The human ear can be functionally divided into four main sections: the external ear, the middle ear, the inner ear, and the hearing processing centers of the brain. All four parts work together to conduct sounds from outside the head to the brain and to process it. This article will explain the function of each part of the hearing system. The hearing mechanisms of the brain (central processing centers) will not be discussed in this article because of the extreme complexity.
The Middle Ear
The middle ear consists of an air-filled space between the tympanic membrane and the inner ear that contains three tiny bones linked together that connect the tympanic membrane to an opening into the inner ear. It also contains tiny ligaments and muscles that support and adjust tension of the bony chain.
The TM is a concave shaped layer of membrane at the end of the ear canal. Sounds travel down the ear canal and strike the TM, causing it to vibrate. These vibrations are then transferred through the "OSSICLES" (the tiny bones previously mentioned) to the "COCHLEA" (inner ear). The first bone is the "MALLEUS" (hammer) and is attached to the inside surface of the TM. The second bone is the "INCUS" (anvil) and the innermost bone is the "STAPES" (stirrup). Sound sets this whole structure into vibration and the footplate of the stapes vibrates within the oval window opening of the cochlea, transferring sound energy to the fluids and tissues of the cochlea.
There is a small tube that connects the middle ear space to the back of the throat and is known as the "EUSTACHIAN TUBE." This tube is normally closed but opens momentarily with yawning, swallowing, etc. This periodic opening maintains equalization of the air pressure between the middle ear and the outside air pressure. This pressure must be equalized for most effective transfer of sound through the middle ear. If it becomes unequal as in relatively rapid altitude change, the sudden opening of the Eustachian tube produces a "pop" and improved hearing because pressure balance is restored.
The Inner Ear
The inner ear is comprised of two functionally separate sections: The "VESTIBULAR" or balance part and the cochlea, which is the hearing part. These two parts are interconnected and each serves its own vital function.
The purpose of the vestibular portion is to help us sense acceleration/deceleration of both rotational and linear motion and to sense head position in relation to gravity. Equally important, it is part of a reflex arc that makes it possible for us to maintain sharp visual focus with the many small and rapid motions of the head that occur as we engage in walking, riding, chewing, etc. Vestibular function is described in more detail in another article so will not be covered in further detail here.
The cochlea is a coiled canal in the dense bone tissue of the skull. The shape of this canal somewhat resembles a snail shell and houses three fluid-filled membranous canals extending its full length. The central canal houses the "ORGAN OF CORTI" which is comprised of specialized cells and their supporting tissues. Vibratory energy propagated through the fluid produces deformation of the organ of Corti, in turn resulting in shearing forces on tiny tufts of "hairs" or cilia extending from the upper surfaces of the "hair cells." This shearing action triggers an electro-chemical signal that travels upward through the auditory nervous pathway which passes through the internal auditory canal to the brainstem and then upward to the auditory processing centers in the temporal lobes of the brain. This is a simplified description of a fascinatingly complex activity that ultimately results in "hearing."