Abstract
The impressive precision of mammalian limb movements relies on internal feedback pathways that convey information about ongoing motor output to cerebellar circuits. The spino-cerebellar tracts (SCT) in the cervical, thoracic and lumbar spinal cord have long been considered canonical neural substrates for the conveyance of internal feedback signals. Here we consider the distinct features of an indirect spino-cerebellar route, via the brainstem lateral reticular nucleus (LRN), and the implications of this pre-cerebellar “detour” for the execution and evolution of limb motor control. Both direct and indirect spino-cerebellar pathways signal spinal interneuronal activity to the cerebellum during movements, but evidence suggests that direct SCT neurons are mainly modulated by rhythmic activity, whereas the LRN also receives information from systems active during postural adjustment, reaching and grasping. Thus, while direct and indirect spino-cerebellar circuits can both be regarded as internal copy pathways, it seems likely that the direct system is principally dedicated to rhythmic motor acts like locomotion, while the indirect system also provides a means of pre-cerebellar integration relevant to the execution and coordination of dexterous limb movements.
Highlights
Cerebellar circuits are of major importance in the control of movements, providing a neural basis for pattern recognition and motor behavioral correction and adaptation (Ito, 2006)
In teleosts there are abundant axon collaterals from direct spino-cerebellar pathways to the lateral reticular nucleus (LRN) (Szabo et al, 1990), whereas in cats the DSCT does not provide collateral excitation to LRN neurons (Ekerot and Oscarsson, 1975). These phylogenetic differences suggest that direct spino-cerebellar and indirect spino-LRN-cerebellar pathways may have originated as cooperative systems, which became progressively separated as more advanced motor repertories evolved
A prominent population of excitatory propriospinal neuron (PN) involved in goal-directed reaching movements was identified within the Chx10-expressing V2a interneuron class (Azim et al, 2014); notably, only cervical but not lumbar V2a interneurons project to the LRN, indicating that indirect LRN-cerebellar pathways originating in the lumbar cord have distinct genetic identities
Summary
Cerebellar circuits are of major importance in the control of movements, providing a neural basis for pattern recognition and motor behavioral correction and adaptation (Ito, 2006). In teleosts there are abundant axon collaterals from direct spino-cerebellar pathways to the LRN (Szabo et al, 1990), whereas in cats the DSCT does not provide collateral excitation to LRN neurons (Ekerot and Oscarsson, 1975) These phylogenetic differences suggest that direct spino-cerebellar and indirect spino-LRN-cerebellar pathways may have originated as cooperative systems, which became progressively separated as more advanced motor repertories evolved. The location of ascending axonal projections in the white matter of the spinal cord and the pattern of mossy fiber termination zones within the cerebellar cortex differ across individual systems, but broad comparison of direct and indirect pathways to each other reveals no clear differences (cf review Ito, 1984). At least at the gross anatomical level, direct spino-cerebellar and indirect spinoLRN-cerebellar pathways target overlapping cerebellar cortical circuits
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