Hearing and the Ear
Ear is organ of hearing and is also involved in balance of body.
It is supplied by VIII cranial nerve i.e cochlear part of vestibulocochlear nerve, which is stimulated by vibration caused by sound wave.
Ear consist of three parts -
1. External Ear
Auricle or pinna
Auditory canal
Tympanic membrane (Eardrum)
2. Middle ear
Tympanic cavity
Auditory ossicles; malleus, incus, stapes
3. Inner ear
Bony labyrinth
Membrane labyrinth
Outer ear collects sound waves and directs them to middle ear, which in turn transfer them to inner ear, where they are converted to nerve impulses and transmitted to hearing area in cerebral cortex.
1. External Ear (Outer ear)
1. Auricle (pinna)
The auricle is visible part of ear that projects from side of head.
It is composed of fibroelastic cartilage covered with skin.
It is deeply grooved and ridged; the most prominent outer ridge is Helix.
Ear lobe (lobule)
Ear lobe is soft pliable and without cartilage, part of composed of fibrous tissue and adipose tissue richly supplied with blood covered by skin.
2. External acoustic meatus (auditory canal)
This is slightly S shaped tube about 2.5cm long extending from auricle to tympanic membrane (eardrum).
The canal is about 24 nm long, of which 16nm is bony in temporal bone and lateral 8 nm is cartilaginous
Canal and tympanic membrane are covered within skin.
There are numerous ceruminous gland and hair follicles with associated sebaceous glands.
Ceruminous gland secretes earwax is sticky material that contains lysozyme and immunoglobulins.
Forign material such as dust, insect, microbes are prevented from reaching tympanic membrane by wax, hair and curvature of meatus.
Movement of temporomandibular joint during chewing and speaking massages cartilaginous meatus moving wax towards the exterior.
3. Middle ear (tympanic cavity)
It is small chamber between tympanic membrane and inner ear.
This is irregular shaped are filled cavity within petrous portion of temporal bone.
The cavity, it's contents and air sac which open out it are lined with either simple squamous or cuboidal epithelium.
1. Lateral wall of middle ear is formed by tympanic membrane.
Roof and floor are formed by temporal bone.
2. Median wall
Median wall is thin layer of temporal bone in which there are two opening.
1. Oval window (opening by part of small bone called stapes).
2. Round window (opening by fine sheet of fibrous tissue).
3. Anterior wall
Auditory tube or eustachian tube.
Anterior wall of tympanic cavity is opening of auditory tube/ eustachian tube.
Air reaches cavity, through auditory or eustachian tube, which exten from nasopharynx.
It is about 4cm long and is lined with cilia columnar epithelium.
The presence of air at atmospheric pressure on both side of tympanic membrane is maintained by auditory tube and enables membrane to vibrate when sound waves strike it
The auditory tube is normally closed but when there is unequal pressure across tympanic membranes.
For example, at high attitude, it is opened by swallowing or yawning and ears pop equilizing pressure again.
4. Posterior wall
Posterior wall of cavity has irregular aperture called aditus. This opening in posterior wall leads into mastoid antrum.
2. Auditory ossicles
Auditory ossicles are 3 very small bones that extend across middle ear from tympanic membrane to oval window.
They form a series of movable joints with each other and with medial wall of cavity at oval window.
1. Malleus
This is lateral hammer shaped bone. The handle is in contact with tympanic membrane and head forms a movable joint within incus.
It is about 8 to 9 nm long.
2. Incus
Incus is middle anvil shaped bone. It's body articulates with malleus, the long process with stapes, and it is stablished by short process, fixed by fibrous tissue to posterior wall of tympanic cavity.
3. Stapes
This is medial stirrups shaped bone.
It is shortest bone of body.
It's head articulates with incus and it's footplate fits into oval window.
Three ossicles are held in position by fine ligaments.
3. Inner ear
Inner ear or labyrinth contain organ of hearing and balance.
It consist of two structural parts;
1. Bony labyrinth; cavity within temporal bone lined with periosteum.
2. Membranous labyrinth ; lies with in bony labyrinth
1. Bony labyrinth
This is cavity within temporal bone lined with periosteum.
Bony labyrinth consist of -
Vestibule
Cochlea
semicircular canals;
1. Anterior semicircular canal
2. Posterior semicircular canal
3. Lateral semicircular canal
1. Vestibule
Vestibule is expanded part nearest part of middle ear.
Oval and round window are located in it's lateral wall.
It contain two membranous sac; Utricle , Saccule which are important in balance.
2. Cochlea
Anterior to vestibule is spiral cochlea.
This resembles a snails shell.
It has broad base where it is continuous with vestibule and it spirals round a central bony column.
3. Semicircular canals
There are 3 tube arranged semicircular duct, inner connected tube located in inner ear.
Size;
There are all about 0.8mm in diameter have terminal swelling called ampulla and open into vestibule by 5 openings. One common opening between anterior and posterior semicircular canals.
They are continuous with vestibule.
2. Membranous labyrinth
Membranous labyrinth lies within body labyrinth.
It is filled with endolymphatic fluid.
