Transcript Tympanic membrane

In The Name Of God
Hearing System physiology
Characteristics of Sound Waves
1) Frequency: number of cycles of sound waves passing a
stationary point per second (Hertz = Hz; sec-1)
- range of human hearing is ~ 20 - 20,000 Hz
- range of human voice is ~ 350 - 3500 Hz
2) Intensity: amplitude of a sound wave (decibels = dB)
- expressed in terms of sound pressure
- sound intensity is proportional to square of pressure
dB = 10 log
intensity of a particular sound wave
intensity threshold for human hearing
Normal conversation 60 dB
Airplane
100 dB
Damaging sound
120 - 140 dB
Anatomy of the Ear
Outer ear consists of:
1) Pinna - collects
sound waves
2) External Auditory
Canal - conducts
sound waves to
middle ear
Middle Ear consists of:
1) Tympanic membrane
(eardrum) - vibrates
in response to
sound waves
2) Auditory ossicles 3 bones which
mechanically
transduce tympanic membrane vibrations to the inner ear
a. Malleus (hammer) - tympanic membrane to incus
b. Incus (anvil) - malleus to stapes
c. Stapes (stirrup) - incus to oval window of cochlea
3) Eustacheon (auditory) tube - equalizes pressure between middle
ear and the environment
4. Two muscle
• Tensor tympani muscle: anchored to bone
at one end is attached to the malleus
• stapedius muscle: extend from a fixed
anchor of bone and attached to stapes
contraction reduces ossicular mobility
• contraction therefore: reduce the intensity of
lower frequency sound transmission by 30 to40
decibels that its called Attenuation reflex
•
function attenuation reflex
1. Protect the cochlea from damaging
vibrations caused by extensively loud
sound
2.To mask low frequency sound in loud
environments
3. Decrease a person hearing sensitivity to
his or her own speech
Function of the Ossicular Chain
I. Impedance Matching - effective transfer of sound energy from air
to fluid
Sound pressure in the middle ear is amplified by 2 mechanisms:
1. Ossicular act like a lever system : displace oval window
against cochlear fluid
Force increase = 1.3 fold
2. Surface area of oval window = 1/17 surface area of
tympanic membrane
Force increase = 17 fold
NET RESULT: Ossicles increase sound pressure 22 fold
Inner ear consists of:
1) Semicircular canals important for sense
of balance and
equilibrium
2) Cochlea - responsible for sound detection, discrimination and
transduction into neural signal
From: W.F. Ganong, Review of Medical Physiology, 19th ed. Appleton & Lange, 1999
Cochlear structures:
BASE
1) Cochlear duct - fluid-filled
tube within cochlea
2) Scala media - endolymph
filled space containing the
sensory apparatus; delimited
by Reissner’s and basilar
membranes (high [K+] !)
3) Scala vestubuli - perilymph filled
space above scala media
4) Scala tympani - perilymph filled
space below scala media
From: Berne & Levy, Physiology, 3rd ed., Mosby Year Book, 1993.
apex
Length: 34 mm
Dia. 2mm
5. Helicotrema - distal opening
between scala vestibuli and
scala tympani
6. Oval window - membranous
opening of scala vestibuli,
joins with stapes
7. Round window - membranous opening of scala tympani
8. Basilar membrane: Organ of Corti - sensory detection apparatus
From: Berne & Levy, Physiology, 3rd ed., Mosby Year Book, 1993.
Properties of basilar membrane
0.5 mm
0.04
The basilar membrane of the cochlear duct is stiff and narrow
close to the oval window. It becomes wider and more flexible
near its distal end.
Some important features of the Organ of Corti:
1) Auditory hair cells (AHC) sensory receptor cells,
have stereocilia projecting
from their apical surface
2) Tectorial membrane glycoprotein-rich flap,
stereocilia tips imbedded
in its surface
3) Afferent nerve fibers - synapse with AHCs, carried in the
vestibulocochlear nerve
From: Berne & Levy, Physiology, 3rd ed., Mosby Year Book, 1993.
