The somatotopically organized rubrospinal pathway is the major
component of the laterally descending brainstem pathways, and is
especially involved in steering of fractionated movements of the
distal parts of the limbs. Electrophysiological studies in cat
showed that this fiber system, in contrast to the medially descending
pathways, has a limited degree of collateralization in the
spinal cord (Abzug et al., 1973 and 1974; Shinoda et al., 1977). The
red nucleus projects also to the contralateral cerebellum (Brodal
and Gogstad, 1954; Courville and Brodal, 1966). The collateralization
of the rubrospinal neurons to the contralateral cerebellar
interpositus nucleus is relatively high as indicated by anatomical
and electrophysiological findings (Anderson, 1971; Brodal and Gogstad,
1954). Thus, the findings in these studies suggested that
almost all rubrocerebellar fibers are collaterals from rubrospinal
neurons.
In view of the above data it appears that the rubrospinal pathway
represents a focussed system, which distributes its fibers to specific
groups of spinal segments. On the other hand, the rubrocerebellar
pathway probably represents a direct, tightly coupled return
projection to the interpositus nucleus, which is the main source of
afferents to the rubrospinal neurons (Courville, 1966b; Dekker, 1981;
Flumerfelt et al., 1973; King et al., 1973; Tsukahara et al., 1967).
In the present anatomical study an attempt has been made to
demonstrate anatomically the existence of collaterals in the rubrospinal
pathway and to compare quantitatively the degree of this collateralization
in rat, cat and monkey. This was done with the aid of the multiple retrograde fluorescent tracer technique. This technique will be
described in chapter II. In this anatomical study one fluorescent tracer was
injected in the cervical grey and another in more caudal segments of the cord.
In these experiments the distribution of sin3le and double labeled neurons in
red nucleus was studied. The descending pathways from the ventrolateral pontine
tegmentum and from the raphe magnus, including the adjoining ventral reticular
formation, also descend in the dorsolateral funiculus in rat, cat and monkey
and also terminate in the dorsal grey (Basbaum and Fields, 1978, 1979; Holstege
et al., 1979; Kuypers and Maisky, 1977; R.F. Martinet al., 1978; G.F. Martinet
al., 1979 and 1981a; Tohyama et al., 1979a+b). Therefore, in these retrograde
fluorescent double labeling studies the degree of the collateralization ~n
these descending tracts was compared to that of the rubrospinal pathway ~n
each of three mammalian species (i.e. rat, cat and monkey). In addition, in
cat an attempt was made to determine the location of the rubro-olivary neurons
in relation to the rubrospinal ones and to establish whether the rubro-olivary
neurons give collaterals to the spinal cord. These studies will be presented
in Chapter III and IV. In these studies attention was also paid to the rubrocerebellar
connections. Specifically an attempt has been made to determine
whether these connections are established by collaterals of rubrospinal
neurons. These findings will be presented ~n Chapter V. Electron microscopic
studies in rat, combining the anterograde and retrograde intra-axonal transport
techniques, showed that many rubrospinal neurons receive afferents from the
cerebellar interpositus nucleus (Dekker, 1981). Some of the fluorescent
tracers used in the present study can be employed both retrogradely to demonstrate
parent cell bodies as well as anterogradely to demonstrate the fiber
terminals. Using the fluorescent tracers in this fashion it could be demonstrated
in light microsc0py that the cerebellar interposito-rubral fibers
establish contact with rubrospinal neurons. These data are also presented ~n
Chapter V