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Hearing is one of the five human senses. The others are vision, smell, taste, and touch. Hearing is the process of picking up sound and attaching meaning to it.
The outer ear (pinna and ear canal) collect sound pressure and direct it toward the eardrum. Acoustically, the canal concentrates incoming sound pressure creating a slight 5-15 decibel enhancement primarily in the 2000-3000Hz range. Pulling the ear forward and/or putting one's hand to the ear further concentrates incoming sound pressure, thereby making sound louder (i.e., 10-15 decibels higher) by the time it reaches the eardrum. Hearing loss in the outer ear usually involves blockage due to ear wax, but can involve anatomical disorders that restrict sound transmission. Most, but not all, abnormalities of the outer ear can be rectified through medical and/or surgical means to improve hearing. Less than 5% of hearing losses are caused by this type of hearing loss, known as conductive hearing loss.
The middle is comprised of the eardrum and three middle ear bones (hammer, anvil and stirrup). The middle ear further concentrates sound pressure, but the degree of concentration is significant - by 30-50 decibels. The middle ear is the part that "pops" when we equalize air pressure. The eustachian tube communicates between the back of the throat and middle ear and normally remains closed. If air pressure within the middle ear is different compared to the air pressure outside the body, hearing will be affected. Hearing loss in the middle ear can result from eardrum perforations, extreme stiffness or disarticulation of the middle ear bones, poor eustachian tube function, fluid, tumors, or infection. Most, but not all, abnormalities of the middle ear can be rectified through medical and/or surgical means. Less than 5% of hearing losses are caused by this type of hearing loss, known as conductive hearing loss.
The inner ear is a fluid-filled space made up of the cochlea and vestibular system. The vestibular system is part of the body's balance system and contributes very little to hearing. The cochlea, however, is a very important part of the hearing mechanism with its function to change incoming acoustic sound pressure into nerve impulses. The acoustic-to-nerve transfer is created in the hair cells of the cochlea. Most people with hearing loss have abnormalities in the inner ear. With advancing age, overexposure to noise, or hereditary/genetic pre-dispositions (as the most common factors), the important hair cells die and never recover. Anatomically, hair cells associated with high frequency reception are more vulnerable to damage. As such, most people with hearing problems have "high frequency hearing loss." This reduces one's ability to perceive unvoiced consonant sounds, making word clarity increasingly more difficult. Very few abnormalities of the inner ear can be effectively treated through medical and/or surgical methods, requiring most to resort to hearing aids to augment hearing ability. More than 90% of all hearing losses are caused by this type of hearing loss, known as sensory hearing loss.
The acoustic nerve (cranial nerve #8) is the nerve bundle that communicates the inner ear with the body's brainstem. Hearing problems involving the acoustic nerve result in extremely reduced word clarity and usually involve benign tumors through the internal auditory meatus and/or the aging process This type of hearing loss is referred to as neural hearing loss.
BRAINSTEM AND CORTICAL PROCESSING CENTERS
The least understood of the hearing process is the brain. When nerve impulses reach the brainstem, there are projections to many areas of the brain's cortex, but most tend to navigate to the areas sensitive to speech reception and speech motor function. This is usually located in the left temporal lobe. Hearing problems due to abnormalities of the brainstem and/or cortical processing centers cause distortions in sound clarity and are the result of tumors, stroke, aging, and/or head trauma. The type of hearing loss is referred to as central hearing loss.
CAN MORE THAN ONE AREA BE INVOLVED?
Yes, and this is often the case. Patients with hearing problems tend to be older and have abnormalities involving the acoustic nerve, brainstem, and/or cortical processing centers, in addition to sensory hearing loss. Hearing loss involving the inner ear is often described as sensori-neural hearing loss.