Auditory Hair Cells (AHC)
The Specialized Auditory Receptor Cells
2 Populations:
1) Inner AHCs
- ~ 3500 in number
- arranged in a single row
- provide basic auditory
info to CNS
2) Outer AHCs
- ~ 12000 in number
- arranged in 3 parallel rows
- fine tuning of auditory signal
- have a limited motility, and shorten slightly in response to
certain tones
- amplifies the sound wave
Sound Wave Transduction in the Cochlea
1. Sound waves enter via oval window; round window bulges in response
(fluid is not very compressible)
2. Basilar membrane and Organ of Corti vibrate in response
AHC Stereocilia Structure / Function
1) Rows of stereocilia are of constant diameter and taper at their base
2) Stereocilia act as rigid rods
3) Have several stretch-activated
nonselective cation channels
4) Stereocilia tips are joined by
a protein tip link
Mechanism of transduction
Upward Bending:
- stereocilia bend away from
limbus, toward tallest stereocilia
- cation channels open,
AHC depolarizes
- VG Ca++ channels open;
AHC releases NT (glutamate)
Downward Bending:
- stereocilia bend toward limbus,
away from tallest stereocilia
- cation channels close,
AHC hyperpolarizes
- VG Ca++ channels closed;
no NT release
Discrimination of Auditory Signals
The two physical characteristics of sound that we can discriminate
are frequency and intensity.
1) Place Principle of Frequency
High frequency - cause vibration of the basilar membrane at the base of
the cochlea where the membrane is narrow and stiff
Low frequency - cause vibration of the basilar membrane at the apex of
the cochlea where the membrane is wide and more compliant
2. Volley or frequency principle:
It was used for determination low
freq. sound from 20 to 200 HZ It cause volley of impulse synchronized at
same at same freq.
Discrimination of Auditory Signals (continued)
3) Mechanisms for discrimination of LOUDNESS
Louder sound
firing rate of AHCs
stimulation of special
“high threshold” AHCs
amplitude of basilar
membrane vibration
number of stimulated AHCs
Determination of the direction sound
1.By the time lag between the entry
of sound into one ear and into the
opposite area.
• it is benefit at Frq.<3000 Hz
•The medial superior olivary nucleus has
important role in determination time lag
between acoustic signal entering the tow
ear
2.Differnce between the intensity of
the sound in the two ear
• the intensity mechanism operated best
at Frq.>3000 Hz
• it is concern with Lateral superior
olivary nucleus
Figure 9.19 Pathways in the
auditory system
Subcortical Mechanisms of Sound
Localization
The lateral and medial superior olives
react to differences in what is heard by
the two ears
Medial – arrival time differences
Lateral – amplitude differences
Both project to the superior colliculus
The deep layers of the superior colliculus are laid out
according to auditory space, allowing location of sound
sources in the world; the shallow layers are laid out
retinotopically
Tonotopic mapping/organization
• The organization of frequency in terms of place.
http://tonks.disted.camosun.bc.ca/courses/psyc290/brain/tonotopic.GIF
• Info is carried through auditory system in frequency channels.
Two Streams of Auditory
Cortex
Auditory signals are conducted to two areas
of association cortex
Prefrontal cortex
Posterior parietal cortex
Anterior auditory pathway may be more
involved in identifying sounds (what)
Posterior auditory pathway may be more
involved in locating sounds (where)
Perception of different
characteristics of sound
Frequency
Starts at the basilar membrane and frequency sharpening occurs
throughout the auditory pathway
Intensity
Starts at the hair cells (OHC are stimulated by weaker stimulus)
Frequency of impulses
Direction
Inter-aural time difference
Pattern recognition
Cortical function
Interpretation of speech
Complex cortical phenomenon
Electrical stimulation in Wernicke’s area of a conscious person
occasionally causes a highly complex thought.
. The types of thoughts that might be experienced include complicated
visual scenes that one might remember from childhood, auditory
hallucinations such as a specific musical piece, or even a statement
made by a specific person.
For this reason, it is believed that activation of Wernicke’s area can
call
forth complicated memory patterns that involve more than one sensory
modality even though most of the individual memories .may be
stored elsewhere
Hearing abnormality
A. Conductive Defects
1. Otitis - inflammation of the external or middle ear
2. Otosclerosis - calcification of the stapes
3. Tympanic perforation (broken eardrum)
4. Foreign body insertion
B. Sensorineuronal Defects
1.
2.
3.
4.
Drug-induced - aminoglycoside antibiotics
Congenital
Infections - syphilis, measles, mumps, meningitis, flu
Acoustic neuroma - benign tumor of vestibulocochlear nerve