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Tuesday, 21 August 2012

Human Ear : Auditory Organ of Hearing of Human Body

Human Ear Page of my Bio-World Software

The ear is the organ of hearing.It is supplied by 8th cranial nerve, i.e. the cochlear part of the vestibulo cochlear nerve, which is stimulated by vibrations caused by sound waves.

With the exception of the auricle(pinna), the structures that form the ear are encased within the petrous portion of the temporal bone.

The ear is divided into 3 dictinct parts
1) Outer ear
2) Middle ear (Tympanic cavity)
3) Inner ear.

3D Picture of Different Parts of Human Ear created by me (Manash Kundu)

Outer ear:
The outer ear consists of the auricle(pinna) and the external acoustic meatus(auditory canal).

The auricle(pinna):

The auricle is expanded portion that projects from the side of the head.It is composed of fibroelastic cartilage covered with skin.It is deeply grooved and ridged; the most prominent outer ridge is the helix.
The lobule(earlobe) is the soft pliable part at the lower extremity, composed of fibrous and adipose tissue richly supplied with blood.

External acoustic meatus(Auditory Canal):

This is a slightly S-shaped tube about 2.5 cm long extending from the auricle to the tympanic membrane(eardrum).The lateral third is cartilaginous and the remainder is a canal in the temporal bone.The meatus is lined with skin continuous with that with the auricle.There are numerous ceruminous glands and hair follicles, with associated sebaceous glands. In the skin of lateral third.Ceruminous glands are modified sweat glands that secrete cerumen(Earwax), a sticky material containing lysozyme and immunoglobins.Foreign materials e.g. dust, insects and microbes, are prevented from reaching the tympanic membrane bu wax, hair and the curvature of meatus.Movement of temporomandibular joint during chewing and speaking `massage` the cartilaginous meatus , moving the wax towards the exterior.

3D Picture of Three Parts of The Human Ear created by me (Manash Kundu)

Middle ear(Tympanic Cavity):

This is an irregular-shaped air-filled cavity within the petous portion of the temporal bone.The cavity, its contents and the air sacs which open out of it are lined with either simple squamous or cuboidal epithelium.The lateral wall of the middle ear is formed by the tympanic membrane.The roof and floor are formed by the temporal bone.The posterior wall is formed by the temporal bone with openings leading to the mastoid antrum through which air passes to the air cells within the mastoid process.The median wall is a thin layer of temporal bone in which there are 2 openings:

  •      Oval window
  •      Round window 
The oval window is occluded by part of a small bone called the stapes and the round window, by a fine sheet of fibrous tissue.Air reaches the cavity through the pharyngotympanic (Auditory or Eustachian) tube, which extends from the nasopharynx.It is about 4 cm long and is lined with ciliated columner epithelium.The presence of air at atmospheric pressure on both sides of the tympanic membrane is maintained by the pharyngotympanic tube and enables the membrane to vibrate when sound waves strike it.The pharyngotympanic tube is normally closed but when there is unequal pressure across the tympanic membrane, e.g. at high altitude,it is opened by swallowing or yawning and the ears `pop`,equalising the pressure again.

Auditory Ossicles:

These are 3 very small bones that extend across the middle ear from the tympanic membrane to the oval window.They form a series of movable joints with each other and with the medial wall of the cavity at the oval window.They are named
according to their shapes.

The Malleus:

This is the lateral hammer-shaped bone. The handle is in contact with the tympanic membrane and the head forms a movable joint with the Incus.

The incus:

This is the middle anvil-shaped bone. Its body articulates with the malleus, the long process with stapes, and it is stabilised by the short process, fixed by fibrous tissue to the posterior wall of the tympanic cavity.

The stapes:

This is medial stirrup-shaped bone.Its head articulates with the incus and its footplate fits into the oval window. The 3 ossicles are held in position by fine ligaments.

3D Picture Different Parts of The Inner Ear created by me (Manash Kundu)

Inner ear:
The inner ear or labyrinth contains the organs of hearing and balance.It is described in 2 parts, the bony labyrinth and the membranous labyrinth.

Inner Ear

Bony Labyrinth:

This is a cavity within the temporal bone lined with periosteum. It is larger than,and encloses, the membranous labyrinth of the same shape that fits into it, like a tube within a tube.Between the bony and membranous labyrinth there is a layer of watery fluid called perilymph and within the membranous labyrinth there is a similarly watery fluid, endolymph.

The bony labyrinth consists of :

  • The vestibule.
  • The cochlea.
  • Three semicircular canals.

This is the expanded part nearest the middle ear. It contains the oval and round windows in its lateral wall.

Membranous labyrinth:

This contains endolymph and lies within its bony counterpart. It comprises

  • The vestibule Which contain utricle and saccule
  • The cochlea.
  • Three semicircular canals.
A cross section of the cochlea contains three compartments:

  • The scala vestibuli.
  • The scala media, or cochlear duct
  • The scala tympani. 
In the cross section the bony cochlea has 2 compartments containing perilymph;The scala vestibuli, which originates at the oval window,The compartments are continuous with each other the cochlear duct is the part of the membranous labyrinth and its triangular in shape. On the basilar membrane or the base of the triangle there are supporting cells and specialised hair cells containing auditory receptors. These cells form the spiral organs(of corti).Then sensory organ that responds to vibration by initiating nerve impulses that are then perceived as hearing by the brain. The auditory receptors are  dendrites of the efferent nerves that combine forming  the cochlear part of the vestibocochlear nerve (8Th cranial nerve), which passes through a foramen in the temporal lobe of the cerebrum.


These nerves are composed of two distinct sets of fibres,vestibular nerves and cochlear nerves.
The Vestibular nerves arise from the semicircular canals of the inner ear and convey impulses to the cerebellum. they are associated  with the maintenance of posture and balance.
The Cochlear nerves originate in the spiral organ in the inner ear and convey impulses to the hearing area in the cerebral cortex where sound is perceived.

Detail Picture of Different Parts of The Ear

Semicircular canals:

These are 3 tubes arranged so that one is situated in each of the 3 planes of space.They are continuous with the vestibule

3D Picture of Three Parts of The Human Ear created by me (Manash Kundu)

Every sound produces sound waves or vibration in the air, which travel at about 332 metres per sec.The auricle, because of its shape, collects and concentrates the waves and directs  them along the external acoustic meatus causing the Tympanic membrane to vibrate.Tympanic membrane vibrations are transmitted and amplified through the middle ear by movement of the ossicles.At their medial end the footplate of the shape rocks to and fro in the oval window, setting up fluid waves in the perilymph of the scala vestibuli.Some of the force of these waves is transmitted along the length of the scala-vestibuli and scala tympany, but most of the pressure is transmitted into the cochlear duct.This causes a corresponding wave motion in the endolymph,resulting in vibration of the basilar membrane and stimulation of the auditory receptors in the hair cells of the spiral organ.

Physiology or Process of Hearing

The nerve impulse generated pass to the brain in the cochlear of the vestibo cochlear nerve(8th cranial nerve).The fluid wave is finally expended into the middle ear by vibration of the membrane of the round window. The 8th cranial nerve transmit the impulses to the auditory nuclei in the medulla ,where they synapse before they are conducted to the auditory area in the temporal lobe of the cerebrum.Because some fibres cross over in the medulla and others remain on the same side, the left and right auditory areas of the cerebrum receives impulses from both ears.
Sound waves have the properties of pitch and volume, or intensity.Pitch is determined by the frequency of the sound waves and is measured in Hertz(Hz).Sounds of different frequencies stimulate the basilar membrane at different places along its length. allowing discrimination of pitch.

Human Ear Post

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