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Lecture 2

HTHSCI 1H06 Lecture Notes - Lecture 2: Thalamus, Endolymph, Inferior Colliculus

Health Sciences
Course Code
Peter Helli

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CNS - The Ear and Hearing
Cranial Nerves Associated with Hearing
- Vesibulo-cochlear CN (VIII)
-hearing & balance
- Facial Nerve (VII)
- controls stapedius muscle which puts tension on the bones of the
middle ear to reduce the amount of sound when it’s really loud
- Vagus Nerve (X) – sensory fibers in the ear
- Muscles of Masication (Trigeminal Nerve V)
- Tensor Tympani Muscle
- drowns out sounds from chewing
The Ear
1) External Ear – extends from the oracle, allows for sounds to funnel into the ear
2) Middle Ear – behind tympanic membrane
3) Inner Ear – filled with CSF
a. Vestibular apparatus
b. Cochlea
4) Tympanic Membrane
5) Oval Window – separates air filled space from fluid filled space
6) Round Window – relief valve
7) Auditory Tube – place for air sinuses to communicate with outside air pressure,
extends from nasal pharynx
a. Usually closed but will open when yawning
External Ear
-Auricle: made of skin and cartilage
-Ends with tympanic membrane
-External Auditory Canal – contains wax that trap foreign bodies
oSeruminous Glands in the skin produce ear wax
oBad smell repels insects
-Thin skin firmly attached to periosteum of bone closer to middle ear
Eardrum (Tympanic Membrane)
-connective tissue with skin on the outside, mucus membrane on the other side
-cone shaped
Middle Ear
-Mechanical relay of vibrations of the tympanic membrane are relayed to oval
window membrane where they set up a series of vibrations in the fluid of the
inner ear
-Muscles in ear dampen vibrations from tympanic membrane to oval window
oStapedius (CN VII) – attached to stapes
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Prevent high amplitude of sounds
oTensor Tympani (CN V) – puts tension on tympanic membrane
Pressure Equilibrium
-occurs through the auditory tube
-allows for air to get in ear to equilibrate pressure
-Mastoid process – big bone at back of ear, attachment site for
oContains air cells
Middle Ear Infections
-occurs in children (smaller auditory tube)
-raging upper respiratory tract infection in their nasal pharynx
-coughing and sneezing opens auditory tube allowing for the virus to go into
the middle ear
-infection of mucus membrane of middle ear
-no place for virus to drain
obuilds up pressure = pain
-To relieve pressure:
o(1) rupture of tympanic membrane and drainage of fluid into external
o(2) infection spreads into mastoid air cells leading to mastoiditis
(needs tubes)
o(3) if infection gets past mucus membrane, erosion of temporal bone
above middle ear leading to meningitis
Inner Ear
a system of channels in the petrous portion of temporal bone
oVestibular system
-filled with cerebral spinal fluid
-connected with subarachnoid space
-round and oval windows prevent leakage
-Cochlear duct: floats in perilymph (around cochlear duct) filled with
endolymph (inside cochlear duct)
-Bony labyrinth: filled with CSF – perilymph
oIf there is overproduction of CSF in the perilymph and endolymph, it
can put pressure on hair cells, causing glaucoma of the ear resulting in
damage to the hair cells and hearing loss
Transmission of Vibrations
1) sound waves enter into the ear and cause vibrations of the tympanic membrane
2) causes movement in the ossicles that can be modulated by the contraction of the
Stapedius and tensor tympani
3) the stapes is resting on the oval window
4) vibrations are then set up in the perilymph: Scala vestibule (next to oval window)
 Scala tympani (next to round window)
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