ANATOMY AND PHYSIOLOGY OF THE AUDITORY SYSTEM Flashcards
(53 cards)
1
Q
Structure of the auditory system
Location
A
- Contained in the temporal
bone - Located on the sides of
the skull
2
Q
Divisions of the ear
A
- ____peripharal_________ Auditory System
- Outer Ear
- Middle Ear
- Inner Ear
- Cochlea
- Vestibular System
- _____central_________ Auditory System
- Auditory Nerve
- Auditory Nuclei and Pathways (8CSLIMA)
3
Q
The Peripheral Auditory System
A
- ___outer____Ear
- __acoustic ____energy
- ___middle____Ear
- ___mechanic___energy
- __inner_____ Ear
- ____hydro/neuro__energy
4
Q
Outer ear
A
*Pinna (Auricle)
*External auditory
meatus
5
Q
Pinna
A
- Pinna
- Funnel shaped
- Composed of cartilage
- Attached to cranium by
ligaments - Landmarks
- Concha
- Helix
- Tragus
- Lobe
6
Q
Pinna Physiology
A
Acoustic
* Collects and funnels sound from the
environment into the ear canal
* Enhances high frequency sounds
Sound localization
* Is sound above or below, front or back or
from right side or left side
N
on Acoustic
* Protects entrance into EAC
7
Q
External Auditory Canal – Anatomy
A
- 2.5 cm long (1”)
- S-shaped
- Outer 1/3
- Cartilaginous
- Skin contains hair follicles
- Cerumenous / sebaceous
glands - Inner 2/3
- Bony & narrower
- Osseocartilaginous junction
8
Q
External Ear Physiology
Acoustic vs non acoustic
A
Non acoustic:
* Protection of Tympanic Membrane
* Cerumen
* Length and shape of ear canal
* Lubricates ear
* Protects ear from foreign objects
Acoustic:
* Sound Collection – Directs sound to TM
* High Frequency amplifie
9
Q
Combined External Ear Resonance
A
- Influence of pinna (p)
- Influence of ear canal (m)
- Combine influence (t)
- At about 3000 Hz, the final
amplification (t) is 20 dB - Helpful in discriminating high
frequency fricative
consonants such as __ __ __
and ___?
10
Q
Neuro-reflexes of the EAC
3
A
- Vagus Reflex -Arnold’s branch of the vagus (CN X)
▪ Cough or gag reflex
▪ Evoked during insertion of otoblock, wax removal, otoscopy
▪ Trigeminal Reflex – (CN V)
▪ Red Reflex
▪ Can cause excessive vascularization and
thickening of TM during otoscopy
▪ Lymphatic Reflex - (Cranial V and VII)
▪ Excessive swelling of tissues and soreness while wearing a custom mold or HA
▪May be due to allergic reaction or result of a poor fit
11
Q
Outer Ear Malformations
A
• Microtia: A condition where the outer ear is smaller than normal.
• Anotia: A condition where the outer ear is completely missing. • Atresia: The ear canal is either closed or missing, affecting hearing. • Low set ears: Ears that are positioned lower than usual on the head. • Abnormally formed ears: Ears that have an unusual shape or structure. • Pits and tags: Small holes (pits) or extra bits of skin (tags) near the ears.
12
Q
External Ear Disorders
A
- Cerumen Impaction
- Foreign Bodies Occlusion
- Infections
- Growths/Tumors
- Exostoses - benign, associated
with exposure to cold water
13
Q
Syndromes Associated with OE Malformation
A
◼ Down Syndrome
◼ Turner Syndrome,
◼ Treacher Collins
◼ DiGeorge Syndrome
◼ Charge Syndrome
◼ Trisomy 13 and 18 Syndrome
◼ Goldenhar Syndrome (oculo-auriculo-vertebral spectrum)
14
Q
Tympanic Membrane
A
AKA
* Eardrum
* Tympanum
* Border between
outer ear and middle
ear
15
Q
Tympanic Membrane Anatomy
A
- Semi transparent oval, cone shaped membrane
- Composed of 3 layers
*Outer – continuous with skin of bony canal
*Middle – Fibrous connective tissue
*Inner – Continuous with mucous membrane of ME
16
Q
Disorders of the Tympanic Membrane
A
- Perforation
- Trauma
- Barotrauma
- Infection
- Acoustic Trauma
17
Q
The Middle Ear
What’s the name of the roof of the middle ear?
