Download Structure of Ear Lesson Name : Structure of Ear

Structure of Ear
Lesson Name : Structure of Ear
Lesson Developer : Dr Mahtab Zarin and Dr. Zubeda
College/Department : Department of Zoology
Institute of Life Long Learning, University of Delhi
Structure of Ear
Table of Contents
•
Introduction
•
Structures of Ear
1.
2.
3.
External (outer) ear
•
Auricle
•
Ear canal
•
Tympanic membrane
Middle Ear
•
Auditory ossicles
•
Skeletal muscles
•
Eustachian tube
Inner (Internal) Ear
•
Bony labyrinth
•
Membranous labyrinth
•
Cochlea
•
Organ of Corti
•
Semicircular Canals
•
Utricle and Saccule
• Structures of Neural Pathways
1.
Structure of auditory nerve pathways
2.
Structure of equilibrium (vestibular) nerve pathways
• Summary
• Exercises
• Glossary
• References
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Structure of Ear
Learning objectives
•
Explain the structures of the ear;
• Understanding the functions of each part of the ear;
•
Describe structures and function of receptor organs for equilibrium
•
To discuss neural pathways/parts of ear.
Introduction
Hearing is among the five senses in our body that is vision, taste, smell, and
touch. The ears collect sound and transmit it to brain, where it is construed, so
that speech, music, and other signals can be listened. So, the auditory system
needs a source of sound, mechanism for detecting this sound, mechanisms for
transmitted sounds to the central nervous system, and pathways of central
nervous systems in order to send this sensory information to the brain where it
can be processed, interpreted and accumulated.
Structure of Ear
The ear is the part of auditory system, which perceives sound and helps in
maintenance of balance and body position.
Ear is partitioned into three major parts (Fig. 1):
1. External ear or outer ear
• It collects sound waves and channels them inside of the ear.
• It is the most outer part of the ear.
• It is subdivided into the auricle, ear canal and tympanic membrane.
2. Middle ear
• It is cavity filled with airin the temporal bone, which opens through the
eustachian tube into nasopharynx and from nasopharynx to the external.
• Three auditory ossicles: the mallaeus, incus and stapes are placed in
the middle ear.
• Manubrium or handle of the malleus is connected to backside of the
tympanic membrane.
• The head of the malleus is connected to wall of middle ear and is attached
to the incus.
• Incus articulates with the head of the stapes.
• The foot plate of stapes is connected through an annular ligament to the
walls of the oval window.
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Structure of Ear
• The middle ear also contains two small skeletal muscles named as tensor
tympani and the stapedius. These muscles control the vibrations of the
tympanic membrane.
• It pass on the sound waves to the oval window.
3.
Internal ear or inner ear
• It is also known as labyrinth and consists of two parts i.e. bony and
membranous labyrinth, placed one within the other.
• Bony labyrinth is a sequence of channels in petrous portion of the
temporal bone of the skull. Inside these channels, enclosed by a fluid
known as perilymph, is the membranous labyrinth.
• Membranous labyrinth encloses a fluid called endolymph.
• Labyrinth is provided with a system of passages includes two chief
functional divisions that is the cochlea and vestibular system.
• Cochlea is a coiled tube, which is 35mm long and makes 2.5 in humans
and devoted to hearing.
• Vestibular system comprises of three semicircular canals and saccule and
utricle. These vestibular apparatus are dedicated to maintain the balance
or equilibrium.
• The sensory receptors involved are hair cells. There are six groups of hair
cells in each inner ear i.e. in the cochlea, each of 3 semicircular canals,
saccule and utricle.
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Structure of Ear
Fig. 1. Cross-section of ear consists of outer, middle and
inner ear.
Source: http://cnx.org/contents/b375ea7d-22d5-4f47-b10a-41dd93637896@4
CC
External ear
The external ear is the outer part of the ear. It contains the fleshy noticeable
outer ear termed as auricle, ear canal, and the tympanic membrane.
a) Auricle (Fig. 2)
The visible and flap like part is called the auricle or pinna. It is composed of a fine
sheet of yellow elastic cartilage, enclosed by skin. It isconnected to the
neighboring parts through ligaments and muscles and leading to external acoustic
meatus through fibrous tissue.
Auricle consists of the curved rim, the helix and it is extended up to the ear canal
known as the external acoustic meatus. The inferior portion of the auricle is
lobule. Ligaments and muscles connect the auricle to the head. The center of
the pinna named as the concha that leads to the external auditory
canal.
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Structure of Ear
Fig. 2. The Auricle.
Source:http://en.wikipedia.org/wiki/Outer_ear#mediaviewer/File:Gray
904.png
Image Credit: This image is in the public domain because its copyright has expired.
This applies worldwide.
Function of auricle:
Outer ear and head have a submissive but significant role in hearing due to their
acoustic qualities. The concha, or bowl of the auricle, has a resonance of about 5
kHz, and the irregular surface of the pinna introduces other resonances and
antiresonances. These acoustic features are useful to help differentiate whether
sound sources are in front of the listener or behind.
b) Ear canal
The ear canal or external auditory canal is 2.5 cm elongated curved tube,
which is placed in temporal bone. It comprised of inner segment enclosed by
bone, and an outer segment enclosed by cartilage. The skin adjacent to the ear
canal (external acoustic meatus) includes few hairs and specific sweat glands
known as ceruminous
glands, which release ear wax (cerumen). The
combination of hairs and cerumen helps to prevent dust and foreign objects from
entering the ear. The ear canal extended upto the external surface of the ear
drum (tympanic membrane).
