Outer wave within the inner – ear



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Outer ear (external ear):

Serves to collect and channel sound to the middle ear.

Middle ear:

Serves to transform the energy of a sound wave into the internal vibrations of the bone structure of the middle ear and ultimately transform these vibrations into a compressional wave in r the inner ear.

Inner ear:

Serves to transform the energy of a compressional wave within the inner – ear fluid into nerve impulses which can be transmitted to the brain.

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External ear:

It consists of pinna, external auditory canal and tympanum.

Pinna is designed to collect sound waves and is the visible part of the outer ear. The latter is a curved passage which is lined by ceruminous glands. The glands secrete cerumen, a waxy material which entraps dust and also lubricates tympanum. External auditory meatus is a fairly deep passage from the ear hole down to the tympanic membrane. Tympanum or ear drum is a circular membrane present on the inner end of the external auditory canal and partitions it from the tympanic cavity of the middle ear.

Middle ear:

The middle ear is represented by air-filled tympanic cavity which consists of eardrum and three interconnected tiny bones. This cavity communicates with the pharynx by a passage called eustachian canal.

This connection allows for the equalization of pressure within the air filled cavities of the ear. When this tube become clogged during the cold, then the ear cavity is unable to equalize its pressure, which will often leads to ear aches and other pains. Present in the inner wall of the tympanic cavity are two openings, the upper fenestra ovalis and the lower, fenestra rotunda, each covered by a membrane.

The three small bones, the ear ossicles from outside to inside) include hammer shaped malleus, anvil shaped incus and stirrup shaped stapes. The outer arm of malleus is in contact with inner surface of tympanum, while the inner end of stapes forms contact with the membrane on fenestra ovalis. These three tiny bones of the middle ear act as levers to amplify the vibrations of the sound wave and are also called as auditory ossicles.

Internal ear:

It is also called membranous labyrinth and is surrounded by bony labyrinth of almost similar shape. The space between the membranous labyrinth and bony labyrinth is filled with a watery fluid, the perilymph. The membranous labyrinth contains endolymph. The internal ear is a delicate organ and differentiated into vestibule, semicircular canals and cochlear duct.

Vestibule:

The vestibule is the central body proper, composed of two chambers – lower and smaller sacculus and upper large utriculus. Both chambers contains a special group of sensory cells called maculae which have hair like processes embedded in the otolith membrane that contains granules of calcium carbonates (called otoliths or otoconia or ear stones).

Maculae are similar to cristae, but there is no cupula.

Semicircular canals:

They are three arched structures which emerge from utriculus and open back into it. They include anterior and posterior vertical canals and a horizontal canal. The vertical canals join to form a common passage before they open into utriculus. Each semicircular canal is dilated at the base to form ampulla which contains sensory spot called crista formed of receptor cells and supporting cells. The receptor cells bear sensory hair which are embedded into a dome shaped gelatinous cupula above. It lacks otolith. The cristae and maculae are the receptors of balance

Cochlea:

The auditory region of internal ear is represented by a spirally coiled structure called cochlea.

It consists of cochlear duct arising from the sacculus, which is surrounded by similarly shaped cochlear canal, a part of bony labyrinth. The cochlear duct is fused with cochlear canal on lateral sides, but is free laterally therefore. In T.S., the cochlea shows three chambers, the upper scala vestibuli, the middle scala media and the scala tympani. The scala media is partitioned from the scala vestibuli by Reissner’s membrane and from the scala tympani by basilar membrane. Scala vestibuli and scala tympani contain perilymph while scala media is filled with endolymph. The upper and lower chambers communicate through helicotrema, a narrow opening present at the distal end of cochlea.

Present on the basilar membrane is a sensory ridge called organ of Corti formed of receptor cells, Deiter’s cells (basal cells) and supporting cells (cells of Hensen). The sensory hair of the receptor cells are embedded above in gelatinous tectorial membrane whereas cristae and maculae are innervated by the vestibular branch of auditory nerve, the organ of corti is supplied by its cochlear branch. Cochlea is the main hearing organ.