How Hearing Works

Anatomy of the Human Ear

                                                                                                                             Photo by Mark Thomas Acosta

The ear is comprised of three portions: an outer ear (external), a middle ear, and an inner ear. Each part performs an important function in the process of hearing and balance.

The outer ear consists of the auricle and the ear canal. These structures gather the sound and direct it towards the eardrum (tympanic membrane).

The middle ear chamber lies between the external and inner ear behind the eardrum. This chamber is normally air filled and connected to the back of the throat (pharynx) by the eustachian tube, which serves as a pressure-equalizing valve. The middle ear consists of an eardrum and three small ear bones (ossicles): the malleus (hammer), incus (anvil), and stapes (stirrup). These structures transmit sound vibrations to the inner ear. In so doing they act as a transformer, converting sound vibrations in the external ear canal into fluid waves in the inner ear. A disturbance of the eustachian tube, eardrum, or the ear bones may result in a conductive hearing impairment, meaning impairment of sound conduction to the inner ear. This type of impairment is usually correctable medically or surgically.

The inner ear contains microscopic hair cells connected to hearing nerve endings. The hair cells are bathed in fluid. Inner ear fluid waves move the delicate hair cells, which in turn excite the hearing nerve endings and transmit sound energy information by the hearing nerve to the brain, where it is interpreted into sound. A disturbance in the inner ear fluids, hair cells, nerve endings, or hearing nerve may result in a sensorineural hearing impairment. Most often, this type of hearing impairment is due to a hair cell loss. This type of impairment is not correctable with surgery. The inner ear is also responsible for sensation of motion and helps with balance. The three semi-circular canals and two otolithic organs orientate our body in three dimensional space.

The Mechanics of Hearing

Hearing occurs as sound enters the outer ear canal and causes vibrations of the tympanic membrane (eardrum). This subsequently produces movement of the middle ear bones. The piston-like action of the stapes bone (stirrup) initiates a fluid wave within the perilymph of the scala vestibule. This “traveling wave” in turn activates the hair cells of the organ of Corti, causing the hearing nerve to be activated. The hair cells are responsible for converting the mechanical energy of the fluid wave into an electrical signal, which will be processed by the brain. There are actually two different types of hair cells in the cochlea: inner and outer hair cells. Inner hair cells are the special sensory receptors that receive the traveling wave information and relay it to the brainstem. The outer hair cells are responsible for amplifying the traveling wave signal, and “fine tuning” the signal to a frequency specific region of the cochlea. Hearing loss may arise from the outer, middle, and inner ear, as well as from the cochlear nerve, brainstem, or temporal lobe. A loss that arises as a result of a blockage of sound energy from reaching the inner ears is referred to as conductive; whereas, a loss that results from injury to the inner ear, hearing nerve, or brain is called “sensori-neural.” Although a clinical exam will often identify the type of hearing loss, an audiogram (hearing test) is usually necessary to pinpoint the location, as well as the severity. Specialized hearing tests and (sometimes) imaging studies are sometimes performed if the cause of the loss is still unknown.