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The Journal of Neuroscience, April 15, 2009, 29(15):4993-4999; doi:10.1523/JNEUROSCI.3720-08.2009

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Behavioral/Systems/Cognitive
Direct and Indirect Connections with Upper Limb Motoneurons from the Primate Reticulospinal Tract

C. Nicholas Riddle,¹ Steve A. Edgley,¹ and Stuart N. Baker²

¹Department of Physiology, Development, and Neuroscience, University of Cambridge, Cambridge CB2 3DY, United Kingdom, and ²Institute of Neuroscience, Medical School, Newcastle University, Newcastle upon Tyne NE2 4HH, United Kingdom

Correspondence should be addressed to Dr. Stuart N. Baker, Institute of Neuroscience, Medical School, Newcastle University, Henry Wellcome Building, Framlington Place, Newcastle upon Tyne NE2 4HH, UK. Email: stuart.baker{at}ncl.ac.uk

Although the reticulospinal tract is a major descending motor pathway in mammals, its contribution to upper limb control in primates has received relatively little attention. Reticulospinal connections are widely assumed to be responsible for coordinated gross movements primarily of proximal muscles, whereas the corticospinal tract mediates fine movements, particularly of the hand. In this study, we used intracellular recording in anesthetized monkeys to examine the synaptic connections between the reticulospinal tract and antidromically identified cervical ventral horn motoneurons, focusing in particular on motoneurons projecting distally to wrist and digit muscles. We found that motoneurons receive monosynaptic and disynaptic reticulospinal inputs, including monosynaptic excitatory connections to motoneurons that innervate intrinsic hand muscles, a connection not previously known to exist. We show that excitatory reticulomotoneuronal connections are as common and as strong in hand motoneuron groups as in forearm or upper arm motoneurons. These data suggest that the primate reticulospinal system may form a parallel pathway to distal muscles, alongside the corticospinal tract. Reticulospinal neurons are therefore in a position to influence upper limb muscle activity after damage to the corticospinal system as may occur in stroke or spinal cord injury, and may be a target site for therapeutic interventions.

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Received Aug. 5, 2008; revised March 3, 2009; accepted March 5, 2009.

Correspondence should be addressed to Dr. Stuart N. Baker, Institute of Neuroscience, Medical School, Newcastle University, Henry Wellcome Building, Framlington Place, Newcastle upon Tyne NE2 4HH, UK. Email: stuart.baker{at}ncl.ac.uk

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