Brain Behav. Evol. 13: 142-153 (1976)
Spinocerebellar Tracts in the Brush-Tailed Possum,
Trichosurus vulpecula C harles
R. R. W atso n , A lan B roomhead and M ary -C lare H olst
School of Anatomy, University of New South Wales, Kensington, N.S.W.
Key Words. Spinocerebellar tracts • Marsupials • Comparative neuroanatomy
The anatomy of the dorsal and ventral spinocerebellar tracts in the American marsupial opossum (Didelphis marsupialis virginiana) was re cently studied experimentally by H azlett et al. [1971], who demonstrat ed that these two tracts and their cerebellar terminations were similar in marsupials and eutherian mammals. A distinctive feature of the spinocer ebellar termination in Didelphis was the presence of five rows of spino cerebellar fibres in the deep cerebellar white matter. V oogd [1969] showed that sagittally oriented rows of spinocerebellar fibres were present in all of a variety of eutherian mammals he studied, but the number of such rows was never less than seven and in some species numbered as many as eleven. In the present study we have examined these same tracts in an Austra lian marsupial brush-tailed possum (Trichosurus vulpecula) in order to
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Abstract. The direct projections of the spinal cord onto the cerebellar cortex were traced using the Nauta method following the placement of cervical or thoracic spinal cord hemisections in six brush-tailed possums. Degenerating fibres reached the cerebellum via typically placed dorsal and ventral spinocerebellar tracts. Al though complete differentiation of the terminations of ventral and dorsal tracts was not possible, it was found that the dorsal tract terminates mainly in the ipsilateral anterior lobe vermis and in the pyramis and paraflocculus of the ipsilateral poster ior lobe. The ventral tract ends almost entirely in the anterior lobe with the majori ty of fibres terminating contralateral to the side of the hemisection. Within the anterior lobe, degenerating fibres were distributed fairly symmetrical ly about the midline in five sagittal rows. Three such rows were found in the poster ior lobe. The relatively small number of rows in the anterior lobe (five) may be a characteristic feature of marsupials when compared with eutherian mammals.
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obtain some indication of the range of variation of spinocerebellar tract anatomy that exists between geographically separated groups of marsupi als. The brush-tailed possum has been the subject of many contemporary neurological studies [C lezy et al., 1961; H ayh o w , 1966; H eath and J ones , 1971; H ore et al., 1973; M artin et al., 1970, 1971; M artin and M egirian , 1972; R ees and H ore , 1970; R ockel et al., 1972; W arner and W atson , 1972; W eller , 1972],
Materials and Methods In each animal, an attempt was made to hemisect the spinal cord, either at a cervical level (4 cases) or at a thoracic level (2 cases). The operations were carried out aseptically while the animals were anaesthetised with intraperitoneal pentobarbi tone sodium (60 mg/kg-1) or intramuscular amylobarbitone sodium (90 mg/kg-1). The hemisection was made with fine jeweller’s forceps with the spinal dura mater intact, after the method of H a and Liu [1968]. The animals were allowed to survive for 10-15 days after the operation and were then deeply anaesthetised and perfused first with saline and then with 10% formol saline. The brain and spinal cord were later removed and in some cases embedded in egg yolk [Snodgrass and D orsey, 1963]. Coronal sections 30 ^m thick were cut with a freezing microtome from blocks of selected spinal cord segments and of the brain stem and cerebellum. Each 10th or 20th section was stained with the F ink and H eimer [1967] modification of the Nauta method. In some cases a series of adjacent sections was stained with cresyl violet or neutral red for identification of cell groupings. Degenerating fibres in Nauta sections were plotted with a microscope and entered in enlarged projection drawings of the sections. There was little difficulty in recognising degenerating ax ons and their terminal ramifications within the cerebellum; artifactual silver depo sits were uncommon in these preparations. A series of coronal sections through the spinal lesion area was stained with cresyl violet so that the extent of tissue destruc tion and gliosis could be mapped. To assist in the identification of cerebellar lobules and deep nuclei, use was made of standard horizontal and sagittal sections of pos sum cerebellum that had been alternately stained with neutral red and the Weil method. Cerebellar cortical areas are referred to by traditional names as well as the Roman numeral system of L arsell [1936]. The relationship between the classical nomenclature for the vermis and L arsell’s numeral system is shown in figure 1.
Lesions The extent and level of each of the six spinal cord lesions is shown in figure 2. The course and pattern of termination of ascending degenerating fibres was found to be essentially the same in each of the six animals with
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Results
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D
PC
Fig. 1. A projection drawing at X 10 magnification of a parasagital section through the cerebellum and brain stem of Trichosurus vulpecula. The relationship between the classical descriptive nomenclature for the vermis and L arsell’s [1936] numbering of cerebellar lobules is as follows: Lingula (L) = lobule I; preculmen (PC) = lobules II and III; culmen (C) = lobules IV and V; declive (D), folium (F) and tuber (T) = lobules VI and VII; pyramids (P) = lobule VIII; uvula (U) = lob ule IX; nodule (N) = lobule X. The posteriolateral fissure (plf) and the primary fis sure (fp) are also Indicated.
1-C4
3-T5
4- T9
5-C6
6-C2
7 - C5
Fig. 2. Maximum transverse extent of necrosis and gliosis after spinal cord le sion placements in six brush-tailed possums (PCH-1, 3, 4, 5, 6 and 7). The level of the lesion (C4, T5, etc.) is indicated in each case.
Spinal and Brain Stem Course of the Spinocerebellar Tracts The degenerating dorsal and ventral spinocerebellar tracts were prima rily identified as they entered the cerebellum, in typical mammalian
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spinal lesions. The quantity of degeneration was, however, notably small er in the two cases with thoracic lesions (PCH-3 and PCH-4). Because the pattern of termination was consistent, the results in only one animal (PCH-5) will be presented in detail.
