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5.1. Structure of the Spiral Ganglion

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5.1. Structure of the Spiral Ganglion CHAPTER 5. INNERVATION OF THE ORGAN OF CORTI The investigation of nerve components of the acoustic system’s periph- eral part is so difficult methodologically that a whole series of questions which were solved long ago during the investigation of the other sensory system have not yet been clarified. The basic difficulty for the morphologists lies in the fact that the organ of Corti, together with its nerve elements, is located within the osseous tissue. In addition, it is in the shape of a spirally involut- ed geometrical figure. These structural peculiarities create considerable dif- ficulties during the determination of the connections between the types of peripheral and central neuron’s processes and bodies of the spiral ganglion of the cochlea. Therefore, most of the work on the cochlea’s innervation and the computation of the different element’s quantity demands an application of special methods, including graphical reconstruction of the serial sections. The Golgi method was and still remains the basic histological method of the organ of Corti’s innervation study, which has been supplemented by the cochlea’s electron-microscope investigations in the normal conditions and during the experimentally induced degenerations. 5.1. Structure of the spiral ganglion The neurons which innervate the auditory receptor cells form a spiral ganglion: a nerve-knot of the VIII pair’s acoustic part of the craniocerebral nerves. The ganglion fills the Rosental’s canal in the cochlea’s axis and re- peats the number of its spiral turns. The ganglionic neuron has, as a rule, a widened body with two processes: peripheral and central (Diagram 7). The body is covered with a complicated myelinated membrane or capsule. The peripheral process (dendrite) penetrates to the organ of Corti through an aperture in habenula perforata; a cat has 2500 of such apertures [Spoend- lin, 1966]. The dendrite is covered with a myelin (medullated) membrane, which disappears only at the entrance of the habenula’s aperture. In terms of electrophysiology, the peripheral process as well as the central one is an 112 PART I. PERIPHERAL PART OF THE AUDITORY SYSTEM OF MAMMALS IN POSTNATAL ONTOGENY axon, since the place of the myelin membrane’s termination is morphologi- cally similar to the Ranvie’s interception [Engström, Wersall, 1958]. Accord- ing to existing conceptions, this is exactly the area that is analogous to the initial neuron’s segment, where the action potential is generated. The cen- tral processes (axons) form the auditory nerve, the fibers of which, com- bined with vestibular ones, enter the CNS in the area of the medulla’s and pons’s boundary (Diagram 7). In the perikaryon of the cat’s and the rat’s bipolar ganglionic neurons, the same organelles as in other nerve cells are described [Spoendlin, 1966]. The cytoplasm contains cisterns with slit-like outgrowths. They are surround- ed by a 10-nm thick membrane, on the surface of which granules of a 10-nm diameter are situated. In addition to these membranes of the granular en- doplasmic reticulum, there is a system of non-granular membranes, mainly C C 10 µm N P P Diagram 7. Two neurons of the spiral ganglion form the middle turn of the cochlea of a six-day-old rat, showing atypical ramification of the central processes (C) near the cellular body. P — peripheral processes (dendrites), N — cellular nucleus. Golgi method, trans-modiolar (horizontal) shear plane [Perkins, Morest, 1975]. CHAPTER 5. INNERVATION OF THE CORTI’S ORGAN 113.
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