After 25-30 seconds of continuous rotation it
  catches up and stimulation ceases

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Strange and weird

• Vestibular nystagmus-eye movements after you spin
  around
• Motion sickness-sensory mismatch
• Conductive deafness-interference with
  transmission of vibrations to inner ear
• Sensorineural deafness-damage to cochlea or
  auditory nerve
• Otitis media-middle ear infection