Exp. Brain Res. 30, 511-526 (1977)
Experimental Brain Research 9 Springer-Verlag1977
The Pontine Projection to the Flocculonodular Lobe and the Paraflocculus studied by means of Retrograde Axonal Transport of Horseradish Peroxidase in the Rabbit G.H. Hoddevik Anatomical Institute, University of Oslo, Karl Johansgt. 47, Oslo 1, Norway
Summary. The occurrence and distribution of labeled cells in the pontine nuclei were mapped following injections of small amounts of horseradish peroxidase (0.05-0.5 ~1, 50 % suspension) in the flocculus, nodulus and the dorsal and ventral paraflocculus in adult albino rabbits. While no labeled cells were found in the pontine nuclei following injections in the nodulus, some were present following injections in the flocculus and a great number following injections in the paraflocculus. The projections onto the flocculus and paraflocculus are precisely organized. Following injections in the paraflocculus labeled neurons are arranged in four columns (E and G in the paramedian pontine nucleus, F in the peduncular and H in the dorsolateral nucleus). Following injections in the ventral paraflocculus labeled cells are present only in parts of column E and F, while columns G and H and parts of E and F project onto the dorsal paraflocculus. Following injections in the flocculus labeled cells occur in the rostral part of column E only. A comparison between the sites of termination of pontine afferents and the areas giving origin to floccular and parafloccular fibers shows that only few fibers mediating visual impulses end in these pontine areas, while they receive numerous fibers from gyrus cinguli and areas 18 and 19 of the cerebral cortex. Key words: P o n s - Flocculus - Nodulus - Paraflocculus - Rabbit
Introduction The present paper is the fourth in a series aiming at determining the areas within some precerebellar nuclei which project onto the cerebellar visual areas. So far the projection of the inferior olive and the pontine nuclei onto the vermal visual area (Hoddevik et al., 1976; Hoddevik et al., 1977) and the projection of the inferior olive onto the flocculonodular lobe and the paraflocculus (Hoddevik and Brodal, 1977) have been described.
512
G.H. Hoddevik
Since Maekawa and Simpson (1972, 1973) described activation of climbing fibers in the vestibulocerebellum following visual stimulation, a pathway via the inferior olive has been extensively studied. The recent observation of mossy fiber activation in a small part of the flocculus following electrical stimulation of the optic disc in rabbits (Maekawa, 1975; Maekawa and Takeda, 1975 and 1976) has prompted a search for precerebellar nuclei which give rise to mossy fibers terminating in the vestibulocerebellum. Since the pontine nuclei are an important source of mossy fibers to the cerebellum and in addition receive afferents from several cortical and subcortical visual areas (see Discussion), it is a relevant problem to investigate whether these nuclei project onto the vestibulocerebellum. This is the primary aim of the present study. In addition the projection to the paraflocculus has been mapped because of the close anatomical relation between the paraflocculus and the flocculus and because there are reasons to believe that even the paraflocculus may have some teleceptive function. The projection of the pontine nuclei proper only will be described. The reticular tegmental pontine nucleus (of Bechterew), a part of the reticular formation (Brodal, 1957), is not included. Its projection onto the cerebellar visual areas will be the subject of the fifth paper in this series. Furthermore, the present study is a link in a more extensive investigation of the whole pontocerebellar projection by means of the method of retrograde transport of horseradish peroxidase (HRP). So far the projection onto the paramedian lobule (Hoddevik, 1975), the vermal visual area (Hoddevik et al., 1977) and the anterior lobe (Brodal and Walberg, 1977) in the cat have been described. The main reason for using rabbits in the present study is that physiological experiments on activation of the vestibulocerebellum via visual pathways have been in rabbits. However, it is considered likely (see Discussion) that the results obtained in the present study are valid for the cat as well. The method of retrograde transport of HRP has proved to be valuable for determining precisely the pontine regions giving