Biology THE EAR


`star` Parts of an Ear
`star` Mechanism of Hearing


● The ears performs two sensory functions, `color{violet}"hearing"` and `color{violet}"maintenance of body balance"`.

● Anatomically, the ear can be divided into three major sections called the `color{brown}"outer ear"`, the `color{brown}"middle ear"` and the `color{brown}"inner ear."`

● The outer ear consists of the `color{brown}"pinna"` and `color{brown}"external auditory meatus"` (canal).

● The pinna collects the `color{violet}"vibrations in the air"` which produce sound.

● The external auditory meatus leads inwards and extends up to the `color{brown}"tympanic membrane (the ear drum)"`.

● There are very `color{brown}"fine hairs"` and `color{brown}"wax-secreting sebaceous glands"` in the skin of the pinna and the meatus.

● The tympanic membrane is composed of `color{violet}"connective tissues"` covered with skin outside and with `color{violet}"mucus membrane"` inside.

● The middle ear contains `color{brown}"three ossicles"` called `color{violet}"malleus, incus and stapes"` which are attached to one another in a chain-like fashion.

● The malleus is attached to the `color{violet}"tympanic membrane"` and the stapes is attached to the `color{brown}"oval window"` of the cochlea.

● The ear ossicles increase the `color{violet}"efficiency of transmission"` of sound waves to the inner ear.

● An `color{brown}"Eustachian tube"` connects the middle ear cavity with the pharynx.

● The Eustachian tube helps in `color{violet}"equalising the pressures"` on either sides of the ear drum.

● The `color{violet}"fluid-filled inner ear"` called `color{brown}"labyrinth"` consists of two parts, the `color{brown}"bony and the membranous "`labyrinths.

● The bony labyrinth is a `color{violet}"series of channels"`.

● Inside these channels lies the `color{violet}"membranous labyrinth"`, which is surrounded by a fluid called `color{brown}"perilymph"`.

● The `color{violet}"membranous labyrinth"` is filled with a fluid called `color{brown}"endolymph"`.

● The `color{violet}"coiled portion"` of the labyrinth is called `color{brown}"cochlea"`.

● The membranes constituting cochlea, the `color{brown}"reissner’s and basilar,"` divide the surounding perilymph filled bony labyrinth into an `color{brown}"upper scala vestibuli"` and a `color{brown}"lower scala tympani."`

● The space within cochlea called `color{brown}"scala media"` is filled with `color{violet}"endolymph"`.

● At the `color{violet}"base of the cochlea"`, the `color{violet}"scala vestibuli"` ends at the oval window, while the `color{violet}"scala tympani"` terminates at the round window which opens to the middle ear.

● The `color{brown}"organ of corti"` is a structure located on the `color{violet}"basilar membrane"` which contains `color{brown}"hair cells"` that act as `color{brown}"auditory receptors"`.

● The `color{violet}"hair cells"` are present in rows on the `color{violet}"internal side"` of the organ of corti.

● The `color{violet}"basal end"` of the hair cell is in close contact with the `color{violet}"afferent nerve fibres"`.

● A large number of processes called `color{brown}"stereo cilia"` are projected from the apical part of each hair cell.

● Above the rows of the hair cells is a `color{violet}"thin elastic membrane"` called `color{brown}"tectorial membrane"`.

● The inner ear also contains a complex system called `color{brown}"vestibular apparatus"`, located above the cochlea.

● The vestibular apparatus is composed of `color{brown}"three semi-circular canals"` and the `color{brown}"otolith organ"` consisting
of the `color{brown}"saccule and utricle"`.

● Each semi-circular canal lies in a `color{violet}"different plane at right angles"` to each other.

● The `color{violet}"membranous canals"` are suspended in the `color{violet}"perilymph"` of the bony canals.

● The `color{violet}"base of canals is swollen"` and is called `color{brown}"ampulla"`, which contains a projecting ridge called `color{brown}"crista ampullaris"` which has hair cells.

● The `color{violet}"saccule and utricle"` contain a projecting ridge called `color{brown}"macula"`.

● The `color{violet}"crista and macula"` are the specific receptors of the `color{violet}"vestibular apparatus"` responsible for maintenance of `color{violet}"balance of the body and posture"`.


● The `color{violet}"external ear"` receives `color{brown}"sound waves"` and directs them to the `color{violet}"ear drum"`.

● The `color{violet}"ear drum vibrates"` in response to the sound waves and these vibrations are transmitted through the `color{violet}"ear ossicles"` (malleus, incus and stapes) to the `color{violet}"oval window"`.

● The vibrations are passed through the oval window on to the `color{violet}"fluid of the cochlea"`, where they generate `color{violet}"waves in the lymphs."`

● The waves in the lymphs induce a `color{violet}"ripple in the basilar membrane"`.

● These movements of the basilar membrane `color{violet}"bend the hair cells,"` pressing them against the `color{violet}"tectorial membrane"`.

● As a result, `color{violet}"nerve impulses"` are generated in the `color{brown}"associated afferent neurons"`.

● These impulses are transmitted by the `color{violet}"afferent fibres"` via `color{brown}"auditory nerves"` to the `color{brown}"auditory cortex"` of the brain, where the `color{violet}"impulses are analysed"` and the `color{violet}"sound is recognised"`.