Homework Help: Science: Biology: Human Ear

The External Ear

The external ear consists of the outer portion of the ear that is visible on the side of the head, called the pinna, and the ear canal that leads to the tympanic membrane, also known as the ear drum. The pinna is made up of skin and cartilage, with muscular attachments in the back. Its shape helps us localize where sounds in the environment are coming from. Sounds coming from the front are more easily directed into the ear canal. The pinna can be injured in trauma, which if untreated can lead to permanent deformity. Various infections can also involve the pinna. Individuals who wear pierced earrings may develop tears in the lower portion of the pinna, the ear lobe, which can be corrected in a minor operation.

The ear canal is a tube about 2.5 cm (one inch) leading to the ear drum. The outer portion of the tube is made of cartilage, and the inner portion is bone. A thin layer of skin forms the wall of the cartilage. This region can become infected, causing external otitis, also called swimmer's ear. Wax is produced by small glands in the ear canal. The wax helps to prevent infections of the external ear. It normally falls out on its own, and one should not try to remove it with a cotton tip applicator or other object as this often pushes the wax farther into the canal or causes injury to the canal wall skin.

The Middle Ear

The middle ear is that part of the ear between the external and the inner ear. Its outer border is the thin membrane known as the ear drum, or tympanic membrane. The ear drum is a thin layer of tissue about 1 cm (0.4 inches) in diameter. As sound strikes the ear drum, it vibrates and in turn transmits these vibrations to the three small bones of the middle ear: the malleus, the incus, and the stapes. The combination of the ear drum and the lever action of these bones helps to amplify the sound vibration. The end of the stapes is called the footplate and it is connected to the inner ear. As the stapes vibrates, the footplate moves in and out like a piston and transmits the sound vibrations into the inner ear.

The middle ear is normally filled with air. An opening called the Eustachian tube connects the middle ear to the back of the upper throat. When this tube is opened, for example during yawning, the pressure in the middle ear is equalized to atmospheric pressure. If the tube does not open properly, a negative pressure develops in the middle ear. This causes fluid to build up in the middle ear, which in turn can lead to infection. This it the type of infection, called otitis media, that so often affects young children.

Trauma to the ear can cause a hole in the eardrum, called a tympanic perforation. A blow to the head can also cause the bony chain to become disconnected, caused an ossicular ("bony") discontinuity. This will produce a dramatic hearing loss that is quite easily corrected with surgery.

Another common type of hearing loss due to a middle ear problem is otosclerosis. In this condition, bony overgrowth occurs near the stapes footplate and prevents it from moving. Otosclerosis can also be corrected with surgery.

The Inner Ear

The inner ear is the delicate structure which transforms the sound vibrations from the stapes into nerve signals that are transmitted to the brain. It also plays an important role in maintaining our balance. The inner ear consists of tiny fluid-filled canals encased in some of the hardest bone in the body.

The hearing portion of the inner ear is the cochlea, a snail shaped structure that is connected to the stapes. As the stapes moves in and out, it produces fluid waves within the cochlea. These wave in turn cause movement of tiny cells within the cochlea called the hair cells. As these hair cells vibrate, they send signals to the brain which can then be interpreted as sound.

Other parts of the inner ear are responsible for helping maintain balance. The three semicircular canals are oriented at right angles to each other. When we turn our head, the relative movement of fluid in these canals lets our brain know which way and how much we are turning. Another part of the inner ear responds to the gravitational force and lets our brain know the static position of our head.

The cells of the inner ear are exquisitely sensitive to injury from, for example, a virus infection or blood loss. Unfortunately the loss is often permanent and does not correct itself. Depending on the cells involved and the type of loss, this can produce hearing loss, tinnitus (ringing), or balance problems. The hair cells also tend to degenerate as we get older, often causing a high frequency loss.

Homework Help: Science: Biology

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