Assessment Tools: Introduction to the Anatomy and Physiology of the Auditory System
|By: Sarah E. Ervin, M.A., CCC-A
The following article is written to provide a general understanding of the structures within the auditory system and how they function. The auditory system is comprised of three components; the outer, middle, and inner ear, all of which work together to transfer sounds from the environment to the brain.
THE OUTER EAR
The outer ear includes the portion of the ear that we seethe pinna/auricle and the ear canal.
The ear canal maintains the proper conditions of temperature and humidity necessary to preserve the elasticity of the tympanic membrane. Glands, which produce cerumen (earwax) and tiny hairs in the ear canal, provide added protection against insects and foreign particles from damaging the tympanic membrane.
The middle ear is composed of the tympanic membrane and the cavity, which houses the ossicular chain.
The eardrum is very sensitive to sound waves and vibrates back and forth as the sound waves strike it. The eardrum transmits the airborne vibrations from the outer to the middle ear and also assists in the protection of the delicate structures of the middle ear cavity and inner ear.
Middle Ear Cavity
The ossicular chain consists of the three smallest bones in the body: the malleus, incus, and stapes. The malleus is attached to the tympanic membrane. The footplate of the stapes inserts into the oval window of the inner ear. The incus is between the malleus and the stapes.
Attached to the ossicular chain are two tiny muscles, the stapedius and tensor tympani muscles. These muscles contract to protect the inner ear by reducing the intensity of sound transmission to the inner ear from external sounds and vocal transmission.
THE INNER EAR
The inner ear is composed of the sensory organ for hearingthe cochlea, as well as for balancethe vestibular system. The systems are separate, yet both are encased in the same bony capsule and share the same fluid systems.
Vestibular or Balance System
The cochlea is composed of three fluid-filled chambers that extend the length of the structure. The two outer chambers are filled with a fluid called perilymph. Perilymph acts as a cushioning agent for the delicate structures that occupy the center chamber. It is important to note that perilymph is connected to the cerebrospinal fluid that surrounds the brain and the spinal column. The third fluid filled chamber is the center chamber, called the cochlear duct. The cochlear duct secretes a fluid called endolymph, which fills this chamber.
The cochlear duct contains the Basilar membrane upon which lies the Organ of Corti. The Organ of Corti is a sensory organ essential to hearing. It consists of approximately 30,000 finger-like projections of cilia that are arranged in rows. These cilia are referred to as hair cells. Each hair cell is connected to a nerve fiber that relays various impulses to the cochlear branch of the VIIIth cranial nerve or auditory nerve. The "pitch" of the impulse relayed is dependent upon which areas of the basilar membrane, and hence, which portions of the Organ of Corti are stimulated. The apical portion of the basilar membrane (the most curled area of the cochlea) transfers lower frequency impulses. The basal end relays higher frequency impulses.
The VIII cranial nerve (VIII C.N.) or auditory C.N. carries the impulses generated from the Organ of Corti to the brainstem. From the brainstem, nerve pathways extend through numerous nuclei to the cerebral cortex in the temporal lobes of the brain. It is in the temporal lobes of the brain that meaning is associated with the various patterns of nerve impulses.
THE PHYSIOLOGY OF HEARING
The process of hearing begins with the occurrence of a sound. Sound is initiated when an event moves and causes a motion or vibration in air. When this air movement stimulates the ear, a sound is heard.
In the human ear, a sound wave is transmitted through four separate mediums along the auditory system before a sound is perceived: in the outer earair, in the middle ear mechanical, in the inner ear liquid and to the brainneural.
Sound Transmission through the Outer Ear
Sound Transmission through the Middle Ear
Sound Transmission through the Inner Ear
Sound Transmission to the Brain
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