origin to fibers terminating in the cerebellum. Following the demonstration of Brodal that lesions in the cerebral cortex give rise to circumscribed columns of degeneration in the pontine nuclei (Brodal, 1968a and b, 1971a and b, 1972a, b and c), columns of labeled cells have likewise been shown to be present in the pons following injections of HRP in the paramedian lobule (Hoddevik, 1975), the vermal visual area (Hoddevik et al., 1977) and the anterior lobe (Brodal and Walberg, 1977). If the pontine nuclei project to the vestibulocerebellum and the parafiocculus, it appears likely that the fibers arise from similar, limited areas. It will further be of interest to compare pontine areas found to project onto the flocculonodular lobe and/or the paraflocculus with the pontine regions already determined as receiving afferents from visual "centers". Materials and Methods The material in this study is largely the same as that used in the study of the olivocerebellar projection onto the flocculonodular lobe and the paraflocculus (Hoddevik and Brodal, 1977). Of 58 adult albino rabbits weighing 1.7-2.9 kg, 28 could be used in the present study. With a 1 ~xl Hamilton syringe 0 . 0 5 - 0 . 5 ~tl of a 50 % suspension of H R P (either Sigma VI or Serva) was injected
Pontocerebellar Projection
513
under Mebumal anesthesia. The superficial injections (in the paraflocculus) were performed either manually or by means of a hydraulic perfusor, the deep ones (in the flocculus and nodulus) always by a hydraulic perfusor. After injection, the needle was left in situ for half an hour. One to three days postoperatively the animals were perfused under deep Mebumal anesthesia with a mixture of 0.4 % formaldehyde and 1.25 % glutaraldehyde in phosphate buffer. The brains were dissected free, the blocks isolated and immersed in the fixative for 24 hours, then transferred to a 30% sucrose/phosphate buffer solution for another 24 hours. The blocks were cut on a freezing microtome (50 ~m sections, the pons transversally or horizontally, the cerebellum transversally or sagittally). The sections were activated with 3-3' diaminobenzidine and H202 in groups of five according to Graham and Karnovsky (1966). From each group two sections were selected and mounted. One series was left unstained, the other lightly counterstained with cresyl violet. Labeled cells were plotted in drawings of the pontine nuclei made in a projection apparatus. As a control some series were also drawn by means of a pantograph. To facilitate comparison among cases, the findings in each case were transferred to a diagram of the pontine nuclei, prepared from a series from an unoperated adult rabbit (used in Figs. 3 and 5). This diagram is almost identical to that of Brodal and Jansen (1946), based on newborn rabbits, and their nomenclature of the nuclear subdivisions is adopted. A series of transverse sections through the cerebellum of normal rabbits (Brodal, 1940b) together with drawings of sagittally cut sections of the cerebellum (Larsell, 1970), served as guides for the identification of the cerebellar lobules.
Results Following injections of HRP in the flocculonodular lobe and the paraflocculus a f a i r n u m b e r o f l a b e l e d n e u r o n s is f o u n d i n t h e p o n t i n e n u c l e i o n l y w h e n t h e p a r a f l o c c u l u s is i n v o l v e d . T h e d i s t r i b u t i o n o f l a b e l e d cells is n o t r e s t r i c t e d t o p a r t i c u l a r p o n t i n e n u c l e i . H o w e v e r , t o f a c i l i t a t e t h e d e s c r i p t i o n r e f e r e n c e will b e m a d e t o t h e v a r i o u s n u c l e i as d i s t i n g u i s h e d b y B r o d a l a n d J a n s e n ( 1 9 4 6 ) . A c a s e with a large injection which covers most of the left paraflocculus and the f l o c c u l u s , will b e p r e s e n t e d first, f o l l o w e d b y s o m e c a s e s w i t h i n j e c t i o n s restricted to parts of the flocculus, nodulus or paraflocculus. In rabbit R 59 (Fig. 1) following two injections in the dorsal and one in the ventral paraflocculus on the left side there is staining of the molecular and granular layer of the entire flocculus and heavy staining of these layers in the dorsal and ventral paraflocculus except for the medialmost folium of the dorsal paraflocculus. There is weak staining and some labeled cells in the left lateral and interposed cerebellar nuclei. Purkinje cells in