Sensations
1
2
3
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.
OPEN
OPEN
OPEN
The sense of hearing
Definition of hearing
Reading only
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
Reading only
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
Reading only
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
Reading only
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
1
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
3
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
1
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
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
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
The sense of hearing
The inner ear
Organ of corti
stria
vascularis
The basilar
membrane
The sense of hearing
The sense of hearing
The organ of corti
(the cochlear mechano-receptive organ)
➢ Located on the basilar membrane.
➢ It contains the mechanically sensitive hair cells, which are the auditory receptors (there are 20,000
outer hair cell and 3500 inner hair cells in the human cochlea).
➢ this organ extends from the base of the cochlea and consequently has a spiral shape.
The sense of hearing
The organ of corti
(the cochlear mechano-receptive organ)
➢ Covering the rows of hair cells is tectorial membrane in which the tips of the hairs (stereocilia)
of hair cells are embedded.
The sense of hearing
The organ of corti
(the cochlear mechano-receptive organ)
➢ Neurons of the spiral ganglion within the
modiolus send its processes that arborize the bases of hair cells,
these fibers transmit electrical changes generated by hair cells to the
afferent ascending auditory nerve.
The sense of hearing
The Steria vascularis
- It is a highly vascular structure surrounding the Scala media
The sense of hearing
The Steria vascularis
➢ Endocochlear potential:
✓ Cells of the stria vascularis have a high
concentration of Na+-K+ ATPase. It
continuously secretes K+ in the
endolymph.
✓ This account for the fact that the Scala
media is electrically positive by 80 mV
relative to the Scala vestibuli and Scala
tympani at rest.
✓ This difference is called endocochlear
potential. It serves as a major driving
force for signal transduction.
The sense of hearing
The Steria vascularis
▪ - Intracellular recording reveals that
the hair cells have a resting potential
of approximately - 40 to -60 mv.
▪
Therefore, the net potential
difference across the hair cells apical
membrane is 120 to 140 mv. The high
electrical potential at the tips of the
stereocilia increases their ability to
respond to the slightest sound, they
are highly excitable.
The sense of hearing
The basilar membrane
The sense of hearing
The basilar membrane
contains about 25000 basilar fibers:
➢ The length of the fibers increases progressively
from the base (oval and round windows) towards
the apex of the cochlea.
➢ The diameter of the fibers, decreases from the
base towards the apex.
➢ The thick, short fibers near the oval window will
vibrate at a high frequency.
➢ The long thin fibers near the apex will vibrate at a
low frequency.
The sense of hearing
The basilar membrane
01
Transmission
of sound
waves from
the oval
window to
the organ of
Corti.
02
Transduction:
changing
sound
pressure into
an action
potential.
03
Frequency (Pitch)
analysis: each
group of fibers
in the basilar
membrane
responds to
specific
frequency.
04
Intensity analysis:
by degree of
displacement of
basilar
membrane which
is directly
proportional to
amplitude of
action potential.
The sense of hearing
Transmission of sound waves in the cochlea (Traveling waves)
The sense of hearing
Transmission of sound waves in the cochlea (Traveling waves)
➢ The movement of the footplate of stapes set up a series of traveling waves in the perilymph of the Scala
vestibuli.
As the wave moves up the cochlea, its height
increases to a maximum and then drops off
rapidly. The distance from the stapes to this
point of maximum height varies with the
frequency of the vibrations initiating the wave:
✓ High pitched sounds: generate waves that
reach maximum height near the base of the
cochlea
✓ low pitched sounds: generate waves that
peak near the apex.
The sense of hearing
Transmission of sound waves in the cochlea (Traveling waves)
Mechanism of transmission of sound waves:
➢ Bony walls of the Scala vestibuli are rigid, but the Reissner's membrane is flexible.
➢ Displacements of the fluid in Scala tympani are dissipated into air at the round window.
➢ Therefore, sound produces distortion of the basilar membrane and the site at which this distortion is
maximal is determined by the frequency of the sound waves.
The sense of hearing
Transduction of sound waves into action potential
Definition
The hair cells transform the mechanical forces of sound
(cochlear fluid vibration) into the electrical impulses of hearing
(action potentials propagating auditory messages to the
cerebral cortex)
‫خالص قربنا نخلص‬
The sense of hearing
The Auditory pathway
1
The auditory nerve is
composed of 30,000
axons in humans; about
95% are myelinated and
carry signals from inner
hair cells.
The remaining %5 are
unmyelinated and
transmit information
from outer hair cells.
These different innervation
patterns suggest that inner and
outer hair cells transmit
different kinds of information.
The large myelinated axons carry signals from a single inner hair cell
so it has a private line to the CNS and arrive earlier by 1 to 2
seconds than that of outer hair cells.
The unmyelinated axon transmits signals from a relatively large
number of outer hair cells and shows considerable convergence.
The sense of hearing
2
3
Auditory nerve (myelinated &
unmyelinated fibers) courses
through the internal auditory
meatus to enter the brainstem.
Fibers enter the dorsal and
ventral cochlear nuclei (the second
order neurons in upper part of the
medulla) where all fibers synapse .
4
Most of the second order neurons
fibers pass to the opposite side to
reach the superior olivary nucleus. A
few second order fibers pass to the
superior olivary nucleus of the same
side.
The sense of hearing
5
6
From the superior olivary nucleus
fibers constitute the lateral
lemniscus which ends in the
inferior colliculus of the midbrain.
7
The fibers then reach the medial
geniculate body of the thalamus.
They terminate in the primary
auditory cortex (Brodmann's areas
41 and 42) in the superior gyrus of
the temporal lobe
The sense of hearing
The sense of hearing
The Auditory pathway
➢ From the primary auditory cortex the fibers then pass to the auditory association area (area 22)
then to Wernicke’s area.