A
- Air filled cavity behind the eardrum about 2
cm3. - The roof of the middle ear, the tegmen
tympani, is a thin layer of bone, separating
the middle-ear cavity from the brain. - Surrounded by mastoid bone
- Lined with mucous membrane
18
Q
Middle ear anatomy
A
- Eustachian Tube
- Connects middle ear to nasopharynx
- Ossicular Chain
- Name those bones!
- Stapedial Muscle
o Stapedius – attached to stapes
19
Q
Middle Ear Physiology
A
Conduction
Conduct sound from the outer ear to the inner ear
Transducer
Converts acoustic energy to mechanical energy
Protection
Middle ear muscles may? provide protection from loud sounds
Pressure Equalizer
Equalizes pressure between middle ear cavity and nasopharynx
Amplifier
Impedance Matching Transformer
Matches the transfer of energy between air to fluid
20
Q
Eustachian Tube Physiology
A
- Equalizes pressure in the middle ear
cavity so that it remains air filled. - Fluid drainage from ME into
Nasopharynx - The ET is shorter and wider in length
in children compared to adults
21
Q
Ossicular Chain (Malleus, Incus and
Stapes)
A
- Smallest skeletal bones in the body.
- Connected to the medial wall of the TM
and articulate with each other - Each ossicle is suspended
independently by ligaments - Purpose – delivers sound vibrations
efficiently to the inner ear (cochlea)
22
Q
Middle Ear Muscles
A
Stapedius Muscle
* Attached to the Stapes
* Innervated by CN VII
* Acoustic or Stapedial Reflex –
contracts to loud sounds
* Stiffens the ossicular chain and
decreases sound energy to the inner
ear.
* Absent acoustic reflex could signal CHL
or SNHL
23
Q
The (Impedance Mismatch) Problem
A
- Hearing requires a transfer of acoustic energy from low
impedance air-borne soundwaves to high impedance fluid
vibrations in the cochlea. - Sound intensity is lost due to impedance mismatch created
by the change of sound traveling from air medium to a
(cochlear) fluid medium!
24
Q
The (Impedance Mismatch) Solution
A
- An impedance matching
transformer to allow
efficient transfer of energy
between the two media of
different physical
properties!
25
ME Transformer/Amplifier
* Area Ratio Advantage
Laver action
* Area Ratio Advantage
* The area of the tympanice membrane is much larger than
the oval window
* Area of TM is 17X larger than Ow
* Difference between surface are of TM (55mm) and
Stapes footplate (3.2mm) results in increase in sound
pressure of approximately 25 dB of the impedance
mismatch.
ME Transformer/Amplifier
* Lever Action
* The length of the long arm of the
malleus is much longer than the long
arm of the incus
* Difference in length produces a lever
action that amplifies sound pressure
and accounts for about 2 dB of the
mismatch
31
32
26
ME Transformer/Amplifier
*Curved Membrane
By how much dB amplification?
*Curved Membrane
* TM’s curved shape concentrates
energy toward middle where
TM is attached to malleus and
accounts for 3 – 6 dB
27
Middle Ear Disorders
* Otitis Media
* Cholesteatoma
* Ossicular Disarticulation
* Otosclerosis
1. Otitis Media: An infection or inflammation of the middle ear, often causing ear pain and sometimes fluid buildup behind the eardrum.
2. Cholesteatoma: An abnormal, non-cancerous growth of skin cells in the middle ear that can damage surrounding bones and tissues.
3. Ossicular Disarticulation: A separation or break in the chain of tiny bones (ossicles) in the middle ear, leading to hearing loss.
4. Otosclerosis: A condition where abnormal bone growth in the middle ear impairs the movement of the stapes bone, causing hearing loss.
28
Inner Ear – Anatomy of the Cochlea
The different type of labyrinth
* Osseous (bony)labyrinth
* Membranous labyrinth
-Auditory labyrinth
-Vestibular labyrinth
Factoid - Inner ear reaches its full adult size when the fetus is 20-22 weeks old!
