Innervation of the organ of Corti
Overview / Spiral ganglion / Neurotransmitters
Pictures : R. Pujol


The spiral ganglion is formed from the primitive otocyst. It differentiates very early, before the organ of Corti. In man, it is composed of 30 to 35,000 bipolar neurons of two main types. Large and myelinated type I neurons (accounting for more than 90%) are connected to inner hair cells; small and unmyelinated type II neurons are connected to outer hair cells. Both types have central axons delivering messages to the cochlear nuclei.

The two types of spiral ganglion neurons

Three type I neurons (blue arrows) and one type II neuron (green arrow) are seen on the left, together with sections of myelinated fibers (axons from neighboring neurons). On the right, the bipolar nature of one type I neuron is better shown.
scale bars: 10 µm, left; 5 µm, right.

Type I neuron and its satellite glial cell

This type I neuron is ensheathed by processes from a satellite glial cell which form a thin myelin sheath. Note the dense cytoplasmic content, with numerous mitochondria, indicating the high metabolic status of the cell. On the right is the axon hillock.
scale bar: 3 µm.

Myelinated fiber

Cross-section of a myelinated intra-ganglionic auditory fibre (axon from a type I neuron). Within the axon, 3 mitochondria and numerous microtubules are seen. The myelin sheath is formed of about 30 layers (see electronic zoom).
scale bar: 150 nm.

Early stage of myelination

During development, intra-ganglionic auditory fibres (asterisks), are ensheathed by a glial cell process. Two of these glial cells (yellow arrows) have already formed a myelin sheath of several layers (see electronic zoom).
scale bar: 0.5 µm

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