The ear is an organ of the body that is used for hearing and balance. It is connected to the brain by the auditory nerve and is composed of three divisions, the external ear, the middle ear, and the inner ear. The greater part of which is enclosed within the temporal bone. The ear is looked upon as a miniature receiver, amplifier and signal-processing system. The structure of the outer ear catching sound waves as they move into the external auditory canal. The sound waves then hit the eardrum and the pressure of the air causes the drum to vibrate back and forth.
When the eardrum vibrates its neighbour the malleus then vibrates too. The vibrations are then transmitted from the malleus to the incus and then to the stapes. Together the three bones increase the pressure which in turn pushes the membrane of the oval window in and out. This movement sets up fluid pressure waves in the perilymph of the cochlea. The bulging of the oval window then pushes on the perilymph of the scala vestibuli. From here the pressure waves are transmitted from the scala vestibuli to the scala tympani and then eventually finds its way to the round window.
This causes the round window to bulge outward into the middle ear. The scala vestibuli and scala tympani walls are now deformed with the pressure waves and the vestibular membrane is also pushed back and forth creating pressure waves in the endolymph inside the cochlear duct. These waves then causes the membrane to vibrate, which in turn cause the hairs cells of the spiral organ to move against the tectorial membrane. The bending of the stereo cilia produces receptor potentials that in the end lead to the generation of nerve impulses.
The External or Outer Ear – comprises of the auricle or pinna which is the fleshy part of the outer ear. It is cup-shaped and collects and amplifies sound waves which then passes along the ear canal to the ear drum or tympanic membrane. The rim of the auricle is called the helix and the inferior portion is called the lobule. The external auditory canal is a carved tube and contains a few hair and ceruminous glands which are specialized sebaceous or oil glands. These secrete ear wax or cerumen. Both the hairs and the cerumen help prevent dust and foreign objects from entering the ear.
A number of people produce large amounts of cerumen, and this sometimes cause the build up to be impacted and can bring about muffle incoming sounds. The Middle Ear – is an air-filled narrow cavity or cleft that extends vertically for about 15 mm and the distance horizontally is about the same. It is located in the temporal bone and is lined by epithelium. The middle ear is separated from the outer ear by the eardrum and from the inner ear by a thin bony partition that contains two small membrane-covered openings. These membranes are called the oval window and the round window.
On the other side of the eardrum, embodies the mechanism for the conduction of sound waves to the internal ear. It comprises of three of the smallest bones in the body, the auditory ossicles which are connected by synovial joints. These bones are called the malleus or hammer, incus or anvil and stapes or stirrup. The malleus is attached to the internal surface of the eardrum and the head of the malleus articulates with the body of the incus. This is the middle bone, which then articulates with the head of the stapes. The base (footplate) of the stapes then fits into the oval window.
Just below the oval window is another opening called the round window. This is enclosed by a membrane called the secondary tympanic membrane. The anterior wall of the middle ear consists of an opening which leads directly into the auditory tube. This is called the Eustachian tube. This tube is made up of both bone and hyaline cartilage and connects to the middle ear with the nasopharynx which is the upper part of the throat. This is generally closed at the medial end. When swallowing or yawning occurs, it opens. This allows the air to enter or leave the middle ear until the pressure in the middle ear equals the atmospheric pressure.
When both pressures are balanced, the eardrum then vibrates freely as the sound waves hit it. If these pressures are not balanced, an intense pain, hearing impairment, ringing in the ears or vertigo could develop. The Inner Ear or Labyrinth- consists of two main divisions an outer bony labyrinth which encloses an inner membranous labyrinth and the bony labyrinth which is a series of cavities in the temporal bone and it is divide into three areas. These are the semicircular canales, the vestibule and the cochlea. Both the semicircular canals and the vestibule contain receptors for equilibrium.
The Cochlea contains receptors for hearing. The labyrinth is lined with periosteum and contains perilymph. This fluid surrounds the membranous labyrinth, which is a series of sacks and tubes inside of the bony labyrinth and has the same common form. The vestibule is the oval central proportion of the bony labyrinth. Attached to the vestibule are three bony semicircular canals. They are named based upon their positions, anterior, posterior and lateral semicircular canals. The anterior and posterior semicircular canals are in a vertical orientation and the lateral one is in a horizontal orientation.
The Cochlea is also attached to the vestibular. It is a bony spiral canal and resembles a snails shell. There are three channels that divide from the cochlea. These partitions, together are shaped like the letter Y. The stem of the Y protrudes into the canal while the wings of the Y are made up of mainly membranous labyrinth. Just above the bony partition is the scala vestibuli. This ends at the oval window and the channel below is called the scala tympani and this ends at the round window. The third channel is called the cochlear duct or scala media.
The vestibular membrane divides the cochlear duct from the scala vestibuli. The basilar membrane separates the cochlear duct from the scala tympani. Resting on the membrane is the organ of Corti or spiral organ. This organ is a coiled sheet of epithelial cells as well as supporting cells and in the range of 16,000 hair cells. The hair cells are the receptors for hearing. There are inner and outer hair cells, the inner hairs are arranged in a single row and the outer hairs are arranged in three rows. Equilibrium Equilibrium or balance is controlled by the semicircular canals and the vestibule.
There are two types of equilibrium, static equilibrium which maintenances the position of the body relative to the force of gravity and dynamic equilibrium which maintenance the body position in response to any sudden movements, for example, the rotation, acceleration and deceleration of mainly the head. The receptor organs for equilibrium are called the vestibular apparatus which include saccule, utricle, and semicircular ducts. The saccule and utricle contain a small thickened region called a macula. The maculae are the receptors for static equilibrium and some aspects of dynamic equilibrium.
These receptors provide sensory information on the position of the head in space for static equilibrium making it essential for maintaining appropriate posture and balance, where as dynamic they detect linear acceleration and deceleration. There are two kinds of cells in the two maculae, hair cells and supporting cells. The hair cells are the sensory receptors. Laying over the hair cells are columnar supporting cells that probably secrete the thick, gelatinous, glycoprotein layer called the otolithic membrane and over the membrane is a layer of dense calcium carbonate crystals called otoliths.
When the head is tilted, the otoliths shift, and the hairs beneath respond to the change in pressure and bending the hair bundles. Dynamic equilibrium functions in the three semicircular ducts, the saccule and the utricle. The two ventical ducts are the anterior and posterior semicircular ducts. The lateral semicircular duct is horizontal. In the dilated portion of each duct, the ampulla, is a small elevation called the crista. This contains hair cells and supporting cells which are also covered by a mass of gelatinous material which is called cupula. When the head moves the attached semicircular ducts and hair cells move with it.
