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Projection of cervical dorsal root fibers to the medulla oblongata in the brush-tailed possum Trichosurus vulpecula.

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Projection of Cervical Dorsal Root Fibers to the Medulla Oblongata in
the Brush-Tailed Possum, Trichosurus vulpecula
Department of Anatomy, West Virginia University Medical Center, Morgantown, West Virginia 26506
a highly ordered manner, classically described as a layered, dermatotopic' pattern (Ferraro and Barrera, 1935;
Kahler, 1882; Kodama, 1942; Kruger and Norton, 1973;
Ram6n y Cajal, 1952; Walker and Weaver, 1942). More
recently, re-sorting of these fibers into a somatotopic(see
footnote 1)arrangement has been recognized (Pubols et
al., 1965; Whitsel et al., 1970).
Within the DCN, used here to include nucleus gracilis
(GN), nucleus cuneatus (CN), and external cuneate nucleus (EC),these fibers are now known to reach particular nuclei, subnuclei, andor areas within subnuclei
based on their somatotopic origins (Campbellet al., 1974;
Johnson et al., 1968; Kruger et al., 1961)and, to a lesser
extent, on their modalities (Blum et al., 1975; Dykes et
al., 1982; Millar and Basbaum, 1975; Ostapoff et al.,
1983). With the background provided by these studies of
somatotopy and modality sorting in several mammalian
species, it is possible to examine the distribution of dorsal root fibers to the DCN in any species and to draw
limited conclusions about the locations and relative
densities of classes of receptors in skin vs. muscle supplied by that particular root.
Detailed anatomical descriptions of cervical dorsal root
projections to the medulla have been published for the
following mammals: rat (Basbaum and Hand, 19731,
lesser bushbaby (Albright and Haines, 1978), squirrel
(Albright et al., 1983),rhesus monkey (Ferraro and Barrera, 1935; Rustioni et al., 1979; Shiver et al., 1968),
and cat (Keller and Hand, 1970; Rustioni and Macchi,
1968). In addition, one previous report (Culberson and
Albright, 1984) includes some data on the arrangement
of cervical dorsal root fibers in the fasciculus cuneatus
of three species-the raccoon, potoroo, and brush-tailed
possum; however, details of distribution have not been
published for any of these. In the present work, the
medullary distribution of cervical dorsal root fibers in
the brush-tailed possum is presented in detail and discussed in comparison to placental forms already examINTRODUCTION
ined. This represents the first such study of this system
The principal neural inputs to the mammalian dorsal in a species of the order Marsupialia.
column nuclei (DCN) are the ascending afferent nerve
fibers which enter the central nervous system (CNS)via
segmental dorsal roots. These fibers form part of the
spinal dorsal columns (fasciculusgracilis [FG]and fasciculus cuneatus [FC]) which comprise the beginning of a
major somatic afferent pathway to the brain in all orders of mammals. As such they subserve complex aspects of mechanoreceptive sensation, which depends on
'A dermatotopic (or segmentotopic) arrangement of nerve fibers (or
subsequent transmission to brainstem and cerebral cor- cells) preserves and displays the topographic spinal relations that
tex (Mountcastle, 1984), and they also carry ascending exist between fibers from different spinal nerves. A somatotopic patimplies a continuous three-dimensional map of the body surface
proprioceptive information crucial to motor control tern
which is organized via inputs from multiple spinal levels.
(Hummelsheim et al., 1985; Rosen, 1969). These firstorder afferent projections traverse the dorsal columns in
Received July 29, 1986.Accepted October 29, 1986.
