dorsal nucleus (the deepest zone) or on the boundary between the DCN and the VCN (the ... the ventral nucleus and a zone occupying the posterior zone of the.
RELATIONSHIP BETWEEN THE LEVEL OF ORIGIN OF PRIMARY FIBERS IN THE RAT COCHLEA AND THEIR SPATIAL DISTRIBUTION IN THE RAT COCHLEAR NUCLEI M. Merchan, D. E. Lopez, E. Saldana, and F. Collia Chair of Biology, Faculty of Medicine University of Salamanca C/Fonseca, 37007, Salamanca, Spain INTRODUCTION The distribution pattern of the primary afferents to the cochlear nuclei was proposed by Cajal (1909) and Lorente de N6 (1933). Later it was analyzed by different authors in several different species (for a review, see ~esen et al., 1978). These authors concluded that the primary fibers branch in a "V" shape and that they have a geometric distribution that in all cases is related to a frequency; that is, to a particular origin in the cochlea. OUr group has analyzed the problem with transganglionic HRP transport and silver staining methods (Merchan, 1985 a, b) and has concluded that apart from the tonotopic "V" shapes reported by Lorente de N6 (1933) there are nerve fibers that give rise to two primary plexuses (anterior and posterior, Fig. 1) that have a different distribution. The primary anterior plexus is mainly distributed throughout the more lateral regions of the AVCN (zone with bushy and stellate neurons). The posterior primary plexus is distributed in the posterior territory of the PVCN (zone with octopus neurons) and has two terminal forms, either on the dorsal nucleus (the deepest zone) or on the boundary between the DCN and the VCN (the grain cell zone) and the surface of the AVCN. MATERIAL AND METHODS Thirty Wistar rats weighing 200 g were used. The animals were anaesthetized with equithesin and the left cochlea was exposed, making a small orifice in the apical spiral (lO), another at the beginning of the medial spiral (5) and another at the end of the medial spiral (5). Orifices were also made at the beginning of the basal spiral (5) and another at the end of this spiral (5). A small metal electrode was inserted into the orifice; this was connected to an electrocauterization device, thereby producing small lesions in the organ of Corti. After a degeneration period of 4 days, the rats were perfused with 15% saline formalin and the cochlea were decalcified, embedded in paraffin and serially sectioned to contrast the area and characterist:j.cs of the lesion. The cochlear nuclei were sectioned serially along the parasagittal plane at 25 ~m and observed according to the technique of Fink-Heimer to observe the extension and topographic distribution of the degenerated fibers.
J. Syka et al. (eds.), Auditory Pathway © Plenum Press, New York 1988
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Fig. 1.
Camera lucida drawing of parasagittal sections from lateral to medial (A-D) representing the primary cochlear fibers in the cochlear nuclei after transganglionic tracing with HRP injected into the cochlea. Figure A shows the existence of a dense plexus in the anterior region (asterisk). Figure B illustrates the bunch-like disposition of the primary afferents. Figures C and
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(cont'd) D show the classic "V"-shaped arrangement of the cochlear fibers and the existence of a posterior plexus (asterisk).
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Fig. 2.
Schematic drawing of the cochlear lesion at apical (1), medial (3) and basal (5) level.In the three figures the black area represents the zone of most i ntense lesion (neuronal loss greater than 85%) . The gray area represents the zone of least neuronal loss (neuronal loss greater than 15%). (2) Pattern resulting from apical lesions. The degenerated territories correspond to the posterior-most
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Fig. 2.
(cont'd) region of the posterior plexus and to the anterior plexus. (4) Pattern resulting from medial lesions. The degenerated territories correspond to the central zone of the cochlear nuclei, in particular to the territory (without plexuses) corresponding to the distribution of the tonotopic "V"s. (6) Pattern resulting from basal lessions. An area of degeneration can be seen corresponding to the anterior-most region of the posterior plexus. As a result and according to distribution, most of the fibers of this localization on the boundary between the dorsal and ventral nuclei and the superficial region of the anterior nucleus also appear in this territory.
The results were drawn with a camera lucida and these pictures were used to construct schema accounting for all the affected fibers. RESULTS After focal lesion of the cochlea at different levels, three patterns of degeneration of the primary fibers in the cochlear nuclei are obtained. The lesion in the apical spiral (Fig. 2-1) produces a fibrillar degeneration in the cochlear nuclei that in parasagittal sections covers the anterior and ventral territory of the anterior division of the ventral nucleus and a zone occupying the posterior zone of the posterior division of the ventral nucleus which curves inwards and penetrates the deep portion of the dorsal cochlear nucleus (Fig. 2-2). The lesion in the medial spiral (Fig. 2-3) yields a "V"-shaped imp,ge of degeneration that occupies the boundary between the anterior and posterior division of the ventral cochlear nucleus and later extends, bifurcating in the anterior and posterior direction (Fig.2-4). The zone of degeneration does not reach the surface of the ventral nucleus. The shorter posterior zone of degeneration does not penetrate the territory of the posterior plexus (Fig. 1-1, 2-4). . Finally, the lesion of the basal spiral (Fig. 2-5) produces a zone of degeneration that extends linearly in the ventral dorsal direction along the anterior part of the posterior division of the ventral cochlear nucleus and on reaching the boundary between the ventral and dorsal cochlear nuclei curves in the anterior direction until it reaches the surface of the antero-ventral cochlear nucleus (Fig. 2-6). SUMMARY The anterior-most region of the posterior plexus and the anterior plexus have a common origin, either because they correspond to fibers originating at the same cochlear level that uncoil in different directions or because they are branches of a common fiber whose bifurcation is not possible to observe with the HRP technique (Fig. 1). The classic pattern of Lorente de No coincides only with the degeneration patterns corresponding to the lesions of the medial levels of the cochlea (Fig. 2-2). The degeneration pattern of the more basal level corresponds to the territory of the distribution of dendrites of the octopus neurons (anterior zone of the posterior plexus) and to the area of distribution of the third plexus (boundary of the VCN and DeN and surface of the AVCN) which means that it is located in the zone of the grain cells and superficial stellate neurons.
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The findings of the present work allow us to conclude that the classic tonal scheme should be revised, at least in the case of the rat. REFERENCES Arnesen, A. R., Osen, K. K. and Mugnaini, E., 1978, Temporal and spatial sequence of anterograde degeneration in the cochlear nerve fibers of the cat, J. Camp. Neurol., 178:679-696. Lorente de N6, 1933, Anatomy of the eighth nerve. The central projection of the nerve endings of the internal ear, Laryngoscope, 43:1-38. Merchan, M. et al., 1985a, Distribution of the p~imary afferent fibers in the cochlear nuclei. A silver and horseradish peroxidase (HRP) study, J. Anat., 141:121-130. Merchan, M. et al., 1985b, Distribution of primary cochlear afferents in the bulbar nuclei of the rat: a horseradish peroxidase (HRP) study in parasagittal sections, J. Anat., 144:71-80. Ramon y Cajal, 1909, Histologie du Systeme Nerveux de L'Homme et des Vertebres, Paris: Maloine·.
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