Ear

Ear

—  Human Ear is organ of hearing and equilibrium that detects noises by transduction ( or the convertion of sound waves into electrochemical impulses ) and maintains the sense of balance ( equilibrium )

Function

—  In general , human air serves to hearing and equilibrium .

—  But in fact The ear also  captures sound and helps you recall that sound throughout your life. The ear helps the body respond emotionally, according to the sound heard, like emotions of love on hearing the sound of a loved one, emotions of fear on hearing a threat or a gunshot!

Parts and Functions of the ear

The human ear is divided into five parts. These five parts of human ear, have specific functions that help in the process of hearing.

Parts of Human Ear
The parts of the human ear include:

• Outer Ear
• Middle Ear
• Inner Ear
• Acoustic Nerve
• Central Auditory Processing Centers`

Outer ear is divided into the pinna and the external auditory meatus. The pinna, also known as the auricle is the external ear part that is located and seen on each side of our head. It is made up of cartilage and soft tissue. This helps in maintaining a particular ear shape and remains pliable. The pinna is like a funnel that collects the sound vibrations from around us and funnels them towards the external auditory meatus.

The external auditory meatus is also called the ear canal. The ear canal helps understand and determine the source and direction of the sound. It is only ¼ inch in diameter and extends from the pinna to the tympanic membrane. The tympanic membrane is commonly called the eardrum. Skin and hair cover the outer ear canal foundation and the cerumen gland or the wax gland is present in this area. The ear canal develops into a bony structure tightly covered by skin, near the eardrum.

Fact File: The eardrum is an extremely sensitive organ. It can detect the sounds with an intensity range of approximately 100,000,000 to 1. When the softest sound hits the eardrum, it moves about one-millionth of an inch and this vibration is transferred to the inner ear for further processing within the brain.

The middle ear is the structure that begins at the end of the tympanic membrane. There are three tiny bones known as the ossicles that make up the middle ear. These bones connect the eardrum to the inner ear. Sound waves funneled in through the pinna, hit the eardrum. This causes the eardrum to move back and forth, in other words, vibrate, causing the ossicles to move. This causes the sound waves to convert into mechanical vibration.

The three tiny bones forming the ossicles are malleus, incus and stapes. The malleus also known as the hammer is connected to the eardrum on one side and the incus, known as the anvil on the other side. The anvil is connected to the third bone stapes, also called the stirrup. The sound waves converted into mechanical energy are transferred through this ossicular chain. There is an in and out movement at the stirrup base known as the stapes footplate, that matches the incoming sound waves. The beginning of the inner ear is marked by the oval window that fits in the stapes footplate.

The middle ear is present in the mastoid section of the temporal bone. The temporal bone is the skull bone that is present on each side of the head that is filled with air. The Eustachian tube runs from the middle ear front wall to the back of the nose and nasopharynx, that is, throat. The function of the ear tube, that is the Eustachian tube is, to provide ventilation and access to the external air and balance the air pressure on the both sides of the eardrum.

Fact File: When there is a change in air pressure in our ears, we can chew, swallow and even yawn.

The inner ear houses the sensory organs that help in hearing and maintaining balance. The part of human ear involved in the function of hearing is the cochlea. Another major function of the human ear is to maintain balance of the body. The different parts of the human ear that help in balancing are the semicircular canals consisting of the utricle and the sacculepresent in the inner ear.

The bony structure that is shaped like a snail and filled with endolymph and perilymph fluid is called the cochlea. The sensory receptor called the Organ of Corti is present inside the cochlea. It has hair cells and nerve receptors, required for hearing.

The middle ear movement pushes the mechanical energy in the oval window inside the cochlea. The tiny hair cells are stimulated due to the force that moves the fluids inside the cochlea.Pitches or the specific sound frequencies stimulate specific individual hair cells in the inner ear. Thus, certain frequencies are responded by certain hair cells. The hair cells translate signals into nerve impulses. The cochlear portion of the VIII cranial nerve, the acoustic nerve, transmit the nerve impulses to the brain.

The acoustic nerve is the part of human ear that transmits impulses from the cochlea to the mid brain region, the cochlear nucleus, and further on to other pathways in the brain, that end in the auditory cortex of the brain. The nerve fibers of each ear are divided into two pathways from the cochlear nucleus. Of these two pathways, one ascends towards the auditory cortex in one hemisphere of the brain and the other crosses over and ascends to the other hemisphere of the brain. Thus, the function of the human ear nerve fibers pathway is to transmit data or information received from both ears to both the hemispheres of the brain.

The central auditory system function of human ear is to process auditory information carried to the brain. The central auditory system plays a role in the following functions of human ear:

• The localization and lateralization of the sound
• Differentiating between the different sounds
• Temporal resolution, masking, integration and ordering
• Reducing the auditory performance when there are competing acoustic signals
• Reducing the auditory performance when there is a presence of degraded acoustic signal

Functions of the Ear
We have understood the different parts of a human ear and got an overview of their functions. Now, let us have a look at the functions of the ear, in a little detail.