ABSTRACT This study describes the projection of
cervical spinal afferent nerve fibers to the medulla in
the brush-tailed possum, a marsupial mammal. After
single dorsal roots (between C, and TI) were cut in a
series of animals, the Fink-Heimer method was used to
demonstrate the projection fields of fibers entering the
CNS via specific dorsal roots. In the high cervical spinal
cord, afferent fibers from each dorsal root form a discrete layer in the dorsal funiculus. The flattened laminae from upper cervical levels are lateral and those
from lower cervical levels are medial within the dorsal
columns. All afferent fibers at this level are separated
from gray matter by the corticospinal fibers in the
dorsal funiculus. All cervical roots project throughout
most of the length of the well-developed main cuneate
nucleus in a loosely segmentotopic fashion. Fibers from
rostral roots enter more lateral parts of the nucleus,
and fibers from lower levels pass to more medial areas;
but terminal projection fields are typically large and
overlap extensively. At more rostral medullary levels,
fibers from all cervical dorsal roots also reach the external cuneate nucleus. The spatial arrangement here
is more complex and more extensively overlapped than
in the cuneate nucleus. Rostra1 cervical root fibers
reach ventral and ventrolateral areas of the external
cuneate nucleus and continue to its rostral pole; more
caudal root fibers project to more dorsal and medial
regions within the nucleus. These results demonstrate
that projection patterns of spinal afferents in this marsupial are similar to those seen in the few placental
species for which detailed data concerning this system
are available.
(91987 ALAN R. LISS, INC
Experiments were done on a series of 15 adult brushtailed possums (Trichosurus vulpecula). Feral animals
were collected by hand capture from the immediate environs of Dunedin, New Zealand. They were held approximately 3-5 days before use in experiments.
Animals of both sexes (eight females, seven males)
weighing from 2.7 to 3.9 kg were individually housed
under outdoor ambient lighting conditions and fed ad
libitum (dog food) before and after surgery. Animals
were anesthetized by intraperitoneal injection with sodium pentobarbital (50-60 mgkg, supplemented as
needed to maintain deep surgical anesthesia). Through
a dorsal midline incision, the cervical spine and/or external surface of the occipital bone was exposed, and in
each case, a different cervical spinal nerve was identified and isolated. Spinal levels sampled were C2-T1,
with the exception of Cg. TI is included here because it
is a large, brachial plexus root which, due to its peripheral distribution, has more in common with cervical
roots than with other thoracic levels. After the spinal
ganglion was exposed by enlarging the intervertebral
foramen, the dura was opened and all dorsal rootlets of
a single nerve were sectioned just central to the ganglion. Sterile Gelfoam was placed over the bony defect,
and the wound was closed in two stages; muscle was
closed with gut sutures, and the skin with silk. All
demanding stages of surgery were performed under microscopic control. After 5 to 6 days postoperative survival (based on prior experience with this fiber system
in several species; Culberson and Albright, 1984), animals were anesthetized as described above, injected (intracardiac) with 5,000 units of heparin, and killed by
transcardiac perfusion with warm (28-30°C) 0.9% NaCl
solution followed by a 10% formalin fixative solution.
Brains and spinal cords were removed immediately and
stored in fresh 10% formalin for 8 weeks to 6 months
before further processing. Exact levels and completeness
of dorsal root sections were verified at the time of cord
removal. The medulla from each animal was blocked
into one or two pieces, and each of these was frozen
sectioned (40 pm thickness) serially in the transverse
(11cases) or parasagittal (four cases) plane. Transverse
sections at 0.24-mm intervals or alternate parasagittal
sections were impregnated by using the Fink-Heimer
(1967) procedure; an adjacent section in each case was
stained with cresyl violet acetate. Drawings shown here
were traced from projected images of actual sections;the
exact location of degenerated fibers of passage (large
dots in Figs. 1-5) and projection fields (small dots in
Figs. 1-5) was filled in under microscopic control.
principal target nuclei are relevant to the findings. The
cuneate nucleus extends throughout the C1 spinal cord
segment where it bulges into the base of the dorsal
funiculus (Figs. 1-5, A and B). It increases in crosssectional area beginning about 1.5 mm below the obex;
shifts laterally in position beginning at the obex; and at
about 1.5 mm above this level, its cells display a more
diffuse, reticular arrangement. The rostral end of the
CN comes to lie adjacent to the EC rostral t o the obex.
