Ear anatomy

ear anatomy

Your ears are not just for hearing — they are also important for controlling position sense and balance. Each ear is divided into 3 sections: the outer; middle; and inner ear. The middle and inner parts of the ear are located in hollow cavities on either side of the head within the temporal bones of the skull.

The outer ear

The external part of your ear consists of the pinna and ear lobe. The pinna is the shell-like part of your external ear, and is made of cartilage and skin. It directs sound waves from the outside into your external auditory canal (ear canal), which in turn channels sound waves to the tympanic membrane (also known as the ear drum). The tympanic membrane is a thin, semi-transparent membrane that separates the outer and middle ear.

The middle ear

The middle ear is an air-filled space that contains 3 tiny bones (known as ossicles), called the malleus (hammer), incus and stapes (stirrup). Sound waves that reach the tympanic membrane cause it to vibrate. This vibration is then transmitted to the ossicles, which amplify the sound and pass on the vibration to the oval window (a thin membrane between the middle and inner ear).

The Eustachian tube is a narrow tube that connects your middle ear to the back of your nose and throat (known as the nasopharynx). Its function is to allow air into the middle ear as well as drain mucus from the middle ear into the nasopharynx. When you swallow, your Eustachian tube opens up to allow air into the middle ear, so that the air pressure on either side of the tympanic membrane is the same. In situations when there is a sudden change in air pressure (e.g. during take off and landing when travelling on a plane), the pressure in the middle ear is not the same as the outside air pressure. This can make your ear drum bulge or retract and less able to transmit vibrations, causing temporary hearing problems. By swallowing or ‘popping’ your ears, you can equalise the pressure.

The inner ear

The inner ear (also called the labyrinth) contains 2 main structures — the cochlea, which is involved in hearing, and the vestibular system (consisting of the 3 semicircular canals, saccule and utricle), which is responsible for maintaining balance.

The cochlea is filled with fluid and contains the organ of Corti — a structure that contains thousands of specialised sensory hair cells with projections called cilia. The vibrations transmitted from the middle ear cause tiny waves to form in the inner ear fluid, which make the cilia vibrate. The hair cells then convert these vibrations into nerve impulses, or signals, which are sent via the auditory nerve to the brain, where they are interpreted as sound.

The round window (fenestra cochlea) is a membrane that connects the cochlea with the middle ear. It helps dampen the vibrations in the cochlea.

The semicircular canals also contain fluid and hair cells, but these hair cells are responsible for detecting movement rather than sound. When you move your head, the fluid within the semicircular canals (which sit at right angles to each other) also moves. This fluid motion is detected by the hair cells, which then send nerve impulses about the position of your head and body to the brain to allow you to maintain your balance.

The utricle and saccule work in a similar way to the semicircular canals, allowing you to sense your body’s position relative to gravity and make postural adjustments as required.

References

1. Merck Manual: Home Health Handbook. Ears (reviewed Nov 2013). http://www.merckmanuals.com/home/ear_nose_and_throat_disorders/biology_of_the_ears_nose_and_throat/ears.html (accessed May 2014).
2. Tracey, DJ. Anatomica: The Complete Reference to the Human Body and How it Works. Sydney: Random House; 2000.
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