Editing Sensorineural hearing loss (section)

====Cochlear hair cells====
[[File:Cochlea-crosssection.svg|left|thumb|280px|Figure 3: Cross-section of the cochlea.]]

Outer [[hair cells]] (OHCs) contribute to the structure of the [[Organ of Corti]], which is situated between the [[basilar membrane]] and the [[tectorial membrane]] within the cochlea (See Figure 3). The tunnel of corti, which runs through the Organ of Corti, divides the OHCs and the inner hair cells (IHCs). OHCs are connected to the reticular laminar and the Deiters' cells. There are roughly twelve thousand OHCs in each human ear, and these are arranged in up to five rows. Each OHC has small tufts of 'hairs', or cilia, on their upper surface known as [[stereocilia]], and these are also arranged into rows which are graded in height. There are approximately 140 stereocilia on each OHC.<ref name="Gelfand">Gelfand SA. Hearing: An Introduction to Psychological and Physiological Acoustics. 4th ed. New York: Marcel Dekker; 2004.</ref>

The fundamental role of the OHCs and the IHCs is to function as [[sensory receptors]]. The main function of the IHCs is to transmit sound information via [[afferent neurons]]. They do this by transducing mechanical movements or signals into neural activity. When stimulated, the stereocilia on the IHCs move, causing a flow of electric current to pass through the hair cells. This electric current creates [[action potentials]] within the connected afferent neurons.

OHCs are different in that they actually contribute to the active mechanism of the cochlea. They do this by receiving mechanical signals or vibrations along the basilar membrane, and transducing them into electrochemical signals. The stereocilia found on OHCs are in contact with the tectorial membrane. Therefore, when the basilar membrane moves due to vibrations, the stereocilia bend. The direction in which they bend, dictates the firing rate of the auditory neurons connected to the OHCs.<ref name="Moore-Whurr">Moore BCJ. Cochlear Hearing Loss. London: Whurr Publishers; 1998.</ref>

The bending of the stereocilia towards the [[basal body]] of the OHC causes excitation of the hair cell. Thus, an increase in firing rate of the auditory neurons connected to the hair cell occurs. On the other hand, the bending of the stereocilia away from the basal body of the OHC causes inhibition of the hair cell. Thus, a decrease in firing rate of the auditory neurons connected to the hair cell occurs. OHCs are unique in that they are able to contract and expand (electromotility). Therefore, in response to the electrical stimulations provided by the efferent nerve supply, they can alter in length, shape and stiffness. These changes influence the response of the basilar membrane to sound.<ref name="Gelfand" /><ref name="Moore-Whurr" /> It is therefore clear that the OHCs play a major role in the active processes of the cochlea.<ref name="Gelfand" /> The main function of the active mechanism is to finely tune the basilar membrane, and provide it with a high sensitivity to quiet sounds. The active mechanism is dependent on the cochlea being in good physiological condition. However, the cochlea is very susceptible to damage.<ref name="Moore-Whurr" />

=====Hair cell damage=====

SNHL is most commonly caused by damage to the OHCs and the IHCs.{{disputed inline|date=November 2015}}  There are two methods by which they might become damaged. Firstly, the entire hair cell might die. Secondly, the stereocilia might become distorted or destroyed. Damage to the cochlea can occur in several ways, for example by viral infection, exposure to ototoxic chemicals, and intense noise exposure. Damage to the OHCs results in either a less effective active mechanism, or it may not function at all. OHCs contribute to providing a high sensitivity to quiet sounds at a specific range of frequencies (approximately 2–4&nbsp;kHz). Thus, damage to the OHCs results in the reduction of sensitivity of the basilar membrane to weak sounds. Amplification to these sounds is therefore required, in order for the basilar membrane to respond efficiently. IHCs are less susceptible to damage in comparison to the OHCs. However, if they become damaged, this will result in an overall loss of sensitivity.<ref name="Moore-Whurr" />