Download Introduction

The Sense of Hearing
Sound
It is mechanical waves transmitted through a medium( air, water, and
other matters). The waves are formed of positive and negative
components which comprise a cycle. There are 2 parameters which
identify sound, the frequency and intensity.
The frequency is the number of cycles per second, and is measured by
Hertz(Hz) , one cycle per second is one Hz, and 1000 CPS is 1000 Hz.
The other parameter is the intensity which is measured by the
decibel(dB).
Hearing is a pure sensory science linking man to the world around
him. Human ear can generally hear sounds with frequencies between 20
Hz and 20,000Hz .
Anatomy and Physiology of the Ear
The ear is the organ that detects sound and it is an energy
transducer, which converts acoustic energy into electrochemical energy.
It changes sound pressure waves from the outside world into a signal of
nerve impulses sent to the brain. It does not only receive the sound, but
also aids in balance and body position.
For descriptive purpose, the ear is divided into three parts: The
external ear, the middle ear and inner ear
I-The external ear or Outer ear, the outer ear is composed of:
1-The pinna (Auricle), which is a plate of cartilage covered with skin on
either side of the head. It serves to collect the sound to be processed at
deeper levels.
2-The external auditory meatus (EAM): It is approximately 7mm in
diameter and 2.5 cm long. It is S shaped . The ear canal is very important,
unless the canal is open, hearing will be dampened. Ear wax (cerumen) is
produced by glands in the skin of the outer portion of the ear canal. The
outer ear ends at the most superficial layer of the tympanic membrane
fig.( 1).
Fig (1) Anatomy of the ear.
The tympanic membrane (TM) or (ear drum): it is a window of the
middle ear. It is thin, semitransparent membrane, and makes the boundary
between outer and middle ears. It is 55 mm2 in surface area and is slightly
concave .The tip end of the handle of the malleus is attached to the center
of the tympanic membrane, and this point of attachment is constantly
pulled by the tensor tympani muscle, which keeps the tympanic
membrane tensed as shown in fig (2).
Fig.(2) Anatomical landmark of the left tympanic membrane
II- The middle ear
It is an air-filled cavity behind the tympanic membrane. It is a
small space occupied by three of the smallest bones of the body, known
as the ossicles, which include the malleus, incus, and stapes. These
ossicles convert the sound waves striking the eardrum into mechanical
vibrations. The opening of the Eustachian tube is also within the middle
ear. The Eustachian tube connects from the chamber of the middle ear to
the back of the nasopharynx. It is usually closed and opened during
swallowing, yawning to equalize the pressure in middle ear.
The malleus: It consists of a head ,neck, and three processes: the long
process (the manubrium, or handle) is attached to the mobile portion of
the eardrum ,shown in fig.( 3).
The incus: is the bridge between the malleus and stapes.It consists of a
body and two processes.
The stapes: is the smallest named bone in the human body. The head
(caput) articulates with the incus, and the neck bifurcates to become the
crura of stapes which converges on the footplate (the leg-like portion) of
stapes. The footplate of the stapes helps in hearing and it rests on the oval
window.
Fig. (3) Articulated ossicular chain of the right ear in medial view
The ossicles of the middle ear are suspended by ligaments in such a
way that the combined malleus and incus act as a single lever, having its
fulcrum approximately at the border of the tympanic membrane.(Fig 4)
Fig(4)
The three bones are arranged so that the movement of the tympanic
membrane causes movement of the malleus, which causes the movement
of the incus, which causes the movement of the stapes. When the stapes
footplate pushes on the oval window, it causes the movement of the fluid
within the cochlea. The articulation of the incus with the stapes causes the
stapes to push forward on the oval window every time the tympanic
membrane moves inward, and to pull backward every time the malleus
and tympanic membrane moves outward.
“Impedance
Matching”
The middle ear acts as an impedance matching device to transfer
sound energy efficiently from air to a fluid medium in the cochlea. Sound
pressure is amplified through the middle portion of the ear and is passed
from the medium of air into a liquid medium. Therefore, the ossicular
lever system increases the force of movement this is done by several
simple mechanisms :
The first mechanism is the "lever principle". There is a difference in the
dimensions of the articulating ear ossicles. The length of manubrium is
approximately 9 mm, while that of the long process of incus is about 7
mm. This leads to an increase in the force applied to the stapes footplate
compared with that applied to the malleus by about 1.3 times .
A second mechanism is the surface area of the tympanic membrane is
many times that of the stapes footplate. The surface area of the tympanic
membrane is about 55 mm2, whereas the surface area of the stapes
footplate averages 3.2 mm2. The sound energy strikes the tympanic
membrane and is concentrated to the smaller footplate. This 17-fold
difference in area 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 as is
exerted by the sound waves against the tympanic membrane.
Combination of all these results in a gain about 25- 27 dB. Because
fluid has far greater inertia than air does, it is easily understood that
increased amounts of force are needed to cause vibration in the fluid.
The round window is a membrane-covered round opening between
the cochlea and the middle ear. The round window is actually situated
below and a little to the back of the oval window, from which it is
separated by a rounded elevation, the promontory .
