How Hearing Works
The auditory pathways contain the circuitry of the nerves that lead from the inner ear to the brain stem. From the brain stem these signals are carried to the cerebral cortex at the temporal lobe. Here the brain processes the stimuli as speech and language as well as tonal stimuli such as music.
Anatomy Of The Ear
1. Ear canal
6. Oval window
7. Canal leading to the nose
9. Auditory nerve
How Do We Hear?
How the normal ear works:
1. Sound is transmitted as sound waves from the environment. The sound waves are gathered by the outer ear and sent down the ear canal to the eardrum.
2. The sound waves cause the eardrum to vibrate, which sets the three tiny bones in the middle ear into motion.
3. The motion of the bones causes the fluid in the inner ear or cochlea to move.
4. The movement of the fluid in the inner ear causes the hair cells in the cochlea to bend. The hair cells change the movement into electrical impulses.
5. These electrical impulses are transmitted to the hearing (auditory) nerve and up to the brain, where they are interpreted as sound.
The Middle Ear
The eardrum, or tympanic membrane (abbreviated TM), is the dividing structure between the outer and middle ear. Although a very thin membrane, the eardrum is made up of three layers to increase its strength.
The ossicles are the three tiny bones of the middle ear located directly behind the tympanic membrane. These three bones form a connected chain in the middle ear. One of the bones is embedded in the innermost layer of the tympanic membrane, and the third bone is connected to a membranous window of the inner ear. The ossicles take mechanical vibrations received at the tympanic membrane into the inner ear.
The Eustachian tube is the middle ear’s air pressure equalizing system. The middle ear is encased in bone and does not associate with outside air except through the Eustachian tube. The Eustachian tube is normally closed, but can be involuntarily opened by swallowing, yawning or chewing. It can also be intentionally opened to equalize pressure in the ears, such as when flying in an airplane. When this happens, you might hear a soft popping sound.
The Outer Ear
The part of the outer ear that we see is called the pinna, or auricle. The pinna, with its grooves and ridges, provides a natural volume boost for sounds in the 2000 to 3000 Hz frequency range, where we perceive many consonant sounds of speech.
The ear canal, also called the external auditory meatus, is the other important outer-ear landmark. The ear canal is lined with only a few layers of skin and fine hair, and is a highly vascularized area. This means that there is an abundant flow of blood to the ear canal. Wax, or cerumen, accumulates in the ear canal and serves as a protective barrier to the skin from bacteria and moisture. Ear wax is normal unless it completely blocks the ear canal.
The Inner Ear
The inner ear is an organ located deep within the temporal bone, which is the bone of the skull on both sides of the head above the outer ear. The inner ear has two parts: semicircular canals and the cochlea. The semicircular canals do not contribute to hearing, but assist in maintaining balance as we move.
The hearing organ of the inner ear is called the cochlea. The cochlea is a fluid-filled structure that looks like a snail. The cochlea changes the mechanical vibrations from the tympanic membrane and the ossicles into a sequence of electrical impulses. Sensory cells, called hair cells, bend in the cochlea as the fluid is disrupted by the mechanical vibrations. This bending of the hair cells causes electrical signals to be sent from the cochlea to the brain by way of the the auditory nerve. The cochlea is arranged by frequency, much like a piano, and encodes sounds from 20Hz (low pitch) to 20,000Hz (high pitch) in humans.
Damage to any part of the hearing system may cause a hearing loss. Click here to read about the different types of hearing loss.