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Course and Termination of Spinocerebellar Fibres within the Cerebel lum Degenerating fibres of the ventral spinocerebellar tract enter the deep cerebellar white matter closely applied to the dorsal surface of the brach ium conjunctivum. Large numbers of fibres leave the medial side of this bundle and cross the midline. The tract subsequently loses its well-defined borders as its fibres are distributed to the region of the deep cerebellar nu clei as well as to the cerebellar cortex. It should be noted here that differ entiation of ventral and dorsal spinocerebellar tract fibres is only possible when the tracts can be clearly seen as discrete bundles in spinal cord sec-
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fashion, through the inferior and superior cerebellar peduncles, respective ly. From this initial distinct separation of the two tracts it was possible to follow them caudally to their beginnings in the lateral funiculus of the spinal cord. In the spinal cord the dorsal tract is easily identified as it forms a discrete bundle of large axons which, at the cervical level, lies ex ternal to the rubrospinal tract, the latter being identified by its character istic mixture of coarse and fine fibres [Warner and Watson, 1972]. The dorsal spinocerebellar tract is separated from the ventral spinocerebellar tract by a slight indentation in the margin of the spinal cord and lower medulla. The fibres of the ventral spinocerebellar are coarse and are in termingled with a large number of fine fibres which terminate in the brain stem (fig. 3) and thalamus [Watson and Symons, 1971], The composite bundle of coarse ventral spinocerebellar and fine spinobulbar and spi nothalamic fibres corresponds to the anterolateral Grundbiindel originally described by Bechterew [1885, quoted by Mehler et al., I960]. The ventral spinocerebellar tract remains part of this bundle as it courses lat eral to the olive and inferomedial to the nucleus of the spinal trigeminal tract in the lower medulla. After passing external to the facial nucleus, the ventral spinocerebellar fibres turn sharply dorsally and finally caudally to enter the cerebellum on the dorsal surface of the ipsilateral brachium conjunctivum (fig. 3). The dorsal spinocerebellar tract inclines dorsally away from the antero lateral bundle at about the level of the middle of the inferior olive and passes external to the nucleus of the spinal trigeminal tract to reach the inferior cerebellar peduncle. The tract forms a compact sharply defined bundle within the inferior cerebellar peduncle and passes dorsally with the other fibres of the inferior peduncle into the ipsilateral deep cerebel lar white matter.
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tions. The individual fibres that leave the parent bundle to run in the cer ebellar white matter can often not be easily related to the dorsal or ven tral tract. Because of this difficulty, opinions expressed here on the distri bution of dorsal and ventral tract terminations must be accepted as only general inferences. However, careful inspection and reconstruction of the pattern of degeneration in serial sections suggested that the dorsal tract appears to terminate chiefly in the ipsilateral anterior lobe but also sends
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Fig. 3. A series of projection drawings through the brain stem of possum PCH-5 (A-E from rostra! to caudal) to show the pattern of spinocerebellar degeneration af ter a spinal cord right hemisection at C6 (fig. 2). The course and termination of the ventral and dorsal spinocerebellar tracts (vt and dt) can be followed and the termi nation of spinobulbar fibres in certain brain stem nuclei is also indicated. Degener ating fibres are represented by dashes and terminal degeneration (see text) is repre sented by dots. Abbreviations for cerebellar landmarks are as in figure 1 and 4 and those for brain stem structures are as follows: do = dorsal accessory olive; ec = external cuneate nucleus; fc = fasciculus cuneatus; gc = nucleus reticularis gigantocellularis; ic = nucleus interculatus; lr = lateral reticular nucleus; lrf = lateral reti cular formation.
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a major bundle which runs caudally through the deep cerebellar white matter and which spreads out deep to the cortex of the pyramids. The ventral tract terminates in both sides of the anterior lobe with most of its fibres ending on the side contralateral to the lesion. Few ventral tract fibres seem to travel to the posterior lobe. Following these general state ments regarding the overall distribution of the two tracts, the total termi nation of the spinocerebellar fibres will be described without further sepa rate references to ventral and dorsal tracts. Fine degenerating fibres radiate into the three major subdivisions of the anterior lobe vermis: the lingula, preculmen and culmen (fig. 3A). De generation is most dense in the preculmen (lobules II and III) and lingula and is less heavy in the culmen (IV and V) and these fibres run for the most part in five sagittally oriented streams in the white matter of each lobule. Although the five rows are easy to distinguish in coronal sections, they only represent condensations of fibres and a number of fibres are found in between the five major bundles. From each of these sagittal rows, fibres radiate dorsally and ventrally out to ramify into the adjacent granular layer. Degeneration in the granular layer was judged to be ‘ter minal’ (representing telodendria and boutons) because of the finely frag mented appearance of the silver deposits. Although some fibres reached the most superficial parts of the granular layer, no definite degeneration could be seen in the molecular layer - consistent with the evidence that spinocerebellar fibres terminate as mossy fibres in the granule layer [Eccles et al., 1966; L linas and H jllm an , 1969]. Fibre degeneration in the anterior lobule is heaviest in the preculmen and lightest in the part of lobule V of the culmen that borders the fissura prima (fig. 3A). The small diameter of degenerating fibres in the peri pheral parts of the cerebellar white matter contrasts sharply with the thickness of the fibres of the parent spinocerebellar tracts. This observa tion, together with the presence of an abundance of intracerebellar fibres, when compared with the number in the tract, suggests that the spinocere bellar fibres branch extensively as they radiate out in the deep cerebellar white matter. As fibres sweep posteriorly to reach the posterior parts of the cerebellum, the sagittal rows of fibres tend to amalgamate and in many sections only three distinct concentrations could be delineated (fig. 3C). Elsewhere in the cerebellar cortex, terminal degeneration is very sparse or absent except that a very small number of degenerating fibres terminates in the vermal portions of lobules VI and VII - most notably in the part of the declive (lobule VI) adjacent to the fissura prima.
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The fibres that reach the pyramis are distributed more heavily on the side ipsilateral to the lesion. From the white matter of the pyramis many fibres turned laterally to terminate in the granule layer of the paramedian lobule, most prominently in the ventral parts [H VIII A of J ansen , 1954], A small number of fibres were observed to reach the cortex of the paraflocculus bilaterally. Spinal Projections to Precerebellar Relay Nuclei Spinal afferents in the brush-tailed possum project to certain brain nu clei that are known to relay to the cerebellum in eutherian mammals [see B rodal , 1969, p. 278, for discussion of this point]. Reference to figure 3 shows that degenerating spinal fibres terminate on the lateral reticular nu cleus, the nucleus intercalatus (the other two perihypoglossal nuclei also received spinal afferents but are not pictured here), inferior olivary nucle us (restricted to the lateral part of the dorsal accessory olive and the later al part of the caudal medial accessory olive) and the external cuneate nu cleus. The observation of such spinal connections to cerebellar relay nu clei in the brush-tailed possum agress with the findings of M ehler [1969] and H azlett et al. [1972] in the American opossum. Furthermore, these connections in these two marsupials are strikingly similar to the spinobulbar connections described in eutherian mammals, e.g. the rat [L u n d and W ebster , 1967], the hedgehog [Jane and S chroeder , 1971], the cat [A nderso n and B erry , 1959], the tree shrew [S chroeder and Jane , 1971], the monkey and chimpanzee [M ehler , 1969], the sheep [R ao et al., 1969] and the pig [B reazile and K itchell , 1968], Particularly nota ble among spinobulbar projections in the possum is the heavy termination in the external cuneate nucleus from cervical cord hemisections. Such ter minal degeneration is very sparse in cases of thoracic cord damage. These findings are consistent with the known association of this nucleus with re lay of forelimb proprioceptive information to the cerebellum in cats [G rant , 1962b].
