Sound consists of sinusoidal waves of air pressure, and hearing is the perception of sound. The understanding of speech depends on the perception of complex, time-varying multifrequency sound. Hearing is possible when the pitch or frequency range of sound is between 20 and 20,000 cycles/sec (Hz) and when the sound is appropriately loud. The loudness of sound is measured in decibels. Clinically, such as during audiometry, decibels are expressed as units of hearing level, and are computed by the formula DB = 20 log Ptest /Pref , where Ptest is the sound pressure level in question and P ref is the sound pressure just barely audible by a group of young adults. Tinnitus is the false perception of a sound, or the perception of a sound that is not normally perceived, such as the pulse. A single sinusoidal tone corresponds to a ringing noise. Complex multifrequency sounds may be perceived as hissing, buzzing, roaring, or even as speech or music.
Hearing loss consists of a reduction, relative to normal standards, of the ability to perceive sound. Three subtypes are recognized: conductive, sensorineural, and central. In conductive hearing loss, mechanical transmission of sound into the sensory receptors in the cochlea is impaired. In sensorineural hearing loss, there is a loss of function in the sensory receptors in either the cochlea or the auditory nerve. In central hearing loss there is a lesion in the brain stem or auditory cortex.
Vertigo is a false sensation of movement that is usually caused by disorders of the vestibular system, including the inner ear and/or parts of the central nervous system involved in processing of vestibular signals. Vertigo is often accompanied by imbalance as well as secondary symptoms such as nausea and fatigue. Dizziness is a less specific term often used by patients to indicate vertigo as well as a host of other symptoms such as giddiness or lightheadedness, confusion, and imbalance. The vestibular system senses movement by detecting angular velocity and linear acceleration. The semicircular canals are excellent detectors of angular (rotational) velocity. The otolith organs are detectors of linear acceleration that may be related to either movement of the head or changes in orientation to the earth's gravitational field.
The history of the vestibulocochlear system can be traced from the times of the ancient Greeks. [1 Plato theorized that "hearing is motion initiated by sound in the ears and ending in the region of the liver." Aristotle (384 BC to 322 BC) outlined the concept of "inner ear" in the occiput of the skull as forming the basis of hearing. In 97 AD, Rufus of Ephesus described the auricular anatomy of the inner ear using the words helix, antihelix, and tragus, which are terms that are still in use today. Galen (130-200 AD) introduced the term labyrinth to describe the inner structure of the inner ear in temporal bone.
Vesalius (1564) named two of the ossicles, the malleus (drumstick or hammer) and incus (anvil). Eustachio (1510-1571) discovered the eustachian tube, which maintains equal pressure on both sides of the tympanic membrane. In 1850, Corti described the sensory epithelium of the cochlea and the organ of Corti is his namesake. Rinne, in
1855, developed the use of tuning fork tests in the diagnosis of middle ear disease. Meniere, in 1861, described a disorder of increased pressure in the inner ear, a disease that now bears his name. Barr, in 1901, described the use of a ticking watch to quantify hearing loss. Barany received the Nobel Prize in 1909 for his work describing the caloric test. Fletcher in 1923 described the electric audiometer. Bekesy received the Nobel Prize in the 1950s for his discovery of the traveling wave mechanism for the stimulation of the cochlea. In more recent years, the surgical specialty of neuro-otology has emerged, driven by availability of the operative microscope and the need for advanced training and subspecialization to surgically treat otosclerosis and acoustical neuromas.
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