29
Cochlear Anatomy
Cochlear Anatomy
* Snail shaped fluid filled cavity within
mastoid bone
* 2 5/8 turns; 35mm long
* Coiled around central core of bone called
the modiolus
* Auditory nerve exists through the modiolus
* Contains 3 fluid filled chambers
30
Cochlear ducts
* Scala vestibuli
* Upper compartment behind oval window
* Perilymph (Rich in sodium ions)
* Scala tympani
* Lower chamber exits at round window
* Perilymph (Rich in sodium ion)
* Scala media
* Middle chamber
* Endolymph (Rich in potassium ions)
* Contains the Organ of Corti
* Two membranes
* Reissner’s membrane
* Basilar membrane
31
Organ of Corti
* Located in the scala media
* Rests on basilar membrane
* Tectorial membrane
* Jelly like substance that rests across top of
hair cells
* Hair cells
* Outer hair cells – 3 rows
* 13-15,000 OHC
* Inner hair cells – 1 row
* 3000 – 3500 IHC
* Stereocilia
* Hairlike projections on top of hair cells
32
Inner Ear Physiology
Traveling Wave
* Stapes footplate pushes oval window in and displaces fluid causing
outward displacement of the round window
* Fluid movement creates a traveling wave on the basilar membrane
* Maximum displacement of the traveling wave is frequency dependent
* The traveling wave progresses from base to apex causing shearing or
bending of the hair cells in the tectorial membrane
33
Traveling Wave
Tonotopic organization
Tonotopic Organization
* Different areas of the basilar
membrane are sensitive to different
frequencies
* Base: High frequency
* Basilar membrane is narrow and
stiff
* Apex: Low frequency
* Basilar membrane is wide and
flexible
34
Traveling Wave
Tonotopic Organization
Tonotopic Organization
* Different areas of the basilar
membrane are sensitive to different
frequencies
* Base: High frequency
* Basilar membrane is narrow and
stiff
* Apex: Low frequency
* Basilar membrane is wide and
flexible
35
Shearing of the Haircells
* Tip links between cilia of the hair cells act
as mechanical gates.
* The endolymph surrounding the hair cells
contain charged potassium ions
* No sound – tip link remains closed and
charged potassium ions are not allowed in
36
Shearing of the Haircells
Sound is present - traveling wave causes the hair cells to
bend, tip links (gates) open, and charged potassium ions
flow into the haircells.
* Neurotransmitter vesicles drop to the base of the cell and
and release glutamate into the synaptic space and stimulates
the auditory nerve fibers.
* Neural impulses transmit the signal through the nerve,
up the auditory pathways, towards the brain.
37
Outer Hair Cells
3 rows, 13,000 OHCs, test tube shaped
*Cilia are imbedded in tectorial
membrane
*Cochlear amplifier
* Sensitive to soft input sounds up to 60 dBHL
*Expand and contract to mechanically amplify low level sounds
to stimulate IHC
*Frequency resolution – Sharpens peak of traveling
wave
*Efferent fibers (generates otoacoustic emissions)
*Encode sound intensity
38
Inner Hair Cells
*1 row, 3500 IHCs, pear or flask shaped
*Cilia are not touching tectorial membrane
*Sends signals to auditory N. when
stimulated
*Do not sense sounds softer than
conversational speech above 60 dB HL
*Afferent nerve fibers
*Encode sound clarity
39
OHC Damage
HL up to 60 dB HL
* Recruitment – Abnormal growth in
loudness
* Soft sounds below conversational
speech (50-60dBHL) are inaudible
* Loud sounds are perceived as loud
as to someone with normal
hearing
* Difficulty understanding speech in noise
due to poor frequency resolution
40
IHC Damage
* Losses greater than 60dB HL,
most likely involve outer and IHC
damage
* Greater sound distortion
41
Disorder of the Inner Ear
*Congenital
*Head Trauma
*Noise Induced HL
*Infections
*Presbycusis
*Ototoxicictiy
*Meniere’s Disease
*Sudden SNHL (idiopathic)
42
Acoustic (8th C.) Nerve
* Two branches
* Cochlear portion
* Vestibular portion
* Auditory nerve fibers exit cochlea through modiolus
* Cochlear branch joins with vestibular branch, passes
through internal auditory meatus, and terminates at base
of brain
43
Auditory Nerve
* Extends 17-19 mm beyond the internal auditory
canal
* Attaches to the brainstem at the cerebellopontine
angle (CPA-area in the brainstem where cerebellum,
medulla and pons are joined)
* Auditory nerve fibers terminate in the cochlear
nucleus of the brainstem
* Tonotopic Organization is preserved
* High frequencies – outer part of auditory nerve
* Low frequencies – middle part of auditory nerve
44
CANS FEATURES
* Decussation
* Crossing over from one side to the other
* Each hemisphere of the brain processes information from both ipsilateral (same
side) and contralateral (opposite side) sides
* Fibers that originate in the left ear synapse on the right side of the brain and
vice versa
* Thus, information from one ear, reaches both sides of the brain
* Brain damage in one hemisphere has little effect on peripheral hearing.