Attenuation of Sound by Contraction of the Tensor Tympani
and Stapedius Muscles:
When loud sounds are transmitted through the ossicular system and
from there into the central nervous system, a reflex occurs after a latent
period of few milliseconds to cause contraction of the stapedius muscle
and the tensor tympani muscle. The tensor tympani muscle pulls the
handle of the malleus inward while the stapedius muscle pulls the stapes
outward. These two forces oppose each other and thereby cause the entire
ossicular system to develop increased rigidity, thus greatly reducing the
ossicular conduction of low frequency sound.
The function of this mechanism is to:
1-To protect the cochlea from damaging vibrations caused by excessively
loud sound.
2-To mask low-frequency sounds in loud environments.
3-To decrease a person’s hearing sensitivity to his or her own speech.
III-The inner ear
The inner ear it is called labyrinth and it is made of two parts one
within the other the bony labyrinth is encased in the hardest bone of the
body, the petrous bone. Within this ivory hard bone, there is a
membranous labyrinth is surrounded by perilymph, which by itself
contains a fluid called endolymph.
There are three major sections of the inner ear
1-The front portion is the snail-shaped cochlea, which functions in
hearing.
2-The posterior portion, the semicircular canals helps maintain balance.
3-Interconnecting the cochlea and the semicircular canals is the vestibule.
It contains the sense organs responsible for balance, the utricle and
saccule.
The cochlea: it is approximately 35 mm in length and coild 2.5
times into a snail shape about the size of large pea.Within the cochlea are
three fluid filled spaces (1)the scala vestibuli, (2) the scala media, and (3)
the scala tympani. The scala vestibuli and scala media are separated from
each other by Reissner’s membrane (also called the vestibular
membrane); the scala tympani and scala media are separated from each
other by the basilar membrane. On the surface of the basilar membrane
lies the organ of Corti, which contains the hair cells as shown in fig.( 5).
The scala tympani and the scala vestibuli are filled with perilymph and
communicate with each other at the helicotrema. At the base of the
cochlea, the scala vestibule ends at the oval window which is closed by
the foot plate of the stapes. The scala tympani end at the round window
that is closed by secondary TM. The scala media is continuous with the
membranous labyrinth and does not communicates with the other two
scala. It is filled with endolymph and there is no communication between
endolymph and perilymph. Fig (6)
Fig (5) Section through the cochlea.
Fig (6)
The Basilar Membrane: is a fibrous membrane that separates the scala
media from the scala tympani. These fibers are stiff, elastic reed like
structures that are fixed at their basal ends but are free at their distal ends,
so they can vibrate like the reeds of a harmonica. If the cochlea is
unwounded and stretched straight, we can see that it tapers from base to
apex. The basilar membrane tapers in an opposite direction. It is wider at
the apex and narrower at the base (resemble a harp). More important is
the basal end which is about 100 fold stiffer than its apical end. At the
basal end, it has short and taut fibers that vibrate best at a very high
frequency, while the long, looser fibers near the tip of the cochlea vibrate
best at a low frequency.
The organ of Corti lies on the surface of the basilar membrane. The
actual sensory receptors in the organ of Corti are two specialized types of
cells called hair cells .
The hair cells are mechanoreceptors that release a chemical
neurotransmitter when stimulate by mechanical pressure or distortion,
two type of hair cell:
A single row of internal (or “inner”) hair cells and three or four
rows of external (or “outer”) hair cells. Their cilia (steriocilia) project into
the tectorial membrane.
The bases and sides of the hair cells synapse with a network of
cochlear nerve endings. Between 90 and 95 per cent of these endings
terminate on the inner hair cells. This emphasizes their special
importance for the detection of sound
The outer hair cells, in some way, control the sensitivity of the
inner hair cells at different sound pitches.
Traveling Wave
The initial effect of a sound wave entering at the oval window is to
cause the basilar membrane at the base of the cochlea to bend in the
direction of the round window. However, the elastic tension that is built
up in the basilar fibers as they bend toward the round window initiates a
fluid wave that “travels” along the basilar membrane toward the
helicotrema, and is called Traveling Wave
Endocochlear potential
The scala vestibuli and scala tympani contain perilymph, so that
the perilymph is almost identical with cerebrospinal fluid and has a
relatively low potassium
The scala media is filled with endolymph. The endolymph that fills the
scala media is an entirely different fluid secreted by the stria vascularis,
and contains a high concentration of potassium and a low concentration
of sodium, which is exactly opposite to the contents of perilymph. An
electrical potential of about +80 millivolts exists all the time between
endolymph and perilymph, with the positivity inside the scala media
and the negativity outside. This is called the endocochlear potential.
The importance of the endocochlear potential is that the tops of the
hair cells project through the reticular lamina are bathed by the
endolymph of the scala media, whereas perilymph bathes the lower
bodies of the hair cells. Furthermore, the hair cells have a negative
intracellular potential of –70 millivolts with respect to the perilymph but
–150 millivolts with respect to the endolymph at their upper surfaces
where the hairs project. It is believed that this high electrical potential at
the tips of the stereocilia sensitizes the cell by increasing its ability to
respond to the slightest sound.
