The auditory system is a sensory system that is responsible for the sense of hearing. The ability to hear allows animals to be able to detect sounds in their surroundings without direct contact or without seeing the source of the sound. All mammals including humans have bilateral auditory systems, meaning that there are two sets of auditory structures (ears) on opposite sides of the head. The bilateral orientation of the auditory system helps to localize sound direction by using time delay. Specifically, the time of the arrival of the sound at each ear occurs at slightly different epochs, which is used to find the location of the sound. Overall the auditory system allows mammals to detect, locate, and interpret a plethora of sounds. The auditory system comprises several organs and structures, all of which collect and concentrate sound waves through each anatomical segment to the cochlea (see below) where the waves are converted into electrical impulses. The electrical impulses produced by the cochlea are then sent to auditory processing centers in the brain and are subsequently interpreted as sound.
Anatomy and Physiology
The auditory system is divided into three anatomical segments: the outer ear, middle ear, and inner ear. The outer segment of the ear acts like a cone and captures sound waves; the middle segment of the ear concentrates and directs the vibrations to the inner segment of the ear, which converts the wave into electrical impulses. The neural impulses that are created in the inner ear are then sent to the brain and interpreted as sound in the auditory cortex. As mentioned, the first segment is the outer ear, which is the only portion of the auditory system that is visible externally. On each side of the head there are cartilaginous skin folds that are commonly recognized as the ear. The scientific name for this structure is the pinna, which is Latin for “wing.” At the center of the cone-shaped pinna is the external auditory meatus, which is the opening to the auditory canal. The auditory canal is a small tube that is about 2.5 centimeters long and extends from the external auditory meatus to the internal auditory meatus. Within the auditory canal earwax, or cerumen, is produced by modified sweat glands to help protect the canal from various infections and parasite infestations. The structure found at the end of the auditory canal is a thin tissue layer called the tympanic membrane, more commonly known as the eardrum, which is attached to the internal auditory meatus.
The eardrum is slightly conical shaped and taut, similar to the head of a drum that is stretched over the drum’s opening. The eardrum is attached to the internal auditory meatus and is the first structure of the middle ear. The three smallest bones in the body, the ossicles, are directly after the eardrum. Each of the three ossicles is connected to one another; in order they are the malleus, incus, and stapes. Two small muscles attach the malleus and incus to the walls of the middle ear. These muscles help dampen the vibrations of the ossicles and prevent damage to the middle ear. A small tube, also a part of the middle ear, called the auditory tube or eustachian tube, extends from the middle ear down to the back of the throat or nasopharynx. The auditory tube helps equalize the pressure between the middle ear and the outer ear and prevent eardrum damage. It is this tube that is responsible for the “ear popping” experience when changing altitudes. The “pop” occurs at the eardrum when the pressure between the outer ear and the middle ear equalizes.
The final segment of the ear is the inner ear; it begins at the oval window, which is found attached to the footplate of the stapes. A thin membrane covering the oval window separates the middle and inner ear. Within the inner ear is the organ responsible for converting the mechanical sound waves into an electrical impulse, the cochlea. There are two noticeable landmarks on the external surface of the cochlea adjacent to each other, the oval window and the round window. Externally the cochlea looks like the shell of a snail; internally it consists of three fluid-filled cavities. Within one of the cavities is the organ of Corti, which contains specialized sensory receptors called hair cell receptors. These hair cells convert sound waves into electrical impulses that are sent to the brain to be interpreted into sound.
Collectively hearing can be divided into two processes: transmission and transduction. Transmission is the collection of sound waves from the outer ear to the inner ear, whereas transduction is the conversion of sound waves to electrical impulses. In the process of transmission, sound is captured by the pinna and then funneled toward the auditory canal. As the sound vibrations travel down the auditory canal they are concentrated, then at the end of the canal the waves strike the tympanic membrane. The eardrum vibrates in response to the airwave at the same frequency as the sound. This vibration is then transferred to the ossicles of the middle ear; the small muscles attached to the ossicles tighten in response to the vibrations and dampen any loud or unexpected sounds. Next the vibration reaches the oval window via the footplate of the stapes where the vibrations are transferred to the fluid-filled cavities of the cochlea. In the cochlea the basilar membrane vibrates at the same frequency as the fluid of the cochlea. The organ of Corti, with its hair cell receptors, lies on top of the basilar membrane. Hair cells are so named because they contain tiny hair-like projections on their top surface called cilia. The cilia of the hair cells are embedded into a shelf-like membrane called the tectorial membrane, which does not move. It is important to note that the hair cells are “trapped” between two different membranes, one that moves with the fluid of the cochlea and the other that does not move at all. Thus, when a sound wave causes the basilar membrane to move, the hair cells move as well. However, since the cilia are stuck in place, the base of the cilia bends back and forth. It is this back and forth movement of the cilia that produces transduction of sound. The bending force on the cilia changes the resting membrane voltage of the hair cell, causing the voltage to be more positive, which then creates an action potential at the cochlear nerve. The cochlear nerve joins the vestibular nerve to form the eighth cranial nerve (vestibulocochlear nerve or cranial nerve VIII). Auditory signals produced in the cochlea are sent to several areas of the brain for processing such as the auditory cortex, thalamus, and the brainstem. The auditory cortex, located within the temporal lobe of the brain, interprets the signal as the conscious perception of sound, whereas in the brainstem, signals are used in reflexes and feedback in the auditory system.
In a normal functioning auditory system, sound transmission and sound transduction are not impeded. This means that a human with normal hearing should be able to hear sounds within the 20 to 20,000 Hz (hertz) range. There are several disease processes and pathologies that can reduce this range of hearing permanently; however, some disease processes may change the normal range of human hearing only temporarily. Diseases that perpetually affect the ability to hear are referred to as deafness. Typically, deafness is caused by damage at or near the cochlea where sound is transduced. Hearing loss is categorized as either conductive deafness or neural deafness. Issues that impede the conduction of sound waves typically cause conductive deafness such as sinus congestion, earwax buildup, middle ear infection, or even eardrum perforation. Conductive deafness can easily be treated by various medical interventions. Sinus infections and ear infections can often be cleared with antibiotic treatment, and once treated the normal hearing range should return. Chronic ear infections caused by buildup of fluid in the middle ear can damage the cochlea if not treated. The typical treatment for chronic ear infections is surgical placement of artificial auditory tubes, which help drain excess fluid. Eardrum perforations generally occur directly from a perforating instrument, like a cotton-tipped swab being inserted too deeply, or indirectly via a blow to the head or other head trauma. When the eardrum is perforated an individual will experience sharp pain and mild conductive hearing loss. The membrane can heal on its own with full recovery of hearing; however, some severe tears require surgical repair before hearing returns to normal.
See also: Cochlea; Cochlear Implants; Deafness; Inferior Colliculus; Sound Localization; Thalamus; Tonotopic Map; Vestibulocochlear Nerve
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