Direct and Indirect Connections with Upper Limb Motoneurons from the Primate Reticulospinal Tract

Abstract

Although a major descending motor pathway in mammals, the reticulospinal tract’s 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 employed intracellular recording in anaesthetised monkeys to examine the synaptic connections between the reticulospinal tract and antidromically identified cervical ventral horn motoneurons, focussing in particular on motoneurons projecting distally to wrist and digit muscles. We found that motoneurons receive mono- 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.

Spinal Interneurons Facilitate Coactivation of Hand Muscles during a Precision Grip Task in Monkeys

“…Therefore, we suggest that spinal INs have a significant role in generating and coordinating finger muscle activities during grasping movements. 1983; Buys et al, 1986;Maier et al, 1993;McKiernan et al, 1998), red nucleus (Mewes and Cheney, 1991;Sinkjaer et al, 1995), and reticular formation (Riddle et al, 2009). According to the output effect pattern, it has been proposed that these neurons have specialized functions for the control of grasping.…”

The Spinal Substrate of the Suppression of Action during Action Observation

“…Premotor cortical areas can influence spinal activity either through the primary motor cortex (Dum and Strick, 2005) or through their corticospinal (Dum and Strick, 1991;He et al, 1993) and cortico-brainstem-spinal projections (Kuypers, 1981;Keizer and Kuypers, 1989). The reticulospinal tract, which has long been considered to control motoneurons innervating axial and proximal muscles (Kuypers, 1981), has been recently demonstrated to also influence motoneurons projecting to distal limb muscles, including intrinsic hand muscles, and thus forming a pathway parallel to the corresponding corticospinal tract (Riddle et al, 2009). Premotor cortical areas have proven to facilitate the primary motor cortex (Shimazu et al, 2004;Schmidlin et al, 2008) and inhibit the spinal cord (Moll and Kuypers, 1977;Sawaguchi et al, 1996).…”

Spinal interneuron circuits reduce approximately 10-Hz movement discontinuities by phase cancellation

“…The first is the pontomedullary reticular formation (PMRF), which gives rise to the reticulospinal tract. In primates, the PMRF is involved in control of shoulder and forearm muscles (9); more recent work suggests that it can also influence distally projecting motoneurons innervating hand muscles (10). Two findings are consistent with a role for the PMRF in generating discontinuities during slow finger movements.…”