Download Physiology / sheet 7 The ear consists of three parts : external

Physiology / sheet 7
The ear consists of three parts : external , middle and inner ear .
Up till tympanic membrane it’s considered as external ear .
External ear has the auricle ( the outside part) which is mobile in animals , it collects the sound
waves and bring them to vibrate the tympanic membrane .
But humans have a developed neck movement that we don’t need to move our auricles like
animals . however 3% of people can move them just for fun .
Tympanic membrane(ear drum) is the border between external and middle ear .
Middle ear : it contains the three smallest bones in our body (malleus , incus and stapes) and
the two smallest muscles ( tensor tympani and stapedius ) .
it is a close chamber, because it is only opened to nasopharynx through eustachian tube , which
is the source of infections and secondary otitis media especially in children because it is very
short and wide while in adults it is longer and less liable to be affected by secondary otitis
media .
External auditory meatus is directed anteriorly and forward , so when you put your stethoscope
in your ear it should be directed in this direction .
The last thing is the inner ear : it is the most complicated structure in the whole body , it has
vestibular system ( semicircular canals , utricle and saccule ) and cochlea . in this lecture we will
talk about cochlea .
When the sound waves hit the tympanic membrane , it will vibrate in and out … then it will
move the lever system (malleus , incus and stapes) after that stapes which has the footplate will
push oval window deep inside the cochlea to vibrate endolymph and perilymph in order to
stimulate the hair cells.
The surface area of the tympanic membrane equals 55 mm² while the oval window no more
than 3.2 mm² so there is 17 times reduction in the surface area where the pressure is applied,
when the sound moves from higher surface area to a lower surface area the pressure will
increase , lever system increases the force of movement by 1.3 , and the intensity of sound
waves increases 22 times . The question here , will any sound from the external environment be
magnified 22 times in our ears ??
We hear almost the same intensity because the sound waves transmitted very easily in air
however it is much more difficult when it is in a watery environment ( perilymph and
endolymph inside the cochlea). This magnification will be dissipated when the sound waves
start moving within the endolymph and perilymph rocking reissner’s (vestibular) membrane
and the basilar membrane . so we magnify to over come the reduction in the intensity that is
going to occur within the inner ear , so the intensity will be the same .
NOTE : (1) Reissner’s (vestibular) membrane : It separates scala media from scala vestibuli.
(2) Tympanic reflex , when you hear gunshot sound or jet noise the stapedius muscle in
the middle ear will try to lower the intensity of the sound by contraction (this doesn’t occur in
response to normal sounds) .
Now , when you examine a person affected by a car accident, you should examine behind his
ear because you may find Battle’s sign (Ecchymosis behind ear “ bluish color “ ) . if you see
pandaeyes (Ecchymosis around the eyes ) and you suspected a base skull fracture you should
look behind his ear if you find Battle’s sign and it is associated with otorrhea ( tearing of the
tympanic membrane which will lead to leakage of CSF from the ear ) it is a powerful indicator of
a basal skull fracture .
Example : If a teacher hit a student on his ear , and the blood flow out from his ear it is an
evidence of tympanic membrane tearing , so you should suture it and give him antibiotic . if you
don’t do this , otitis media will occur and it may lead to deafness , so you should be carful .
Sound wave : it means vibration of molecules in the space . the wave sound is associated with
condensation and rarefaction in the space .
Parameters are :
1)
Frequency .
2)
Amplitude .
Frequency : normally from 20-20,000 Hz . if it is less than 20 Hz it’s called infrasound
while more than 20,000 Hz it’s called ultrasound . ultrasound (which is used for
diagnostic and surgical procedures e.g :cataract treatment) is much better than
infrasound ( which is dangerous . they use it in wars in Iraq eg: sound bombs, their
frequencies between 7-8 Hz that will damage the tissues because all the particles inside
these tissues will vibrate at this low frequency then people will suffer from these
symptoms: headache, tightness , chest pain , nausea , vomiting , abdominal cramps …
etc )
How we can differentiate between a musical sound and noise ??
By Octaves which mean harmony between frequencies ( meaning : 2-4, 4-8, 8-6 not 1-5-2), The
ear will consider it as half octave, one octave or two octaves which means that this is a musical
sound. But if it happenes without harmony , then CNS will consider it as noise.
** Female’s and Male’s Voice :
Female’s voice : high frequency and low amplitude (e.g : Fairouz) it is called Soprano sound
which’s frequency between 250-255 Hz .
Male’s voice : low frequency and high amplitude . it is called Alto sound which’s frequency is
less than 150 Hz . (( an indicator of the depth of the voice))
Animals have much better hearing sense than humans .
1)Dogs and cats  50,000 Hz
2)Dolphins  200,000 Hz (according to them they discover the ultrasound !)
3)Bats  120,000 Hz
4)Elephants  <5 Hz
First three animals their voices’ frequencies is higher than 20,000 Hz ,so it is Ultrasound . while
the last one is below 20 Hz so it is infrasound (because elephants can hear any insect’s sound at
low frequency that these infrasound waves will not affect their tissues).
The best sense in humans as a part of animals kingdom is Vision while animals is much better
than us in all other senses for survival .
