Lack of recurrent depression from motor axon collaterals of IaIPSPs in dorsal spinocerebeller tract neurones

Proprioceptive Recognition with Artificial Neural Networks Based on Organizations of Spinocerebellar Tract and Cerebellum

Muscle kinematics and kinetics are nonlinearly encoded by proprioceptors, and the changes in muscle length and velocity are integrated into Ia afferent. Besides, proprioceptive signals from multiple muscles are probably mixed in afferent pathways, which all lead to difficulties in proprioceptive recognition for the cerebellum. In this study, the artificial neural networks, whose organizations are biologically based on the spinocerebellar tract and cerebellum, are utilized to decode the proprioceptive signals. Consistent with the controversy of the proprioceptive division in the dorsal spinocerebellar tract, the spinocerebellar tract networks incorporated two distinct inferences, (1) the centralized networks, which mixed Ia, II, and Ib and processed them together; (2) the decentralized networks, which processed Ia, II, and Ib afferents separately. The cerebellar networks were based on the Marr-Albus model to recognize the kinematic states. The networks were trained by a specific movement, and the trained networks were subsequently required to predict kinematic states of six movements. The results demonstrated that the centralized networks, which were more consistent with the physiological findings in recent years, had better recognition accuracy than the decentralized networks, and the networks were still effective even when proprioceptive afferents from multiple muscles were integrated.

Information to cerebellum on spinal motor networks mediated by the dorsal spinocerebellar tract

The main objective of this review is to re-examine the type of information transmitted by the dorsal and ventral spinocerebellar tracts (DSCT and VSCT respectively) during rhythmic motor actions such as locomotion. Based on experiments in the 1960s and 1970s, the DSCT was viewed as a relay of peripheral sensory input to the cerebellum in general, and during rhythmic movements such as locomotion and scratch. In contrast, the VSCT was seen as conveying a copy of the output of spinal neuronal circuitry, including those circuits generating rhythmic motor activity (the spinal central pattern generator, CPG). Emerging anatomical and electrophysiological information on the putative subpopulations of DSCT and VSCT neurons suggest differentiated functions for some of the subpopulations. Multiple lines of evidence support the notion that sensory input is not the only source driving DSCT neurons and, overall, there is a greater similarity between DSCT and VSCT activity than previously acknowledged. Indeed the majority of DSCT cells can be driven by spinal CPGs for locomotion and scratch without phasic sensory input. It thus seems natural to propose the possibility that CPG input to some of these neurons may contribute to distinguishing sensory inputs that are a consequence of the active locomotion from those resulting from perturbations in the external world.

Interactions between spinal interneurons and ventral spinocerebellar tract neurons

Recent evidence indicates that ventral spinocerebellar tract (VSCT) neurons do not merely receive information provided by spinal interneurons but may also modulate the activity of these interneurons. Hence, interactions between them may be mutual. However, while it is well established that spinal interneurons may provide both excitatory and inhibitory input to ascending tract neurons, the functional consequences of the almost exclusively inhibitory input from premotor interneurons to subpopulations of VSCT neurons were only recently addressed. These are discussed in the first part of this review. The second part of the review summarizes evidence that some VSCT neurons may operate both as projection neurons and as spinal interneurons and play a role in spinal circuitry. It outlines the evidence that initial axon collaterals of VSCT neurons target premotor inhibitory interneurons in disynaptic reflex pathways from tendon organs and muscle spindles (group Ia, Ib and/or II muscle afferents) to motoneurons. By activating these interneurons VSCT neurons may evoke disynaptic IPSPs in motoneurons and thus facilitate inhibitory actions of contralateral muscle afferents on motoneurons. In this way they may contribute to the coordination between neuronal networks on both sides of the spinal cord in advance of modulatory actions evoked via the cerebellar control systems.

