· It is supplied by the VIIIth cranial nerve which is stimulated by the vibration of sound waves.
· Except pinna, other structures of the ear lie within the petrous portion of the Temporal bone.
· Except pinna, other structures of the ear lie within the petrous portion of the Temporal bone.
Structure of the Human Ear
The ear is divided into 3 distinct parts:a. The External ear- Pinna and Auditory canal
b. The Middle ear- Tympanic cavity and Ear ossicles
c. The Inner ear- Cochlea and Semicircular canals
A. Pinna (Auricle)
· It is the expanded portion that projects from the side of the head.
· It is composed of fibro-elastic cartilage covered with skin.
· It is deeply grooved and ridged, the most prominent is a helix.
· The ear lobe (lobule) is the pliable part at the lower end, composed of fibrous and adipose tissue richly supplied with blood.
B. Auditory canal
· This is a slightly ‘S’-shaped tube, about 2.5 cm long and extends from pinna to tympanic membrane.
· It has numerous ceruminous glands and hair follicles.
· Ceruminous glands secrete cerumen (ear wax).
· Dust, microbes and insects are prevented from reaching the tympanic membrane (a membrane that completely separates the external auditory canal from the middle ear) by wax, hairs and curvature of the auditory canal.
· Oval-shaped tympanic membrane consists of the outer covering of hairless skin, a middle layer of fibrous tissue and the inner lining of mucous membrane continuous with that of the middle ear.
C. Tympanic Cavity
· This is an irregular air-filled cavity. Its medial wall consists of two openings: oval and a round window.
· Oval window is the occluded by part of stapes and round window by a fine sheet of fibrous tissue.
· Air reaches the cavity through the eustachian tube, which extends from the nasopharynx.
· It is about 4 cm long and is lined with ciliated columnar epithelium.
· It maintains the pressure on both sides of the tympanic membrane and enables the membrane to vibrate when sound waves strike it.
D. Ear Ossicles
· These are three very small bones that extend from the tympanic membrane to the oval window.
· There are Three ear ossicles. They are:
a. The Malleus:
b. The Incus:
c. The Stapes:
E. Bony labyrinth (Labyrinth- maze)
· It is the cavity within the temporal bone that encircles the membranous labyrinth of the same shape that fits into it.
· The layer of watery fluid between the bony and membranous labyrinth is called perilymph and within the membranous labyrinth is endolymph.
· Bony labyrinth consists of vestibule, cochlea and semicircular canals.
a. Vestibule: It contains oval and round windows.
b. Cochlea: It resembles a snail’s shell. It has a broad base that is continuous with the vestibule and spirally coiled narrow apex.
c. Semicircular canals: These are three tubes arranged so that one is situated in each of the three planes of space. They are continuous with the vestibule.
G. Cochlea
· A cross-section of cochlea consists of three parts– the scala vestibuli, the scala media and the scala tympani.
· In cross-section, the bony cochlea consists of two compartments containing perilymph: the scala vestibule that originates at the oval window and the scala tympani, which ends at the round window.
· These two compartments are continuous with each other.
· The scala media is triangular in shape.
· On the basilar membrane or base of the triangle, there are supporting cells and hair cells containing auditory receptors. These cells form the spiral organ (of Corti); the sensory organ that responds to vibrations by initiating the nerve impulses which are perceived as hearing by the brain.
· Auditory receptors combine to form the cochlear part of the vestibulocochlear nerve that passes through a foramen in the temporal bone to reach the hearing area in the temporal lobe of the cerebrum.
· Tympanic membrane vibrations are transmitted and amplified through the middle ear by ear ossicles.
· The footplate of stapes rocks to and fro in the oval window, setting up fluid waves in the perilymph of scala vestibule.
· Most of the force of waves is transmitted into the scala media (cochlear duct).
· This creates a corresponding wave motion in the endolymph, resulting in vibration of basilar membrane and stimulation of auditory receptors in the hair cells of spiral organ.
· The nerve impulses generated pass in the cochlear portion of the vestibulocochlear nerve.
· The fluid wave is finally expanded into the middle ear by the vibration of the membrane of the round window.
· Prolonged very loud noise causes hearing loss, as it damages the sensitive hair cells of spiral organs.
· The semicircular canals provide information about the position of the head in the space, contributing to the maintenance of posture and balance.
· Semicircular canals are situated above, beside and behind the vestibule of the inner ear and open into it.
· The semicircular canals have outer bony walls (perilymph) and inner membranous tubes (endolymph).
· Three semicircular canals at their dilated ends open into the utricle.
Physiology of Balance
· Any change in the position of the head causes movement of the perilymph that bends hair cells and stimulates sensory receptors in utricle, saccule and ampullae.
· The resultant nerve impulses are transmitted by the vestibular nerve, which joins with the cochlear nerve to form the vestibulocochlear nerve.
· This is a slightly ‘S’-shaped tube, about 2.5 cm long and extends from pinna to tympanic membrane.
· It has numerous ceruminous glands and hair follicles.
· Ceruminous glands secrete cerumen (ear wax).
· Dust, microbes and insects are prevented from reaching the tympanic membrane (a membrane that completely separates the external auditory canal from the middle ear) by wax, hairs and curvature of the auditory canal.
· Oval-shaped tympanic membrane consists of the outer covering of hairless skin, a middle layer of fibrous tissue and the inner lining of mucous membrane continuous with that of the middle ear.
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Human Ear and its parts |
C. Tympanic Cavity
· This is an irregular air-filled cavity. Its medial wall consists of two openings: oval and a round window.
