Morphology, synaptology and electrophysiology of the developing cochlea.

The development of synapses clearly differentiates the two kinds of cochlear receptors inner (IHC) and outer (OHC) hair cells. An adult-like pattern of innervation can be seen very early below the IHCs: both afferent and efferent (mainly axo-dendritic) synapses are formed before the onset of function. At the OHC level, drastic changes occur: numerous afferent dendrites retract as the large efferent endings arrive and synapse with the hair cell. This synaptic OHC remodelling coincides with maturation of compound action potential (AP) parameters, such as the "S" -shape of the input-output curves and the sharpening of AP tuning curves.