The Human Ear

Date: 2015-10-07; view: 401.

Pre-text assignment

Unit 18

Learn the key words and phrases:

pinna, cerumen, tympanic membrane, ossicle, Eustachian tube, acoustic nerve

The human ear is the most complex sensory system in the human body. Vision and smell are extraordinary senses, but they do not match in complexity the ear's process of turning minute waves of sound pressure from air molecules banging against the eardrum into the neural signals that get sent to the brain and interpreted as sound.

With our sense of hearing we can hear the exquisite sounds of nature, enjoy all kinds of music, and understand the complexities of one or more spoken languages.

The ear is divided into four parts: the external or outer ear, the middle ear, the inner ear, and the neural ear. Each part serves an important and unique function in the process of hearing.

The external ear consists of the most visible ear structure, called the pinna or auricle, and the ear canal. This part of the ear acts as a sound collector to guide sound waves down the ear canal so that they impinge against the eardrum or tympanic membrane causing it to vibrate. The shape and features of the pinna help us in identifying where sounds are coming from. In locating the direction of sounds, we also depend on the fact that we have two ears so the brain can compare the sound arriving at one ear versus the other.

The pinna, of course, is useful to keep eyeglasses from sliding down our face and is often used to attach decorative jewelry.

Lying beneath the outermost part of the ear canal are glands that produce earwax or cerumen. The skin covering the ear canal is very thin, especially farther down the canal, near the tympanic membrane. Because of this, one should avoid pushing cotton swabs or anything else down the ear canal since abrasion of the skin can cause bleeding and increase the risk of injury or infection.

The middle ear lies between the tympanic membrane and the inner ear. The middle ear space is filled with air and within it are the three smallest bones in our body, called the ossicles. These are known as the malleus (hammer), incus (anvil) and stapes (stirrup).

The ossicles serve to mechanically amplify the pressure waves from the vibrating tympanic membrane to provide an efficient transfer of the sound energy into the fluid filled spaces of the inner ear.

In addition, the relative difference in the area of the tympanic membrane and the much smaller oval window lying beneath the stapes footplate allows for a concentration of the pressure wave much the way a spiked heel on a shoe concentrates force compared to a flat heel. These two mechanisms multiply the pressure wave at the tympanic membrane about 22 times.

Also in the middle ear is the opening of the Eustachian tube that allows for the equalization of air pressure on both side of the tympanic membrane. We all experience the opening and closing of the Eustachian tube when our ears pop going up or down in an elevator or airplane.

The inner ear consists of two functional systems, both encased in the temporal bone of the skull. One, called the vestibular system is responsible for our sense of balance and equilibrium. The three semi-circular canals, set at right angles to each other, respond to motion and the forces of gravity. The other system, the coiled tube called the cochlea, is the most extraordinary structure of the ear. It is here that sound waves, now propagating as waves in the fluid-filled spaces of the cochlea, get converted to the neural impulses that are sent to the brain.

In the cochlea, thousands of tiny structures called hair cells are set into motion by the moving fluid wave. The hair cells trigger the attached nerve fibers into activity. It is within the cochlea that the most common type of hearing loss, sensory hearing loss, occurs when exposure to loud noise or diseases cause the destruction of many hundreds of these delicate hair cells.

Once the sound waves, balance and equilibrium information is encoded into neural signals by the inner ear, the neural impulses are sent to the brain along a nerve fiber bundle called the vestibulo-cochlear nerve, sometimes called the acoustic nerve. This nerve is numbered eight of the twelve cranial nerves that enter the brainstem. Other cranial nerves include the optic nerve, the olfactory nerve, the facial nerve and more.

In the brain's central auditory cortex, the neural impulses are interpreted as sound based on our learning and cognitive processes associated with our auditory experiences.