Function of ear canal:
The external auditory canal (EAC) is basically a tube that is open at one side and
closed at the other. Thus the EAC behaves like a quarter-wave resonator. The
resonant frequency (f0) is determined by the length of the tube whereas the
curvature of the tube is irrelevant. For a tube of 2.5 cm, the resonant frequency
is approximately 3.5 kHz.
c) Tympanic membrane
The eardrum or tympanic membrane is a fine, semi-translucent division between
the outer audiory canal and middle ear. The tympanic membrane is surrounded
by epidermis and covered through simple cuboidal epithelium tissue.
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Structure of Ear
Value addition: Did you Know
Heading text: Perforated eardrum and Otoscope
Body text: Tearing of tympanic membrane is known as perforated eardrum
and caused by changes in air pressure, generally heals within period of
month. It can occur as a consequence of
(i)
ear infection e.g. otitis media,
(ii) trauma e.g. cleaning the ear with pointed instrument,
(iii) loud noise e.g. explosion or
(iv) surgery e.g. accidental creation of a rupture.
(v) Flying or landing when suffering from severe cold can also leads to
perforation in ear drum as changes occurs in air pressure and
blocked eustachian tubes resulting from the cold.
It results in conductive hearing loss that is generally temporary. The tympanic
membrane may be examined by an otoscope, a viewinginstrument that light ups
and magnifies the external auditory canal and tympanic membrane.
Fig. 3a. A view through the Otoscope
Source: ILLL in house
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Structure of Ear
Fig.3b. Large perforated eardrum
Source: http://commons.wikimedia.org/wiki/File:
PerforationTympan.jpg#mediaviewer/File: PerforationTympan.jpg CC
Middle Ear(Fig 4.)
Middle ear is made up of an air-filled cavity just at the back of the tympanic
membrane, consists of three auditory ossicles: the malleus, incus, and stapes.
The middle ear also attached to the upper throat through the Eustachian tube
(Fig 2).
a. Auditory ossicles
It is separated from inner ear through a fine bony dividing wall having two small
and membrane-enclosed openings termed as oval window and round window. It
is attached through ligaments to the three-minute bones called as auditory
ossicles. These bones are called as malleus, incus, and stapes and also usually
named as the hammer, anvil, and stirrup correspondingly as named attributed to
their shapes.
The handle of the malleus connected to the inner layer of the tympanic
membrane. The head of the malleus articulates along with body of incus. The
incus, the central bone in the sequence, articulates along with the head of the
stapes. The base or footplate of the stapes fixed into the oval window. Another
opening beneath the oval window termed as round window is covered through a
membrane known as secondary tympanic membrane.
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Structure of Ear
b. Eustachian tube
Front wall of the middle ear contains an aperture that directs into the auditory
(phagyngotympanic)tube; commonly we call it as the eustachian tube. It is
composed of both bone and elastic cartilage. The Eustachian tubeattaches
middle ear with the nasopharynx. When we swallow and yawn, it opens in order
to allow air to move in and outside the middle ear until the pressure in middle ear
equilibrates with the new atmospheric pressure.
Fig. 4.Middle Ear Anatomy
Source: http://en.wikipedia.org/wiki/Middle_ear#mediaviewer/File:
Blausen_0330_EarAnatomy_MiddleEar.png CC
"Blausen gallery 2014". Wikiversity Journal of Medicine. DOI:
10.15347/wjm/2014.010. ISSN 20018762. - Own work
Image Credit: CC BY-SA 3.0
c. Skeletal muscles
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Structure of Ear
There are also two tiny skeletal muscles, which are connected to the ossicles.
(1)
Tensor tympani muscle is among the tinyskeletal muscles that are
supplied with the mandibular branch of the trigeminal (V) nerve. It is
enclosed in the bony canal above the bone part of the eustachian tube. It
dampens sounds such as those produced from chewing. This restricts
movement and enhances tension on the eardum or tympanic membrane
to avoid damage to the internal ear because of loud sounds.
(2)
Second skeletal muscle is stapedius muscle that is supplied with the
facial (VII ) nerve. It is known to be the smallest skeletal muscle in
the body of human. It controls the intensity of vibrations in the stapes
arises from loud sounds; in turn it protects the oval window, although it
minimizes the receptivity of hearing.
Value addition: Did you Know
Heading text: Infection of Eustachian tube
Body text:The purpose of the Eustachian tube within the ear is to drain the
middle ear so that air pass through to clean it and also to protect it. The tube
attaches itself from the middle ear to the pharynx. If this will not occur the ear
can cause to middle ear inflammation. Infection in the middle ear more common
to children as they possess shorter Eustachian tube, which results weak
protection. As the humans grow they get the longer tube.
With the closure of the Eustachian tube by infection such as sore throat or
common cold, the swallowing mechanism becomes ineffective. The mucous
membrane absorbs the air within the middle ear and its pressure decreases; as a
result the tympanic membrane bulges inwards and its vibrations get decreased or
abolished, causing discomfort and loss of hearing.
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Structure of Ear
Fig.5.Blockage ofEustachian tube
3.
Source:http://www.webmd.com/a-to-z-guides/eustachian-tubes
Image Credit: WebMD for non-commercial purpose only
Inner (Internal) Ear(Fig.6.)
Inner (internal) ear is also termed as the labyrinth because of its complex series
of canals (Fig 3). Anatomically, it has two major divisions: (i) external bony
labyrinth and (ii) internal membranous labyrinth. It resembles to long balloons
put inside a rigid tube.
a. Bony labyrinth
The bony labyrinth has the sensory organs for sense of balance and movement,
viz. (i) the vestibules and (ii) the semicircular canals and (iii) sensory organ for
hearing, the cochlea. The bony labyrinth encloses perilymph, a fluid that is
chemically equivalent to cerebrospinal fluid that lies around the membranous
labyrinth. The membranous labyrinth is a sequence of epithelial sacs and tubes
contains endolymph, situated within the bony labyrinth. Membranous labyrinth
contains the receptors for hearing and equilibrium.
b. Membranous labyrinth
The membranous labyrinth in the vestibule composed of two sacs known as
utricle and the saccule that are filled with endolymph. There are three bony
semicircular canals protruding superiorly and posteriorlyfrom the vestibule.