The present study serves to emphasize the similarity of long tract anat omy that exists between marsupial and eutherian mammals [see for exam ple R ockel et al., 1972; W arner and W atson , 1972]. However, the presence of five sagittal rows of spinocerebellar fibres within the anterior
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Discussion
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Fig. 4. A diagram [after the style of H azlett el at., 1971] showing the cerebel lar cortex of the brush-tailed possum as if it was laid out flat. The distribution of spinocerebellar termination is indicated by dots. This pattern is discussed in the text. The paramedian (PM) and ansiform (A) lobules and the paraflocculus (PF) and flocculus (FC) are labelled. Other abbreviations are the same as in figure 1.
Overall Distribution of Spinocerebellar Fibres The separation of anterior and posterior areas of cerebellar degenera tion is as clear in the brush-tailed possum as in the eutherians [B eck , 1927; A nderso n , 1943; Yoss, 1952, 1953; G rant , 1962a] and the opos
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lobe of the cerebellum in the brush-tailed possum contrasts with findings in eutherians. V oogd [1969] drew attention to the concentration of spinocerebellar fibres in sagittally oriented rows in the deep cerebellar white matter of a variety of different eutherians (ferret, cat, hedgehog, tree shrew, loris and macaque). He showed that up to eleven such rows are present in the an terior lobe of the cerebellum of some eutherians and the smallest number (seven) is found in the tree shrew. Some variations in the number of rows may be due to differences in the complexity of cerebellar lobulation in some eutherians. Despite this, it is clear that the five sagittal rows present in the opossum [H azlett et al., 1971] and in the brush-tailed possum (present study) are a distinct feature of these marsupials. It seems reasonable to infer that this feature is relat ed to the phylogeny of the species rather than to their particular degree of cerebellar development because even the poorly lobulated cerebellum in the eutherian hedgehog has eleven such rows [V o ogd , 1969].
W atson/B roo.v head /H olst
sum [H azlett et al., 1971], This separation is shown diagrammatically in figure 4. The anterior area of spinocerebellar termination in the brush-tailed possum comprises lobules I-V (lingula, preculmen and the culmen). G rant [1962a] found (in cats) a ‘sharp posterior border’ delineating the degenerating spinocerebellar fibres in the anterior part of lobule V from those in the posterior part. Such a clear distinction is not seen in Trichosurus - a situation which parallels the findings of H azlett et al. [1971] in the opossum. However, the latter authors seem to have misinterpreted the findings of G rant [1962a] in their discussion of the anterior area of lobe degener ation in the opossum; H azlett et al. [1971] infer that the physiologically defined hindlimb and forelimb areas of the anterior lobe are distinct in the cat, which they are not [O scarsson , 1965] and they also imply that the spinocerebellar terminations in the anterior part of the cerebellum in the opossum are quite different from those in the cat [G rant , 1962a], which is probably not the case. We feel that the apparent differences be tween marsupials and the cat in terms of the extent of anterior lobe termi nation are only slight and may be due to differences in the interpretation of the density of degeneration in parts of the culmen. The posterior area of spinocerebellar termination is concentrated in the pyramis and paramedian lobule (and to a lesser extent, the uvula) in the brush-tailed possum. An important difference between this posterior terminal area and the one found in the anterior lobe is that degeneration in the latter area is bilaterally symmetrical following spinal cord hemisection, whereas in the posterior area the fibres are more heavily concentrat ed on the ipsilateral side, particularly in the paramedian lobule. Within the white matter of the pyramis, the sagittally coursing fibres are concen trated in three rows (fig. 3C) which are, however, not nearly as distinct as the sagittal rows in the anterior lobe. The distribution of fibres to the pyr amis and paramedian lobule is similar to that found in the rat [A n d e r so n , 1943], cat [G rant 1962a], macaque [Yoss, 1952, 1953] and opos sum [H azlett et al., 1971], Sagittal rows have been found in the pyramis of a number of eutherians by V oogd [1969] but the number of rows was invariably greater in these anim als than in the brush-tailed possum . H azlett et al. [1971] did not note the presence of sagittal rows in the pyramis of the opossum but reference to their figure 3C shows a pattern of three rows in the region of the cerebellum.
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Intracerebellar Course of the Ventral and Dorsal Tracts The separate distribution of each of the two spinocerebellar tracts within the cerebellum of cats has been mapped in Nauta stained material by G rant [1962a] and electrophysiologically by L undberg and O scars son [1962] and O scarsson et al. [1964], These studies show that the dorsal spinocerebellar tract has an almost entirely ipsilateral distribution to the anterior lobe and the pyramis and paramedian lobule. On the other hand, about 75% of the ventral spinocerebellar fibres terminate contralaterally in the cerebellar cortex with 10% ipsilateral and 15% branching to end in both sides [O scarsson , 1965]. Ventral spinocerebellar fibres have a similar distribution in the anterior lobe as their dorsal counterparts, but relatively few ventral spinocerebellar fibres reach the pyramis and para median lobule [G rant , 1962a], In the present study on Trichosurus, the intracerebellar termination of each of these two tracts could only be in ferred from data on the intracerebellar course of each tract. However, the clear impression was gained that dorsal spinocerebellar fibres spread out largely towards the ipsilateral anterior lobe and pyramis and paramedian lobule, whereas large numbers of ventral tract fibres are seen to cross the midline. No evidence was found of any major differences between the ter minations of spinocerebellar fibres in the brush-tailed possum and eutherians. Acknowledgements We are grateful to Dr. J ohn I. J ohnson, jr., Mr. Brian F reeman, Ms. J an P rovis, Dr. J onathan Stone , Professor F red R ost and Ms. L orraine Brooks who all assisted in various ways in producing this manuscript.
References A nderson , R. F.: Cerebellar distribution of the dorsal and ventral spinocerebellar
tracts in the white rat. J. comp. Neurol. 79: 415-423 (1943). systems in the cat brainstem. J. comp. Neurol. I l l : 195-229 (1959). Beck, G.: The cerebellar terminations of the spinocerebellar fibres of the lower lumbar and sacral segments of the cat. Brain 50: 60-98 (1927). Breazile , J. E. and K itchell , R. L.: Ventrolateral spinal cord afferents to the brain stem in the domestic pig. J. comp. Neurol. 110: 363-372 (1968). Brodal, A.: Neurological anatomy in relation to clinical medicine; 2nd ed. (Oxford University Press, New York 1969).