the lateralmost portion of Crus II and in the posterior vermis (lobules IX and X) are faintly stained. The right half of the pons is cut horizontally, the left transversally. Numerous labeled neurons occur in the rostral 3/4 of the pontine nuclei on both sides, with a slight contralateral preponderance. The labeled cells are aggregated in four main groups or columns (referred to below as columns E, F, G and H). Column E (horizontal sections I-VI, transverse levels 6 - 1 0 in Fig. 1) is the most conspicuous group with a great number of labeled cells. It is localized mainly to the paramedian and the ventral pontine nuclei, but extends a little into the peduncular nucleus as well Somewhat fewer cells are present in column F (horizontal sections I-IV, transverse levels 2-7 in Fig. 1). At some levels the labeled cells are arranged in slender, transversally oriented, bands (horizontal sections I and II, transverse levels 4 and 5 in Fig. 1). The column is mainly localized in the paramedian and the lateral pontine nuclei, but extends into the peduncular and ventral nuclei as well. A smaller number of labeled cells are present in column G (horizontal sections IV-VI, transverse levels 5 - 8 in Fig. 1) situated close to the midline in the paramedian and in parts in the medial pontine nuclei (Fig. 2). As column H are indicated a few labeled neurons, which are found in the dorsolateral pontine nucleus (horizontal sections V and VI, transverse levels 3 - 8 in Fig. 1). Findings corresponding to those described above are made in two other cases (rabbits R 5 and 57, not illustrated). In the former the staining of the cerebellum is restricted to the lateral three folia of the dorsal paraflocculus and the rostralmost folium of the ventral paraflocculus as well as the
514
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Fig. 1. Diagrammatic representation of findings in rabbit R 59 with a large injection covering most of the left flocculus and paraflocculus. A Staining of the cerebellar cortex transferred to a diagram of the unfolded cerebellar surface (taken from Brodal, 1940b) and as seen in a transverse section through the caudal part of the cerebellum. Black denotes heavy staining, hatchings weak staining. Dotting in the cerebellar nuclei indicates labeled cells. B Labeled neurons are indicated by dots in equally spaced horizontal sections of the right half of the pons. Different densities of dotting indicate different densities of labeled cells. Rectangle in section IV refers to the photomicrograph (Fig. 2). C Labeled cells are indicated by dots in equally spaced transverse sections through the left half of the pons. Broken lines in B and C indicate borders between pontine nuclei. D A diagram to show four main columns (E, F, G and H) of labeled cells (see text) and their longitudinal extent as reconstructed from the drawing above. Since the distribution of labeled cells is similar on both sides of the pontine nuclei, only one half is illustrated in the diagram. Different densities of dotting refer to different densities of labeled cells. The numbers refer to the transverse levels in C. For abbreviations see list on p
Pontocerebellar Projection
515
List of Abbrevations
Flocc.: flocculus L. pro.: paramedian lobule N. dl.: dorsolateral pontine nucleus NI: nucleus interpositus NL: nucleus lateralis N. 1.: lateral pontine nucleus NM: nucleus medialis N. m.: medial pontine nucleus Nod.: nodulus N. p.: peduncular pontine nucleus N. pm.: paramedian pontine nucleus N. v.: ventral pontine nucleus P. fl. d.: dorsal paraflocculus P. fl. v.: ventral paraflocculus Uv.: uvula VIIIB, IX, X: vermal lobules of Larsell
Fig. 2. Photomicrograph showing density of labeled cells in column G in rabbit R 59 (see rectangle in section IV in Fig. 1B). The sharp limitation of the column is clearly seen. Dark field illumination. Magnification X 140
caudalmost folium of the flocculus. There is no spreading to the cerebellar nuclei. In rabbit R 57 there is heavy staining of all parts of the dorsal and ventral paraflocculus and the caudal portion of the flocculus, but in addition some staining of the lateral third of the dentate nucleus and the white matter and adjoining parts of the granular layer of Crus II and the paramedian lobule. Cases with injections covering one only of the lobules labeled in the above cases give information of whether the columns described project onto different lobules.
