Sensations
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Organic Sensations
General Sensations
Special Sensations
Sensations important
for life.
Somatic or Visceral
sensations
Special sense is one that is
localized into a specific organ.
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The sense of hearing
Definition of hearing
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Hearing is the neural perception of sound energy, it is a mechanoreceptive sense.
Functioning parts of the ear
Each ear has three functioning parts :
-External ear.
-Middle ear.
- Cochlea of inner ear.
Physical nature of sound
• Any device capable of producing disturbance of air
molecules in form of regions of compression alternating with
regions of rarefaction is a source of sound .
The sense of hearing
Character of sound
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❶ Pitch ( Frequency )(number of cycles /sec) :
The audible range extends in humans from (20 - 20.000 Hz)
‫( ز‬
- the greatest sensitivity is from( 1000 to 4000 Hz)
The ear can discriminate sounds ( ‫تمي وتفهم الصوت ال انت سامعه‬
- discrimination decreases above 4000 Hz.
Low Frequency
Sound
‫صوت غليظ‬
High Frequency
Sound
‫صوت حاد‬
The sense of hearing
Character of sound
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❷Intensity (loudness or volume):
The intensity, or loudness, of a sound depends on the amplitude of the sound waves.
Within the range, the greater the amplitude, the louder the sound .
- Unit of measurement : decibels (dB).
- minimal sound pressure detected by the human ear (the threshold value = 0.000204 dynes/squ.cm = 0 dB )
- Sounds of intensity greater than 130 dB can permanently damage the sensitive sensory apparatus .
𝐼𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑠𝑜𝑢𝑛𝑑
𝑆𝑜𝑢𝑛𝑑 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 𝑑𝐵 = 10 𝑙𝑜𝑔
𝐼𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑠𝑜𝑢𝑛𝑑
The sense of hearing
Character of sound
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❸ Timbre ( quality ) of sound :
- depends on its overtones : additional frequencies
superimposed on the fundamental pitch or tone.
- It is analyzed by high cortical centers .
◼Types of sound transmission:
• Air conduction.
• Bone conduction.
The sense of hearing
The sense of hearing
Function of external ear
The pinna
Tympanic membrane
a sound collector
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The external auditory canal
a. tortuous shape and secretes wax → protection against physical trauma and entry of
foreign materials.
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b. The effect of resonance (vibration) it vibrates when wave length of sound is 4 times the length
of the tube.
The external ear resonance adds 15-20db to the sound pressure at the tympanic membrane
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The sense of hearing
b. The effect of resonance (vibration) it vibrates when wave length of sound is 4 times the length
of the tube.
The external ear resonance adds 15-20db to the sound pressure at the tympanic membrane
The sense of hearing
Function of external ear
4
Sound localization by
changes in pressure on the
tympanic membrane.
The pinna
Tympanic membrane
a sound collector
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2
The external auditory canal
a. tortuous shape and secretes wax → protection against physical trauma and entry of
foreign materials.
3
b. The effect of resonance (vibration) it vibrates when wave length of sound is 4 times the length
of the tube.
The external ear resonance adds 15-20db to the sound pressure at the tympanic membrane
The sense of hearing
Function of tympanic membrane
- The anatomical surface area is 62 mm2.
- The physiological surface area is 55 mm2.
- The tympanic membrane moves in and out in response to pressure changes
produced by sound waves on its external surface so it acts as a resonator.
