Overview / Cochlea / Auditory pathway / Audiograms
Scientific concept: Jérôme Ruel, Jean-Luc Puel, Nuno Trigueiros Cunha
Drawings: P. Minary, S. Blatrix

Cochlear potentials
There are 2 types of cochlear potentials : individual unit potentials recorded directly from a sensory cell or nerve cell, and compound potentials, recorded at a distance, reflecting the activity of several cells.

Single unit potentials
Single unit potentials recorded from the sensory cells are also known as receptor potentials.
These potentials are recorded via a micro-electrode placed directly into the cell (intracellular recordings).Single unit potentials of the primary auditory neurones can be recorded either at the level of the dendrites (as shown), the cell bodies in the spiral ganglion, or at the level of the auditory nerve fibres.

Hair cell receptor potentials
The electrical response of the hair cells to an acoustic stimulus (shown at the bottom of the figure) is made up of two components : a continuous component (CC), duplicating the acoustic stimulus envelope, and an alternating component (AC), superimposed on the CC, corresponding to the pure sound frequency. These continuous and alternating components depend on the frequency of sound stimulation. The amplitude of the CC grows with the frequency, whereas the CA is more important for low frequencies. The CA is more important in the outer hair cells than the inner hair cells, and the CC is more dominant in the inner hair cells (ref. Russell et al.).

Auditory nerve single unit potentials

A sound stimulation (click) leads to an increase inthe numbre of single unit potentials in the individual fibres of the auditory nerve. This increase in the rate of discharge, synchronised with the sound stimulation, is called an evoked potential. The presence of action potentials during periods of silence is a reflection of the basal activity of the nerve fibre, also known as its spontaneous activity.

Composite cochlear potentials

Electrocochleography (ECochG) is the name given to the recording of cochlear potentials. Under local anaesthetic, a thin needle electrode is placed through the tympanic membrane onto the promontory, near the round window niche.(ref. Aran et al.)
The gross cochlear response complex is recorded in response to a sound stimulus (i.e. it is an evoked response), and it is made up of various components. The use of appropriate filters allows the identification of these components, which in turn correspond to the various anatomical structures implicated in the mechanotransduction of sound and in the transmission of auditory information.

Compound action potential (CAP)
The compound action potential (CAP) is the result of synchronous activity of the auditory nerve fibres. The CAP amplitude is measured between N1 and P1. The summating potential (SP) reflects the continous component of the sensory hair cells, mainly the IHCs.

Microphonic potential
The cochlear microphonic, which closely resembles the sound stimulus, is a reflection of the alternating component, which mainly originates from the OHCs..


Oto-acoustic emissions (OAEs)
OAEs reflect the activity of outer hair cells, which are the most sensitive cells of the organ of Corti. They are very useful for screening newborns, but are also widely used as as an objective test in subjects of any age.

see details on the specific OAEs page


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