325x325px The outer ear receives sound, transmitted through the ossicles of the middle ear to the inner ear, where it is converted to a nervous signal in the cochlear and transmitted along the vestibulocochlear nerve<unk>  occurs when too much sound intensity is transmitted into and through the auditory system. An acoustic signal from a sound source, such as a radio, enters into the external auditory canal (ear canal), and is funneled through to the tympanic membrane (eardrum), causing it to vibrate. The vibration of the tympanic membrane drives the middle ear ossicles, the malleus, incus, and stapes to vibrate in sync with the eardrum. The middle ear ossicles transfer mechanical energy to the cochlea by way of the stapes footplate hammering against the oval window of the cochlea, effectively amplifying the sound signal. This hammering causes the fluid within the cochlea (perilymph and endolymph) to be displaced. Displacement of the fluid causes movement of the hair cells (sensory cells in the cochlea) and an electrochemical signal to be sent from the auditory nerve (CN VIII) to the central auditory system within the brain. This is where sound is perceived. Different groups of hair cells are responsive to different frequencies. Hair cells at or near the base of the cochlea are most sensitive to higher frequency sounds while those at the apex are most sensitive to lower frequency sounds. There are two known biological mechanisms of<unk>  from excessive sound intensity: damage to the structures called stereocilia that sit atop hair cells and respond to sound, and damage to the synapses that the auditory nerve makes with hair cells, also termed "hidden hearing loss". Physiological response The symptoms mentioned above are the external signs of the physiological response to cochlear overstimulation. Here are some elements of this response: * Damaged sensory hairs (stereocilia) of the hair cells; damaged hair cells degenerate and die. In humans and other mammals, dead hair-cells are never replaced; the resulting hearing loss is permanent. * Inflammation of the exposed areas. This inflammation causes poor blood flow in the exposed blood vessels (vascular stasis), and poor oxygen supply for the liquid inside the cochlea (endolymphatic hypoxia) Those noxious conditions worsen the damaged hair cell degeneration. * Synaptic damages, by excitotoxicity.<unk>  over-stimulation causes an excessive release of glutamate, causing the postsynaptic bouton to swell and burst. However the neuron connection can be repaired, and the hearing loss only caused by the "wiring" (i.e. excitotoxicity) can thus be recovered within 2–3 days. Hair cell damage or death When the ear is exposed to excessive sound levels or loud sounds over time, the overstimulation of the hair cells leads to heavy production of reactive oxygen species, leading to oxidative cell death. In animal experiments, antioxidant vitamins have been found to reduce hearing loss even when administered the day after<unk>  exposure. They were not able to fully prevent it. Antioxidants however do not seem to be effective in protecting the human ear. Damage ranges from exhaustion of the hair (hearing) cells in the ear to loss of those cells.<unk>  is, therefore, the consequence of overstimulation of the hair cells and supporting structures. Structural damage to hair cells (primarily the outer hair cells) will result in hearing loss that can be characterized by an attenuation and distortion of incoming auditory stimuli. During hair cell death 'scars' develop, which prevent potassium rich fluid of the endolymph from mixing with the fluid on the basal domain. The potassium rich fluid is toxic to the neuronal endings and can damage hearing of the entire ear. If the endolymph fluid mixes with the fluid on the basal domain the neurons become depolarized, causing complete hearing loss. In addition to complete hearing loss, if the area is not sealed and leakage continues further tissue damage will occur. The 'scars' that form to replace the damaged hair cell are caused by supporting hair cells undergoing apoptosis and sealing the reticular lamina, which prevents fluid leakage. The cell death of two supporting hair cells rapidly expands their apical domain, which compresses the hair cell beneath its apical domain. Nerve damage Recent studies have investigated additional mechanisms of<unk>  involving delayed or disabled electrochemical transmission of nerve impulses from the hair cell to and along the auditory nerve. In cases of extreme acute acoustic trauma, a portion of the postsynaptic dendrite (where the hair cell transfers electrochemical signals to the auditory nerve) can rupture from overstimulation, temporarily stopping all transmission of auditory input to the auditory nerve. This is known as excitotoxicity. Usually, this sort of rupture heals within about five days, resulting in functional recovery of that synapse. While healing, an over-expression of glutamate receptors can result in temporary tinnitus, or ringing in the ears. Repeated ruptures at the same synapse may eventually fail to heal, leading to permanent hearing loss. Prolonged exposure to high intensity<unk>  has also been linked to the disruption of ribbon synapses located in the synaptic cleft between inner hair cells and spiral ganglion nerve fibers, leading to a disorder referred to as cochlear synaptopathy or hidden hearing loss. This disorder is cumulative and over time, leads to degeneration of the spiral ganglion cells of the inner ear and overall dysfunction in the neural transmission between auditory nerve fibers and the central auditory pathway. The most common symptom of cochlear synaptopathy is difficulty understanding speech, especially in the presence of competing<unk> . However, this type of hearing impairment is often undetectable by conventional pure tone audiometry, thus the name "hidden" hearing loss. Acoustic over-exposure can also result in decreased myelination at specific points on the auditory nerve. Myelin, an insulating sheath surrounding nerve axons, expedites electrical impulses along nerves throughout the nervous system. Thinning of the myelin sheath on the auditory nerve significantly slows the transmission of electrical signals from hair cell to auditory cortex, reducing comprehension of auditory stimuli by delaying auditory perception, particularly in<unk>  environments. Individual susceptibility towards<unk>  There appear to be large differences in individual susceptibility to<unk> . The following factors have been implicated: * missing acoustic reflex * previous sensorineural hearing loss * a bad general health state: bad cardiovascular function, insufficient intake of oxygen, a high platelet aggregation rate; and most importantly, a high viscosity of the blood * cigarette smoking * exposure to ototoxic chemicals (medication or environmental chemicals that can damage the ear), including certain solvents and heavy metals * type 2 diabetes
Noise-induced hearing loss
Vibroacoustic disease
Conductive hearing loss
Cochlear hydrops
Nasodigitoacoustic syndrome
Musical ear syndrome
Autoimmune inner ear disease
Sensorineural hearing loss