Cells of the CN are pale-staining, round and irregular
multipolar neurons of medium size; some slightly larger,
more intensely staining cells occur in the large part of
the nucleus near the obex and scattered through its
more caudal levels. The EC is easily recognized by its
large, intensely stained, multipolar cells. It is first seen
at obex levels as scattered neurons at the dorsolateral
angle of the medulla. From there it expands in size and
extends rostrally for about 4 mm (Figs. 1-5, D-F). Its
medial border abuts on the rostral part of the CN.
The Ascending Fibers
Just above their spinal cord entry level, fibers from
each dorsal root form a compact bundle of degenerated
fibers in the lateral edge of FC. As they course rostrally,
they spread into a larger area of FC and mix with
adjacent normal fibers. Figure 6A shows an example of
degenerated FC fibers. Fibers from most cervical dorsal
roots tend to separate into two bundles, one medial and
deep in the FC, the other more laterally and superficially placed (cf. Fig. 3A-C). There are differences in the
distribution of fibers from these two bundles which may
relate to different functions (see Discussion). Beginning
at caudal medullary levels and continuing throughout
the length of CN, terminals or collaterals of FC axons
form small bundles which drop vertically through the
FC and the pyramidal tract to reach their targets for
distribution. Many axons from the lateral bundle continue rostrally, shift laterally, and finally extend directly into targets in the EC and the spinal trigeminal
Distribution of Upper Cervical Afferent Fibers (C2-C4}
Degenerated fibers from these roots form adjacent,
overlapping laminae in the lateral part of FC in the
caudal medulla. Fibers from C2 are most lateral, with
C3 (Fig. 1A) and then C4 (Fig. 2A) fibers medial to them.
As fibers continue rostrally, separation into a deeper
and a more superficial population is seen, most apparently in the C4 case Fig. 2B,C). Preterminal degeneration is seen to enter and distribute diffusely in the rostral
The principal results of this study are summarized by
Figures 1-5, which illustrate the course and medullary
distribution of the dorsal root fibers degenerated after
rhizotomy in five representative experimental cases. The
following text treats features common to projections from
all spinal cord levels and points out differences among
projections of different dorsal roots.
Architecture of Medullary Target Nuclei
Although we did not undertake cytoarchitectural t n
analysis of the possum medulla, brief comments on the tt
area postrema
central cervical nucleus
nucleus cuneatus
external cuneate nucleus
fasciculus cuneatus
fasciculus gracilis
nucleus gracilis
hypoglossal nucleus
nucleus intercalatus
dorsal motor nucleus of vagus
pyramidal tract fibers
solitary complex (nucleus and tract)
spinal trigeminal nucleus
spinal trigeminal tract
Fig. 1. Drawings of the dorsolateral quadrants of sections through the medulla of a possum
with a C3 dorsal root lesion. Sections shown are spaced at approximate 1.25-mm intervals and
are arranged from caudal (A) to rostra1 (F). In this and subsequent figures, large dots represent
fibers of passage; small dots represent apparent projection fields or terminal fields of fibers.
Fig. 2. Drawings of the dorsolateral quadrants of sections through the medulla of a possum
with a C4 dorsal root sesion. Sections shown are spaced at approximate 1.25-mm intervals and
are arranged from caudal (A) to rostra1 (F).
Fig. 3. Drawings of the dorsolateral quadrants of sections through the medulla of a possum
with a C, dorsal root lesion. Sections shown are spaced at approximate 1.25-mm intervals and
are arranged from caudal (A) to rostra1 (F).
Fig. 4. Drawings of the dorsolateral quadrants of sections through the medulla of a possum
with a C7 dorsal root lesion. Sections shown are spaced at approximate 1.25-mmintervals and
are arranged from caudal (A) to rostra1 (F).
Fig. 5. Drawings of the dorsolateral quadrants of sections through the medulla of a possum
with a T, dorsal root lesion. Sections shown are spaced at approximate 1.25-mm intervals and are
arranged from caudal (A) to rostra1 (F).