Membranous labyrinth is separated from bony labyrinth by perilymphatic fluid.
Membranous labyrinth comprises;
1. Vestibule
Vestibule which contains
Utricle (3 semicircular duct open into it at their ampulae)
Sacule ( communicate with Utricle and cochlea)
2. Cochlea; cochlear duct
3. 3 semicircular canals; semicircular duct
1. Cochlea duct
Cochlea duct is an endolymph filled cavity inside cochlea, located between tympanic duct and vestibular duct separated by basilar membrane and vestibular membrane respectively.
Cochlear duct houses organ of corti.
Roof of cochlear duct is vestibular membrane.
Floor of cochlear duct is basilar membrane, which support organ of corti.
1. Vestibular membrane
Cochlear duct is separated from vestibular duct by vestibular membrane.
2. Basilar membrane
Cochlear duct is separated from tympanic duct by basilar membrane.
Resting on basilar membrane is organ of corti or organ of hearing.
3. Organ of corti
Spiral organ is sense organ of hearing located on basilar membrane.
Organ of hearing include;
Hair cells; receptor of hearing and transduce sound energy into electrical energy.
Hair cells are in between tectorial and basilar membrane.
Supporting cell; provide support hair cell.
Tectorial membrane; it overlies organ of corti
Sharing force between hair cells and tectorial membrane produces stimulus of hair cells.
Organ of Corti
Organ of corti is sensory organ that respond to vibration by initiating nerve impulse that are then perceived as hearing witin brain.
Auditory receptors, efferent (sensory nerves) that combine forming auditory part of vestibulo cochlear nerve (VIII cranial nerve) which passes through foramen in temporal bone to reach hearing nerve area in temporal lobe of cerebrum.
3. Semicircular duct
Semicircular duct of membranous labyrinth located within semicircular canals of bony labyrinth.
Perilymph is located in between duct and bony walls of canals.
Semicircular ducts communicate Utricle anteriorly.
Auditory apparatus
All of components of organ of hearing outer and middle and inner ear.
Auditory system for sense of hearing.
Physiology of Hearing
Sound
Sound is created by vibration that may occur in air, fluid or solid material. It's speed in dry air is only about 331m/s.
The frequency range of human hearing from 20 to 20000 waves per second or Hertz (HZ).
Pitch of sound determined by frequency of sound waves .
Higher frequency - Higher pitch
Amplitude is sound wave determines intensity of sound hence loudness intensity of sound is measured is in decibles (dB).
Mechanism of Hearing
Sound wave enter the outer ear and travel through external auditory canal they reach tympanic membrane causing membrane and attached chain of auditory ossicles to vibrate.
Sound waves
Auricle
Auditory canal
Causing tympanic membrane to vibrate
Vibration of ossicles
Oval window
Vibration of endolymph
Vibration into basilar membrane
Stimulate of auditory reception (hair cell)
Organ of corti
Nerve impulse generated pass to brain in auditory portion of vestibulo cochlea nerve (VIII cranial nerve).
Fluid wave finally expended into middle ear by vibration of membrane of round window.
Conduction of nerve impulse to primary Auditory area of cerebral cortex.
Perception of sound.
1.Every sound produce sound wave or vibration in air, to vibration of air molecules enters external ear.
After wave reach tympanic membrane (eardrum) and vibrate.
2. Low frequency sound wave produce slow vibration. High frequency produce rapid vibrations.
These vibration moves malleus.
3. Handle of malleus articulate with incus, causing it to vibrate.
4. Vibrating incus move stapes into and out at oval window.
5. Sound waves reach inner ear through oval window, set up pressure changes that vibrate perilymph in vestibular duct.
Resulting in Vibration basilar membrane and stimulation of auditory receptors in hair cells in organ of corti.
6. There hair cells act as transducers and convert mechanical engery into electrical impulse.
7. Vibration of perilymph are transmitted across vestibular membrane to endolymph of cochlear duct and also up vestibular duct and down typanic duct.
Basilar membrane vibrate, hair cell attached to membrane move to tectorial membrane.
8. Causing stimulation of hair cell in organ of corti . There potential excite cochlear nerve to generate action potential.
9.Nerve impulse generated pass to brain in cochlear portion of vestibulo cochlear nerve.
10. The nerve transmits impulses to primary Auditory area in temporal lobe of cerebral cortex where sound is perceived.
Physiology of Balance
Semicircular canals and vestibule (Utricle and Saccule) are concerned with balance.
Any changes of head cause move movement in perilymph and endolymph, which bends hair cells and stimulate Sensory receptors in Utricle, Saccule and ampulae.
Nerve impulses are transmitted by vestibular nerve which joints cochlear nerve to form vestibulo cochlear nerve.
Vestibular branch passes first to vestibular nucleus, then to cerebellum, cerebral cortex.
Utricle and Saccule
Responsible for static equilibrium.
Semicircular Duct
3 semicircular duct along with Saccule and Utricle maintained kinetic/ dynamic equilibrium.
Equilibrioception or sense of balance is physiological sense. It allow human to walk without falling.
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