* Improves processing of complex speech
45
CANS FEATURES
* Tonotopic organization
* Preserved throughout the entire auditory pathway
* Afferent Pathway
* Sound travels from the cochlea UP to the auditory
nerve, then to the brainstem and then to the brain.
* Efferent Pathway
* Descending nerve fibers from the brain to the
brainstem and cochlea (not well understood)
* Primary and secondary auditory Cortex
46
Afferent Central Auditory
Pathways - 8CSLIMA
* 8th Craniel Nerve
* Cochlear Nucleus
* Superior Olivary Complex
* Lateral Lemniscus
* Inferior Colliculus
* Medial Geniculate Body
* Auditory Cortex
47
Central Auditory Pathways
Cochlea nucleus
Superior Olivary Complex or Nucleus
* Cochlear Nucleus
* Auditory nerve fibers terminate in the
ipsilateral cochlear nucleus
* Superior Olivary Complex or Nucleus
* First relay station that receives ipsilateral
and contralateral fibers from both cochlear
nuclei
* Fx - Localization (time and intensity cues)
* Fx- Mediates acoustic reflex activity
48
Central Auditory Pathways
Lateral Lemniscus
Inferior Colliculus
Medial Geniculate Body
* Lateral Lemniscus
* Carries auditory fibers from the superior
olive to the inferior colliculus.
* Involved in acoustic startle reflex
* Inferior Colliculus
* Obligatory relay station of the ascending
auditory pathway
* Medial Geniculate Body
* Last subcortical relay station in ascending
pathway
49
Primary Auditory Cortex
* Temporal Lobe
* Heschl’s Gyrus)
* Tonotopically mapped
* Auditory Perception
50
Secondary (Associated) Auditory Cortex
* Surrounds primary area and receives impulses
from the primary auditory area
* Integrates and associates sound with other
sensory areas of for interpreting and
understanding the meaning of sounds.
* Interhemispheric fiber tracts – delivers sound
to different areas of the brain
* Arcuate Fasciculus delivers sound to the frontal
lobe.
51
Landmark Studies of Auditory Brain Development
Green et al., 2005
* Measured PET scan while post-lingually deafened adult
cochlear implant users listened to a story.
* Showed activation of right and left primary and association
auditory areas.
* The normal listening brain shows bilateral activation of the
primary and secondary auditory cortex
52
Landmark Studies of Auditory Brain Development
Nishimura et al., 1999
* Used PET to evaluate areas of brain that are active in pre-lingually deafened adults who
use ASL and received CI in adulthood
* Story told via hand movements in meaningless fashion showed responsiveness in the
occipital area of the brain
* Story via sign language showed robust activity in secondary cortex
* Story with audition alone showed activity in the primary auditory cortex only.
* Evidence of cross modal plasticity due to sound deprivation during the formative years
of life
53
Key Points - Auditory Deprivation and Cross
Modal Plasticity
* Primary auditory cortex is reserved for hearing sound. There is
little evidence of cross-modal plasticity.
* Cross Modality plasticity – the brain reorganizes itself to receive
input from other senses, primarily vision
* Cross Modal plasticity occurs in the second auditory cortex.
* When an individual who is deprived of sound in the formative years
hears sound, it is stuck in the primary auditory cortex.
* The secondary auditory cortex is processing other types of stimuli.
Sound will not be delivered to other areas of the brain for higher-
order meaning
