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

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.

Fractionation of muscle activity in rapid responses to startling cues

“…StartReact acceleration of reaction time is not seen in isolated movements of the index finger (Carlsen et al 2009), which may be consistent with a presumed proximal bias of the reticulospinal tract (but see Baker 2011; Kuypers et al 1960). However, StartReact effects can be observed if subjects respond with a precision grip (Honeycutt et al 2013), which is consistent with recent data showing reticulospinal connections to muscles acting on the hand (Riddle and Baker 2010; Riddle et al 2009) and modulation of reticular formation cells with finger movements (Soteropoulos et al 2012). …”

Organization of pontine reticulospinal inputs to motoneurons controlling axial and limb muscles in the neonatal mouse

“…Most electrophysiological studies of reticulospinal pathways in the cat and monkey have focused on the MRF and the caudalmost parts of the PRF without much distinction within this large region (Davidson and Buford 2006;Drew and Rossignol 1990a,b;Peterson et al 1975;Soteropoulos et al 2012). Moreover, the practice of stimulating the MLF as a means of activating the reticulospinal pathway (Alstermark and Ogawa 2004;Edgley et al 2004;Floeter et al 1993;Grillner et al 1968;Jankowska et al 2003;Riddle et al 2009) contributes little information about regional differences within the RF.…”

Startling acoustic stimuli can evoke fast hand extension movements in stroke survivors

“…Therefore, an intact reticulospinal tract to the hand following stroke suggests that the reticulospinal tract may serve as an appropriate alternative pathway for voluntary movement of the hand and should be considered as a potential therapeutic target following stroke. Though reticular neurons are strongly mediated during fine finger movements (Soteropoulos et al , 2012), startReact does not appear to be intact during individuated movements of the fingers (Honeycutt, Kharouta, 2013) Therefore, the usefulness of the startReact phenomenon may not extend to fine finger control.…”