Rhythmic activity of feline dorsal and ventral spinocerebellar tract neurons during fictive motor actions

Neurons of the dorsal spinocerebellar tracts (DSCT) have been described to be rhythmically active during walking on a treadmill in decerebrate cats, but this activity ceased following deafferentation of the hindlimb. This observation supported the hypothesis that DSCT neurons primarily relay the activity of hindlimb afferents during locomotion, but lack input from the spinal central pattern generator. The ventral spinocerebellar tract (VSCT) neurons, on the other hand, were found to be active during actual locomotion (on a treadmill) even after deafferentation, as well as during fictive locomotion (without phasic afferent feedback). In this study, we compared the activity of DSCT and VSCT neurons during fictive rhythmic motor behaviors. We used decerebrate cat preparations in which fictive motor tasks can be evoked while the animal is paralyzed and there is no rhythmic sensory input from hindlimb nerves. Spinocerebellar tract cells with cell bodies located in the lumbar segments were identified by electrophysiological techniques and examined by extra- and intracellular microelectrode recordings. During fictive locomotion, 57/81 DSCT and 30/30 VSCT neurons showed phasic, cycle-related activity. During fictive scratch, 19/29 DSCT neurons showed activity related to the scratch cycle. We provide evidence for the first time that locomotor and scratch drive potentials are present not only in VSCT, but also in the majority of DSCT neurons. These results demonstrate that both spinocerebellar tracts receive input from the central pattern generator circuitry, often sufficient to elicit firing in the absence of sensory input.

A trans-spinal loop between neurones in the reticular formation and in the cerebellum

Voluntary limb movements are initiated in the brain but the neurones responsible for activating the muscles (motoneurones and interneurones) are located in the spinal cord. The spinal cord also contains neurones that provide the brain, and especially the cerebellum, with continuous information on effects of the descending commands. We show that one population of such neurones provide the cerebellum with information on how likely the brain's commands (mediated by descending reticulospinal neurones) are to be executed as planned, depending on the degree of inhibition of motoneurones. They may therefore play an important role in preventing errors in activation of motoneurones and thereby help the brain to correct its signals to the spinal cord before such errors have been committed.

Interneuronal activity in reflex pathways from group II muscle afferents is monitored by dorsal spinocerebellar tract neurons in the cat

The main aim of the study was to investigate whether group II muscle afferents contribute to the inhibition of dorsal spinocerebellar tract (DSCT) neurons and thereby modulate information provided by these neurons in the cat. In intracellular recordings, we found disynaptic IPSPs from group II afferents in the majority of DSCT neurons, most often in parallel with IPSPs evoked from group I afferents. In an attempt to identify interneurons that mediate these IPSPs, the second aim of the study, laminas IV-VII in midlumbar segments were searched for interneurons antidromically activated by stimuli applied within Clarke's column. Such interneurons were found in regions in which focal field potentials were evoked by group I and II afferents, or ventral to them, and most were coexcited by these afferents. The input to these interneurons and their location indicate that they belonged to the previously identified population of premotor interneurons in disynaptic pathways between group I and II afferents and hindlimb motoneurons. The study leads thus to the conclusion that inhibitory actions of group II afferents on DSCT neurons are collateral to actions on motoneurons and that DSCT neurons monitor inhibitory actions of group II afferents on motoneurons as closely as they monitor actions of group I afferents. The results also indicate that interneurons mediating disynaptic reflex actions from tendon organ (group Ib) afferents and those mediating disynaptic actions from secondary muscle spindle (group II) afferents to motoneurons may be parts and parcel of the same interneuronal population rather than constitute distinct interneuronal populations.

The ascending projection of interneurones activated by group I muscle afferent fibres of the cat hindlimb

1. The level of the ascending projection of lumbosacral interneurones with a monosynaptic input from group I muscle afferents of the cat hindlimb has been investigated. The study was concerned particularly with the interneurones mediating group I non-reciprocal inhibition of motoneurones. 2. In chloralose-anaesthetized cats, single-unit microelectrode recordings were made, in the lateral funiculus at L6, from the ascending axons of lumbosacral interneurones. The axons studied were discharged by group I afferents primarily from extensor muscles of the knee and ankle. Some units were discharged in addition by cutaneous and/or joint afferents. 3. The extent of the ascending projection of the interneurones was assessed by antidromic activation of their axons in the lateral funiculus at different spinal levels. The great majority of axons tested (thirty-four out of thirty-six) were found to have an ascending projection to at least the L4 level and of these most (90%) did not project beyond the L4 or L3 segments of the cord. 4. The projection to at least L4, together with monosynaptic input from group I afferents and a pattern of peripheral input characteristic of interneurones in laminae V-VI of lumbosacral segments, identified thirty-four of the thirty-six units as interneurones mediating group I non-reciprocal inhibition. 5. These results suggest that the ascending group I inhibitory pathway, formed by these interneurones, is associated specifically with the group I relay of the dorsal spinocerebellar tract in Clarke's column, rather than being conterminous with group I afferents, which project throughout the rostral lumbar and lowest thoracic segments.