➢ Impulse from either ear are transmitted through the auditory pathway of both sides of the
brainstem with more projection along the contralateral pathway.
➢ Many collateral branches are given off to the reticular activating system of the brainstem.
➢ The tonotopic organization present in the organ of Corti is preserved within the cochlear nuclei,
the inferior colliculus and the primary auditory area
The sense of hearing
Function of specific parts of The Auditory pathway
1
Dorsal cochlear nucleus
➢ its neurons analyze sound frequencies (lateral neurons receive low frequency sound waves, medial
neurons for high frequency waves) as arranged on basilar membrane.
➢ This spatial organization continues all the way up to the cerebral cortex.
➢ Cochlear nuclei of both sides are interconnected.
2
superior olivary nucleus
Its function is determination of sound direction in space through difference in amplitude and time lag of
signals arriving to the two ears + Masking effect + Sends connections to the reticular formation leads to
brain arousal .
The sense of hearing
Function of specific parts of The Auditory pathway
3
The efferent olivary-cochlear pathway
➢ Connection to the reticular formation leads to brain arousal.
➢ Fibers descend from olivary nucleus through auditory nerve to innervate and decrease the cochlear
amplifier (masking effect) by triggering hyperpolarization of the outer hair cells:
✓ this decreases the auditory response to background noises that partially prevents acoustic trauma. Also
this pathway can discriminate transient sounds from background noise
4
inferior colliculus
It is a center for auditory reflexes (audio-spinal reflexes) lt processes information received from previous
nuclei then sends fibers to the geniculate body of thalamus.
The sense of hearing
Function of specific parts of The Auditory pathway
5
medial geniculate body
it plays a role in processing of complex vocal communications as human speech.
6
primary auditory cortex
➢ Pitch discrimination by presence of different tonotopic maps similar to those of
(cochlear nuclei, the low frequencies are presented laterally while the high
frequencies are presented medially.
➢ Integration and processing of complex auditory signals.
➢ Localization of sound direction in space (by time lag neurons)
➢ Discrimination of timbre
The sense of hearing
Function of specific parts of The Auditory pathway
7
auditory association cortex (secondary auditory cortex)
❖ Interpretation of the meaning of the sound heard:
➢ it plays an important role in speech perception due to its connection with
the language reception center; Wernicke’s area.
➢ It associates sound information with information coming from other
cortical sensory areas (as visual cortex)
➢ Auditory information from subcortical structures also projects to the
amygdala which is a part of limbic system so emotional responses could
be evoked by sounds.
The sense of hearing
Auditory code
1
The place principle of pitch analysis
It states that:
➢ Vibration of fibers of the basilar membrane depends
upon their length and diameter,
➢ i.e. the short thick fibers at the base are maximally
activated by the high frequency sounds
➢ The long thin fibers at the apex are maximally
activated by the low frequency sounds and the
➢ Fibers between the apex and base respond to
intermediate frequencies so each pitch (or frequency)
would cause vibration of its own particular "place" on
the basilar membrane.
The sense of hearing
➢ There is spatial organization of the cochlear nerve
fibers from the cochlea to the cochlear nuclei in the
brainstem i.e. fibers from each respective area of
the basilar membrane terminate in a corresponding
area of cochlear nuclei.
➢ This spatial arrangement continues all the way up
the brainstem to the cerebral cortex. So specific
neurons are activated by specific sound frequencies
i.e. there is no mixing of fibers, each area has its
own fibers in the nerve pathway up to the auditory
cortex
The sense of hearing
2
Determination of loudness of sound
➢ It is determined by the degree of displacement of the
basilar membrane.
➢ Thus increasing the amplitude stimulates the nerve
endings at more rapid rates. More and more of the
receptors (hair cells) become stimulated.
3
Sound localization
➢ In the horizontal plane (Require bilateral hearing).
➢ It is a function of the superior olivary nucleus and the
primary auditory cortex.
➢ They detect the time lag between the entry of sound into
one ear and its entry into opposite ear through the
presence of time lag neurons. They also detect the
difference in the intensity of sound between the two ears.
Questions
MCQs
1
Amplification of the sound by the middle ear serves to overcome:
01
Hyperpolariz
ation of the
hair cells
02
03
04
Inertia of fluid
in the cochlea
Inertia of air in
the middle ear
Hyperpolarizati
on in the
cochlear nerve
MCQs
2
Which is TRUE about inner hair cells of organ of Corti ?
01
They are
considered
as true
neurons.
02
03
The
Hydrodynamic
Calcium influx
causes glutamate forces of basilar
membrane have
release at the
no effect on hair
synaptic area
cells
04
Sodium influx is
the first event in
transduction
after bending of
stereocilia
MCQs
3
What is the range of frequencies that the human ear is most sensitive
to?
01
50 Hz to 500
Hz
02
12,000 Hz to
20,000 Hz
03
04
20 Hz to 20,000
Hz
1000 Hz to 4000
Hz
MCQs
4
Sound waves are conducted from the air outside the ear to the inner ear by the
processes of:
01
02
03
04
Absorption,
transmission
and refraction.
Reflection,
transmission
and scattering
Resonance,
leverage and
amplification.
Resonance,
diffraction and
refraction.
MCQs
5
Which membrane lies over the hair cells found in the organ of Corti?
01
02
03
04
Basilar
Vestibular
Tectorial
Cochlear
MCQs
6
Which of the following is the center of auditory reflexes ?
01
Medial
geniculate
body
02
03
04
Inferior
colliculus
superior olivary
nucleus
Dorsal cochlear
nucleus
MCQs
7
The thick, short fibers near the oval window will vibrate at a low
frequency.
01
True
02
False
MCQs
8
The physiological surface area of tympanic membrane is 55 cm2.
01
True
02
False