Downloaded by: Nagoya University 133.6.82.173 - 1/15/2019 12:24:15 PM
A nderson , F. D. and B erry, C. M.: Degeneration studies of long ascending fiber
152
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maesthetic afferent system of the marsupial phalanger, Trichosurus vulpécula. Aust. J. exp. Biol. med. Sci. 39: 19-28 (1961). E ccles , J. C ; L unas , R., and Sasaki, K.: The excitatory synaptic action of climbing fibres on the Purkinje cells of the cerebellum. J. Physiol., Lond. 182: 268-296 (1966). Fink, R. P. and H imer, L.: T wo methods for selective silver impregnation of degen erating axons and their synaptic endings in the central nervous system. Brain Res. 4: 369-374 (1967). G rant, G.: Spinal course and somatotopically localized termination of the spinocer ebellar tracts. An experimental study in the cat. Acta physiol, scand. 56: suppl. 193, pp. 1-61 (1962a). G rant, G.: Projection of the external cuneate nucleus onto the cerebellum in the cat. An experimental study using silver methods. Expl Neurol. 5: 179-195 (1962b). Ha, H. and Liu, C. N.: Cell origin of the ventral spinocerebellar tract. J. comp. Neurol. 133: 185-206 (1968). H ayhow , W. R.: The accessory optic system in the marsupial phalanger, Trichosu rus vulpécula. J. comp. Neurol. 126: 653-672 (1966). H azlett , J. C.; D om , R., and M artin , G. F.: Spinobulbar, spinothalamic and medi al lemniscal connections in the American opossum (Didelphis marsupialis virginiana). J. comp. Neurol. 146: 95-118 (1972). H azlett , J. C ; M artin, G. F., and D om, R.: Spinocerebellar fibres of the opossum Didelphis marsupialis virginiana. Brain Res. 33: 257-271 (1971). H eath, C. J. and J ones , E. G.: Interhemispheric pathways in the absence of a cor pus callosum. An experimental study of commisural connections in the marsupi al phalanger. J. Anat. 109: 253-270 (1971). H ore, J.; P hillips , C. G., and P orter , R.: The effects of pyramidotomy on motor performance in the brush-tailed possum Trichosurus vulpécula. Brain Res. 49: 181-184 (1973). Jane, J. A. and Schroeder , D. M.: A comparison of dorsal column nuclei and spin al afferents in the European hedgehog (Erinaceus europeaus). Expl Neurol. 30: 1-17 (1971). J ansen, J.: On the morphogenesis and morphology of the mammalian cerebellum; in J ansen and Brodal Aspects of cerebellar anatomy (Grundt Tatum, Oslo 1954). L arsell, O.: The development and morphology of the cerebellum in the opossum. II. Later development and adult. J. comp. Neurol. 63: 251-291 (1936). L linás, R. and H illman, D. E.: Physiological and morphological organization of the cerebellar circuits in various vertebrates; in L linás Neurobiology of cere bellar evolution and development, pp. 43-73 (Am. med. Ass., Chicago 1969). L und , R. D. and W ebster, K. E.: Thalamic afferents from the spinal cord and trigeminal nuclei. An experimental anatomical study in the rat. J. comp. Neurol. 139: 313-328 (1967). L undberg , A. and O scarsson, O.: Functional organization of the ventral spinocere bellar tract in the cat. IV. Identification of units by antidromic activation from the cerebellar cortex. Acta physiol, scand. 54: 252-269 (1962).
Downloaded by: Nagoya University 133.6.82.173 - 1/15/2019 12:24:15 PM
C lezy, J. K. A.; D ennis , B. J., and K err, D. 1. B.: A degeneration study of the so-
Marsupial Spinocerebellar Tracts
153
M artin, G. F. and M egirian, D.: Corticobulbar projections of the marsupial phal-
anger (Trichosurus vulpécula). II. Projections to the mesencephalon. J. comp. Neurol. 144: 165-192 (1972). Martin, G. F.; M egirian, D., and R oebuck, A.: The corticospinal tract of the mar supial phalanger (Trichosurus vulpécula). J. comp. Neurol. 139: 245-258 (1970). M artin, G. F.; M egirian, D., and R oebuck, A.: Corticobulbar projections of the marsupial phalanger (Trichosurus vulpécula). I. Projections to the pons and medulla oblongata. J. comp. Neurol. 142: 275-296 (1971). M ehler , W. R.: Some neurological species differences - a posteriori. Ann. N.Y. Acad. Sei. 167: 424-468 (1969). M ehler , W. R.; F efferman, M. E., and N auta, W. J. H.: Ascending axon degener ation following anterolateral cordotomy. An experimental study in the monkey. Brain S3: 718-750 (1960). G scarsson, O.: Functional organization of the spino- and cuneocerebellar tracts. Physiol. Rev. 45: 495-522 (1965). O scarsson, O.; R osen, I., and U ddenberg , N.: A comparative study of ascending spinal tracts activated from hindlimb afferents in monkey and dog. Archs ital. Biol. 102: 137-155 (1964). Rao, G. S.; Breazile, J. E., and K itchell , R. L.: Distribution and termination of spinoreticular afferents in the brain stem of sheep. J. comp. Neurol. 137: 185-196 (1969). R ees, S. and H ore, J.: The motor cortex of the brush-tailed possum (Trichosurus vulpécula): motor representation, motor function and the pyramidal tract. Brain Res. 20: 439-451 (1970). R ockel, A. J.; H eath, C. J., and J ones, E. G.: Afferent connections to the dience phalon in the marsupial phalanger and the question of sensory convergence in the ‘posterior group’ of the thalamus. J. comp. Neurol. 145: 105-130 (1972). Schroeder, D. M. and J ane, J. A.: Projection of dorsal column nuclei and spinal cord to brain stem and thalamus in the tree shrew. J. comp. Neurol. 142: 309-350 (1971). Snodgrass, A. B. and D orsey, C. H.: Egg albumin embedding: A procedure com patible with neurological staining techniques. Stain Technol. 33: 149-155 (1963). Voogd , J.: The importance of fibre connections in the comparative anatomy of the mammalian cerebellum; in L linäs Neurobiology of cerebellar evolution and de velopment, pp. 407-459 (Am. med. Ass., Chicago 1969). W arner, G. and W atson, C. R. R.: The rubrospinal tract in a diprotodont marsupi al (Trichosurus vulpécula). Brain Res. 41: 180-183 (1972). W atson, C. R. R. and S ymons, M.-C.: Ascending pathways to the brain stem of the phalanger (Trichosurus vulpécula). J. Anat. 110: 501 (1971). W eller , W. L.: Barrels in somatic sensory neocortex of the marsupial Trichosurus vulpécula (brush-tailed possum). Brain Res. 43: 11-24 (1972). Yoss, R. E.: Studies of the spinal cond. I. Topographic localization within the dor sal spinocerebellar tract in Macaca mulatto. J. comp. Neurol. 97: 5-20 (1952). Yoss, R. E.: Studies of the spinal cord. II. Topographic localization within the ven tral spinocerebellar tract in the macaque. J. comp. Neurol. 99: 613-638 (1953).