The same interneurones mediate inhibition of dorsal spinocerebellar tract cells and lumbar motoneurones in the cat

The aim of the study was to investigate whether inhibition of dorsal spinocerebellar tract (d.s.c.t.) cells evoked from group I afferents is mediated by the same interneurones which mediate the non-reciprocal inhibition of hind-limb motoneurones. The origin of inhibition of d.s.c.t. cells from group I afferents was compared in intact preparations, after lesions of the dorsal funiculi (when it could only be mediated by lower lumbar interneurones) and after lesions of the lateral and ventral funiculi (when it would be expected to be evoked by upper lumbar interneurones). In all three preparations extensors were the most common source of inhibition, as in motoneurones. Lower lumbar interneurones inhibiting d.s.c.t. cells were found to be co-excited by group I (Ia and/or Ib) and cutaneous and joint afferents, and by rubrospinal tract fibres, as are interneurones mediating inhibition of motoneurones. Co-excitation by group I and rubrospinal fibres was also found for upper lumbar interneurones. I.p.s.p.s were evoked in hind-limb motoneurones from within Clarke's column in cats with the dorsal funiculi cut between L4 and L5 segments; they were evoked at thresholds as low as 2 microA, i.e. by stimuli with very local actions. The latencies of these i.p.s.p.s were short enough to allow them to be evoked monosynaptically via axonal branches of the same interneurones which projected to Clarke's column. Correspondingly, i.p.s.p.s were evoked in d.s.c.t. cells from within motor nuclei in L7 segments; they were evoked at similarly low thresholds and with similar latencies. In confirmation of previous reports i.p.s.p.s of Ia origin evoked in d.s.c.t. cells were not found to be depressed by Renshaw cells, which excludes their mediation by interneurones responsible for Ia reciprocal inhibition. The study leads to the conclusion that the inhibition of d.s.c.t. cells from group I afferents is, at least in part, collateral to the non-reciprocal inhibition of lumbar motoneurones.

Spinocerebellar projections to the vermis of the posterior lobe and the paramedian lobule in the cat, as studied by retrograde transport of horseradish peroxidase

Spinal neurons projecting to the posterior lobe of the cerebellum were identified with the retrograde horseradish peroxidase technique in the cat. In four cases with the injections, which were preceded by hemisections at cervical or thoracic levels, it was determined whether in the spinal cord the identified neurons give rise to crossed ascending axons or uncrossed ascending ones. The main groups of neurons projecting to sublobule VIIIB were located in the central cervical nucleus (with crossed ascending axons), Clarke's column (with uncrossed ascending axons), and the medial part of lamina VII of L6 to the caudal segments (with crossed ascending axons). Additional labeled neurons were found in the medial part of lamina VI between C2 and T1 and L5 and L6 (with uncrossed ascending axons), and in the ventral as well as dorsal horns of the sacral-caudal segments (with crossed ascending axons). On the other hand, neurons projecting to sublobule C (the copular part) of the paramedian lobule, which appeared always ipsilaterally to the side of the injections, were located in lamina V of C8 to L4 (with uncrossed ascending axons). Marginal neurons of Clarke's column (with uncrossed ascending axons) and spinal border cells (with crossed ascending axons that recross in the cerebellum) projected specifically to this part. At L1 and L2 or L2 and L3 labeled large and medium-sized neurons were also found within Clarke's column. The present study suggests that there are segregated projections of spinal neurons to the cortex of the cerebellar posterior lobe.