· Oval window is the occluded by part of stapes and round window by a fine sheet of fibrous tissue.
· Air reaches the cavity through the eustachian tube, which extends from the nasopharynx.
· It is about 4 cm long and is lined with ciliated columnar epithelium.
· It maintains the pressure on both sides of the tympanic membrane and enables the membrane to vibrate when sound waves strike it.
D. Ear Ossicles
· These are three very small bones that extend from the tympanic membrane to the oval window.
· They form a series of movable joints with each other at the oval window and with the medial wall of the cavity.
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| Three ear ossicles |
· There are Three ear ossicles. They are:
a. The Malleus:
· It is a lateral hammer-shaped bone.
· Its handle is in contact with the tympanic membrane and the head forms a movable joint with the incus.
b. The Incus:
· It is a middle and anvil-shaped bone.
· Its body articulates with the malleus; the long process with the stapes and is stabilised by the short process, fixed by fibrous tissue to the posterior wall of the tympanic cavity.
c. The Stapes:
· It is a stirrup-shaped bone.
· Its head articulates with the incus and the footplate fits into the oval window.
· The ear ossicles are held in position by ligaments.
Inner Ear
· It consists of a bony and membranous labyrinth.
· It consists of a bony and membranous labyrinth.
E. Bony labyrinth (Labyrinth- maze)
· It is the cavity within the temporal bone that encircles the membranous labyrinth of the same shape that fits into it.
· The layer of watery fluid between the bony and membranous labyrinth is called perilymph and within the membranous labyrinth is endolymph.
· Bony labyrinth consists of vestibule, cochlea and semicircular canals.
a. Vestibule: It contains oval and round windows.
b. Cochlea: It resembles a snail’s shell. It has a broad base that is continuous with the vestibule and spirally coiled narrow apex.
c. Semicircular canals: These are three tubes arranged so that one is situated in each of the three planes of space. They are continuous with the vestibule.
F. Membranous labyrinth
· It also consists of the vestibule (utricle and saccule), the cochlea and semicircular canals.
· The fluid present in the utriculus and sacculus consists of small calcareous particles, called otoliths.
· It also consists of the vestibule (utricle and saccule), the cochlea and semicircular canals.
· The fluid present in the utriculus and sacculus consists of small calcareous particles, called otoliths.
· Their function is balancing.
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T.S. Cochlea |
G. Cochlea
· A cross-section of cochlea consists of three parts– the scala vestibuli, the scala media and the scala tympani.
· In cross-section, the bony cochlea consists of two compartments containing perilymph: the scala vestibule that originates at the oval window and the scala tympani, which ends at the round window.
· These two compartments are continuous with each other.
· The scala media is triangular in shape.
· On the basilar membrane or base of the triangle, there are supporting cells and hair cells containing auditory receptors. These cells form the spiral organ (of Corti); the sensory organ that responds to vibrations by initiating the nerve impulses which are perceived as hearing by the brain.
· Auditory receptors combine to form the cochlear part of the vestibulocochlear nerve that passes through a foramen in the temporal bone to reach the hearing area in the temporal lobe of the cerebrum.
Physiology of Hearing
· Ear pinna (auricle) collects and concentrates the sound waves and then directs them to the auditory canal causing the tympanic membrane to vibrate.· Tympanic membrane vibrations are transmitted and amplified through the middle ear by ear ossicles.
· The footplate of stapes rocks to and fro in the oval window, setting up fluid waves in the perilymph of scala vestibule.
· Most of the force of waves is transmitted into the scala media (cochlear duct).
· This creates a corresponding wave motion in the endolymph, resulting in vibration of basilar membrane and stimulation of auditory receptors in the hair cells of spiral organ.
· The nerve impulses generated pass in the cochlear portion of the vestibulocochlear nerve.
· The fluid wave is finally expanded into the middle ear by the vibration of the membrane of the round window.
· Prolonged very loud noise causes hearing loss, as it damages the sensitive hair cells of spiral organs.
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Physiology of hearing |
Balance
· Balancing is by semicircular canals and vestibule· The semicircular canals provide information about the position of the head in the space, contributing to the maintenance of posture and balance.
· Semicircular canals are situated above, beside and behind the vestibule of the inner ear and open into it.
· The semicircular canals have outer bony walls (perilymph) and inner membranous tubes (endolymph).
· Three semicircular canals at their dilated ends open into the utricle.
· In the walls of the utricle, saccule and ampullae, there are fine specialized epithelial cells with the minute projections called hair cells.
· Amongst the hair cells, there are receptors on sensory nerve endings, which combine and form the vestibular part of the vestibulocochlear nerve.
· Amongst the hair cells, there are receptors on sensory nerve endings, which combine and form the vestibular part of the vestibulocochlear nerve.
Physiology of Balance
· Any change in the position of the head causes movement of the perilymph that bends hair cells and stimulates sensory receptors in utricle, saccule and ampullae.
· The resultant nerve impulses are transmitted by the vestibular nerve, which joins with the cochlear nerve to form the vestibulocochlear nerve.
· The vestibular branch passes to the cerebellum.
· Nerve impulses from the eyes, skeletal muscles and joints are transmitted to the cerebellum and pass to the cerebrum and to the skeletal muscles.
· Nerve impulses from the eyes, skeletal muscles and joints are transmitted to the cerebellum and pass to the cerebrum and to the skeletal muscles.
· This results in awareness of body position, maintenance of upright posture and fixing of eyes on the same point, independently of head movements.