Glossary
is a bundle of approximately 30,000 nerve fibres that carries hearing information between the cochlea and the brain.
treatment of the cause of the impairment, as opposed to treating the symptom. A cochlear implant, for example, treats the symptom of hearing loss. Unfortunately, the same still applies to all forms of tinnitus treatment.
the snail-shaped bony duct containing the sensory organ of hearing, the organ of Corti. The word "cochlea" is a Latin word derived from the Greek "kokhlos" designating the land snail.
The extraordinary sensitivity, frequency selectivity and dynamic range of the cochlea are achieved by a so-called cochlear amplifier. This is an amplifier which produces mechanical force, to act against damping forces in the cochlea. Severe damage to the cochlear amplifier causes deafness.
a device for electrical stimulation of the nerves by means of wires inserted into the cochlea.
decibel sound pressure level, a logarithmic measure of the pressure of a sound. It is defined as the logarithm to the base 10 of the ratio of the sound pressure to a reference pressure. By convention, the reference pressure is 20 µPa. This reference value was chosen because, on average, it is the smallest amount of pressure required for a young adult to detect the presence of a sinusoid of frequency between 1000 Hz and 4000 Hz. That is, on average, the hearing threshold is 0 dB SPL for this frequency range.
diagnosis of the underlying causes of the hearing impairment, with precise localization of the injured tissue in the cochlea, as opposed to simply the diagnosis that the patient is suffering from sensorineural hearing loss. Prerequisites for a differential diagnosis are that the mechanisms are understood, and that the technology is available for diagnosis.
is the process which forms vesicles from the plasma membrane and allows particles to enter into the cell within the vesicles (endocytosis) or release the content of vesicles into the extracellular fluid (exocytosis).
see endocytic activity
are the sensory receptor cells responsible for hearing. Along the organ of Corti, there is one row of inner hair cells and three rows of outer hair cells. The human cochlea has about 3,500 inner hair cells and 12,000 outer hair cells. The upper end of a hair cell contains stereocilia, which transduce their deflection into a current. The lower portion of a cell is innervated: the nerves of the inner hair cells send information to the brain, whereas the nerves of the outer hair cells receive information from the brain. The inner hair cells are, therefore, the true sensory cells of hearing. The outer hair cells are electromotile; they transduce a change of potential across their wall to a longitudinal displacement of the entire cell. It is as if the human cochlea contains 12,000 piezoelectric microphones. There is strong evidence that this type of electromotility is the basis of mechanical amplification in the cochlea. The neural input from the brain is capable of modulating the electromotility, thus enabling a control of amplification by the brain.
contains two sensory systems: the cochlea for detecting sound and the vestibular system for detecting accelerations to provide sense of balance.
see hair cell
form multiple, highly ordered layers beneath the plasma membrane. They make up a substantial fraction of the endoplasmic membrane of the outer hair cell.
a device for transducing a mechanical signal into an electrical signal. A microphone is such an example. In the inner ear, the transducer is located in the stereocilia, which are located at the upper surface of the sensory cells. Mechanical deflection of the stereocilia modulates the opening probability of ion channels in the stereocilia, thus modulating the flow of ions into the stereocilia and then into the cell.
recycling of the membrane by means of vesicle transport.
an air-filled cavity extending from the ear drum to the bony cochlear wall. It contains a chain of three bones, which transform sound-induced vibration of the ear drum into motion of the fluids in the cochlea.
the sensory receptor of the organ of hearing; it is named after Alfonso Corti, who first described it. The organ extends along the entire length of the cochlear duct. It consists of the sensory cells and so-called supporting cells, which are required for structural and biochemical support. It transforms changes of fluid pressure in the cochlea, induced by vibration of the middle ear, into electrical signals in the auditory nerve.
are acoustic signals generated by the normal cochlea, either in the absence of acoustic stimulation (spontaneous emissions) or in response to acoustic stimulation (acoustically-evoked emissions) or electrical stimulation (electrically-evoked emissions). They are a by-product of the electromechanical action of the outer hair cells. Acoustically evoked otoacoustic emission testing gives an objective indication of whether the cochlear amplifier is functioning normally.
toxic (poisonous) to the ear. Some pharmaceuticals are known to have an ototoxic side effect when given in therapeutic doses.
see hair cell
Strictly speaking, this relates to the sensory cells and the components of the nerves. However, when describing hearing loss it commonly refers to damage to all structures in the cochlea.
tiny hairs inserted into the upper surface of each sensory cell; hence, the name “hair” cell. A stereocilium has a diameter of about 150 nm and a length of 2-8 um, dependent on the position of the cell along the cochlea and also on the position of the stereocilium on the cell. The stereocilia are mechanoelectrical transducers.
is a semigelatinous ribbon-like structure overlying the hair cells. The tallest stereocilia of the outer hair cells are embedded in the tectorial membrane, whereas the stereocilia of the inner hair cells appear to be either entirely free-standing or only tenuously attached to the tectorial membrane.
is the name given to the condition of noises “in the ears” and/or “in the head” with no external source. It is estimated that 1 out of every 5 people experience some degree of tinnitus.