What we think of as the ear is really just a very small part of the system responsible for our hearing. The anatomy of the ear can be divided into four major structures:
- outer ear
- middle ear
- inner ear
- central auditory pathways
Like most anatomy the ear is remarkably complex.
Outer Ear. The outer ear is made up of several parts: the pinna, or auricle, is the frame of cartilage with skin. The pinna has folds, depressions and a prominent bowl, each with their own name:
Helix. Fold surrounding the auricle.
Scapha. Hollow at the end of the helix.
Root of the helix. Part of the helix.
Anthelix. Curved ridge in front of the helix.
Concha. Deep fossa of the external ear.
Tragus. Prominence over and in front of the acoustic meatus.
Antitragus. Ear projection opposite the tragus.
External auditory meatus. Mouth of the canal of the temporal bone that carries sounds to the eardrum.
Lobe. Fleshy part at the base of the ear.
The shape of an individual’s ear is unique and can be used like finger or retinal prints. The pinna collects and funnels sound down the ear canal. It also enhances some sounds because of its resonance characteristics.
The ear canal is curved, almost “S” shaped and averages about 1 inch in length in adults. The ear canal is two-thirds cartilaginous framework, and the inner one-third is bony. The skin of the external ear canal is continuous with the skin of the pinna. The skin of ear canal has hairs and glands that produce wax called cerumen that protects the ear canal. Cerumen also helps lubricate the skin and keep it moist.
The middle ear begins at the end of the external auditory canal with a special membrane called the eardrum or tympanic membrane. This thin membrane stretches across the entire inner end of the ear canal separating it from the middle ear. The tympanic membrane is constantly working (even when we are sleeping) to transform fluctuations in air pressure (sound) into exact vibrational copies.
On the inner side of the tympanic membrane is an air-filled space called the middle ear cavity which contains the three smallest bones in the human body, two muscles, a number of ligaments, and the opening of the Eustachian tube.
The vibrations of the tympanic membrane are transmitted to the bones in the middle ear, which are known as the ossicles or the ossicular chain. This ossicular chain articulates with the tympanic membrane through the malleus (hammer) to the incus (anvil), which in turn communicates with the inner most ossicle called the stapes (stirrup).
The ossicles are suspended from the roof of the middle ear cavity by tiny ligaments, and the malleus is also connected to the tympanic membrane by a ligament. There are two muscles located in the middle ear, one of which, the stapedius, is attached to the stapes and contracts when very loud sounds are detected.
The Eustachian tube is a muscular tunnel that opens and closes to provide fresh air to and drain debris from the middle ear space and is very important to maintaining the health of the middle ear. It also serves to equalize the pressure between the outer and middle ear space.
The inner ear has two functions and divisions for hearing and balance. The hearing division consists of the cochlea and nerves. The cochlea is a snail-shaped, bony structure made up of three fluid-filled compartments that run the cochlea’s entire length. The middle compartment contains the sensory organ for hearing called the organ of Corti. The organ of Corti responds when the cochlear fluid activates its tiny hair cells and releases chemical messengers that stimulate the nerves that carry sound stimuli to the brain.
The balance mechanism is also called the vestibular system. It is also made up of a series of fluid-filled compartments that contain the sense organs for balance and movement.
Central Auditory Pathways
The central auditory pathways are short and long segments organized like circuits. This system begins as the nerves enter the brainstem. From there the neural pathway makes its way to the cerebral cortex at the temporal lobe of the brain. Right ear information is directed to the left temporal lobe, and left ear information goes to the right temporal lobe. In most people, the left side of the brain processes speech and other complex language functions, while tonal noise and music are processed by the right side of the brain.
How We Hear
Our ears transform acoustic stimulus into the type of neural code that our brains can recognize, process and understand.
Sound waves enter from the outer ear through the ear canal to the middle ear. From there all sorts of mechanical, hydraulic, neurochemical and electrical responses take place.
The tympanic membrane converts sound into a mechanical code which is transmitted through the ossicular chain to the inner ear at the stapes footplate. It’s here the code is transformed again, this time into hydraulic energy, so it can be transmitted through the fluid-filled cochlea. When the fluid waves stimulate the cochlea’s hair cells a neurochemical event takes place which excites the hearing nerves. The characteristics of the original sound are preserved at every energy change along the way.
The ears and the brain work in a truly remarkable way to process sound into the sense of hearing and all that it encompasses. We actually hear with our brain, not with our ears!