Fig. 6 . Photomicrographs showing characteristic appearance of de- caudal nucleus cuneatus, adjacent to cuneate fasciculus. This is on the
generated axons and terminals in the medulla. A: Fibers in medial medial side of the nucleus after a C6 dorsal root section. D: Slightly
fasciculus cuneatus following C8 dorsal rhizotomy. Nearly all fibers ex- coarser, less-dense debris in the ventrolateral part of the nucleus cuneahibit a coarse, beaded appearance. 8: Dense, coarse preterminal debris tus from the same section as C. Fink-Heimer stain; bar in C = 50 pm for
surrounding large neurons in the medial part of the external cuneate all figures.
nucleus after C7 lesion. C: Fine, granular debris in the dorsal shell of the
end of the central cervical nucleus (C), and in caudal
sections through C1 there are extensive endings in the
adjacent reticular formation. This is especially true for
the C2 and C3 (Fig. 1A) cases, where this projection is
an extension of the typically heavy afferent fiber distribution to spinal intermediate gray matter. Within the
caudal CN, fibers from Cz project to the extreme lateral
edge of the nucleus and generate a large preterminal
field in the ventrolateral part of the CN; this projection
resembles that shown in Figure 6D. Fibers from C3 and
C4 have similar distribution; they are most expansive in
the deeper ventrolateral area of CN (Figs. lA,C, 2A,C)
and have a small projection t o the lateral, superficial
zone. At obex levels, all three of these roots (Cz-C,) send
a projection to the superficial dorsal portion of the spinal
trigeminal nucleus (Figs. 1D,E; 2D). This extends far
ventrally in the C3 case but in all cases has a very
limited rostrocaudal extent.
Fiber projections to the EC are extensive; rostrally
coursing fibers from C2 and C3 lie just ventral t o the EC
mixed with fibers of the spinal trigeminal tract. Their
endings are in the extreme ventral and ventrolateral
areas of the EC throughout its length, although with a
heavier projection to its rostral end than more caudally
(Fig. 1D-F). The C4 distribution to the EC arises from
fibers passing forward in this same location and from
fibers of the superficial bundle, which lies at the dorsomedial edge of the nucleus (Fig. 2D-F). The target for
C4 fibers is more caudally placed than that for Cz and
C3, in the central and medial areas of the ventral onethird of the EC.
Distribution of midcervical afferent fibers (C5,
Since a C5 case is lacking in this study, these midcervical levels are represented by Figure 3, which was
drawn from a representative C6 case. There continues
to be a diffuse projection to the central cervical nucleus
(Fig. 3A) from Cg. The CN in this case receives a dense
projection to well-defined areas of its superficial shell
(Fig. 6C); these fibers arise mainly from the more medial
of the two recognizable bundles in the FC. This input to
the CN is medial to the high cervical input to the CN
and is concentrated in the dorsal superficial part of the
nucleus (Fig. 3A,C,D), although there is some shifting
in location (Fig. 3B). There is a second projection field in
the deeper, ventrolateral CN which is well developed
throughout all but the rostral end of the nucleus. These
fibers enter from the larger, lateral bundle in the FC
and end by overlapping the same area as in C 4 4 cases.
The EC receives input mainly from the lateral FC
bundle which lies at the EC-CN interface (Fig. 3D) and
just dorsal to the EC. Fibers in the ventral EC end in
the center of the nucleus at caudal levels; more rostrally
their large terminal field shifts dorsally into the center
of the nucleus. A few fibers extend to the rostral end of
the EC to central and lateral areas.
Distribution of more caudal cervical afferent fibers (C7-T,)
Fibers from these dorsal roots form flattened to crescentic laminae in the extreme medial FC when viewed
in cross section in the caudal medulla (Figs. 4A,B; 5A,B).
As they continue rostrally some fibers retain this relative position while many shift laterally in a superficial
location in FC. The central cervical nucleus receives a
minimal input in the C7 case but contains no degenerated fibers following Cs or T1 lesions. Fiber projections to
the caudal CN arise as collaterals or terminals of the
more medial FC fibers; these radiate toward the nucleus
from several directions (Figs. 4A; 5B,C) to reach and
ramify among the superficial cells of the CN. This projection to dorsal CN is extensive from C7 and Cs and
extends to multiple targets, especially along the medial
edge of the CN (Fig. 4B,C). T1 also projects to multiple
sites in the shell of CN. These three roots also send
many fibers into the deep ventrolateral area of NC where
other cervical roots project. This latter system arises
from the large, laterally situated FC bundle, which also
projects more rostrally into the EC. Within the EC, C7
fibers reach large target areas; a small projection goes
caudal and ventral, more fibers spread out through the
dorsal one-half of EC at its largest extent, and the heaviest projection is to its medial midregion (Figs. 6B, 4D).