Flocculus In rabbit R 28 (Fig. 3) there is staining of the cortex in a restricted part of the rostralmost folium of the left flocctdus without spreading to the paraflocculus or to the cerebellar nuclei. However, in the rostralmost folium of the paramedian lobule, through which the needle was inserted, there is good staining of a tiny patch.. In thepontine nuclei only few labeled cells are found, all of them within the rostral part of column E described above. Most of the cells are located ventrally, only a few dorsally. To see if the spreading to the paramedian lobule could invalidate the result, two cases (rabbits R 52 and 61, not illustrated) with injections in the rostral part of the paramedian lobule were studied. In
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Experimental Brain Research 9 Springer-Verlag1977
The Pontine Projection to the Flocculonodular Lobe and the Paraflocculus studied by means of Retrograde Axonal Transport of Horseradish Peroxidase in the Rabbit G.H. Hoddevik Anatomical Institute, University of Oslo, Karl Johansgt. 47, Oslo 1, Norway
Summary. The occurrence and distribution of labeled cells in the pontine nuclei were mapped following injections of small amounts of horseradish peroxidase (0.05-0.5 ~1, 50 % suspension) in the flocculus, nodulus and the dorsal and ventral paraflocculus in adult albino rabbits. While no labeled cells were found in the pontine nuclei following injections in the nodulus, some were present following injections in the flocculus and a great number following injections in the paraflocculus. The projections onto the flocculus and paraflocculus are precisely organized. Following injections in the paraflocculus labeled neurons are arranged in four columns (E and G in the paramedian pontine nucleus, F in the peduncular and H in the dorsolateral nucleus). Following injections in the ventral paraflocculus labeled cells are present only in parts of column E and F, while columns G and H and parts of E and F project onto the dorsal paraflocculus. Following injections in the flocculus labeled cells occur in the rostral part of column E only. A comparison between the sites of termination of pontine afferents and the areas giving origin to floccular and parafloccular fibers shows that only few fibers mediating visual impulses end in these pontine areas, while they receive numerous fibers from gyrus cinguli and areas 18 and 19 of the cerebral cortex. Key words: P o n s - Flocculus - Nodulus - Paraflocculus - Rabbit
Introduction The present paper is the fourth in a series aiming at determining the areas within some precerebellar nuclei which project onto the cerebellar visual areas. So far the projection of the inferior olive and the pontine nuclei onto the vermal visual area (Hoddevik et al., 1976; Hoddevik et al., 1977) and the projection of the inferior olive onto the flocculonodular lobe and the paraflocculus (Hoddevik and Brodal, 1977) have been described.
512
G.H. Hoddevik
Since Maekawa and Simpson (1972, 1973) described activation of climbing fibers in the vestibulocerebellum following visual stimulation, a pathway via the inferior olive has been extensively studied. The recent observation of mossy fiber activation in a small part of the flocculus following electrical stimulation of the optic disc in rabbits (Maekawa, 1975; Maekawa and Takeda, 1975 and 1976) has prompted a search for precerebellar nuclei which give rise to mossy fibers terminating in the vestibulocerebellum. Since the pontine nuclei are an important source of mossy fibers to the cerebellum and in addition receive afferents from several cortical and subcortical visual areas (see Discussion), it is a relevant problem to investigate whether these nuclei project onto the vestibulocerebellum. This is the primary aim of the present study. In addition the projection to the paraflocculus has been mapped because of the close anatomical relation between the paraflocculus and the flocculus and because there are reasons to believe that even the paraflocculus may have some teleceptive function. The projection of the pontine nuclei proper only will be described. The reticular tegmental pontine nucleus (of Bechterew), a part of the reticular formation (Brodal, 1957), is not included. Its projection onto the cerebellar visual areas will be the subject of the fifth paper in this series. Furthermore, the present study is a link in a more extensive investigation of the whole pontocerebellar projection by means of the method of retrograde transport of horseradish peroxidase (HRP). So far the projection onto the paramedian lobule (Hoddevik, 1975), the vermal visual area (Hoddevik et al., 1977) and the anterior lobe (Brodal and Walberg, 1977) in the cat have been described. The main reason for using rabbits in the present study is that physiological experiments on activation of the vestibulocerebellum via visual pathways have been in rabbits. However, it is considered