• The pinna and the ear canal deliver the sound waves to the middle ear. Foreign bodies like insects, dust, etc. are prevented from gaining entry into the ear due to the presence of wax and hair in this region. This helps in preventing many ear infections.
• The eardrum vibrates according to the frequency and the amplitude of sounds that strike it.
• The middle ear function of human ear is to transmit and amplify the sounds vibrated from the eardrum towards the oval window. It also acts as a dampener to loud sounds that may damage the cochlea.
• The round window is a flexible membrane present at the opposite end of the fluid filled channels from the oval window. The round window function of human ear is to keep the cochlear fluids contained within the scala vestibuli and scala tympani. It also functions as a multiplier of the sound waves generated from the oval window membrane.
• The malleus transmits sound vibrations from the eardrums to the incus.
• The incus transmits the sound vibrations to the stapes.
• The stapes transmit the vibrations to the membrane of the inner ear present inside the fenestra ovalis.
• The semicircular canals function is to maintain the balance by responding to gravity and the acceleration changes of the head.
• The mastoid bone acts as an amplifier of certain sounds that are in the low-frequency range.
• The cochlea, the actual organ that helps in hearing functions as a sound wave interpreter and converter.

Parts and Functions of the Ear Involved in Balancing

The sense of equilibrium is controlled by the vestibular system. This system is present in the inner ear. The temporal bone space is shared between the vestibular and the cochlea. The fluids present in the cochlea are present in the vestibular. In order to maintain balance and equilibrium when standing, sitting, running, walking etc. in relation to gravity without falling over, is taken care by the vestibular system. Many other systems like vision, muscle response, help the vestibular system in performing its balancing function of human ear effectively.

The utricle and the saccule of the semicircular canals lie in anatomically different planes. These planes lie at a right angle to each other. These planes each have a specific function that deals with movement, that is, up and down, side to side and tilting from one side to the other side. These canals contain sensory hair cells and are activated by the movement of the endolymph fluid. When the head tilts to one side, the sensory hair cells send a nerve impulse to the brain with the help of acoustic nerve. The fluid in the semicircular canal acts on calcium carbonate crystals (CaCO₃). These crystals shift on their sensory hair beds at the base of the utriculus and the sacculus. These impulses are stimulated by the crystals present and are processed in the brain stem and the cerebellum area, in relation to the position of head with gravity.

Fact File: When the calcium carbonate crystals break off from their hair beds, they float within the vestibular labyrinth. These floating crystals cause serious balance and vertigo problems.

How the Human Ear Functions

We have seen the parts of human ear and the functions of the ear. But, if you want to understand

how the human ear functions, the following diagram of the ear will prove to be useful. You can see in the diagram that the external ear captures the traveling sound waves. The sound waves enter through the pinna into the ear canal. Through the ear canal the sound waves reach the eardrum. These sound waves cause vibrations of the eardrum that are passed on to the middle ear. The three tiny bones in the middle ear pass the vibrations over to the inner ear. The stapes deliver the sound waves to the cochlea through the round window of the outer part of the inner ear, called the bony labyrinth. These vibrations cause the perilymph fluid present in the cochlea to vibrate. These vibrations are picked up by the sensory hair cells, that translate it into nerve impulses and deliver it to the auditory nerve present at the base of cochlea. The VIII cranial nerve carries the messages to the brain, where it is read and translated into meaningful sounds.

Disease of ear

Frostbite.

The exposed position of the outer ear makes it the part of the body most frequently involved by freezing or frostbite. Humidity, duration of exposure, and, most of all, wind, in addition to degrees of temperature below freezing, predispose to occurrence of frostbite. The frozen area begins along the upper and outer edge of the ear, which becomes yellow-white and waxy in appearance, cold and hard to the touch, and numb with loss of skin sensation.

In treatment of frostbite the victim is placed as soon as possible in a warm room, but the frozen ear is kept cool until the returning blood circulation gradually thaws the frozen part from within. Massage of the frozen ear is avoided, for it is likely to injure the skin. Heat applied to the frozen area before circulation is established can result in clotting of the blood in the blood vessels. This in turn can result in death of that part of the ear, which turns black and eventually falls off, a process called dry gangrene.

Hematoma.

Injury to the outer ear can cause bleeding between the cartilage and the skin, producing a smooth, rounded, nontender purplish swelling called hematoma. The accumulation of clotted blood is removed by a surgeon because, if it is left, it will become transformed into scar tissue and cause a permanent, irregular thickening of the outer ear commonly called cauliflower ear and seen in boxers and wrestlers whose ears receive much abuse.

Perichondritis.

Infection of the cartilage of the outer ear, called perichondritis, is unusual but may occur from injury or from swimming in polluted water. It is due to a particular microorganism, Pseudomonas aeruginosa. There is a greenish or brownish, musty or foul-smelling discharge from the outer ear canal, while the affected outer ear becomes tender, dusky red, and two to three times its normal thickness. Prompt medical treatment is necessary to prevent permanent deformity of the outer ear.