Each one of bony semicircular canals is placed at right angles to the other two.
Based on their positions, they are called as anterior, posterior, and lateral
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Structure of Ear
semicircular canals. There is a swollen enlargement at one end part of each canal
known as the ampulla. The regions of the membranous labyrinth that are placed
within the bony semicircular canals are known as the semicircular ducts.
Fig.6.Internal Ear Anatomy
Source:
n.wikipedia.org/wiki/Inner_ear#mediaviewer/File:Blausen_0329_EarAnatomy_Int
ernalEar.png CC
"Blausen gallery 2014". Wikiversity Journal of Medicine. DOI:
10.15347/wjm/2014.010. ISSN 20018762. - Own work
Image Credit: CC BY-SA 3.0
c. Cochlea(Fig.7)
Cochlea is situated at anterior of the vestibule. It is a bony spiral canal, which
resembles to a snail’s shell and makes around 2.5 turns in the region of a central
and conical bony core called modiolus. In cochlea, waves propagate from the
base (close to the middle ear and the oval window) to the apex (the top or center
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Structure of Ear
of the spiral). Sections through the cochlea reveal that it is partitioned into three
chambers (Scalae):
(vi) Scala vestibuli: It contains perilymph and lies superior to the cochlear duct
and adjacent to the oval window.
(vii) Scala tympani: Contain perilymph and lies inferior to the cochlear duct and
terminates at the round window.
(viii) Cochlear duct or scala media: It has endolymph and high potassium ion
concentration that the stereocilia of the hair cells protrude into helicotrema.
Fig 7.The cochlea and organ of Corti.
a | The cochlea is a part of the internal ear, which also includes the vestibular
apparatus and lies in the temporal bone of the skull. Sounds impinge upon the
eardrum, whose vibrations are communicated through the three miniscule bones
of the middle ear to initiate oscillatory changes in the pressure within the coiled
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Structure of Ear
cochlea. b | As shown in a cross-section, the cochlea comprises three liquid-filled
compartments, the scalae, which are separated by two elastic partitions:
Reissner's membrane and the basilar membrane. The scala vestibuli and scala
tympani contain perilymph, an ordinary extracellular fluid similar to that
surrounding most neurons. The scala media contains K+-rich endolymph, which is
secreted by cells of the stria vascularis and which also endows the scala media
with a potential of about +80 mV with respect to other extracellular
compartments. The somata of the afferent neurons that innervate hair cells lie in
the spiral ganglion. c | Upon the collagenous basilar membrane rests the organ of
Corti, a strip of epithelial cells that have highly varied structures. The human
cochlea includes a single row of inner hair cells, which detect and transmit most
of the afferent information to the brain. The three rows of outer hair cells have a
motor function, implementing the active process that enhances hearing. The
tectorial membrane is a gelatinous strip that is attached at its bottom surface to
the tips of the longest stereocilia in the hair bundles of outer hair cells. Movement
of the membrane relative to the bundles accordingly deflects the outer hair cell
bundles. The hair bundles of inner hair cells are instead deflected by motion of
the liquid beneath the tectorial membrane. d | When the cochlea is excited by
sound, the back-and-forth motion of the stapes produces alternate increases and
decreases in the pressure of the liquid at the base of the scala vestibuli. The
pressure difference across the basilar membrane elicits a series of travelling
waves that progress along the membrane at a speed of some metres per second,
far below the velocity of sound in water.
Source: A. J. Hudspeth (2014). From integrating the active process of hair cells
with
cochlear
function.
Nature
Reviews
Neuroscience15,600–614doi:
10.1038/nrn3786. http://www.nature.com/nrn/journal/v15/n9/fig_tab/nrn3786_F
1.html
Displayed with permission.
The cross section of the cochlear duct shows:
a. Helicotrema : a small opening through which the scala tympani and the scala
vestibuli join together at the apex of the cochlea.
b. The cochlear duct is bounded on three sides through the basilar membrane,
the stria vascularis, and Reissner's membrane.
(i)
Stria vascularis is a rich bed of capillaries and secretory cells.
(ii) The vestibular membrane also called Reissner’s membrane divides
cochlear duct from the scala vestibuli.
(iii) The basilar membrane divides the cochlear duct from the scala tympani.
Spiral lamina isa thin spiral and bony protuberance and an extension of the
modiolus. It serves as the medial support for the basilar membrane.
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Structure of Ear
Fig.8. Cross section of the cochlea.
Source :http://www.med.umich.edu/histology/cns/ear.html#cortiCC
d. Organ of Corti
•
The spiral organ or organ of Cortiis positioned on the basilar membrane.
The spiral organ is a coiled area of epithelial cells extends from apex to the
base of the cochlea.
•
The receptor cells in organ of Corti are known as hair cells. Receptors of
hearing are located on the hair cells. Sensory hair cells are motorized
through the potential variation among perilymph and endolymph.
•
Hair cellsare supported by the phalangeal cells (also called Deiter's
Cells).
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Structure of Ear
•
These hair cells are mechanoreceptors which contains hair like projections
from one end called stereocilia.
•
Stereocilia of the hair cells pierce the tough, membrane like reticular
lamina (also knownas apical cuticular plate orreticular membrane) which is
formed by inner and outer rods of corti or pillar cells.
•
Reticular lamina or reticular membrane divides endolymph in the scala
media from underlying corticolymph and perilymph of scala tympani.
•
The hair cells are arranged in four rows: 3 rows of outer hair cells lateral
to the tunnel shaped by the rods of corti, and one row of inner hair cells
which is medially situated to the tunnel. There are around 20,000 outer hair
cells and 3500 inner hair cells found in each of both (right and left) human
cochlea.
