I. ACTIVITY OF NEURONS OF THE LATERAL
RETICULAR NUCLEUS
(1) Signals transmitted to the cerebellum by
the spino-reticulocerebellar pathway (SRCP)
during scratching were studied. For this
purpose, the activity of neurons of the
lateral reticular nucleus (LRN), which are
the last-order neurons of the SRCP, was recorded
during scratching in thalamic cats. Scratching
was evoked by stimulation of the pinna. LRN
neurons were identified antidromically by
stimulation of the hindlimb area in the
cerebellar anterior lobe. In most experiments,
animals were immobilized with Flaxedil, and
stimulation of the pinna resulted in fictitious
scratching, i.e., in periodical reciprocal
activity of flexor and extensor motoneurons
typical of actual scratching.
(2) During both actual and fictitious
scratching, the discharge frequency of LRN
neurons was rhythmically modulated in relation
with the scratch cycle. Most LRN neurons fired
in short high-frequency bursts of spikes which
coincided (completely or partly) with the
extensor phase of the cycle. In this respect the
SRCP differs from the ventral spinocerebellar
tract (VSCT) which is maximally active in the
flexor phase of the cycle.
(3) The firing pattern of LRN neurons during
fictitious scratching was similar to that during
actual scratching. Therefore, the rhythmical
burst firing of LRN neurons is determined mainly
by the central mechanisms and not by the
rhythmical sensory input.
(4) Rhythmical modulation of LRN neurons
disappeared after transection of the ipsilateral
lateral funiculus of the spinal cord in which
spinoreticular libers are located. On the other
hand, considerable reduction of rhythmical
activity in descending brainstem-spinal pathways
after contralateral hemisection of the spinal
cord did not affect the discharge pattern of LRN
neurons. These two facts indicate that the SRCP
conveys mainly messages about activity of the
central spinal mechanisms, and that influences
of supraspinal motor centers on LRN neurons and
on spinoreticular neurons are of minor
importance.
(5) Axonal terminations of LRN neurons are
distributed rather evenly over the hindlimb area
in the anterior lobe of the cerebellum.
Therefore, messages about the events, which
happen within the spinal cord in the vicinity of
the extensor phase of the cycle, arrive at every
point of the hindlimb area.
II ACTIVITY OF NEURONS OF THE VENTRAL
SPINOCEREBELLAR TRACT
(1) The activity of neurons of the ventral
spinocerebellar tract (VSCT) during scratching
was studied in thalamic and decapitate cats. The
neurons were identified antidromically either by
stimulation of the hindlimb area in the anterior
lobe of the cerebellurn (in thalamic cats) or by
stimulation of the contralateral ventrolateral
funiculus of the spinal cord (in decapitate
cats). The scratch reflex was elicited by
stimulation of either the pinna (in thalamic
cats) or the cervical spinal cord (in decapitate
cats). In most experiments, animals were
immobilized and the activity of VSCT neurons was
recorded during fictitious scratching.
(2) During both actual and fictitious
scratching, the discharge of VSCT neurons was
rhythmically modulated in relation with the
scratch cycle: neurons fired in bursts separated
with periods of silence. Phases of activity of
different neurons were unevenly distributed over
the scratch cycle: most neurons fired within the
limits of the flexor phase of the cycle.
(3) The firing pattern of VSCT neurons
during fictitious scratching was similar to that
during actual scratching. Therefore, rhythmical
burst firing of VSCT neurons is determined
mainly by central mechanisms and not by a
rhythmical sensory input.
(4) The firing pattern of VSCT neurons in
decapitate cats was similar to that in thalamic
cats. Therefore, rhythmical burst firing of VSCT
neurons is determined mainly by the central
spinal mechanism and not by supraspinal motor
centers.
(5) The VSCT neurons which fired in long
bursts during the greater part of the flexor
phase were usually activated during the latent
period of scratching, while those firing later
in the cycle were usually either inhibited or
not affected during this period.
(6) The antidromic response in most VSCT
neurons could be evoked from a large number of
points in the hindlimb area of the cerebellar
anterior lobe, both in the vermis and in the
pars intermedia. Due to such extensive branching
of axons, each point of the cortex receives
signals from neurons firing in different phases
of the cycle. But axons of VSCT neurons firing
in long bursts during the greater part of the
flexor phase terminate more extensively in the
pars intermedia, while axons of neurons firing
later in the cycle terminate more extensively in
the vermis.
(7) The functioning of the VSCT is
essentially similar to that of the
spinoreticulocerebellar pathway (SRCP)5. Both
pathways convey messages about activity of the
central spinal mechanism generating the motor
output pattern of scratching, but the VSCT is
active mainly in the flexor phase of the scratch
cycle and the SRCP in the extensor one. A
hypothesis is advanced that these pathways
monitor activity of different groups of spinal
interneurons.