External otitis.

Infection of the outer ear canal by molds or various microorganisms occurs especially in warm, humid climates and among swimmers. The ear canal itches and becomes tender; a small amount of thin, often foul-smelling material drains from it. If the canal becomes clogged by the swelling and drainage, hearing will be impaired. Careful and thorough cleaning of the outer ear canal by a physician, application of antiseptic or antibiotic eardrops, and avoidance of swimming are indicated to clear up the infection.

Boil in the ear (furuncle).

Infection of a hair follicle anywhere on the body is known as a boil, or furuncle. This can occur in a hair follicle in the outer ear canal, especially when there is infection of the skin of the canal. It always occurs because of a particular type of germ known as staphylococcus. Because the skin of the ear canal is closely attached to the underlying cartilage, a boil in the ear canal is especially painful, with swelling, redness, and tenderness but generally without fever. Heat applied to the outer ear by a hot-water bottle or electric pad helps the infection to come to a head and begin to drain. Antiseptic eardrops and careful cleaning of the outer ear canal are needed to prevent other hair follicles from becoming infected with a series of painful boils in the ear.

Erysipelas of the outer ear.

Erysipelas is an infection in the skin caused by a particular type of streptococcus that causes a slowly advancing red, slightly tender thickening of the skin. It begins at the ear and spreads to the face and neck. Centuries ago erysipelas epidemics caused severe and often fatal infections. In AD 1089 one of the most severe epidemics was known as St. Anthony's fire; those who prayed to St. Anthony were said to recover; others, who did not, died. Today erysipelas is a rather mild and comparatively rare infection that clears up rapidly when sulfanilamide is taken by mouth or penicillin by injection.

Leprosy.

Leprosy, seen rarely outside of the tropics today, was another scourge of ancient times that sometimes affected the outer ear. It is caused by the leprosy bacillus, Mycobacterium leprae, which causes a painless, slowly progressing thickening and distortion of the affected tissues. The diagnosis is made by examining a bit of the infected tissue under a microscope and finding the leprosy bacilli, which in appearance are not unlike the bacilli that cause tuberculosis. Fortunately, the antibiotics effective against tuberculosis are effective today in arresting the ravages of leprosy.

Osteoma of the bony ear canal.

Osteoma of the bony ear canal is a bony knob that grows close to the eardrum membrane, especially in those who swim a great deal in cold water. It is not dangerous and does not need to be removed unless the bony overgrowth becomes large enough to block the ear canal.

Cyst of the ear.

A cyst is a sac filled with liquid or semisolid material. A cyst of the ear is most often caused by a gland that lubricates the skin behind the earlobe, less often at the entrance of the ear canal. If the duct of this gland becomes stopped, the lubricating fatty material accumulates as a soft rounded nodule in the skin. Infection of the cyst causes a tender abscess to form and drain. The cyst will re-form unless it is removed completely by surgery.

Another type of cyst occurs above the ear canal, just in front of the outer ear or, rarely, in the neck behind and below the ear. This is a remnant of the primitive gill of the early embryo, a reminder of our ancient fishy ancestors. It may appear as a tiny pitlike depression that discharges a little moisture from time to time, or a cystic swelling may develop when the opening of the pit is closed, requiring surgical removal.

Keloid of the ear.

In dark-skinned people, overgrowth of scar tissue from any skin incision or injury causes a thickened elevation on the scar called a keloid. Having the earlobes pierced for earrings sometimes results in a large, painless nodular keloid enlargement of the earlobe, harmless but unsightly. Keloids are removed surgically (see also INTEGUMENTARY SYSTEMS).

Deformities.

Absence of the outer ear.

Congenital deformity or absence of the outer ear, usually on one side, sometimes on both, is often accompanied by absence of the outer ear canal. This failure of the primitive gill structures to become properly transformed into the normal outer and middle ear is, in rare instances, hereditary. More often it occurs for no known reason. In some cases it can be traced to the damaging effects on the embryo of rubella (German measles) in the mother during the first three months of her pregnancy. Since the inner ear and nerves of equilibrium and hearing come from the otocyst, separate from the gill structure, in most cases of deformed or absent outer ear the hearing nerve is normal. Surgical construction of a new ear canal and eardrum membrane can then often improve the hearing, which has been impaired by the failure of sound conduction to reach the hearing nerve in the inner ear.

Cancer of the outer ear.

Cancer of the outer ear occurs chiefly in areas exposed for many years to the direct sun. A small and at first painless ulcer, with a dry scab covering it, that slowly enlarges and deepens may be a skin cancer. Removal of a small bit of tissue from the edge (biopsy) and examining it under a microscope comprise the method of diagnosis. Complete removal by surgery or properly applied irradiation is needed for a cure. Cancer that begins in the ear canal is more serious, for it may deepen into the bone before it is diagnosed. It is then more difficult to cure by removal. Cancers of the ear canal are rather rare, while cancers of the skin of the outer ear are more common, as well as more readily cured by removal.

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