•
Outer and inner pillar cells form a triangular shaped tunnel, named as inner
tunnel, thatcontainsperilymph-type fluid known as corticolymph.
•
The organ of Corti is cover through atectorial membranewhich is thin,
gelatinous but elastic in nature. Tectorial membrane is formed and
maintainedthrough the columnar cells positioned on apex of the spiral
limbus just medial to the organ of Corti.
•
Nerve fibers enter the organ of Corti through openings in a bone-shelf
which extends from modiolus. Nerve fibers enter through the supporting
cells so that synapse with hair cells.
Function of outer hair cells: The main role of outer hair cells is essentially to
contract upon stimulation, so that "draging" on tectoral membrane in turn
stimulate inner hair cells
(see
weblink
an
animated
drawing
in
the
following
http://www.neuroreille.com/promenade/english/corti/cofunct/cofunct.ht
m).
This mechanically alters movement of hair cells in a complex mode which
sharpens frequency turning at each point beside the basilar membrane.
Function of inner hair cells: Stereocilia of inner hair cells project into
endolymph fluid and convert sound waves originated by fluid movement in
cochlear duct into receptor potentials.
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Structure of Ear
Fig.9. a. Cross section of Organ of Corti
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Structure of Ear
b. . Pictoral representation of Cross section of organ of Corti.
Source :a.http://www.med.umich.edu/histology/cns/ear.html#corti CC
b.http://en.wikipedia.org/wiki/Organ_of_Corti#mediaviewer/File:Gray931.png CC
Value addition: Did you Know
Heading text: Mechanoelectrical transduction
Body text:“Hair cells, the sensory receptors of the auditory and vestibular
systems, are the best-understood mechanoreceptors in eukaryotes. These cells
are specialized to detect stimuli of amplitudes well below a nanometre and
frequencies far above a kilohertz.
When a stimulus deflects the hair cell's mechanoreceptive organelle, the hair
bundle (which can be due to sound in the auditory system, acceleration in the
vestibular or water motion in the lateral-line system), transduction commences
through an incompletely characterized group of proteins to producea stimulus
force into an electrical response
Fundamentally the hair bundle uses an, the receptor potential, which is a change
in the voltage across the cell's membrane. The receptor potential in turn
modulates the release of the neurotransmitter glutamate from synapses at the
hair cell's base and thus transmits the response to the CNS” (Fig. 9).
Fig. 9: The hair cell and hair bundle.
a | A schematic drawing depicts a cylindrical hair cell surrounded by supporting
cells. A ribbon synapse at the base of the cell releases glutamate, which excites
the afferent nerve terminal and sends information into the brain. Neural centres
in the brainstem can reduce the hair cell's sensitivity by activating an efferent
nerve terminal that releases acetylcholine. The mechanoreceptive hair bundle
consists of 10–300 cylindrical processes, the stereocilia, that project as a
compact cluster from the cell's apical surface. As hypertrophic derivatives of
microvilli, the stereocilia are filled with parallel actin filaments that are held in
register by crosslinks of plasmin, fimbrin, espin and other proteins. The stereocilia
vary systematically in length across each hair bundle so that the structure is
bevelled like the tip of a hypodermic needle. Tip links interconnect successive
stereocilia along the bundle's axis of mirror symmetry. A single true cilium, the
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Structure of Ear
kinocilium, stands at the bundle's tall edge. During development, this structure
has a role in organizing and orienting the bundle. b | In a lateral view, a scanning
electron micrograph shows an individual hair bundle from the frog's sacculus, an
organ that detects gravity and ground-borne vibration. About 8 μm in height, the
bundle comprises a single kinocilium with a bulbous swelling at its tip and about
60 cylindrical stereocilia with tapered basal insertions. c | A top view shows the
specialized hair bundle of an outer hair cell from the bat's cochlea, which is Vshaped and has only three ranks of stereocilia, the shortest of which is 0.5 μm in
height.
Source: A. J. Hudspeth (2014). From integrating the active process of hair cells
with
cochlear
function.
Nature
Reviews
Neuroscience15,600–614doi:
10.1038/nrn3786http://www.nature.com/nrn/journal/v15/n9/fig_tab/nrn3786_F2
.html
Displayed with permission.
Tether in stereocilia:(Fig.10)
The stereocilia are collection of microvilli-type structures arranged in an order of
tallest to shortest. Protein fibers tether neighboring hairs collectively within each
group, in a way that this group will bend in reaction to movements of basilar
membrane.
Fig.10. Tether in stereocilia of hair cells.
Source: http://cnx.org/resources/0dfe4b4792b7cde18817f853a971455
6/1407_The_Hair_Cell.jpg CC
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Structure of Ear
Function of cochlea
The perilymph in cochlea moves in response to vibrations, which processed and
comes from middle ear through the oval window. When the perilymph moves,
cochlear partbetween basilar membrane and organ of Cortimoves. Evantually,
thousands of hair cells detect this motion through their stereocilia, and transduce
that
motion
into
electrical
signals.
These
signals
aretransformed
into
electrochemical impulses action potentials that travel through the auditory nerve
to the brainstem for further processing.
The perilymph
Perilymph is an extracellular fluid, which fills the bony labyrinth of internal
ear. It is placedwithin two of three chambers in cochlea of internal ear: scala
tympani and scala vestibuli.The ionic composition of perilymph can be compared
with that of plasma and cerebrospinal fluid. The major cation in perilymph is
sodium ion.
The endolymph
Endolymph is the fluid which fills the membranous labyrinth of internal ear. It is
also known asScarpa's fluid.
Main constituent of this unique extra-cellular fluid is potassium, which is
released from stria vascularis. Due to the high content of potassium in the
endolymph, potassium is carried as the de-polarizing electrical current in hair
cells. This is called the mechano-electric transduction (MET) current.