Few fibers extend into the rostral quarter of EC. The Ti
root sends fewer fibers into the EC, where they end in
its dorsal one-half (Fig. 5E,F).
The brush-tailed possum is a cat-sized, diprotodont
marsupial with a strong, thick neck; large, mobile ears;
and a large, furry tail. The animal is largely arboreal
and moves quickly and with great agility through the
trees. It possesses very strong limb (especially forelimb)
musculature and displays reasonable but not facile dexterity in manipulation of objects with the forepaws (see
Ride, 1970, for more details). Although few studies of
somatosensory pathways in this or other Australasian
marsupials have been published, Clezy et al. (1961) and
Dennis and Kerr (1961)have demonstrated the presence
of major classical pathways (dorsal column-medial lemniscus and anterolateral systems) in the brainstem in
Trichosurus. The forebrain components of these pathways that relate thalamus and cortex in this species
have also been studied by Rockel et al. (1972)and Haight
and Neylon (1978); and, a t least in general outline, they
resemble the placental arrangement. Since neither the
primary afferent nor the spinomedullary tracts have
been examined, the present data are considered in relation to the few studies on American marsupials @idelphis sp.) and the more extensive work on several
placental forms.
The qualitative observations included here concerning
cytoarchitecture and nuclear subdivisions of DCN are
necessarily preliminary in nature as they rely only on
Nissl material and some data concerning afferent fiber
projections. Two longitudinal zones are differentiateda rostral reticular area, beginning 1-5 mm rostral to the
obex, and the remaining caudal zone which extends
down through the medulla and the C1 spinal segment.
This breakdown coincides with the developmental description of the CN provided by Ulinski (1969) for the
opossum and with other authors’ description of the CN
in the rat (Basbaum and Hand, 1973) and cat (Kuypers
and Tuerk, 1964; Keller and Hand, 1970). The caudal
region in Trichosurus is enlarged in size for about 1.5
mm above and below the obex, and although there is
little obvious change in cytology and cell density (in our
Nissl-stained sections), this is the area which corresponds to the third subdivision of the CN described in
most detailed cytoarchitectural studies (Albright and
Haines, 1978, bushbaby; Albright et al., 1983, squirrel;
Biedenback, 1972, rhesus monkey; Cheema et al., 1983,
cat; Penny, 1982, opossum). This obex region is likely to
be the major target for low-threshold cutaneous afferents from the forepaw (see below);but in the brush-tailed
possum, it is not obviously distinguished by presence of
cell “clusters” (Keller and Hand, 1970; Kuypers and
Tuerk, 19641, “bricks” (Basbaum and Hand, 19731, or
cell “columns” (Albright and Haines, 1978). There also
is no separation into obvious subnuclei as in the monkey
(Cheema et al., 1983; Ferraro and Barrera, 1935; Rustioni et al., 1979).
