title: Direct and indirect spino-cerebellar pathways: shared ideas but different functions in motor control.
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.
authors.affiliation: Department of Integrative Medical Biology, Section of Physiology, Umeå University Umeå, Sweden.
authors.forename: Juan
authors.initials: J
authors.lastname: Jiang
authors.affiliation: Departments of Neuroscience and Biochemistry and Molecular Biophysics, Howard Hughes Medical Institute, Kavli Institute for Brain Science, Mortimer B. Zuckerman Mind Brain Behavior Institute, Columbia University New York, NY, USA.
authors.forename: Eiman
authors.initials: E
authors.lastname: Azim
authors.affiliation: Department of Experimental Medical Science, University of Lund Lund, Sweden.
authors.forename: Carl-Fredrik
authors.initials: CF
authors.lastname: Ekerot
authors.affiliation: Department of Integrative Medical Biology, Section of Physiology, Umeå University Umeå, Sweden.
authors.forename: Bror
authors.initials: B
authors.lastname: Alstermark
keywords: internal feedback; lateral reticular nucleus (LRN); motor control; spino-LRN-cerebellar pathways; spino-cerebellar pathways