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Anatomy of the Human Ear
The ear is comprised of three portions: an outer ear (external), a middle ear, and an inner ear. Each part performs animportant function in the process of hearing and balance.
The outer (external) 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 air filled chamber is 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 hearing nerve endings (hair cells) bathed in fluid. Inner ear fluid waves move the delicate nerve endings, which in turn 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 balance, the three semi-circular canals 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) and subsequently the 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 nerve to discharge. 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 hear 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 or central structures are 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 (uncommonly) imaging studies are sometimes performed if the cause of the loss is still unknown.
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The Mechanics of Balance
The balance portion of the inner ear is made up of three semicircular, fluid filled canals which join a larger, globular structure called the vestibule. Similar to the cochlea, the semicircular canals are tubular structures filled with endolymph and surrounded by perilymph. The hardest bone in the body the labyrinthine bone, surrounds the entire structure. Each semicircular canal is oriented at right angles to the others, comprising vertical, horizontal, and posterior (behind) canals. The right and left semicircular canals are mirror images of each other, so that every direction of angular head motion is represented by both ears, oppositely. At the junction of each semicircular canal and vestibular is a special receptor for angular rotational movements of the head, referred to as the crista. The crista contains hair cells embedded in a gelatinous matrix, with accompanying nerve fibers. As the head turns in a particular direction, the fluid within that semicircular canal turns in the opposite direction bending the hair cells and inducing a neural discharge. That signal is sent through the vestibular (balance) nerve to the brain where it is interpreted, and adjustments are made in eye movements and postural control. This ensures that the eyes remain on a given target, and that the arms and legs remain in a good position for maintaining stable posture. Within the vestibule are hair cells that respond to changes in head and body movements in the horizontal and vertical planes. These “otolithic” hair cells are embedded in a layer of calcium carbonate, making them top heavy and therefore motion sensitive for both linear accelerations and gravitational forces.
Nerve fibers from the crista and the otolithic organs form two large balance nerves, the superior and inferior vestibular nerves. They travel from the inner ear to the brainstem along with the cochlear and facial nerves. Within the brainstem they form an extensive neural network involving nerves from the eyes, ears, the cerebellum, and positional receptors “proprioceptors” located in the arms, legs and neck. The brain interprets this information, and makes modifications in eye, head and body position to maintain a fixed eye position, and erect posture. Unfortunately, there are also connections to the thalamic region of the brain, which is responsible for the nausea and vomiting which accompanies most disturbances within the vestibular system. A sensation of disequilibrium may accompany any imbalance or dysfunction within this neural network. Therefore, similar symptoms of imbalance or “dizziness” may be experienced by the injury to the eye, ear, brain, and proprioceptors from the extremities. Therefore, it is often difficult to determine the exact site of injury based on symptoms alone, and further diagnostic testing is necessary.
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