Tuesday 29 November 2022

THE STRUCTURE AND FUNCTIONS OF THE EAR

 The ear is the organ of hearing and balancing. The organ of earing hearing and balance are divided into three parts:

-          The external ear

-          The middle ear

-          The inner ear

 

The external and middle ear are involved in hearing only, whereas the inner ea functions in hearing and balance.

 

The external ear includes the auricle and external auditory canal. The external ear terminates medially at the tympanic membrane or eardrum.

 

The middle is an air-filled space within the petrous portion of the temporal bone hat contains the auditory ossicles

 

The inner ear houses the sensory cells organ for both hearing and balance. It consists of inner connecting fluid-filled tunnels chambers within the petrous portion of the temporal bone.

 

External ear

The auricles or pinna is the fleshly part of the external ear. On the outside of the head. It consists primarily of elastic cartilage covered by skin, it shape helps collects sound waves and directs them towards the external auditory canal. The eternal auditory canal is linked with hairs and ceruminous gland which produce cerum a modified sebum commonly called earwax. The hairs and cerum helps prevent foreign bodies from reaching the delicate tympanic membrane.

 

The tympanic membrane is a thin, semi-transparent nearly oval membrane hat separates the external ear from the middle ear. It consists of three layers: a low simple cuboidal epithelium on the outer surface and a thin stratified squarous epithelium on the outer surface with a layer of connective tissue between. Sound waves reaching the tympanic membrane through the external auditory canal causes it to vibrate. Rupture of the tympanic membrane can be caused by foreign objects thrust into the ear, an infection of the middle ear or sufficient differential pressure between middle ear and outside ear as occurs when changing altitude n an airplane or diving into a deep water. Rupture of the tympanic membrane may result in hearing impairment.

 

Middle ear

Medial to the tympanic membrane is the air-filled cavity of the middle ear. Two covered openings, the round and oval window, on the medial side of the inner ear. Two additional openings provide air passage from the middle ear. One opens into the mastoid air cells into the mastoid process of the temporal bone. The other passage way, the auditory tube, or pharyngotympanic tube also called Eustachian tube opens into the pharynx and equalizes air pressure between the outside air and he middle ear causing unequal pressure between the middle ear and outside environment can distort he tympanic membrane, dampens its vibration and makes hearing difficult.

 

Distortion of the tympanic membrane which occurs under these conditions also stimulates pain fibres associated with it. Because of this distortion, when a person changes altitude, sound seem muffled and eardrum may become painful. Swallowing, yawning, chewing and holding the nose and mouth shut gently forcing air out of the lungs can relieve distorting of the tympanic membrane these actions open the auditory tube which allows air to pass through the auditory tube and equalizes air pressure on each side of the eardrum.

 

The middle ear contains three auditory ossicles: the malleus, the incus and the stapes which transmits vibration from the tympanic membrane to the oval window. The handle of the malleus is attached to the inner surface of the tympanic membrane and vibration of membranes causes the malleus to vibrate as well.

 

The head of the malleus is attached by a very small synovial joint to the incus, which in turn is attached by a small synovial joint. To the stapes. The foot plate of the stapes fit into the oval window and is held in place by a flexible annular ligament. Two skeletal muscles originates from bone around the middle ear and insert into auditory ossicles.

 

The inner ear

The inner (internal ear) or labyrinth (meaning maize) contains the organ of hearing and balance. It is described into two parts: the bony labyrinth and the membranous labyrinth.

 

Bony labyrinth

This is the cavity within the temporal bone lined with periosteum. It is larger than and encloses he 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, there is a similar watery fluid, endolymph.

 

The bony labyrinth consists of:

-          Vestibule

-          Cochlea

-          Three semi-circular canal.

 

The vestibule

This s he expanded part nearest the middle ear, he oval and round window are located in its lateral wall. It contains two membranous sacs: the utricle and baccule, which are important in balance.

 

The cochlea

This resembles a snail’s shell. It has a broad base where it is continuous with the vestibule and a narrow apex and it spirals around a central bony column.

 

The semi-circular canals

 These are three tubes arranged so that one is situated in each of the three planes of space. Hey are continuous with the vestibules.

 

Membranous labyrinth

This is a network of delicate tubes, filled with endolymph. It comprises:

-          The vestibule which contains he utricle and saccule

-          The cochlea

-          The three semi-circular canal

 

The cochlea

The cross-section of the cochlea contains three compartments:

-          The scala vestibule

-          The scala medial or cochlea duct

-          The scala tympanic

 

In cross section, the bony cochlea has two compartment containing perilymph: the scala vestibule which originates at oval window and the scala tympanic which ends at the round window. The two compartments are continuous with each other. The cochlea duct is part of the membranous labyrinth and is triangular in shape on the basilar membrane or base of the triangle are supporting each cell and specialized cochlea hair cells containing auditory receptors. The sensory organs that responds to vibration by initiating nerve impulses that are perceived as hearing within the brain. The auditory receptors are dendrites of efferent (sensory) nerves that combines forming the cochlea (auditory) part of the vestibules cochlea nerves (8th cranial nerve) which passes through a foramen in the temporal bone to reach the hearing area in the temporal lobe of the cerebrum.

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