The sense of hearing
Function of middle ear
The sense of hearing
Function of middle ear
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Conduction of sound from the tympanic membrane to the cochlea
a- Amplification of sound intensity
Two mechanical devices are built
in the middle ear that increase or
amplify sound pressure that
arrives at the oval window
Lever action of the malleus and incus
b- Impedance matching by the
ossicular system
C- Sound protection of the
round window
The areal ratio of the tympanic membrane and oval window
The sense of hearing
More distance
Less force
Less distance
More force
The sense of hearing
The sense of hearing
𝑀𝑎𝑙𝑙𝑒𝑢𝑠
𝐼𝑛𝑐𝑢𝑠
=
𝟏. 𝟑
𝟏
= 1.3
The sense of hearing
Function of middle ear
1
Conduction of sound from the tympanic membrane to the cochlea
a- Amplification of sound intensity
Two mechanical devices are built
in the middle ear that increase or
amplify sound pressure that
arrives at the oval window
Lever action of the malleus and incus
b- Impedance matching by the
ossicular system
C- Sound protection of the
round window
The areal ratio of the tympanic membrane and oval window
The sense of hearing
Function of middle ear
The areal ratio of the tympanic membrane and oval window
- The force exerted over the large tympanic
membrane (55sq.mm) is concentrated on the tiny
footplate of the stapes (3.2 sq.mm) with a gain in
pressure equals the surface area of the larger divided
by the smaller (55/ 3.2) = 17.
The sense of hearing
This 17 fold difference times the 1.3 fold ratio of the
lever system causes about 22 times as much total force
to be exerted on the fluid of the cochlea which has
greater inertia than air.
The sense of hearing
Function of middle ear
1
Conduction of sound from the tympanic membrane to the cochlea
a- Amplification of sound intensity
b- Impedance matching by the
ossicular system
➢ Definition: The function of the middle ear is to
transfer sound energy from a gas to a liquid medium
without great loss of energy.
➢ Therefore, the tympanic membrane and ossicular
system provide Impedance matching between the
sound waves in the air and the sound vibrations in
the fluid of the cochlear fluid; impedance matching
allows utilization of most of the energy of the
incoming sound waves.
C- Sound protection of the
round window
The sense of hearing
Function of middle ear
1
Conduction of sound from the tympanic membrane to the cochlea
a- Amplification of sound intensity
b- Impedance matching by the
ossicular system
C- Sound protection of the
round window
➢ Bulging the round window towards the
middle ear due to pressure energy of sound
in Scala tympani
➢ It serves as a pressure relief hole that
prevents overlap between the successive
waves, so it allows discrimination of sound
waves inside the cochlea
The sense of hearing
Function of middle ear
2
Functions of middle ear muscles (Tympanic muscle reflexes, attenuation reflex):
receptor:
the cochlea
stimulus:
▪ sound of high
intensity (80 db)
▪ low frequency (<
1 KHz =
1000cyc/s).
CN
Efferent: through motor
nuclei of the 5th and
7th cranial nerves
afferent: through the cochlear
nuclei → superior olivary nucleus
SO
IC
Center:
inferior colliculucs
The sense of hearing
Function of middle ear
2
Functions of middle ear muscles (Tympanic muscle reflexes, attenuation reflex):
➢ Effectors:
✓ the tensor tympani and stapedius muscles
respectively.
➢ Effect:
✓ The tensor tympani muscle → pulls the handle of the
malleus inward.
✓ The stapedius muscle → pulls the stapes outward.
✓ These two forces → cause the entire ossicular system
to develop a high degree of rigidity → greatly
reducing the ossicular transmission of loud sounds.
This is called attenuation reflux arc.
Note
➢ The reflex is concensual i.e. bilateral regardless of
whether the sound stimulus is applied to one or both
ears.
➢ the reflex occurs after a latent period 40-80
milliseconds.
Function of this reflex
It partially protects cochlea
from damaging vibrations
caused by very loud sound,
by providing partial
reduction of only 40 db.
It decreases a person's
hearing sensitivity to his own
speech. (N.B. collateral
signals are transmitted to
these muscles during speech).
It minimizes transmission of
low frequency sounds in a
loud environment.
▪ This usually removes a major share of background noise and allows
the person to concentrate on sounds > 1000 Hz where most of the
pertinent information in voice communication is transmitted.
▪ Also efferent fibers from olivary neurons (which are sensitive to low
frequency sounds) passes to cochlea can decrease cochlear response to
background noise by triggering hyperpolarization of outer hair cells
through an acetylcholine mediated pathway (making effect).
This effect raises auditory threshold
The sense of hearing