Abstract
Recent work has shown that the primate reticulospinal tract can influence spinal interneurons and motoneurons involved in control of the hand. However, demonstrating connectivity does not reveal whether reticular outputs are modulated during the control of different types of hand movement. Here, we investigated how single unit discharge in the pontomedullary reticular formation (PMRF) modulated during performance of a slow finger movement task in macaque monkeys. Two animals performed an index finger flexion–extension task to track a target presented on a computer screen; single units were recorded both from ipsilateral PMRF (115 cells) and contralateral primary motor cortex (M1, 210 cells). Cells in both areas modulated their activity with the task (M1: 87%, PMRF: 86%). Some cells (18/115 in PMRF; 96/210 in M1) received sensory input from the hand, showing a short-latency modulation in their discharge following a rapid passive extension movement of the index finger. Effects in ipsilateral electromyogram to trains of stimuli were recorded at 45 sites in the PMRF. These responses involved muscles controlling the digits in 13/45 sites (including intrinsic hand muscles, 5/45 sites). We conclude that PMRF may contribute to the control of fine finger movements, in addition to its established role in control of more proximal limb and trunk movements. This finding may be especially important in understanding functional recovery after brain lesions such as stroke.
Highlights
The reticulospinal tract is a major descending pathway by which the brain controls spinal motor output
The reticular formation is traditionally associated with locomotion and postural control (Mori et al 2001; Schepens et al 2008), some recent studies (Davidson & Buford, 2006; Riddle et al 2009; Riddle & Baker, 2010) suggest involvement in controlling more distal musculature in primates
Stimuli within pontomedullary reticular formation (PMRF) can activate muscles acting on the digits, and PMRF cells can respond to peripheral input following finger movement
Summary
The reticulospinal tract is a major descending pathway by which the brain controls spinal motor output. Spinal cord interneurons involved in the control of hand movements frequently receive convergent input from reticulospinal and corticospinal tracts (Riddle & Baker, 2010). Our previous work on the reticulospinal tract has shown only connectivity, indicating a potential influence on hand muscles. An alternative would be that reticular inputs to spinal circuits are active during finer finger movements, with the final movement produced by a combined output from both brainstem and cortical systems. Such dual control by reticulospinal and corticospinal cells has previously been suggested for locomotion in the cat (Drew et al 2004; Edgley et al 2004)
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