likely (see Discussion) that the results obtained in the present study are valid for the cat as well. The method of retrograde transport of HRP has proved to be valuable for determining precisely the pontine regions giving origin to fibers terminating in the cerebellum. Following the demonstration of Brodal that lesions in the cerebral cortex give rise to circumscribed columns of degeneration in the pontine nuclei (Brodal, 1968a and b, 1971a and b, 1972a, b and c), columns of labeled cells have likewise been shown to be present in the pons following injections of HRP in the paramedian lobule (Hoddevik, 1975), the vermal visual area (Hoddevik et al., 1977) and the anterior lobe (Brodal and Walberg, 1977). If the pontine nuclei project to the vestibulocerebellum and the parafiocculus, it appears likely that the fibers arise from similar, limited areas. It will further be of interest to compare pontine areas found to project onto the flocculonodular lobe and/or the paraflocculus with the pontine regions already determined as receiving afferents from visual "centers". Materials and Methods The material in this study is largely the same as that used in the study of the olivocerebellar projection onto the flocculonodular lobe and the paraflocculus (Hoddevik and Brodal, 1977). Of 58 adult albino rabbits weighing 1.7-2.9 kg, 28 could be used in the present study. With a 1 ~xl Hamilton syringe 0 . 0 5 - 0 . 5 ~tl of a 50 % suspension of H R P (either Sigma VI or Serva) was injected
Pontocerebellar Projection
513
under Mebumal anesthesia. The superficial injections (in the paraflocculus) were performed either manually or by means of a hydraulic perfusor, the deep ones (in the flocculus and nodulus) always by a hydraulic perfusor. After injection, the needle was left in situ for half an hour. One to three days postoperatively the animals were perfused under deep Mebumal anesthesia with a mixture of 0.4 % formaldehyde and 1.25 % glutaraldehyde in phosphate buffer. The brains were dissected free, the blocks isolated and immersed in the fixative for 24 hours, then transferred to a 30% sucrose/phosphate buffer solution for another 24 hours. The blocks were cut on a freezing microtome (50 ~m sections, the pons transversally or horizontally, the cerebellum transversally or sagittally). The sections were activated with 3-3' diaminobenzidine and H202 in groups of five according to Graham and Karnovsky (1966). From each group two sections were selected and mounted. One series was left unstained, the other lightly counterstained with cresyl violet. Labeled cells were plotted in drawings of the pontine nuclei made in a projection apparatus. As a control some series were also drawn by means of a pantograph. To facilitate comparison among cases, the findings in each case were transferred to a diagram of the pontine nuclei, prepared from a series from an unoperated adult rabbit (used in Figs. 3 and 5). This diagram is almost identical to that of Brodal and Jansen (1946), based on newborn rabbits, and their nomenclature of the nuclear subdivisions is adopted. A series of transverse sections through the cerebellum of normal rabbits (Brodal, 1940b) together with drawings of sagittally cut sections of the cerebellum (Larsell, 1970), served as guides for the identification of the cerebellar lobules.
Results Following injections of HRP in the flocculonodular lobe and the paraflocculus a f a i r n u m b e r o f l a b e l e d n e u r o n s is f o u n d i n t h e p o n t i n e n u c l e i o n l y w h e n t h e p a r a f l o c c u l u s is i n v o l v e d . T h e d i s t r i b u t i o n o f l a b e l e d cells is n o t r e s t r i c t e d t o p a r t i c u l a r p o n t i n e n u c l e i . H o w e v e r , t o f a c i l i t a t e t h e d e s c r i p t i o n r e f e r e n c e will b e m a d e t o t h e v a r i o u s n u c l e i as d i s t i n g u i s h e d b y B r o d a l a n d J a n s e n ( 1 9 4 6 ) . A c a s e with a large injection which covers most of the left paraflocculus and the f l o c c u l u s , will b e p r e s e n t e d first, f o l l o w e d b y s o m e c a s e s w i t h i n j e c t i o n s restricted to parts of the flocculus, nodulus or paraflocculus. In rabbit R 59 (Fig. 1) following two injections in the dorsal and one in the ventral paraflocculus on the left side there is staining of the molecular and granular layer of the entire flocculus and heavy staining of these layers in the dorsal and ventral paraflocculus except for the medialmost folium of the dorsal paraflocculus. There is weak staining and some labeled cells in the left lateral and interposed cerebellar nuclei. Purkinje cells in the lateralmost portion of Crus II and in the posterior vermis (lobules IX and X) are faintly stained. The right half of the pons is cut horizontally, the left transversally. Numerous labeled neurons occur in the rostral 3/4 of the pontine nuclei on both sides, with a slight contralateral