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Dr. C harles, R. R. W atson, School of Anatomy, University of New South Wales, P. O. Box 1, Kensington, NSW 2053 (Australia)
Spinocerebellar Tracts in the Brush-Tailed Possum,
Trichosurus vulpecula C harles
R. R. W atso n , A lan B roomhead and M ary -C lare H olst
School of Anatomy, University of New South Wales, Kensington, N.S.W.
Key Words. Spinocerebellar tracts • Marsupials • Comparative neuroanatomy
The anatomy of the dorsal and ventral spinocerebellar tracts in the American marsupial opossum (Didelphis marsupialis virginiana) was re cently studied experimentally by H azlett et al. [1971], who demonstrat ed that these two tracts and their cerebellar terminations were similar in marsupials and eutherian mammals. A distinctive feature of the spinocer ebellar termination in Didelphis was the presence of five rows of spino cerebellar fibres in the deep cerebellar white matter. V oogd [1969] showed that sagittally oriented rows of spinocerebellar fibres were present in all of a variety of eutherian mammals he studied, but the number of such rows was never less than seven and in some species numbered as many as eleven. In the present study we have examined these same tracts in an Austra lian marsupial brush-tailed possum (Trichosurus vulpecula) in order to
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Abstract. The direct projections of the spinal cord onto the cerebellar cortex were traced using the Nauta method following the placement of cervical or thoracic spinal cord hemisections in six brush-tailed possums. Degenerating fibres reached the cerebellum via typically placed dorsal and ventral spinocerebellar tracts. Al though complete differentiation of the terminations of ventral and dorsal tracts was not possible, it was found that the dorsal tract terminates mainly in the ipsilateral anterior lobe vermis and in the pyramis and paraflocculus of the ipsilateral poster ior lobe. The ventral tract ends almost entirely in the anterior lobe with the majori ty of fibres terminating contralateral to the side of the hemisection. Within the anterior lobe, degenerating fibres were distributed fairly symmetrical ly about the midline in five sagittal rows. Three such rows were found in the poster ior lobe. The relatively small number of rows in the anterior lobe (five) may be a characteristic feature of marsupials when compared with eutherian mammals.
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obtain some indication of the range of variation of spinocerebellar tract anatomy that exists between geographically separated groups of marsupi als. The brush-tailed possum has been the subject of many contemporary neurological studies [C lezy et al., 1961; H ayh o w , 1966; H eath and J ones , 1971; H ore et al., 1973; M artin et al., 1970, 1971; M artin and M egirian , 1972; R ees and H ore , 1970; R ockel et al., 1972; W arner and W atson , 1972; W eller , 1972],
Materials and Methods In each animal, an attempt was made to hemisect the spinal cord, either at a cervical level (4 cases) or at a thoracic level (2 cases). The operations were carried out aseptically while the animals were anaesthetised with intraperitoneal pentobarbi tone sodium (60 mg/kg-1) or intramuscular amylobarbitone sodium (90 mg/kg-1). The hemisection was made with fine jeweller’s forceps with the spinal dura mater intact, after the method of H a and Liu [1968]. The animals were allowed to survive for 10-15 days after the operation and were then deeply anaesthetised and perfused first with saline and then with 10% formol saline. The brain and spinal cord were later removed and in some cases embedded in egg yolk [Snodgrass and D orsey, 1963]. Coronal sections 30 ^m thick were cut with a freezing microtome from blocks of selected spinal cord segments and of the brain stem and cerebellum. Each 10th or 20th section was stained with the F ink and H eimer [1967] modification of the Nauta method. In some cases a series of adjacent sections was stained with cresyl violet or neutral red for identification of cell groupings. Degenerating fibres in Nauta sections were plotted with a microscope and entered in enlarged projection drawings of the sections. There was little difficulty in recognising degenerating ax ons and their terminal ramifications within the cerebellum; artifactual silver depo sits were uncommon in these preparations. A series of coronal sections through the spinal lesion area was stained with cresyl violet so that the extent of tissue destruc tion and gliosis could be mapped. To assist in the identification of cerebellar lobules and deep nuclei, use was made of standard horizontal and sagittal sections of pos sum cerebellum that had been alternately stained with neutral red and the Weil method. Cerebellar cortical areas are referred to by traditional names as well as the Roman numeral system of L arsell [1936]. The relationship between the classical nomenclature for the vermis and L arsell’s numeral system is shown in figure 1.
Lesions The extent and level of each of the six spinal cord lesions is shown in figure 2. The course and pattern of termination of ascending degenerating fibres was found to be essentially the same in each of the six animals with
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Results
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PC
Fig. 1. A projection drawing at X 10 magnification of a parasagital section through the cerebellum and brain stem of Trichosurus vulpecula. The relationship between the classical descriptive nomenclature for the vermis and L arsell’s [1936] numbering of cerebellar lobules is as follows: Lingula (L) = lobule I; preculmen (PC) = lobules II and III; culmen (C) = lobules IV and V; declive (D), folium (F) and tuber (T) = lobules VI and VII; pyramids (P) = lobule VIII; uvula (U) = lob ule IX; nodule (N) = lobule X. The posteriolateral fissure (plf) and the primary fis sure (fp) are also Indicated.
1-C4
3-T5
4- T9
5-C6
6-C2
7 - C5
Fig. 2. Maximum transverse extent of necrosis and gliosis after spinal cord le sion placements in six brush-tailed possums (PCH-1, 3, 4, 5, 6 and 7). The level of the lesion (C4, T5, etc.) is indicated in each case.
Spinal and Brain Stem Course of the Spinocerebellar Tracts The degenerating dorsal and ventral spinocerebellar tracts were prima rily identified as they entered the cerebellum, in typical mammalian
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spinal lesions. The quantity of degeneration was, however, notably small er in the two cases with thoracic lesions (PCH-3 and PCH-4). Because the pattern of termination was consistent, the results in only one animal (PCH-5) will be presented in detail.
