3. INTRODUCTION
The ear is the organ of hearing and equilibrium that detects and
analysis sound by the conversion of sound waves into electroch-
emical impulses.
The sensory receptors can have ability to transduce sound vibra-
tions into electrical signals 1000 times faster than photorecepto-
rs that can respond to light.
The auditory range extends from 20 to 20,000 hertz.
The sound intensity is measured in units called decibels(dB).
4. ANATOMY OF THE EAR
Ear is divided into three main regions
The External ear
consists of auricle, external auditory canal and ear drum.
The auricle or pinna is an elastic cartilage covered by skin.
The external auditory canal is curved tube about 2.5cm long
that lies in the temporal bone and leads to ear drum.
The ear drum is a thin, semitransparent partition between
external auditory canal and middle ear.
The tympanic membrane is covered by epidermis and lined
simple cuboidal epithelium. Between epithelial layers is
connective tissue composed of collagen elastic fibres and fibro
blasts.
5.
6. Middle ear
The middle ear is air filled cavity that is lined by epithelium
Middle ear contains three small ossicles malleus incus and stapes,
• that are attached by each other by ligaments and are connected
• by synovial joints
The handle of the malleus attaches to the internal surface of the
• tympanic membrane, the incus attaches with the head of the
• stapes and the stapes fittes into oval window
Besides the ligaments, the two tiny skeletal muscles also attaches
• to ossicles, the tensor tympani muscle attached to the malleus
• and the stapedius attache to the stapes.
The auditory tube are eustachian tube which connects the middle
• ear with the nasopharynx which consists of bone and cartilage.
7.
8. Internal ear
• The inner ear is also called the labyrinth because of its complicated
series of canals.
• Structurally consists of two main divisions, an outer bony labyrinth
and an inner membranous labyrinth.
• The inner membranous labyrinth is long tube like structure.
• The inner ear is divided into three parts, semicircular canals,
vestibule and cochlea.
• The vestibule is oval central portion of the labyrinth.
• The membranous labyrinth in the vestibule consists of two sacs
called utricle and the saccule connected by small duct.
• There are three semicircular canals, based on their position they
are called anterior, posterior and lateral. At one end of each canal is
swollen enlargement called ampula
9. Cont….
• Anterior of the vestibule is the cochlea a bony spiral canal that
resumbles a snail shells and makes almost three turn around a
central boney core called modiolus.
• If we draw the section of cochlea reavels that it is divided into three
compartments, scala vestibuli, scala media and scala tympani.
• The scala tympani are separated with the scala media by vestibular
membrane.
• The basilar membrane separates cochlear duct and scalar tympani.
• The organ of corti are present on the basilar membrane that contains
hair cells and supporting cells. Almost 16,000 hair cells are present in
the organ of corti.
• The vestibulocochlear(viii) nerve are attached with the inner ear by
two branches, vestibular branch and cochlear branch.
10.
11. Physiology of hearing
• When the sound waves travels through the auditory canal hits the
tympanic membrane the tympanic membrane vibrates and send
them towards the middle ear where the ossicles amplify sound
towards oval window.
• The oval window bulges inwards and round window moves outwards
due to fluid pressure wave.
• The pressure wave travels through scala vestibuli, then scala tympani
and then move in the cochlear duct.
• The pressure wave in the endolymph causes the basilor membrane to
vibrate hair cells move against tectorial membrane to generate nerve
impulses that finally reachres in the brain where it is interpreted.
12. Cont….
• High pitched sound is absorbed at the base of cochlea because
of stiffness. Low pitched sound absorbed at the apex region of
the cochlea
• Louder sound also may stimulate large number of hair cells.
Therefore leads to higher action potential.
• The hair cells transduce the mechanical vibrations into
electrical signals.
• As the basilar membrane vibrates the hair cells bend back and
forth and slide one another.
• The tiplink protien connects the tip of stereocilium to ion gated
channels called trandiction channels.
13. Cont….
• As the stereocilia band in the direction of kinocilium the tip link
tug on the transduction channels and open them.
• The channels allow the potassium and calcium ions enter in the
hair cells where the depolarization occurs.
• The calcium ion helps the synaptic vessicle to fuse with plasma
membrane leads to exocytosis of neurotransmitters (gultamate)
that stimulates afferent neurons in which action potential is
generated and ultimately reaches in the brain where the
impulse is interpreted.
14.
15.
16. Physiology of equilibrium
Utricle and Saccule
• These two structures are located in the vestibular portion of inner ear in
the membraneous labyrinth.
• Basically outer bony labyrinth contain perilymph whereas inner
membraneous labyrinth contains endolymph.
• The utricle and saccule contains special detectors called macula.
• The macula are the small thickned region.
• Macula are two in number one is present in utricle and one is present in
saccule.
• The macula of utricle is present on the floor whereas macule of saccule on
the wall.
• Maculae contains receptors for static equilibrium.
• The maculae detects linear acceleration but in the horizontal direction.
• The macula of saccule detects the linear acceleration but in vertical
direction.
17. Cont...
• The macula of both utricle as well as saccule consists of two kinds
of cells.
• The hair cells as well as supporting cells. And also contains
vestibular dark cells.
• A gelatinous, glycoprotein layer called otolithic membrane that rest
some hair cells.
• The otolithic membrane sits on the top of the macula.
• The otolithic membrane pushes the stereocilia towards the
kinocilium due to which ion gated channels open and the Potassium
and Calcium cations enter into the hair cells.
• The calcium ion attaches with the neurotransmitter vesicles and
helps to fuse the vesicle with the plasma membrane of hair cells.
• Which may leads to release of neurotransmitters that sends action
potential in the vestibular branch of 8th nerve that reaches in the
brain.
18.
19.
20. Semicircular canals
• They are responsible for dynamic equlibrium. They detects
rotational acceleration.
• The semicircular ducts contains special detectors that is called
crista ampularis present in the ampula.
• Crista contains a group of hair cells and supporting cells.
• The cupula are present on the hair cells.
• As the head rotates in one direction, the cupula is dragged
through endolymph and bend in opposite direction.
• Bending of hair bundles produces receptor potentials. In turn
the receptor potentials leads to nerve impluses that pass
along the vestibular branch of the vestibulocochlear(viii)
nerve.
21. Cont....
• The action potentials or impulses stimulates vestibular
nuclei that sends commands to nuclei of cranial verves
that controls coupled movement of eyes with those of
head.
• Nuclei of accessory nerves controls head and neck
movements and maintaining equilibrium.
• Vestibulospinal tract conveys impulses down the spinal
cord to maintain muscle tone in skeletal muscles to
help maintain equilibrium.
• Ventral posterior nucleous provide us with conscious
awareness of position and movement of head and
limbs.
22.
23.
24. CONCLUSION
• The ear is the organ of hearing and equilibrium.
• The ear converts the mechanical energy into
electrical energy.
• The ear uses resonance, impedance and analysis.
• By understanding the physiology of hearing and
balance, we came to know that every organ in
our body is performing complex functions.