preponderance. The labeled cells are aggregated in four main groups or columns (referred to below as columns E, F, G and H). Column E (horizontal sections I-VI, transverse levels 6 - 1 0 in Fig. 1) is the most conspicuous group with a great number of labeled cells. It is localized mainly to the paramedian and the ventral pontine nuclei, but extends a little into the peduncular nucleus as well Somewhat fewer cells are present in column F (horizontal sections I-IV, transverse levels 2-7 in Fig. 1). At some levels the labeled cells are arranged in slender, transversally oriented, bands (horizontal sections I and II, transverse levels 4 and 5 in Fig. 1). The column is mainly localized in the paramedian and the lateral pontine nuclei, but extends into the peduncular and ventral nuclei as well. A smaller number of labeled cells are present in column G (horizontal sections IV-VI, transverse levels 5 - 8 in Fig. 1) situated close to the midline in the paramedian and in parts in the medial pontine nuclei (Fig. 2). As column H are indicated a few labeled neurons, which are found in the dorsolateral pontine nucleus (horizontal sections V and VI, transverse levels 3 - 8 in Fig. 1). Findings corresponding to those described above are made in two other cases (rabbits R 5 and 57, not illustrated). In the former the staining of the cerebellum is restricted to the lateral three folia of the dorsal paraflocculus and the rostralmost folium of the ventral paraflocculus as well as the
514
G.H. Hodde,vik
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Fig. 1. Diagrammatic representation of findings in rabbit R 59 with a large injection covering most of the left flocculus and paraflocculus. A Staining of the cerebellar cortex transferred to a diagram of the unfolded cerebellar surface (taken from Brodal, 1940b) and as seen in a transverse section through the caudal part of the cerebellum. Black denotes heavy staining, hatchings weak staining. Dotting in the cerebellar nuclei indicates labeled cells. B Labeled neurons are indicated by dots in equally spaced horizontal sections of the right half of the pons. Different densities of dotting indicate different densities of labeled cells. Rectangle in section IV refers to the photomicrograph (Fig. 2). C Labeled cells are indicated by dots in equally spaced transverse sections through the left half of the pons. Broken lines in B and C indicate borders between pontine nuclei. D A diagram to show four main columns (E, F, G and H) of labeled cells (see text) and their longitudinal extent as reconstructed from the drawing above. Since the distribution of labeled cells is similar on both sides of the pontine nuclei, only one half is illustrated in the diagram. Different densities of dotting refer to different densities of labeled cells. The numbers refer to the transverse levels in C. For abbreviations see list on p
Pontocerebellar Projection
515
List of Abbrevations
Flocc.: flocculus L. pro.: paramedian lobule N. dl.: dorsolateral pontine nucleus NI: nucleus interpositus NL: nucleus lateralis N. 1.: lateral pontine nucleus NM: nucleus medialis N. m.: medial pontine nucleus Nod.: nodulus N. p.: peduncular pontine nucleus N. pm.: paramedian pontine nucleus N. v.: ventral pontine nucleus P. fl. d.: dorsal paraflocculus P. fl. v.: ventral paraflocculus Uv.: uvula VIIIB, IX, X: vermal lobules of Larsell
Fig. 2. Photomicrograph showing density of labeled cells in column G in rabbit R 59 (see rectangle in section IV in Fig. 1B). The sharp limitation of the column is clearly seen. Dark field illumination. Magnification X 140
caudalmost folium of the flocculus. There is no spreading to the cerebellar nuclei. In rabbit R 57 there is heavy staining of all parts of the dorsal and ventral paraflocculus and the caudal portion of the flocculus, but in addition some staining of the lateral third of the dentate nucleus and the white matter and adjoining parts of the granular layer of Crus II and the paramedian lobule. Cases with injections covering one only of the lobules labeled in the above cases give information of whether the columns described project onto different lobules.
Flocculus In rabbit R 28 (Fig. 3) there is staining of the cortex in a restricted part of the rostralmost folium of the left flocctdus without spreading to the paraflocculus or to the cerebellar nuclei. However, in the rostralmost folium of the paramedian lobule, through which the needle was inserted, there is good staining of a tiny patch.. In thepontine nuclei only few labeled cells are found, all of them within the rostral part of column E described above. Most of the cells are located ventrally, only a few dorsally. To see if the spreading to the paramedian lobule could invalidate the result, two cases (rabbits R 52 and 61, not illustrated) with injections in the rostral part of the paramedian lobule were studied. In
516
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