Table 1: Difference between Perilymph and endolymph
Features
Perilymph
Endolymph
Location in labyrinth
Within the osseous or bony
In the membranous labyrinth
labyrinth of the inner ear
of the inner ear
the scala tympani and scala
the cochlear duct
Location
in
cochlea
chamber
Composition
vestibuli of cochlea
K+
4.8 mmol/L
144.8mmol/L
Na+
150.3 mmol/L
15.8 mmol/L
Cl-
121.5 mmol/L
107.1 mmol/L
Protein
50mg/dL
15mg/dL
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Structure of Ear
Electric potential
Less than that of endolymph
80–120 mV in the cochlea
Function of perilymph
•
The perilymph enclosing the saccule, utricle, and semicircular canals, provides
a padding support for the membranous labyrinth.
•
Perilymph of scala vestibuli and scala tympani in the cochlea conveys sound
waves across the scala media. Sound waves bend the basilar membrane so
that stimulate hair cells of the organ of Corti.
Function of endolymph
Movement occurs in the endolymph in different sections of the membranous
labyrinth in reaction to the movement in the perilymph due to sound vibrations
from middle ear. For example in Cochlear duct, Sound waves-induced
movement in the endolymph excite the receptor cells, so that itconvert their
movement into nerve impulses whichis perceived as sound by brain.
Semicircular canals:Angular movement of endolymph in semicircular ducts
excites the vestibular nerve receptors. The semicircular canals of both inner ears
act in order to coordinate balance.
Vestibular system
Collectively, the receptor organs to maintain the equilibrium/balance called
asvestibular apparatuswhich includessaccule, utricle, and semicircular
ducts Hair cells are also present in the vestibular apparatus of the inner ear.
The hair cells perceive alterations in the movement and head position through
mechanism of stereocilia transduction like that discussed for hair cells in cochlea.
(i) Semicircular Canals
The three semicircular canalsare a part of the bony labyrinth in inner ear on each
side of head.
•
Inside the bony canals, the membranous canals are suspended in
perilymph.
•
The three canals are the horizontal, superior and posterior semicircular
canals.
•
The three canals/ducts are oriented in different planesso that different ducts
stimulated through the head rotation in different planes (Fig. 11).
•
Among various species of mammals, the dimension of the semicircular ducts
is interrelated to their different kinds of locomotion. Particularly, species
that are responsive and have speedy, jerky movementspossess larger
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Structure of Ear
semicircular canals in relation to their body size as compared tothose, which
move more cautiously.
•
Dilated portion of each membranouscanalcalledampullain which there is a
small elevation termed as crista. Receptor cells i.e. hair cells and stereocilia
are contained in the crista. Each crista is encapsulated within a gelatinous
mass, the cupula.Endolymph contained in the ampulla plays a role in
detecting rotational movement.The streocillia of the hair cells lies in the
cupula and the bases of the hair cells are innervate by the afferent
neurons of the vestibular division of the vestibulocochlear nerve(Fig.
11).
Fig 11. Cross section of a semicircular canal
Source: http://en.wikipedia.org/wiki/Endolymph#mediaviewer/File:Vestibular_sy
stem%27s_semicircular_canal-_a_cross-section.jpgCC
Table 2: Features ofSemicircular canals
Features
Semicircular canals
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Different
Lateral or horizontal
or
external
semicircular canal
Superior or anterior
semicircular canal
Posterior
semicircular canal
Position
At an angle of 30degree
from
the
horizontal plane.
Positioned vertically
at a right angle to
posterior
semicircular canal
Positioned vertically
at a right angle to
superior semicircular
canal
Length
12 to 15 mm
15 to 20 mm
18 to 22 mm
Shortest of the three
canals
Medium of the three
canals
Longest of the three
Movement of
fluid
inside
canal
in
response to
Rotation
of
head
around
a
vertical
axis
Rotation
of
head
around a anteriorposterior axis
Rotations of head in
around a lateral axis
Detects
Rotation
in
transverse
plane
Rotation
in
the
coronal plane plane
Rotation
names
of
canals
the
body
canals
in
the
sagittal body plane
Click the hyperlink to view the different body planes
http://en.wikipedia.org/wiki/Transverse_plane#mediaview
er/File:BodyPlanes.jpg
Example
When you turn your
head to the left and
right
hand
sides
before crossing a
road or
when you spin in a
rotating chair
When you move your
head to touch your
shoulders, or when
doing a cartwheel.
when nodding your
head. or
when
you
bend
forward
to
pick
something up from
the floor
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Structure of Ear
Pictoral
depictionof
function
of
semicircular
canals
in
different
directions of
head
movement
Fig.
12.
Orientation
and
function
of
three
semicircular canals
Source: ILLL in house
Function
of
equilibrium.
semicircular
The
canals:Semicircular canalsfunction in dynamic
semicircular
canals
perceive
information
about
angular
accelerations in rotational movements and position of head in three perpendicular
axes.
(ii) Utricle and Saccule
The utricle and saccule are slightly more complex than ampullae in semicircular
canals in inner ear.
•
Within each membranous labyrinth, on the surface of the utricle, there is an
otolithic organ called macula.
•
Another macula placed in on the surface of the saccule in a semi-vertical
position.
•
Maculas constitute supporting cells and hair cells.
•
Hair
cells
in
maculaswork,
as
mechano-receptorscomprise
of
40-70
stereocilia and one true cilium known askinocilium. The kinocilium is only
sensory part of hair cell that causespolarization of hair cell.
•
The tip of stereocilia and kinocilium protruding outward from the hair cells are
enclosed by gelatinous otolithc membrane.
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Structure of Ear
•
There are minute stones termed as otoliths, embedded in gelatinous otolithic
membrane.
