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The lateral reticular nucleus; integration of descending and ascending systems regulating voluntary forelimb movements.

Alstermark B, Ekerot CF - Front Comput Neurosci (2015)

Bottom Line: Individual motoneurones do not have projections to spino-cerebellar neurons.The LRN projections to the deep cerebellar nuclei exert a direct excitatory effect on descending motor pathways via the reticulospinal, vestibulospinal, and other supraspinal tracts, and might play a key role in cerebellar motor control.Our results support the hypothesis that the LRN provides the cerebellum with highly integrated information, enabling cerebellar control of complex forelimb movements.

View Article: PubMed Central - PubMed

Affiliation: Department of Integrative Medical Biology, Section of Physiology, Umeå University Umeå, Sweden.

ABSTRACT
Cerebellar control of movements is dependent on mossy fiber input conveying information about sensory and premotor activity in the spinal cord. While much is known about spino-cerebellar systems, which provide the cerebellum with detailed sensory information, much less is known about systems conveying motor information. Individual motoneurones do not have projections to spino-cerebellar neurons. Instead, the fastest route is from last order spinal interneurons. In order to identify the networks that convey ascending premotor information from last order interneurons, we have focused on the lateral reticular nucleus (LRN), which provides the major mossy fiber input to cerebellum from spinal interneuronal systems. Three spinal ascending systems to the LRN have been investigated: the C3-C4 propriospinal neurones (PNs), the ipsilateral forelimb tract (iFT) and the bilateral ventral flexor reflex tract (bVFRT). Voluntary forelimb movements involve reaching and grasping together with necessary postural adjustments and each of these three interneuronal systems likely contribute to specific aspects of forelimb motor control. It has been demonstrated that the command for reaching can be mediated via C3-C4 PNs, while the command for grasping is conveyed via segmental interneurons in the forelimb segments. Our results reveal convergence of ascending projections from all three interneuronal systems in the LRN, producing distinct combinations of excitation and inhibition. We have also identified a separate descending control of LRN neurons exerted via a subgroup of cortico-reticular neurones. The LRN projections to the deep cerebellar nuclei exert a direct excitatory effect on descending motor pathways via the reticulospinal, vestibulospinal, and other supraspinal tracts, and might play a key role in cerebellar motor control. Our results support the hypothesis that the LRN provides the cerebellum with highly integrated information, enabling cerebellar control of complex forelimb movements.

No MeSH data available.


Summary of direct pyramidal, rubral and indirect pathways via C3-C4 PN to LRN neurons. Open synapses are excitatory and filled synapses are inhibitory. The arrowhead from the LRN neurons indicated multiple terminations (in deep cerebellar nuclei and granule cell layers) in the cerebellum.
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Figure 9: Summary of direct pyramidal, rubral and indirect pathways via C3-C4 PN to LRN neurons. Open synapses are excitatory and filled synapses are inhibitory. The arrowhead from the LRN neurons indicated multiple terminations (in deep cerebellar nuclei and granule cell layers) in the cerebellum.

Mentions: Our results show that disynaptic excitation and inhibition in LRN neurons from cortico- and rubrospinal fibers could be mediated by convergent input to neurons in the C3-C4 segments, as demonstrated by the persistence of disynaptic effects after C5 lesion, but their elimination after C2 lesion. In addition, stimulation of the isolated DLF in C3 (following DLF transection in both C2 and C5) evoked disynaptic EPSPs and IPSPs in LRN neurones at low threshold, also showed transmission via neurons in the C3-C4 segments. It was previously shown that the vast majority (>84%) of the PNs have bifurcating axons projecting to the LRN and MNs (Alstermark et al., 1981a), which is in contrast to medial segmental interneurons (no projection to MNs) recorded in the same segments as the PNs (Alstermark et al., 1984c). These authors found that about 20% of these medial segmental interneurons have ascending collaterals to the LRN. Second, these interneurons, have weak or no convergent excitatory input from NR and tectum (Alstermark et al., 1984c). Since we demonstrate a strong facilitation of pyramidal EPSPs and IPSPs from NR after C5 DLF (Figures 3A,D), these effects must have been mediated via excitatory and inhibitory C3-C4 PNs. Furthermore, a majority of these LRN cells receiving either disynaptic excitation or inhibition via the C3-C4 PNs, also had monosynaptic excitatory inputs from the cortico-reticular neurons, as illustrated in Figure 9.


