Audition Promenade round Cochlea autour Cochlée oreille ear organ of Corti C.R.I.C. CRIC Montpellier Remy Pujol

	Outer Hair Cells (OHCs)
	Overview / Coupling / Membrane / Synapses / Active mécanism / Oto-émissions
	Drawings: S. Blatrix; Pictures: R. Pujol

**OHC synapses**
R. Pujol	The basal pole of an OHC is contacted by large vesiculated terminals of the medial efferent system. Amongst these efferent synapses can be found one or two little dendritic boutons (green arrow) of a spiral afferent fibre (type II neurons). This type of organisation is characteristic of most of the OHCs in that part of the cochlea that codes for the mid- and high frequencies (2 to 20 kHz). d = Deiters' cell; o = OHC Electronic zoom scale bar: 1 µm.

High magnification of an efferent synapse

R. Pujol

OHC in the basal turn of a rat cochlea.

A large vesiculated terminal contacts the base of an OHC (o), which is itself in close contact with the Dieters' cell (d); green arrows indicate two small afferent dendritic boutons.

The zoom allows good visualisation of the postsynaptic cistern (blue arrows) and the presynaptic microvesicles, often arranged in small packets close to the membrane.

scale bar: 1 µm.


high magnification of synapses between OHC (o) and the type II afferent dendrites (a)
R.Pujol	R.Pujol
On serial sectioning, one often encounters an invagination of a spiral afferent dendritic bouton (a) in the OHC (o). On both sides, one can see membrane densifications (yellow arrows). Some vesicles are seen on the postsynaptic side (blue arrow). On the right, one can see the internal covering of the endocytic vesicle (red arrow). scale bars: 250 nm


Base / apex differences
R.Pujol	Synaprtic poles of an OHC (o) at the apex of the cochlea: start of the fourth turn (above) and extreme apex (below) in the guinea pig. In contrast to the basal turn, the synaptic pole ot the OHC is essentially surrounded by afferent dendritic boutons. The vesiculated efferent terminals (red arrows) arevery infrequent and the postsynaptic differentiation incomplete (blue arrow). d = Deiters' cell. Electronic zoom scale bar: 1 µm

R.Pujol	N.B. This type of synaptic organisation is characteristic of the part of the cochlea that codes for low frequencies: it is more widespread in man (almost the whole of the apical turn of the cochlea). This base / apex difference can be explained by the development of the OHCs: see the diagram of synaptogenesis below. Electronic zoom

Synaptogenesis below the OHC (after ref.b5) diagrams by Stéphan Blatrix after Marc Lenoir et Rémy pujol
Stage 1 - The base of the immature OHC is only connected to afferent terminals, of the spiral system (type II, in green) and of the radial system (type I, in blue); opposite the latter, many synaptic bodies are visible. Compare this stage with the synaptic organisation of an OHC in the extreme apical portion of an adult cochlea .
Stage 2 - At the time the cochle first begins to function, the first efferent synapses are visible (red), while the number of radial afferent terminals (blue) regresses. Compare this stage with the synaptic organisation of an OHC at the apex of an adult cochlea.
Stage 3 - At the end of maturation, one finds the classic synaptic organisation of an OHC: large efferent terminal (red) and small type II afferent (green). This type of arrangement is typical of those OHCs in the region coding for mid- to high frequencies, i.e. most of the cochlea.

In conclusion, everything occurs as if the development of the synapses under the OHCs were adapted to their physiological properties: the OHCs of the apex keep a classical sensory function (with an innervation which is little different to that of the IHCs), whilst the OHCs in the basal part of the cochlea, having become electro-motile, are equiped with a regulatory system for this new function.