Determination of Sound Frequency
1- Place Principle: low-frequency sounds cause maximal activation of the
basilar membrane near the apex of the cochlea, and high-frequency
sounds activate the membrane near the base of the cochlea. Intermediate
frequency sounds activate the membrane at intermediate distances
between the two extremes.
2-Frequency principle or volley principle, by which, low frequency
sounds from 20 to 2000 Hz can cause volleys of nerve impulses
synchronized at the same frequencies. These volleys are transmitted by
the cochlear nerve into the cochlear nuclei of the brain. It is further
suggested that the cochlear nuclei can distinguish the different
frequencies of the volleys.
Determination of Loudness
Loudness is determined by three ways.
1-The amplitude of vibration of both the basilar membrane and hair cells
increases, so that the hair cells excite the nerve endings at more rapid
rates.
2-As the amplitude of vibration increases, it causes more and more of the
hair cells to become stimulated; thus causing spatial summation of
impulses, i.e transmitted by many nerve fibers.
3-The outer hair cells do not become stimulated significantly until
vibration of the basilar membrane reaches high intensity.
Localization of the sound
Human beings localize sound within the central nervous system, by
different mechanisms:
1-Comparing arrival-time differences, time lag.
2-The difference between the loudness of the sounds in the two ears.
The principle of hearing and the auditory pathway:
All sound waves funnel down through the ear canal and strike the
eardrum, causing it to vibrate. The vibrations are passed to the small
bones of the middle ear (ossicles), first, vibrations pass to the malleus ,
which pushes the incus , which pushes the stapes. The base of the stapes
rocks in and out against the oval window, this is the entrance for the
vibrations. The stapes agitates the perilymph of the bony labyrinth. At
this point, the vibrations become fluid-borne. The perilymph, in turn,
transmits the vibrations to the endolymph of the membranous labyrinth,
The initial effect of a sound wave entering at the oval window is to cause
the basilar membrane at the base of the cochlea to bend in the direction of
the round window this bending of basilare membrane cause the cilia to
bend results into a mechanical transduction that opens 200 to 300 cation
channels, allowing rapid movement of positively charged potassium ions
from the surrounding scala media fluid into the stereocilia, and causes
depolarization of the hair cell membrane .
This, in turn, stimulates the cochlear nerve endings that synapse with the
bases of the hair cells. In this way sound waves are transformed into
nerve impulses (It is the movement of these hair cells which converts the
vibrations into nerve impulses).
Then, the nerve fibers from the spiral ganglion of Corti enter the
cochlear nuclear complex to synapse in the dorsal (higher frequencies)
and ventral (lower frequencies) nuclei in the upper medulla. At this
point, all the fibers synapse, and second-order neurons pass mainly to the
opposite side of the brain stem to terminate in the superior olivary
nucleus. A few second order fibers also pass to the superior olivary
nucleus on the same side. From the superior olivary nucleus, the auditory
pathway passes upward through the lateral lemniscus. Some of the fibers
terminate in the nucleus of the lateral lemniscus, but many bypass this
nucleus and travel onto the inferior colliculus, where all or almost all the
auditory fibers synapse. From the inferior colliculi in the midbrain, fibers
ascend to the medial geniculate bodies in the thalamus and subsequently,
via auditory radiations, to the transverse temporal gyrus of Heschl.
Several important points should be noted.
First,signals from both ears are transmitted through the pathways of both
sides of the brain, with a preponderance of transmission in the
contralateral pathway.
Second in many places in the brain stem, crossing over occurs between
the two pathways:
(1) In the trapezoid body,
(2) In the commissure between the two nuclei of vermis of the cerebellum
Hearing loss :It is a general term for the complete or partial loss of the
ability to hear by one or both ears. It is a major public health problem and
it is usually divided into three types:
(1) Sensorineural hearing loss: problem within the inner ear or the
auditory nerve and higher centers. This is subdivided into:
(a) deafness for low-frequency sounds caused by excessive and
prolonged exposure to very loud sounds (a rock band or a jet airplane
engine).
(b) deafness for all frequencies caused by drug sensitivity of the organ
of Corti—in particular, sensitivity to some antibiotics such as
streptomycin, kanamycin, and chloramphenicol
(c)-high frequency hearing loss occur in old age group people called
(Presbycusis).
(2) Conductive hearing loss is a problem in the outer or middle
ear.There is a block of sound conduction to the inner ear . However, if
the cochlea and nerve are still intact sound waves can still be conducted
into the cochlea by means of bone conduction from a sound generator
applied to the skull over the ear.
(3)Mixed type of hearing loss
Audiometer.
The audiometer is a device for determining the threshold of
hearing and the nature of hearing disabilities .Pure tone audiometry
(PTA) is described as the gold standard for assessment of hearing level.
Pure tones at various frequencies are generated, and their levels are
increased and decreased until thresholds are found.