•
The otoliths are composed of calcium carbonate crystals that make the
gelatinous membrane heavier than the surrounding fluid. The otoliths also
called otoconiaor statoconia or ear dustrange from 3 to 19 µm in length in
humans and more dense than the endolymph.
•
The nerve fibers from the hair cells join those from the cristae in the
vestibulocochlear nerve.
Fig. 13. The membranous labyrinth of the vestibular system,
which contains the organs of balance
(Lower left) the cristae of the semicircular ducts and (lower right) the maculae of
the utricle and saccule.
Source: "macula". Art. Encyclopædia Britannica Online. Web. 04 Mar. 2015.
<http://www.britannica.com/EBchecked/media/68878/The-membranouslabyrinth-of-the-vestibular-system-which-contains-the>
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Structure of Ear
Fig 14. Macula of utricle and saccule and crista of
semicircular canal form vestibular system.
Source: http://www.med.umich.edu/histology/cns/ear.html#vestibule CC
Table 3: Features of utricle and saccule
Features
Utricle
Saccule
Meaning
Leather bag
Money bag
Function
It senses linear accelerations
in
the
and
head
tilts
horizontal plane.
It senses linear accelerations
and head tilts in the vertical
plane.
Size
comparison
The utricle is larger than the
saccule.
It is smaller than the utricle.
Shape
Oblong shaped, compressed
transversely.
Globular
Location
Located in upper and back
part of the vestibule.
Located near the opening of
scala vestibuli of cochlea.
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Structure of Ear
Position
macula
of
Hair cells in
macula
Macula of
horizontally
utricle.
utricle placed
on
floor
of
Macula of saccule placed in a
nearly vertical position.
Hair cells comprise of 40-70
stereocilia
and
one
kinocilium.
Hair cellscomposed of many
(at least 70) stereocilia and
single kinocilium.
Importance of otoliths:
Otoliths
composed
gelatiouns
otolithic
ofcalcium
carbonate-protein
membrane.
Eventually,
granules
gelatinous
adds
otolithic
weight
to
membrane
increasethe weight of the ends of hair cells and enhances their inertia. The
addition in weight and inertia is essential to the capability of utricle and saccule to
sense linear acceleration and to detect theorientation of head.
Function ofUtricle and Saccule:
The utricle and saccule are part of the equilibrium apparatuswithin the vestibule.
The utricle and saccule convey sensory massage related to speed of positioning
up and down, back and forth and alterations in head position in response to the
forces of gravity. This is important to detects linear accelerationsand maintain
the static equilibrium.
Structure of Neural Pathways
It consists of vestibular nerve and cochlear nerve pathways.
(I) Structure of auditory nerve pathways
Cochlear (auditory) nerve pathways represent spiral ganglia, cochlear nerve and
central auditory pathway.
a. Spiralganglion
The spiral (cochlear) ganglion is an assembly of nerve cells which functions in
the sense of hearing through sending a depiction of sound from the cochlea to the
brain.
•
Hair cells synapse with first order sensory neurons as well as with motor
neurons arise from the cochlear branch of vestibulocochlear (VIII)
nerve.
•
The cell bodies of the afferent (sensory) neurons that branched around
the bases of the hair cells are located in the spiral ganglion within the
modiolus. 90-95% of these afferent neurons innerwate the inner hair cells
whereas only 5-10% innervate the more and numarous outer hair cells.
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Structure of Ear
There are between 35,000 and 50,000 neurons which exist in the spiral
ganglion.
•
Most of efferent (motor) neurons in the auditory nerve ends at the outer
hair cells rather on the inner hair cells.
b. Cochlear (Auditory) Nerve
•
The fiber that connectsthe spiral ganglion with the central nervous system
constitutes the cochlear nerve.
•
The axons of the neurons that innervate hair cells(which form cochlear
(auditory) division of the vestibulocochlear acoustic nerve)terminate in the
dorsal and ventral cochlear and ventral cochlear nuclei of the medulla
oblongata.
•
The total number of afferent and efferent fibers in each auditory nerve is
around 28,000.
Fig. 15. Spiralganglion and Cochlear (Auditory) Nerve
Source: http://www.med.umich.edu/histology/cns/ear.html#spiralGanglionOrient
CC
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Structure of Ear
c. Central auditory pathway (Fig. 16)
•
After moving through cochlear nuclei, axons carrying auditory impulses
move
1. Throughvarious pathways to the inferior colliculi which are the
centers for auditory reflexes
2. Through the medial geniculate body in the thalamus to the
auditory cortex.
Fig. 16. Auditory pathway from cochlea to brain
Source: http://cnx.org/contents/c672e6c1-83aa-4c17-82f5da5be3643a5f@3#eip-id1170527734470 CC
•
The auditory cortex is partitioned into three divisions: Primary Auditory
Cortex, Secondary Auditory Cortex and Tertiary Auditory Cortex. These
structures are located concentrically to one another with the primary
auditory cortex placed in middle and tertiary auditory cortex placed on the
outside. The primary auditory cortex is located in Broadmann areas 41
and 42in the superior portion of the temporal lobe (Fig 17).
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Structure of Ear
Fig 16. Primary auditory cortex
Source: http://cnx.org/contents/c672e6c1-83aa-4c17-82f5da5be3643a5f@3#eip-id1170527734470 CC
Image Credit: CC BY-SA 4.0
•
The auditory association areas adjacent to the primary auditory receiving
area are widespread, extending onto the insula.
•
The olivocochlear bundle is a prominent bundle of efferent fibers in each
auditory nerve that arises from ipsilateral as well as contralateral superior
olivary complex and ends primarily around the bases of outer hair cells of
the organ of Corti.
(II) Structure of equilibrium (vestibular) nerve pathways
It represents vestibular ganglia and neural pathways for equilibrium.
a. Vestibular ganglia (Fig. 17)
The vestibular branch of the vestibulocochlear (VIII) nerve is composed of
ampullary,
utricular,
and
saccular
nerves.
