Tests For Hearing
In electrocochleography: [AIIMS May 20 12]
|A||It measures middle ear latency|
|B||Outer hair cells are mainly responsible for cochlear microphonics and summation potential|
|C||Summation potential is a compound of synchronus auditory nerve potential|
|D||Total AP represents endocochlear receptor potential to an external auditory stimulus|
Ref. Mohan Bansal, Textbook of Diseases ofENT tst/ed p24,25 and 145
Electrocochleography (EcoG) measures electrical potentials, which arise in cochlea and CN VIII in response to auditory stimuli within first 5 milliseconds. It consists of following three types of responses
a. Cochlear microphonics
b. Summating potentials
c. Action potential of 8th nerve
Endocochlear potential, cochlear microphonics (CM) and summating potential (SP) are from cochlea while the compound action potential (AP) is from the cochlear nerve fibers. Both CM and SP are receptor potentials similar to other sensory end-organs.
Endocochlear Potential: This resting potential of +80 mV direct current (DC) is recorded from scala mediA. This energy source for cochlear transduction is generated from stria vascularis by Na+/K+ -ATPase pump. Endolymph has high K+ concentration. It acts as a battery and helps in driving the current through the hair cells when they move after exposure to any sound stimulus.
Cochlear Microphonics: Cochlear microphonics (CM) is an alternating current (AC)·potential. Basilar membrane moves in response to sound stimulus. Changes occur in electrical resistance at the tips of OHC. Flow of K+ through the outer hair cells produces voltage fluctuations and called CM.
Cochlear microphonics is absent in the part of cochlea where the outer hair cells are damageD.
Summating Potential: Summating potential (SP) is a DC potential, which may be either negative or positive. It is produced by hair cells. It follows the "envelop" of stimulating sound and is superimposed on cochlear nerve action poten-tial. This is a rectified derivative of sound signal. Probably it arises from IHCs with a small contributionJrom OHCs. Summating potential of cochlea helps in the diagnosis of Meniere's diseases.
Compound.(Auditory Nerve) Action Potential: It is the neural discharge of auditory nerve. It follows all or none phenomena so has all or none response to auditory nerve fibers. Each nerve fiber has optimum stimulus frequency for which the threshold is lowest. Amplitude increases while latency decreases with intensity over 40-50 dB range. The following features differentiate it from CM and SP:
A. No gradation B. Latency
C. Propagation D. Post-response refractory period
The recording electrode (a thin needle) is placed on the prorn-ontory throuqh the tympanic membrane. The testcan be done under local anesthesia however children and anxious uncoop-erative adults need sedation Of general anesthesia, which has no effect on EcoG responses.
The main application of ECOG is to help determine if a patient has Meniere disease. The amplitude of the summating potential (reflecting activity of the hair cells) is compared with that of the compound action potential (reflecting whole nerve activity). If the ratio is larger than normal (0.3-0.5), it is considered indicative of Meniere disease. The procedure is considered valid only the patient is symptomatiC. Now with this background lets analyse each option separately-
Option a - is incorrect as ECOG is a measure of electrical potential of inner ear (and not middle ear latency).
Option b - is correct as explained above - Outer hair cells are mainly responsible for cochlear microphonics and Summation Potential.
Option c - is incorrect as it is not the summating potential but the action potential which is a compound of synchronous auditory nerve potential.
Option d - is incorrect as Action Potential represents neural potential and not the endocochlear receptor potential which is represented by components arising from organ of corti that . SP and cochlear microphonics.