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Both types of hair cells are innervated by specific afferent and efferent systems forming a loop to and from the brainstem.
Innervation of inner (1) and outer hair cells (2)
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The radial afferents (blue) and the lateral efferents (pink) innervate the inner hair cells; the spiral afferents (green) and the medial efferents (red) innervate the outer hair cells. |
IHC innervation
The IHC is synaptically connected to all type I spiral ganglion neurons (refs. a1, c5) forming the radial afferent system (blue) going to the cochlear nuclei (CN). The lateral efferent system (pink) arising from small neurons in the ipsilateral lateral superior olivary complex (LSO) brings a feedback control to the IHC/type I afferent synapse.
OHC innervation
The OHC synapses with a few (at least in basal and mid-portions of the cochlea) small endings from type II spiral ganglion neurons (ref. c1), forming the spiral afferent system (green). In turn, large neurons of the medial efferent system (red), from both sides of the medial superior olivary complex (MSO), form axo-somatic synapses with the OHC.
Schematic representation of the hair cells afferent innervation
Type I (blue) spiral ganglion neurons (95% of the ganglion neurons) have a single ending radially connected to IHCs.
Type II (green) small, unmyelinated neurons spiral basally after entering the organ of Corti and branch to connect about ten OHCs, generally in the same row.
Scheme from Liberman
The spiral (cochlear) ganglion
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.
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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). Scale bar: 10 µm |
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Two type I neurons and one type II (arrow) Scale bar: 5 µm |
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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 osseous spiral lamina fibres
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Cross-section of a myelinated osseous spiral lamina fibre (peripheral 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 |
35 turns of the myelin sheath are visible. The axon cytoplasm is seen to the right. Scale bar: 70 nm
An auditory axon, with microtubules and neurofilaments, is being ensheathed by glial cell processes which are forming about 7 to 8 layers of myelin (compare with the adult sheath).Scale bar: 100 nm