These
nerves
contain
first-
ordersensory neurons as well as motor neurons that synapse with receptors
of equilibrium. The first order sensory neurons transmit sensory information
from the receptors, and the motor neurons transmit responsive signals to the
receptors, so to change their sensitivity. Cell bodies of the 19,000 sensory
neurons innervating the cristae and maculas on each side are situated in the
vestibular ganglia (also called Scarpa'sganglion).
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Structure of Ear
Fig. 17. Cross section showing cochlear and vestibular nerve
of vestibulocochlear (VIII) nerve
Source: http://www.med.umich.edu/histology/cns/ear.html#vestibCochlearNerve
Orient CC
b. Neural pathways for equilibrium (Fig. 17)
Each vestibular nerve terminates in the ipsilateral 4-part vestibular nucleus and
in the flocculonodular lobe of the cerebellum. Second-order neurons pass down
the spinal cord from the vestibular nuclei in vestibulospinal tracts and ascend
through the medial longitudinal fasciculi toward the motor nuclei of cranial nerves
concerned with the control of eye movement. Impulses from the vestibular
receptors are relayed through the thalamus to the cerebral cortex.
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Structure of Ear
Fig. 17. Neural pathways for equilibrium
Source: ILLL in house
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Structure of Ear
Summary
•
Each ear (left or right)has three main regions, external ear, middle ear,
internal ear.
•
External ear contains auricle, ear canal and tympanic membrane. Itreceives
sound waves and directs them inward.
•
Middle ear consists of auditory ossicles (the malleus, incus, and stapes),
skeletal muscles and eustachian tube. It conveys sound waves to oval
window.
•
Internal ear constitutes bony labyrinth and membranous labyrinth. It
contains the receptors for hearing and equilibrium.
•
Membranous labyrinth encloses a fluid known as endolymph. Perilymph
contained in bony labyrinth
•
Labyrinth is provided with two chief functional components i.e. the
cochleaand vesibular system. Cochlea is partitioned into three chambers
(Scalae): Scala vestibuli, Scala tympani and Scala media or Cochlear duct.
•
Helicotrema is a small opening through which the scala tympani and the
scala vestibuli join together at top of the cochlea. Cochlear duct is bordered
on three sides through basilar membrane, stria vascularis and Reissner's
membrane.
•
Cochlear duct contains organ of Corti which is positioned on the basilar
membrane. The organ of Corti is overlaid by a tectorial membrane.
•
The receptor cells for hearing in organ of Corti are known as hair cells. hair
cells areorganized in 3 rows of outer hair cells lateral to tunnel shaped by
the rods of corti, and one row of inner hair cells which is medially situated to
the tunnel.
•
Vestibular system contains saccule, utricle, and semicircular canals or ducts
the receptor organs to maintain the equilibrium. Receptor cells includes hair
cells and stereocilia are contained in the crista of semicircular canals to
detects rotational movement and angular accelaration. Similarly, macula in
the saccule and utricle contains hair cells to detects linear accelaration.
•
The cochlear branch of the vestibulocochlear (VIII) nerve is composed of
spiral
ganglion
and
cochlear
nerves.
Vestibular
branch
of
the
vestibulocochlear (VIII) nerve is composed of nerves from ampulla, utricule,
and saccule.
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Structure of Ear
Exercises
A. Multiple choice question:
1.
Chamber in the cochleawhich encloses the hearing receptors is
A. Vestibular apparatus
B. Scala vestibuli
C. Cochlear duct
D. Semicircular duct
Answers: C
2.
Organs for static equilibrium are placed in the ______________ and
utilize bending of ___________ so togenerate nerve impulse.
A. semicircular canals; gelatinous substance
B. cochlea; perilymph
C. vestibule; ampulla
D. vestibule; otoliths
Answers: D
3.
The auditory canal is part of the ____
A. Internal ear
B. Middle ear
C. External ear
D. Auricle
Answers: C
4.
The auditory ossicle that is attached to oval window of vestibule is
A. Malleus
B. Stapes
C. Incus
D. Stapedius
Answers: B
5.
Hair cells are the hearing receptors which are placed in
A. Cochlea
B. Semicircular canals
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Structure of Ear
C. Vestibule
D. Tympanic membrane
Answers: A
6.
A small aperture which allowscommunication between scala vestibuli to
scala tympani known as
A. Modiolus
B. Spiral lamina
C. Stria vascularis
D. Helicotrema
Answers: D
7.
Organ situated just medial to the oval window is
A. vestibule
B. semi-circular canals
C. cochlea
D. concha
Answers: A
8.
Number of turns in the cochlea is
A. 2
B.
2.5
C.
3
D.
3.5
Answers: B
9.
The last organ for hearing in the inner ear is
A. Crista
B. Helicotrema
C. Macula
D. Organ of Corti
Answers: D
10.
Part of internal ear detects linear acceleration is
A. semi-circular duct
B. utricle and saccule
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Structure of Ear
C. organ of Corti
D. crista
Answers: B
11.
Part of internal ear detects angular acceleration is
A. macula
B. organ of Corti
C. utricle
D. semicircular canals
Answers: D
12.
Organ of corti placed over
A. Tectorial membrane
B. Reticular lamina
C. Basillar membrane
D. Reissner membrane
Answers: C
13.
Perilymph in internal ear:
A. is extra-cellular fluid whichplaced inosseous labyrinth.
B. has high concentration of sodium ions.
C. has low concentration of potassium ions.
D. is directlyconnected with oval window.
E. ALL of the above are correct.
Answer: E
14.
Crista is the organ placed inside
A. utricle and saccule
B.auditory canal
C. cochlea
D. semi-circular canals
Answer: D
15.
