Human Ear

Last Updated on Monday, 13 December 2010 09:32 Written by Sandesh Friday, 10 December 2010 04:05

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Ear is the organ of hearing and balance. Ear can be divided into three regions:

1. External ear
2. Middle ear
3. Internal ear

External ear: It involves- Pinna, the external auditory meatus and tympanic membrane.

1. Pinna: it is outermost visible part of the ear on each side of head. The upper part of pinna is called as helix and lower part as lobule. Helix is made up of cartilage covered with skin. Human pinna is immovable because the ear muscles are vestigial.
2. External auditory meatus (canal): It is an ‘S’ shaped tubular structure extending from pinna to inside of ear. Outer part is cartilaginous while inner part is bony. It contains hairs, sebaceous gland and ceruminous glands. These glands secrete sebum and wax respectively which prevents the entry of dust and bacteria.
3. Tympanic membrane (ear drum): It is a thin membranous structure which separates external ear from middle ear. It is oval shaped membrane which composed of outer stratified squamous epithelium, middle fibro elastic layer and inner cuboidal epithelium.

Middle ear: It is a chamber filled with ear. This chamber is connected with nasopharynx by Eustachean tube which helps in maintaining air pressure in the middle ear. This chamber contains three small bones called as ear ossicles viz.malleus, incus and stapes. They are arranged in chain extending from tympanic membrane to the fenestra ovalis of inner ear. These ear ossicles not only transmit the sound wave, but also amplify it upto 22 times.

Internal ear: Internal ear consists of vestibule, semi-circular canals and cochlea.

1. Vestibule: It is a sac like structure. It is divided into two chambers upper utriculus and ventral sacculus. These chambers are connected by a small sacculo utricular duct. These chambers contain group of sensory cells called as maculae. They are provided with non-motile hair tufts embedded in gelatinous membrane which contains numerous sand particles made up of calcium carbonate called as otoliths.
2. Semi-circular canals: Three semi-circular canals are arising from utriculus. These are one anterior, one posterior and one horizontal canal. Each canal has a swollen terminal ends called as ampulla. Ampulla contains group of sensory cells called as cristae. Cristae consist of sensory cells and supporting cells. Sensory cells are provided with sensory hairs which are embedded in gelatinous substance called as cupula. Basal end of sensory hairs are connected with nerve fibres.
3. Cochlea: cochlea is a spiral canal which makes about 2 and half turns. It looks like a snail’s shell. It arises from sacculus part of vestibule.

In transverse section of cochlea, there are three chambers: upper-scala vestibuli, middle-scala media and lower-scala tympani. These chambers are separated by Reissner’s membrane from upper region and by bassiliar membrane from lower region. Scala media is filled with endolymph while scala vestibuli and scala tympani are filled with perilymph.

Organ of Corti: There is a special type of epithelium in the floor of basiliar membrane called as organ or Corti. It contains four rows of sensory hair cells known as stereocilia or phonoreceptors and supporting cells. There are about 24000 such hair cells connected to the vesticulo cochlear nerve. These stereocilia are covered by a ribbon like tectorial membrane which determines the pattern of sound wave. The organ of Corti is thus specialized to receive sound impulses which are transmitted to auditory nerve endings.

Functions of ear:

There are two functions of the ear: hearing and balance.

Hearing: The sound wave passes into external auditory canal and the tympanic membrane is vibrated. E sound wave passes through ear ossicles- malleus, incus and stapes. They amplify and transmit the sound wave to oval window. Perilymph and endolymph of cochlea are also vibrated with the sound wave. These waves are received by hair cells of organ of Corti in scala media. These hair cells are connected with auditory nerves or vestibule-cochlear nerve. So, the sound wave is transmitted to the hearing centre of medulla oblongata of brain where it is interpreted into sound.

Balance: Organ of balance is three semi circular canals and vesitbules. When the body tilts in any direction, then there is movement of fine sand particles or otolith present in gelatinous fluid of the macula or cristae. This movement stimulates the hair cells of macula or cristae. These hair cells are connected with 8th cranial nerve or vestibule- cochlear nerve through which nerve impulses are passed to brain and the body is balanced.