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Course and Termination of Spinocerebellar Fibres within the Cerebel lum Degenerating fibres of the ventral spinocerebellar tract enter the deep cerebellar white matter closely applied to the dorsal surface of the brach ium conjunctivum. Large numbers of fibres leave the medial side of this bundle and cross the midline. The tract subsequently loses its well-defined borders as its fibres are distributed to the region of the deep cerebellar nu clei as well as to the cerebellar cortex. It should be noted here that differ entiation of ventral and dorsal spinocerebellar tract fibres is only possible when the tracts can be clearly seen as discrete bundles in spinal cord sec-
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fashion, through the inferior and superior cerebellar peduncles, respective ly. From this initial distinct separation of the two tracts it was possible to follow them caudally to their beginnings in the lateral funiculus of the spinal cord. In the spinal cord the dorsal tract is easily identified as it forms a discrete bundle of large axons which, at the cervical level, lies ex ternal to the rubrospinal tract, the latter being identified by its character istic mixture of coarse and fine fibres [Warner and Watson, 1972]. The dorsal spinocerebellar tract is separated from the ventral spinocerebellar tract by a slight indentation in the margin of the spinal cord and lower medulla. The fibres of the ventral spinocerebellar are coarse and are in termingled with a large number of fine fibres which terminate in the brain stem (fig. 3) and thalamus [Watson and Symons, 1971], The composite bundle of coarse ventral spinocerebellar and fine spinobulbar and spi nothalamic fibres corresponds to the anterolateral Grundbiindel originally described by Bechterew [1885, quoted by Mehler et al., I960]. The ventral spinocerebellar tract remains part of this bundle as it courses lat eral to the olive and inferomedial to the nucleus of the spinal trigeminal tract in the lower medulla. After passing external to the facial nucleus, the ventral spinocerebellar fibres turn sharply dorsally and finally caudally to enter the cerebellum on the dorsal surface of the ipsilateral brachium conjunctivum (fig. 3). The dorsal spinocerebellar tract inclines dorsally away from the antero lateral bundle at about the level of the middle of the inferior olive and passes external to the nucleus of the spinal trigeminal tract to reach the inferior cerebellar peduncle. The tract forms a compact sharply defined bundle within the inferior cerebellar peduncle and passes dorsally with the other fibres of the inferior peduncle into the ipsilateral deep cerebel lar white matter.
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tions. The individual fibres that leave the parent bundle to run in the cer ebellar white matter can often not be easily related to the dorsal or ven tral tract. Because of this difficulty, opinions expressed here on the distri bution of dorsal and ventral tract terminations must be accepted as only general inferences. However, careful inspection and reconstruction of the pattern of degeneration in serial sections suggested that the dorsal tract appears to terminate chiefly in the ipsilateral anterior lobe but also sends
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Fig. 3. A series of projection drawings through the brain stem of possum PCH-5 (A-E from rostra! to caudal) to show the pattern of spinocerebellar degeneration af ter a spinal cord right hemisection at C6 (fig. 2). The course and termination of the ventral and dorsal spinocerebellar tracts (vt and dt) can be followed and the termi nation of spinobulbar fibres in certain brain stem nuclei is also indicated. Degener ating fibres are represented by dashes and terminal degeneration (see text) is repre sented by dots. Abbreviations for cerebellar landmarks are as in figure 1 and 4 and those for brain stem structures are as follows: do = dorsal accessory olive; ec = external cuneate nucleus; fc = fasciculus cuneatus; gc = nucleus reticularis gigantocellularis; ic = nucleus interculatus; lr = lateral reticular nucleus; lrf = lateral reti cular formation.
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a major bundle which runs caudally through the deep cerebellar white matter and which spreads out deep to the cortex of the pyramids. The ventral tract terminates in both sides of the anterior lobe with most of its fibres ending on the side contralateral to the lesion. Few ventral tract fibres seem to travel to the posterior lobe. Following these general state ments regarding the overall distribution of the two tracts, the total termi nation of the spinocerebellar fibres will be described without further sepa rate references to ventral and dorsal tracts. Fine degenerating fibres radiate into the three major subdivisions of the anterior lobe vermis: the lingula, preculmen and culmen (fig. 3A). De generation is most dense in the preculmen (lobules II and III) and lingula and is less heavy in the culmen (IV and V) and these fibres run for the most part in five sagittally oriented streams in the white matter of each lobule. Although the five rows are easy to distinguish in coronal sections, they only represent condensations of fibres and a number of fibres are found in between the five major bundles. From each of these sagittal rows, fibres radiate dorsally and ventrally out to ramify into the adjacent granular layer. Degeneration in the granular layer was judged to be ‘ter minal’ (representing telodendria and boutons) because of the finely frag mented appearance of the silver deposits. Although some fibres reached the most superficial parts of the granular layer, no definite degeneration could be seen in the molecular layer - consistent with the evidence that spinocerebellar fibres terminate as mossy fibres in the granule layer [Eccles et al., 1966; L linas and H jllm an , 1969]. Fibre degeneration in the anterior lobule is heaviest in the preculmen and lightest in the part of lobule V of the culmen that borders the fissura prima (fig. 3A). The small diameter of degenerating fibres in the peri pheral parts of the cerebellar white matter contrasts sharply with the thickness of the fibres of the parent spinocerebellar tracts. This observa tion, together with the presence of an abundance of intracerebellar fibres, when compared with the number in the tract, suggests that the spinocere bellar fibres branch extensively as they radiate out in the deep cerebellar white matter. As fibres sweep posteriorly to reach the posterior parts of the cerebellum, the sagittal rows of fibres tend to amalgamate and in many sections only three distinct concentrations could be delineated (fig. 3C). Elsewhere in the cerebellar cortex, terminal degeneration is very sparse or absent except that a very small number of degenerating fibres terminates in the vermal portions of lobules VI and VII - most notably in the part of the declive (lobule VI) adjacent to the fissura prima.
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The fibres that reach the pyramis are distributed more heavily on the side ipsilateral to the lesion. From the white matter of the pyramis many fibres turned laterally to terminate in the granule layer of the paramedian lobule, most prominently in the ventral parts [H VIII A of J ansen , 1954], A small number of fibres were observed to reach the cortex of the paraflocculus bilaterally. Spinal Projections to Precerebellar Relay Nuclei Spinal afferents in the brush-tailed possum project to certain brain nu clei that are known to relay to the cerebellum in eutherian mammals [see B rodal , 1969, p. 278, for discussion of this point]. Reference to figure 3 shows that degenerating spinal fibres terminate on the lateral reticular nu cleus, the nucleus intercalatus (the other two perihypoglossal nuclei also received spinal afferents but are not pictured here), inferior olivary nucle us (restricted to the lateral part of the dorsal accessory olive and the later al part of the caudal medial accessory olive) and the external cuneate nu cleus. The observation of such spinal connections to cerebellar relay nu clei in the brush-tailed possum agress with the findings of M ehler [1969] and H azlett et al. [1972] in the American opossum. Furthermore, these connections in these two marsupials are strikingly similar to the spinobulbar connections described in eutherian mammals, e.g. the rat [L u n d and W ebster , 1967], the hedgehog [Jane and S chroeder , 1971], the cat [A nderso n and B erry , 1959], the tree shrew [S chroeder and Jane , 1971], the monkey and chimpanzee [M ehler , 1969], the sheep [R ao et al., 1969] and the pig [B reazile and K itchell , 1968], Particularly nota ble among spinobulbar projections in the possum is the heavy termination in the external cuneate nucleus from cervical cord hemisections. Such ter minal degeneration is very sparse in cases of thoracic cord damage. These findings are consistent with the known association of this nucleus with re lay of forelimb proprioceptive information to the cerebellum in cats [G rant , 1962b].