Dorsal root fibers generate a dual projection to the CN
up to the rostral zone, where projections are most diffuse. Fibers reaching the dorsal “shell” of the nucleus,
which produce fine, dense debris (Fig. 6C), comprise the
topographically precise projection. Their arrangement is
in accordance with classic descriptions in rat (Basbaum
and Hand, 19731, cat (Kodama, 1942; Rustioni and Macchi, 1968), squirrel (Albright et al., 1983), and primates
(Albright and Haines, 1978; Ferraro and Barrera, 1935;
Rustioni et al., 1979; Shriver et al., 1968; Walker and
Weaver, 1942). Mapping studies of DCN cell responses
to peripheral stimulation in several species confirm this
general arrangement and show that input to this dorsal
part of the nucleus is derived from cutaneous mechanoreceptors which are most numerous in those roots which
innervate the distal extremity or forepaw (Dykes et al.,
1982;Hamilton and Johnson, 1973; Johnson et al., 1968;
Kruger et al., 1961; Millar and Basbaum, 1975). This
observation also has been supported recently by anatomical studies showing cutaneous nerve distribution to the
dorsal (and central) part of the CN in cat (Nyberg and
Blomqvist, 1972). In the present material, there was a
more extensive projection to the dorsal CN from presumed distal (paw)dermatomes (C7-Tl)2 than from proximal ones, and a minimal projection from dermatomes
presumed to supply the neck and shoulder (CZ-C~).
is also more mediolateral overlap of the projections of
adjacent root fibers in Trichosurus than some authors
have reported in other mammals.
The second NC target for spinal afferent fibers is the
ventrolateral/ventrocentral region to which all cervical
roots send an overlapping projection of coarse, degenerated fibers (Fig. 6D). These fibers arise from the large
lateral bundle of FC and are abundant throughout all
but the most rostral part of the CN. They are likely to
be low-threshold muscle afferents, according t o mapping
studies (Rosen, 1969); and appropriately, they are relatively more expansive in CZ-C~cases than from C7 and
Cs. Support for this spatial modality segregation (and
localization) of muscle afferents comes from the recent
studies by Nyberg and Blomqvist (1983) in the cat and
by Hummelsheim et al. (1985) for projections to the
homologous region (pars triangularis) in the monkey CN.
The final major medullary target for primary afferents
is the EC, which is well known to receive muscle afferents (Kruger and Norton, 1973). In the brush-tailed possum, this projection is a large one from all cervical roots;
and, although adjacent root projection fields overlap extensively, a clear segmentotopic pattern is seen. It concurs in general with that shown by Liu (1956) in his
excellent early study of this nucleus in cat. The elegant
muscle-to-EC mapping study by Campbell et al. (1974)
in rat indicates a rostral EC target for neck musculature
(our C2-C4 roots had heaviest rostral projection) and
spatially well-defined areas for fibers from arm, hand,
forearm, and shoulder. Our results, assuming typical
dermatomal representation (see footnote 21, fit reasonably well with their schema of forelimb representation
in the EC. We expect some variation based on the differences in dermatomes and limb usage; and, in addition,
dorsal root projections to the EC also convey some cutaneous aflerents intentionally excluded from consideration by Campbell et al. (1974).
Given the behavioral repertoire of the brush-tailed
possum (see above), it is not surprising to find in an
anatomical study of the dorsal column system a very
extensive, well-developed cuneate system. The FC is in
fact very large compared to the gracile component of the
dorsal column system in this species. Within the cuneate projections are many fibers which distribute to
portions of the CN and EC known to receive proprioceptive inputs. Projections to the dorsal CN, especially in
the region near the obex, are abundant; but neither the
cells in CN nor the fiber projection seem to be arranged
so precisely as those in placental mammals (primates)
with well-developed digital dexterity (Albright and
Haines, 1978; Culberson and Albright, 1984;Rustioni et
al., 1979).
Experimental material upon which this report is based
was colIected while the author was Visiting Senior Lecturer in the Department of Anatomy at University of
Otago, Dunedin, New Zealand. I am very grateful for
facilities, friendship, and assistance graciously provided
by many members of that department, especially Len
Robinson, then Acting Chairman, and C.J. Heath. Excellent technical assistance in Dunedin was provided by
Mr. Ken Turner; and histologic processing (at WVU) was
by Sue Horvat, Dorothy Heritage, and Debbie McIntosh.
Donna Borland typed the manuscript. Some of these
data have been reported in abstract form (Culberson,
1979; Culberson et al., 1983). The work was supported
in part by grants from the West Virginia Medical Corporation-USPHS (5-Sol-RRO5433) and NSF (BNS
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fiber, medulla, dorsal, oblongata, projections, trichosurus, vulpecula, brush, cervical, tailed, possum, roots
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