Fluid filled in the membranous labyrinth is
A.Endolymph
B. Perilymph
C. Corticolymph
D.Blood
Answer: A
16.
Macula is component for equilibrium placed within
A.Organ of corti
B. Semi-circular canals
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Structure of Ear
C.Utricle and saccule
D. Ear drum
Answer: C
17. ___________ does not formboundary of scala media
A.Stria vascularis
B.Tectorial membrane
C. Basilar membrane
D. Reissner’s membrane
Answer: B
18. Endolymph is the fluid found in
A. Stria vascularis
B. Cochlear duct
C. Scala tympani
D. Scala vestibuli
Answer: B
19. A central and conical bony core around which cochlea coils is called as
A. Reissner’s membrane
B. Helicotrema
C. Spiral lamina
D. Modiolus
Answer: D
21.
Stereocilia of outer hair cells are embedded in
A. Reissner’s membrane
B. Reticular lamina
C.Tectorial membrane
D.Basilar membrane
Answer: C
22.
Identify the area in following section
marked with arrow
A. Reissner’s membrane
B.Tectorial membrane
C.Helicotrema
D. Basilar membrane
Answer: B
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Structure of Ear
B. Short answer type question:
1.Define following:
I.
II.
Otoliths
Tympanic membrane
III.
Modiolus
IV.
Reissner’s membrane
V.
tectorial membrane
VI.
Hair cells
VII.
Streocillia
2. Differentiate between followings:
I.
Endolymph and perilymph
II.
Among Three semicircular canals
III.
Uricle and saccule
IV.
Outer and inner hair of organ of corti
3. Write short notes on:
I.
Eustachian tube
II.
Organ of the corti
III.
Cochlear (auditory) nerve
IV.
Spiral ganglion
V.
VI.
Vestibulocochlear nerve
Hair cells
C. Long answer type questions:
1. Explain with the labelled diagram the structures of different parts of
middle ear.
2. Discuss various parts through the section of cochlea and their importance
in the hearing.
3. Discuss about the cochlear nerve pathways
4. What are the vestibular system and their vestibular nerve pathway.
Expalain with the labelled diagrams.
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Structure of Ear
5. Label the parts of the inner ear in the following figure with the help of
names provided in list.
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Structure of Ear
Glossary
•
Auditory ossicles: These are part of the middle ear and are the three
smallest bones namely, malleus, incus, and stapes and plays important
role in perceiving sound waves.
•
Auricle: It is a part of outer ear also known as pinnae and is flap like
visible structure of elastic cartilage projecting outward and covered with
skin.
•
Dynamic equilibrium: It is meant to the maintain body position
(primarily head) in relation to rotational movement and speed of the
movement.
•
ear canal: Part of external ear which leads inside to the eardrum and
middle ear. The ear canal is covered with ear wax and hairs which prevent
from infection and clean the ear.
•
Earwax or cerumen: It is a yellowish wax like substance released from
the sebaceous glands located in the ear canal of outer ear and it helps in
cleaning and lubricating the ear.
•
Organ of Corti: It is a receptive organ of hearing placed within the
cochlear duct. The organ of Corti has specialized cells called hair cells
which transduce sound vibrations into electrical impulses.
•
Otoliths: Stereocilia in saccule and utricle are covered through gelatinous
substances which enclose minute stones termed as otoliths and it is
responsible for maintaining static equilibrium.
•
Scala media: Also known as cochlear duct. This is an area between upper
(Scala vestibuli) and lower (Scala tympani) chambers of the cochlea and
contains hearing receptors in the organ of Corti.
•
Static equilibrium: It is meant to maintain the position of body
(primarily the head) in response to gravitational force.
•
Stereocilium:It is a finger-like projection on the apical surface of sensory
hair cells in the organ of Corti and vestibular apparatus of internal ear and
have an important role in hearing and equilibrium.
•
Tectorial membrane: Situated in the organ of Corti of the cochlea. This
is a sheet of cells lies above the stereocilia of the hair cells. Movement of
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the hair cells attached to basilar membrane leads the stereocilia to move
against the tectorial membrane, thereby initiate a nerve impulse which
pass from hair cell to brain.
•
Tympanic membrane: It is a fine, semi-translucent division between the
ear canal and middle ear.
•
Vestibular apparatus: It is group of the receptor organs to maintain the
equilibrium in response to head position and various body movements and
it includes saccule, utricle, and semicircular canals.
References
1. Tortora, G.J. & Grabowski, S. Principles of Anatomy & Physiology. 13th
Edition, p.642.
2. Moyes, C. D. and Schulte, P. M. (2006). Principles of Animal Physiology, p.
248.
3. Hill, R. W., Wyse, G. A. and Anderson, M. (2006). Animal Physiology.
p.355.
4. Randall, D., Burggren W. and French, Kathleen (2001). Eckert Animal
Physiology.
5. Widmaier, E.P., Raff, H. and Strang, K.T. (2008). Vander’s Human
Physiology, XI Edition, McGraw Hill.
6. Guyton, A.C. and Hall, J.E. (2011). Textbook of Medical Physiology, XII
Edition,
Harcourt Asia Pvt. Ltd./W.B. Saunders Company.
7. Ganong, William F. Review of Medical Physiology. XXI Edition. Mc Graw Hill
8. Textbook of Physiology by Prof. A.K. Jain.
9. Anatomy And Physiology: In health and illness. Ross and Wilson (Tenth
Edition)
Weblinks
•
http://www.med.umich.edu/histology/cns/ear.html#EAR4
•
http://apbrwww5.apsu.edu/thompsonj/AnatomyPhysiology/2010/2010Exa
mReviews/Exam204Review/CH2015SpiralOrganofCorti.htm
•
http://www.austincc.edu/apreview/PhysText/PNSafferentpt2.html
•
http://www.d.umn.edu/~floven/Courses/CSD3103/homework%202.htm
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