What's In This Article...

1. Outer Ear
1. The Auricle (Pinna)
2. Auditory Canal
2. Middle Ear
1. Auditory Ossicles
3. Inner Ear
1. Vestibules
2. The Semicircular Canals
3. The Cochlea
4. Mechanism Of Hearing
5. Disorders Of The Ear

Anatomy Of Human Ear

"Hearing is a form of touch. You feel it through your body, and sometimes it almost hits your face".

The ear is divided into three distinct parts:

• Outer Ear
• Middle Ear (tympanic cavity)
• Inner Ear

The outer ear collects the sound waves and directs them to the middle ear ,which in turn transfers them to the inner ear, where they are converted into nerve impulse and transmitted to the hearing area in the cerebral cortex.

Outer Ear

The Auricle (Pinna)

Visible part of ear projects from the side of the head. Composed from fibroelastic cartilage covered with skin.

Auditory Canal

This is a slightly S shaped tube about 2.5 cm long extending from the auricle to the tympanic membrane (eardrum).

There are numerous ceruminous glands and hair follicles, with associated sebaceous glands in the skin of lateral third.

Ceruminous glands are modified sweat glands that secrete cerumen, a sticky material. Movements of the temporomandibular joint during chewing and speaking massage the cartilaginous meatus moving the wax towards the exterior.

The tympanic membrane completely separated the external acoustic meatus from the middle ear. It is oval shaped with the slightly broader edge upwards covering of hairless skin, the middle layer of fibrous tissue and the inner lining of mucous membrane continuous with that of the middle ear.

Middle Ear

This is an irregular- shaped air filled cavity within the petrous portion of the temporal bone.

The lateral Wall of the middle ear is formed by the tympanic membrane.

The roof and floor are formed by the temporal bone.

The posterior walls are formed by temporal bone with an opening leading to the mastoid antrum through which air passes through the air cells within the mastoid process.

The medial wall is a thin of temporal bone in which there are two opening:

• Oval Window.
• Round Window.

The presence of air at atmospheric pressure on both sides of the tympanic membrane is maintained by the pharyngotympanic tube.

Auditory Ossicles

These are three very small bones only a few millimeters in size.

The ossicles are held in place by fine ligaments.

• The Malles: This is the lateral hammer shaped bone .The handle is in contact with the tympanic membrane and the head forms a movable joint with the incus.
• The Incus: This is the middle anvil shaped bone. Its body articulates with the malleus, the long process with the stapes and it is stabilized by the short process, fixed by fibrous tissue to the posterior wall of the tympanic cavity.
• The Stapes: This is the medial stirrup – shaped bone. Its head articulated with the incus and its footplate fits into the oval window.

Inner Ear

The inner ear or labyrinth (meaning ‘maze’) contains the organs of hearing and balance. It is described in two parts, the bony labyrinth and the membranous labyrinth and is divided into three main regions:

• The vestibule, containing the utricle and saccule.
• Three semicircular canal.
• The cochlea.

The inner ear is formed from a network of channels and cavities in the temporal bone (the bony labyrinth). Within the bony labyrinth like a tube within a tube is the membranous labyrinth, a network of fluid-filled membranes that lines and fills the bony labyrinth.

• THE BONY LABYRINTH: This is lined with periosteum. Within the bony labyrinth, the membranous labyrinth is suspended in a watery fluid called perilymph.
• THE MEMBRANOUS LABYRINTH: This is filled with endolymph.

Vestibules

This is the expanded part nearest the middle ear. The oval and round windows are located in its lateral wall. It contains two membranous sacs, the utricle and the saccule, which are important in balance.

The Semicircular Canals

These are three tubes arranged so that one is situated in each of the three planes of space. They are continuous with the vestibule and are also 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 round a central bony column.

A cross-section of the cochlea contains three Compartments:

• The Scala Vestibuli
• The Scala Media, or Cochlear Duct
• The Scala Tympani

In cross-section the bony cochlea has two compartments containing perilymph: the Scala Vestibuli, which originates at the oval window, and the Scala tympani, which ends at the round window. The two compartments are continuous with each other and the relationship between these structures. The cochlear duct is part of the membranous labyrinth and is triangular in shape. On the basilar membrane, or base of the triangle, are supporting cells and specialized cochlear hair cells containing auditory receptors. These cells form the spiral organ (of Corte), the sensory organ that responds to vibration by initiating nerve impulses that are then perceived as hearing within the brain. The auditory receptors are dendrites of efferent (sensory) nerves that combine forming the cochlear (auditory) part of the vestibulocochlear nerve (8th cranial nerve), which passes through a foramen in the temporal bone to reach the hearing area in the temporal lobe of the cerebrum.

Mechanism Of Hearing

1. The pinna receives the sound waves and it reaches the tympanic membrane through the meatus.
2. The eardrum vibrates and these vibrations get transmitted to the three ossicles present in the middle ear.
3. Malleus, incus and stapes amplify the sound waves.
4. These vibrations then reach the perilymph (Scala Vestibuli) through the oval window.
5. Then the pressure waves get transferred to the endolymph of Scala media and reach the basilar membrane and then to perilymph of Scala tympani. This movement of fluid is facilitated by the round window present at the end of Scala tympani.
6. The basilar membrane movement causes rubbing of stereocilia against the tectorial membrane.
7. Stereocilia are bent resulting in the opening of ion channels in the plasma membrane of hair cells. Glutamate, a neurotransmitter, is released due to Ca++ ion movement inside the cell.
8. These neurotransmitters bind to the receptors of afferent neurons, which synapse with hair cells causing depolarization of neurons. A nerve impulse is generated and transmitted to the auditory cortex of the brain through the auditory nerve (cranial nerve VIII).
9. The brain analyzes the impulses and we hear the sound. The brain not only recognizes the sound but also judges the direction, loudness and pitch of the sound.

Disorders Of The Ear

Acoustic Neuroma

An acoustic neuroma is a benign tumor that grows in the eighth auditory or vestibulocochlear nerve.

Deafness

Deafness is a complete loss of hearing.

Earache

Earache is common and it’s often the result of an ear infection.

Labyrinthitis

Labyrinthitis is an inflammation of the balancing center in your inner ear.

Tinnitus

Tinnitus is a ringing, buzzing, whistling or hissing in one or both ears when there is no outside sound.

Vertigo

Vertigo is a sensation of movement or spinning, tilting or swaying when your body is not actually moving. It is most commonly caused by benign positional paroxysmal vertigo (BPPV).

Meniere's Disease

Meniere’s disease is a disorder of the ear that causes hearing and balance problems. There’s no cure, but the symptoms can be managed.