The lateral reticular nucleus; integration of descending and ascending systems regulating voluntary forelimb movements.

Alstermark B, Ekerot CF - Front Comput Neurosci (2015)

Summary of direct pyramidal, rubral and indirect pathways via C3-C4 PN to LRN neurons. Open synapses are excitatory and filled synapses are inhibitory. The arrowhead from the LRN neurons indicated multiple terminations (in deep cerebellar nuclei and granule cell layers) in the cerebellum.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4525057&req=5

Figure 9: Summary of direct pyramidal, rubral and indirect pathways via C3-C4 PN to LRN neurons. Open synapses are excitatory and filled synapses are inhibitory. The arrowhead from the LRN neurons indicated multiple terminations (in deep cerebellar nuclei and granule cell layers) in the cerebellum.
Mentions: Our results show that disynaptic excitation and inhibition in LRN neurons from cortico- and rubrospinal fibers could be mediated by convergent input to neurons in the C3-C4 segments, as demonstrated by the persistence of disynaptic effects after C5 lesion, but their elimination after C2 lesion. In addition, stimulation of the isolated DLF in C3 (following DLF transection in both C2 and C5) evoked disynaptic EPSPs and IPSPs in LRN neurones at low threshold, also showed transmission via neurons in the C3-C4 segments. It was previously shown that the vast majority (>84%) of the PNs have bifurcating axons projecting to the LRN and MNs (Alstermark et al., 1981a), which is in contrast to medial segmental interneurons (no projection to MNs) recorded in the same segments as the PNs (Alstermark et al., 1984c). These authors found that about 20% of these medial segmental interneurons have ascending collaterals to the LRN. Second, these interneurons, have weak or no convergent excitatory input from NR and tectum (Alstermark et al., 1984c). Since we demonstrate a strong facilitation of pyramidal EPSPs and IPSPs from NR after C5 DLF (Figures 3A,D), these effects must have been mediated via excitatory and inhibitory C3-C4 PNs. Furthermore, a majority of these LRN cells receiving either disynaptic excitation or inhibition via the C3-C4 PNs, also had monosynaptic excitatory inputs from the cortico-reticular neurons, as illustrated in Figure 9.

Bottom Line: Individual motoneurones do not have projections to spino-cerebellar neurons.The LRN projections to the deep cerebellar nuclei exert a direct excitatory effect on descending motor pathways via the reticulospinal, vestibulospinal, and other supraspinal tracts, and might play a key role in cerebellar motor control.Our results support the hypothesis that the LRN provides the cerebellum with highly integrated information, enabling cerebellar control of complex forelimb movements.

View Article: PubMed Central - PubMed

Affiliation: Department of Integrative Medical Biology, Section of Physiology, Umeå University Umeå, Sweden.

ABSTRACT
Cerebellar control of movements is dependent on mossy fiber input conveying information about sensory and premotor activity in the spinal cord. While much is known about spino-cerebellar systems, which provide the cerebellum with detailed sensory information, much less is known about systems conveying motor information. Individual motoneurones do not have projections to spino-cerebellar neurons. Instead, the fastest route is from last order spinal interneurons. In order to identify the networks that convey ascending premotor information from last order interneurons, we have focused on the lateral reticular nucleus (LRN), which provides the major mossy fiber input to cerebellum from spinal interneuronal systems. Three spinal ascending systems to the LRN have been investigated: the C3-C4 propriospinal neurones (PNs), the ipsilateral forelimb tract (iFT) and the bilateral ventral flexor reflex tract (bVFRT). Voluntary forelimb movements involve reaching and grasping together with necessary postural adjustments and each of these three interneuronal systems likely contribute to specific aspects of forelimb motor control. It has been demonstrated that the command for reaching can be mediated via C3-C4 PNs, while the command for grasping is conveyed via segmental interneurons in the forelimb segments. Our results reveal convergence of ascending projections from all three interneuronal systems in the LRN, producing distinct combinations of excitation and inhibition. We have also identified a separate descending control of LRN neurons exerted via a subgroup of cortico-reticular neurones. The LRN projections to the deep cerebellar nuclei exert a direct excitatory effect on descending motor pathways via the reticulospinal, vestibulospinal, and other supraspinal tracts, and might play a key role in cerebellar motor control. Our results support the hypothesis that the LRN provides the cerebellum with highly integrated information, enabling cerebellar control of complex forelimb movements.

No MeSH data available.