The present study serves to emphasize the similarity of long tract anat omy that exists between marsupial and eutherian mammals [see for exam ple R ockel et al., 1972; W arner and W atson , 1972]. However, the presence of five sagittal rows of spinocerebellar fibres within the anterior
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Discussion
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Fig. 4. A diagram [after the style of H azlett el at., 1971] showing the cerebel lar cortex of the brush-tailed possum as if it was laid out flat. The distribution of spinocerebellar termination is indicated by dots. This pattern is discussed in the text. The paramedian (PM) and ansiform (A) lobules and the paraflocculus (PF) and flocculus (FC) are labelled. Other abbreviations are the same as in figure 1.
Overall Distribution of Spinocerebellar Fibres The separation of anterior and posterior areas of cerebellar degenera tion is as clear in the brush-tailed possum as in the eutherians [B eck , 1927; A nderso n , 1943; Yoss, 1952, 1953; G rant , 1962a] and the opos
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lobe of the cerebellum in the brush-tailed possum contrasts with findings in eutherians. V oogd [1969] drew attention to the concentration of spinocerebellar fibres in sagittally oriented rows in the deep cerebellar white matter of a variety of different eutherians (ferret, cat, hedgehog, tree shrew, loris and macaque). He showed that up to eleven such rows are present in the an terior lobe of the cerebellum of some eutherians and the smallest number (seven) is found in the tree shrew. Some variations in the number of rows may be due to differences in the complexity of cerebellar lobulation in some eutherians. Despite this, it is clear that the five sagittal rows present in the opossum [H azlett et al., 1971] and in the brush-tailed possum (present study) are a distinct feature of these marsupials. It seems reasonable to infer that this feature is relat ed to the phylogeny of the species rather than to their particular degree of cerebellar development because even the poorly lobulated cerebellum in the eutherian hedgehog has eleven such rows [V o ogd , 1969].
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sum [H azlett et al., 1971], This separation is shown diagrammatically in figure 4. The anterior area of spinocerebellar termination in the brush-tailed possum comprises lobules I-V (lingula, preculmen and the culmen). G rant [1962a] found (in cats) a ‘sharp posterior border’ delineating the degenerating spinocerebellar fibres in the anterior part of lobule V from those in the posterior part. Such a clear distinction is not seen in Trichosurus - a situation which parallels the findings of H azlett et al. [1971] in the opossum. However, the latter authors seem to have misinterpreted the findings of G rant [1962a] in their discussion of the anterior area of lobe degener ation in the opossum; H azlett et al. [1971] infer that the physiologically defined hindlimb and forelimb areas of the anterior lobe are distinct in the cat, which they are not [O scarsson , 1965] and they also imply that the spinocerebellar terminations in the anterior part of the cerebellum in the opossum are quite different from those in the cat [G rant , 1962a], which is probably not the case. We feel that the apparent differences be tween marsupials and the cat in terms of the extent of anterior lobe termi nation are only slight and may be due to differences in the interpretation of the density of degeneration in parts of the culmen. The posterior area of spinocerebellar termination is concentrated in the pyramis and paramedian lobule (and to a lesser extent, the uvula) in the brush-tailed possum. An important difference between this posterior terminal area and the one found in the anterior lobe is that degeneration in the latter area is bilaterally symmetrical following spinal cord hemisection, whereas in the posterior area the fibres are more heavily concentrat ed on the ipsilateral side, particularly in the paramedian lobule. Within the white matter of the pyramis, the sagittally coursing fibres are concen trated in three rows (fig. 3C) which are, however, not nearly as distinct as the sagittal rows in the anterior lobe. The distribution of fibres to the pyr amis and paramedian lobule is similar to that found in the rat [A n d e r so n , 1943], cat [G rant 1962a], macaque [Yoss, 1952, 1953] and opos sum [H azlett et al., 1971], Sagittal rows have been found in the pyramis of a number of eutherians by V oogd [1969] but the number of rows was invariably greater in these anim als than in the brush-tailed possum . H azlett et al. [1971] did not note the presence of sagittal rows in the pyramis of the opossum but reference to their figure 3C shows a pattern of three rows in the region of the cerebellum.
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Intracerebellar Course of the Ventral and Dorsal Tracts The separate distribution of each of the two spinocerebellar tracts within the cerebellum of cats has been mapped in Nauta stained material by G rant [1962a] and electrophysiologically by L undberg and O scars son [1962] and O scarsson et al. [1964], These studies show that the dorsal spinocerebellar tract has an almost entirely ipsilateral distribution to the anterior lobe and the pyramis and paramedian lobule. On the other hand, about 75% of the ventral spinocerebellar fibres terminate contralaterally in the cerebellar cortex with 10% ipsilateral and 15% branching to end in both sides [O scarsson , 1965]. Ventral spinocerebellar fibres have a similar distribution in the anterior lobe as their dorsal counterparts, but relatively few ventral spinocerebellar fibres reach the pyramis and para median lobule [G rant , 1962a], In the present study on Trichosurus, the intracerebellar termination of each of these two tracts could only be in ferred from data on the intracerebellar course of each tract. However, the clear impression was gained that dorsal spinocerebellar fibres spread out largely towards the ipsilateral anterior lobe and pyramis and paramedian lobule, whereas large numbers of ventral tract fibres are seen to cross the midline. No evidence was found of any major differences between the ter minations of spinocerebellar fibres in the brush-tailed possum and eutherians. Acknowledgements We are grateful to Dr. J ohn I. J ohnson, jr., Mr. Brian F reeman, Ms. J an P rovis, Dr. J onathan Stone , Professor F red R ost and Ms. L orraine Brooks who all assisted in various ways in producing this manuscript.
References A nderson , R. F.: Cerebellar distribution of the dorsal and ventral spinocerebellar
tracts in the white rat. J. comp. Neurol. 79: 415-423 (1943). systems in the cat brainstem. J. comp. Neurol. I l l : 195-229 (1959). Beck, G.: The cerebellar terminations of the spinocerebellar fibres of the lower lumbar and sacral segments of the cat. Brain 50: 60-98 (1927). Breazile , J. E. and K itchell , R. L.: Ventrolateral spinal cord afferents to the brain stem in the domestic pig. J. comp. Neurol. 110: 363-372 (1968). Brodal, A.: Neurological anatomy in relation to clinical medicine; 2nd ed. (Oxford University Press, New York 1969).
