The inner ear and how we hear
Embedded in the temporal bone, the inner ear contains the crucial organs of hearing and balance. It’s also known as the labyrinth of the ear. The bony labyrinth has three parts—the cochlea, vestibule, and semicircular canals—and is filled with a sodium-rich liquid called perilymph. Inside the bony labyrinth are a membranous labyrinth with three parts—the cochlear duct, the saclike structures known as the saccule and utricle, and three semicircular ducts. The membranous labyrinth is filled with potassium-rich endolymph fluid.
Cochlea: Where hearing happens
The snail-shell-shaped cochlea contains three fluid-filled chambers:
- scala vestibuli (or vestibular duct)
- scala media (or cochlear duct)
- scala tympani (or tympanic duct)
These structures are separated by membranes. The scala vestibuli and the scala tympani, both filled with perilymph, are joined by an opening in the cochlea called the helicotrema. The vestibular membrane, also called Reissner’s membrane, lies between the scala vestibuli and the cochlear duct (scala media). The cochlear duct is filled with endolymph. The basilar membrane separates the cochlear duct from the scala tympani. On the surface of the basilar membrane rests the organ of Corti, named after the Italian anatomist Alfonso Corti. The organ of Corti contains rows of hair cells that produce nerve impulses in response to sound vibrations.
Hair cells in the inner ear
The cochlea has two types of hair cells: inner and outer hair cells. You are born with around 3,500 inner hair cells and 12,000 outer hair cells in each cochlea. Sticking out of these cells are hair-like structures known as stereocilia, or hair bundles, which respond to fluid motion. The outer hair cells amplify low-level sound and the inner hair cells convert sound into electrical signals. These signals are then relayed to the brain via the auditory nerve.
Hair cells in the vestibular (or balance) system respond to gravity and motion. Our hair cells can be damaged by noise-induced injury, infections, certain medications or the natural process of ageing. Because it cannot regenerate, loss of hair cells in the cochlea can lead to some degree of hearing loss. Loss of hair cells in the vestibular system can cause dizziness, vertigo and balance dysfunction.
Vestibular system: A sense of balance
The inner ear’s vestibular system is responsible for our sense of balance. It contains the vestibule and the semicircular canals. The two membranous structures within the vestibule, the utricle and the saccule respond to linear acceleration and tilts of the head relative to gravity.
Otoliths (ear stones), calcium carbonate particles attached to hair cell stereocilia in the utricle and saccule, pull on the stereocilia to bend it in the direction of gravity.
The three fluid-filled semicircular canals—known as the anterior (superior), horizontal and posterior canals—are interconnected and respond to head rotation. The hair cells in these canals are covered by a jelly-like membrane. When the head rotates, the canal fluid lags a little behind the motion of the head and pushes on the ‘jelly’ membrane, which bends the stereocilia structures.
Hair cells in the vestibular system convert information about these motions into neural signals that can go to the brain.
How do we hear?
The inner ear is crucial to how we hear. When we’re exposed to sound, the outer ear collects sound waves and funnels it to the tympanic membrane (eardrum). The eardrum absorbs these sound waves, vibrates, and passes it on to the ossicles (three tiny bones in the middle ear—malleus, incus and stapes). The stapes rocks back and forth and transmits the sound waves from the middle ear to the fluid of the cochlea. In the cochlea, this wave motion allows the basilar membrane to vibrate, the organ of Corti to move and hair cells to send signals to the brain.
The inner ear and hearing aids
Age-related or noise-related damage to hair cells, infections and tumours are only some of the reasons you may experience hearing difficulty. Hearing aids can help improve your quality of life, whether you’re always outdoors and around people or spend most of your time indoors. We can help you find the solution that perfectly fits your needs, budget and lifestyle.