THE EAR

HUMAN EAR

FUNCTIONS: Hearing and maintenance of body balance.

 Anatomically, the ear can be divided into three major sections

·         The outer ear

·         The middle ear

·         The inner ear

THE OUTER EAR

STRUCTURES:

·         PINNA: Collects the vibrations in the air which produce sound.

·         EXTERNAL AUDITORY MEATUS (CANAL): It leads inwards and extends up to the tympanic membrane(the ear drum).

*There are very fine hairs and wax-secreting sebaceous glands in the skin of the pinna and the meatus.

THE TYMPANIC MEMBRANE: It is composed of connective tissues covered with skin outside and with the mucus membrane inside.

THE MIDDLE EAR

STRUCTURES:

OSSICLES: Increase the efficiency of transmission of sound waves to the inner ear.

Ear ossicles that are attached to one another in a chain-like fashion are as follows:

1.    MALLEUS: Attached to tympanic membrane

2.    INCUS

3.    STAPES: Attached to the oval window

EUSTACHIAN TUBE: Connects the middle ear cavity with the pharynx & helps in equalizing the pressures on either sides of the ear drum.

THE INNER EAR

STRUCTURES:

LABYRINTH: The fluid-filled inner ear.

1.    BONY LABYRINTH

2.    MEMBRANOUS LABYRINTH

·         The bony labyrinth is a series of channels inside which lies the membranous labyrinth and it is surrounded by a fluid called perilymph.

·         The membranous labyrinth is filled with a fluid called endolymph.

COCHLEA: Coiled portion of the labyrinth

The membranes constitute:

·         COCHLEA

·         REISSNER’S

·         BASILAR

It divides the surrounding perylymph filled bony labyrinth into

·         UPPER SCALA VESTIBULE :

·         LOWER SCALA TYMPANI

SCALA MEDIA: The space within cochlea & is filled with endolymph.

SCALA VESTIBULE: It ends at the oval window at the base of the cochlea

SCALA TYMPANI: It terminates at the round window which opens to the middle ear. ORGAN OF CORTI: A structure located on the basilar membrane

HAIR CELLS: Auditory receptors present in rows on the internal side of organ of corti.

BASAL END: The basal end of the hair cell is in close contact with the afferent nerve fibres.

STEREO CILIA: A large number of processes which are projected from the apical part of each hair cell.

TECTORIAL MEMBRANE: A thin elastic membrane above the rows of the hair cells.

VESTIBULAR APPARATUS: A complex system located above the cochlea. It is composed of:

1.    THREE SEMI-CIRCULAR CANALS

2.    OTOLITH ORGAN

·         SACCULE

·         UTRICLE

*Each semi-circular canals lies in different planes at right angles to each other.

*Membranous canals are suspended in the perilymph of the bony canals.

AMPULA: The base of canals which is swollen

CRISTA AMPULLARIS: The protecting ridge in ampula containing hair cells.

MACULLA:  The saccule and utricle contain a projecting ridge called macula.

CRISTA AND MACULLA : Specific receptors of the vestibular apparatus responsible for maintenance of body balance and posture.

MECHANISM OF HEARING

The external ear receives sound waves and directs them to the ear drum. The ear drum vibrates in response to the sound waves and these vibrations are transmitted through the ear ossicles (malleus, incus and stapes) to the fluid of the cochlea, where they generate waves in the lymphs. The waves in the lymphs induce a ripple in the basilar membrane. These movements in basilar membrane bend the hair cells, pressing them against the tectorial membrane. As a result, nerve impulses are generated in the associated neurons. These impulses  are transmitted by the afferent fibres via auditory nerves to the auditory cortex of the brain, where the impulses are analysed and the sound is recognized.

DISEASES:

HEARING LOSS

International Symbol For Deafness