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A nderson , F. D. and B erry, C. M.: Degeneration studies of long ascending fiber
152
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maesthetic afferent system of the marsupial phalanger, Trichosurus vulpécula. Aust. J. exp. Biol. med. Sci. 39: 19-28 (1961). E ccles , J. C ; L unas , R., and Sasaki, K.: The excitatory synaptic action of climbing fibres on the Purkinje cells of the cerebellum. J. Physiol., Lond. 182: 268-296 (1966). Fink, R. P. and H imer, L.: T wo methods for selective silver impregnation of degen erating axons and their synaptic endings in the central nervous system. Brain Res. 4: 369-374 (1967). G rant, G.: Spinal course and somatotopically localized termination of the spinocer ebellar tracts. An experimental study in the cat. Acta physiol, scand. 56: suppl. 193, pp. 1-61 (1962a). G rant, G.: Projection of the external cuneate nucleus onto the cerebellum in the cat. An experimental study using silver methods. Expl Neurol. 5: 179-195 (1962b). Ha, H. and Liu, C. N.: Cell origin of the ventral spinocerebellar tract. J. comp. Neurol. 133: 185-206 (1968). H ayhow , W. R.: The accessory optic system in the marsupial phalanger, Trichosu rus vulpécula. J. comp. Neurol. 126: 653-672 (1966). H azlett , J. C.; D om , R., and M artin , G. F.: Spinobulbar, spinothalamic and medi al lemniscal connections in the American opossum (Didelphis marsupialis virginiana). J. comp. Neurol. 146: 95-118 (1972). H azlett , J. C ; M artin, G. F., and D om, R.: Spinocerebellar fibres of the opossum Didelphis marsupialis virginiana. Brain Res. 33: 257-271 (1971). H eath, C. J. and J ones , E. G.: Interhemispheric pathways in the absence of a cor pus callosum. An experimental study of commisural connections in the marsupi al phalanger. J. Anat. 109: 253-270 (1971). H ore, J.; P hillips , C. G., and P orter , R.: The effects of pyramidotomy on motor performance in the brush-tailed possum Trichosurus vulpécula. Brain Res. 49: 181-184 (1973). Jane, J. A. and Schroeder , D. M.: A comparison of dorsal column nuclei and spin al afferents in the European hedgehog (Erinaceus europeaus). Expl Neurol. 30: 1-17 (1971). J ansen, J.: On the morphogenesis and morphology of the mammalian cerebellum; in J ansen and Brodal Aspects of cerebellar anatomy (Grundt Tatum, Oslo 1954). L arsell, O.: The development and morphology of the cerebellum in the opossum. II. Later development and adult. J. comp. Neurol. 63: 251-291 (1936). L linás, R. and H illman, D. E.: Physiological and morphological organization of the cerebellar circuits in various vertebrates; in L linás Neurobiology of cere bellar evolution and development, pp. 43-73 (Am. med. Ass., Chicago 1969). L und , R. D. and W ebster, K. E.: Thalamic afferents from the spinal cord and trigeminal nuclei. An experimental anatomical study in the rat. J. comp. Neurol. 139: 313-328 (1967). L undberg , A. and O scarsson, O.: Functional organization of the ventral spinocere bellar tract in the cat. IV. Identification of units by antidromic activation from the cerebellar cortex. Acta physiol, scand. 54: 252-269 (1962).
Downloaded by: Nagoya University 133.6.82.173 - 1/15/2019 12:24:15 PM
C lezy, J. K. A.; D ennis , B. J., and K err, D. 1. B.: A degeneration study of the so-
Marsupial Spinocerebellar Tracts
153
M artin, G. F. and M egirian, D.: Corticobulbar projections of the marsupial phal-
anger (Trichosurus vulpécula). II. Projections to the mesencephalon. J. comp. Neurol. 144: 165-192 (1972). Martin, G. F.; M egirian, D., and R oebuck, A.: The corticospinal tract of the mar supial phalanger (Trichosurus vulpécula). J. comp. Neurol. 139: 245-258 (1970). M artin, G. F.; M egirian, D., and R oebuck, A.: Corticobulbar projections of the marsupial phalanger (Trichosurus vulpécula). I. Projections to the pons and medulla oblongata. J. comp. Neurol. 142: 275-296 (1971). M ehler , W. R.: Some neurological species differences - a posteriori. Ann. N.Y. Acad. Sei. 167: 424-468 (1969). M ehler , W. R.; F efferman, M. E., and N auta, W. J. H.: Ascending axon degener ation following anterolateral cordotomy. An experimental study in the monkey. Brain S3: 718-750 (1960). G scarsson, O.: Functional organization of the spino- and cuneocerebellar tracts. Physiol. Rev. 45: 495-522 (1965). O scarsson, O.; R osen, I., and U ddenberg , N.: A comparative study of ascending spinal tracts activated from hindlimb afferents in monkey and dog. Archs ital. Biol. 102: 137-155 (1964). Rao, G. S.; Breazile, J. E., and K itchell , R. L.: Distribution and termination of spinoreticular afferents in the brain stem of sheep. J. comp. Neurol. 137: 185-196 (1969). R ees, S. and H ore, J.: The motor cortex of the brush-tailed possum (Trichosurus vulpécula): motor representation, motor function and the pyramidal tract. Brain Res. 20: 439-451 (1970). R ockel, A. J.; H eath, C. J., and J ones, E. G.: Afferent connections to the dience phalon in the marsupial phalanger and the question of sensory convergence in the ‘posterior group’ of the thalamus. J. comp. Neurol. 145: 105-130 (1972). Schroeder, D. M. and J ane, J. A.: Projection of dorsal column nuclei and spinal cord to brain stem and thalamus in the tree shrew. J. comp. Neurol. 142: 309-350 (1971). Snodgrass, A. B. and D orsey, C. H.: Egg albumin embedding: A procedure com patible with neurological staining techniques. Stain Technol. 33: 149-155 (1963). Voogd , J.: The importance of fibre connections in the comparative anatomy of the mammalian cerebellum; in L linäs Neurobiology of cerebellar evolution and de velopment, pp. 407-459 (Am. med. Ass., Chicago 1969). W arner, G. and W atson, C. R. R.: The rubrospinal tract in a diprotodont marsupi al (Trichosurus vulpécula). Brain Res. 41: 180-183 (1972). W atson, C. R. R. and S ymons, M.-C.: Ascending pathways to the brain stem of the phalanger (Trichosurus vulpécula). J. Anat. 110: 501 (1971). W eller , W. L.: Barrels in somatic sensory neocortex of the marsupial Trichosurus vulpécula (brush-tailed possum). Brain Res. 43: 11-24 (1972). Yoss, R. E.: Studies of the spinal cond. I. Topographic localization within the dor sal spinocerebellar tract in Macaca mulatto. J. comp. Neurol. 97: 5-20 (1952). Yoss, R. E.: Studies of the spinal cord. II. Topographic localization within the ven tral spinocerebellar tract in the macaque. J. comp. Neurol. 99: 613-638 (1953).
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Dr. C harles, R. R. W atson, School of Anatomy, University of New South Wales, P. O. Box 1, Kensington, NSW 2053 (Australia)