Theory of hearing :
sound waves enter through external auditory meatus and vibrate the tympanic membrane
(which function as a resonator) moving malleus , incus and stapes to move then it will push the
oval window inside of the cochlea (it is two and three quarters winds ) .
cochlea has three spaces separated by two membranes :
1) Tympanic membrane : no physiological importance just to separate two different
compositions .
2)Basilar membrane : contains organ of corti which Extends along the basilar membrane from
the base of cochlea to its apex . organ of corti is a tunnel shape structure with 3 rows of hair
cells (12,000 cell) at the outer side and there is a single row that represents the inner hair cells
which its number between 3,200 – 3,500 cells at the inner side of this tunnel .
Reticular lamina : when the sound comes ,this lamina moves as a single unit . when sound
waves move the basilar membrane upward the reticular lamina will move inward (toward the
modiolus) while when the basilar membrane moves downward the reticular lamina will move
outward .
The hair cells which are found in the organ of corti it’s projections are impeded in the tectorial
membrane … when the reticular lamina(bony structure) moves and the tectorial membrane still
in it’s position the hair projection will bend !
Basilar membrane structure :
Its base(near the oval window) differs from it’s apex(near the helicotrema). At the base the
fibers are very short and stiff while at the apex the fibers are elongated and elastic .
Figure – page6 in the dr.’s slide- :
If the ear is exposed to very high frequency waves (not amplitude) the wave rises rapidly then
it dies immediately (e.g : 1600 Hz) but when the waves reaches ( 400 Hz) it needs more time to
rise to make the vibration .
The highest point(in the curve) where the basilar membrane movement occurs it contains the
hair cells that will signal the brain telling it that this is the area of the basilar membrane that has
been intensely stimulated and this occurs when the natural frequency of the basilar membrane
fibers equalize the sound wave frequency.
In other words : every tissue in our body vibrates at a specific frequency but the basilar
membrane is different because at it’s base the frequency is higher than at it’s apex so when
the frequency of a specific area of the basilar membrane equal the sound’s wave frequency this
will vibrate that specific area and then it will signal the brain (this is the highest point in the
curve ).
Travelling wave theory : the frequency at the base of the basilar membrane is (20,000 Hz) and
as we move toward the apex the frequency will be lower( 200 Hz -20 Hz) .
Note : the apex transmits frequencies from (200 Hz -20 Hz) then it will dissect high up in the
brain , because cochlea can’t make dissection at it’s level but the full dissection will be made at
the level of the spiral ganglia and cochlea nuclei .
The amplitude of the sound waves that we hear between 0-100 decibels (we can hear from an
amplitude = zero , because it is a logarithmic scale), after 100 decibels the sound is considered
as a profound and leads to damage of ear (figure page 8 in the dr.’s slide).
In old people, they lose their hearing capability either in the form of decibels or Hertz , but in
our age we can hear from 0-100 decibel (100 decibels when you are in a noisy factory or in the
airport listening to the engines of the planes).
*********************************
Deafness :
It is of two types :
1)
Conductive deafness: if the damage occurs From the
external ear up till the hair cells
2)
Nerve deafness (perceptive): it has a bad outcome,
because it occurs due to a tumor or a lesion (Not in all cases but the majority is
dangerous) . if the damage occurs in one of the following (Hair cells  fibers of the
8th cranial nerve  medulla  thalamus  cortex).
Most common causes of :
1)
a)
b)
c)
d)
2)
Conductive deafness:
Accumulation of wax : occurs in elderly people
because of their low hygiene level , then this accumulated wax will become hard and
it will block the external auditory meatus . treatment : ear cleaning the water
which is used in this cleaning should not be hot neither cold because it will lead to
damage of the ear, vertigo and nystagmus. it should be of a moderate temperature.
Tearing of tympanic membrane : due to hitting of the
ear
Otosclerosis : the whole lever system becomes hard
and rigid then it should be replaced .
Infection : by a foreign body like : insect .
Nerve deafness :
a)
Damaging of hair cells . e.g : workers in noisy factories
. this will lead to partial loss of hearing . ( it is not that serious) .
b)
Drugs prescribed : eg: antibiotics
c)
Tumors : in the brain.
Pure single ear deafness is rare !
Tests done to differentiate between conductive and nerve deafness ( a patient with a
nerve deafness should be sent to MRI screening) :
Rinne’s test :
The tuning fork that is used for hearing test must be 512 Hz.
The test : (1) sound the tuning fork and hold it from its handle then put it near the ear
(air conduction) . (2) sound the tuning fork again and put it on a bone e.g :mastoid (bone
conduction) .
In normal cases : Sound waves that are transmitted through air (air conduction) are
stronger than the ones that are transmitted through the bones (bone conduction). So
the screened person will hear from the air conduction but not from bone conduction .
if the ear is not normal : the screened person will hear from bone conduction which
means that he suffers from conductive deafness .
Weber’s test :
Sound the tuning fork then put it on the mid line of the forehead . the distance between
the mid line to each ear = 12 cm . There are three cases:
1)
Normal person : Hearing equally In both ears ( no
localization).
2)
If the right ear is damaged (symptomatic) and the
hear sense is more in this ear  this means that there is conductive deafness in the
right ear .
3)
In non-symptomatic ears but there is facial palsy or
cranial nerve involvement( and you suspected that one of them is damaged) if the
patient hears in his right ear mainly this means that there is nerve deafness in his
left ear .
The End
Done By : MAISAM ALI