Cells in the monkey ponto‐medullary reticular formation modulate their activity with slow finger movements

Key points The reticulospinal tract is an important pathway communicating instructions for movement from the brain to the spinal cord Although the reticulospinal tract is usually associated with gross movements such as postural adjustments and walking, recent work has shown that it also connects to spinal centres involved in hand function In awake monkeys, we recorded from the origin of the reticulospinal tract (the reticular formation) during performance of a fine finger movement task. Cells modulated their firing during finger movements Stimulation of sites within the reticular formation sometimes activated hand muscles, and some cells within the reticular formation responded to the sensory input following externally imposed movements of the digits This work supports a role of the reticulospinal tract in hand function in healthy individuals. Additionally, this tract may be able to mediate some recovery of hand function when other pathways are damaged, such as after stroke 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.