Motor Outputs From the Primate Reticular Formation to Shoulder Muscles as Revealed by Stimulus-Triggered Averaging

Abstract

The motor output of the medial pontomedullary reticular formation (mPMRF) was investigated using stimulus-triggered averaging (StimulusTA) of EMG responses from proximal arm and shoulder muscles in awake, behaving monkeys (M. fascicularis). Muscles studied on the side ipsilateral (i) to stimulation were biceps (iBic), triceps (iTri), anterior deltoid (iADlt), posterior deltoid (iPDlt), and latissimus dorsi (iLat). The upper and middle trapezius were studied on the ipsilateral and contralateral (c) side (iUTr, cUTr, iMTr, cMTr). Of 133 sites tested, 97 (73%) produced a poststimulus effect (PStE) in one or more muscles; on average, 38% of the sampled muscles responded per effective site. For responses that were observed in the arm and shoulder, poststimulus facilitation (PStF) was prevalent for the flexors, iBic (8 responses, 100% PStF) and iADlt (13 responses, 77% PStF), and poststimulus suppression (PStS) was prevalent for the extensors, iTri (22 responses, 96% PStS) and iLat (16 responses, 81% PStS). For trapezius muscles, PStS of upper trapezius (iUTr, 49 responses, 73% PStS) and PStF of middle trapezius (iMTr, 22 responses, 64% PStF) were prevalent ipsilaterally, and PStS of middle trapezius (cMTr, 6 responses, 67% PStS) and PStF of upper trapezius (cUTr, 46 responses, 83% PStS) were prevalent contralaterally. Onset latencies were significantly earlier for PStF (7.0 +/- 2.2 ms) than for PStS (8.6 +/- 2.0 ms). At several sites, extremely strong PStF was evoked in iUTr, even though PStS was most common for this muscle. The anatomical antagonists iBic/iTri were affected reciprocally when both responded. The bilateral muscle pair iUTr/cUTr demonstrated various combinations of effects, but cUTr PStF with iUTr PStS was prevalent. Overall, the results are consistent with data from the cat and show that outputs from the mPMRF can facilitate or suppress activity in muscles involved in reaching; responses that would contribute to flexion of the ipsilateral arm were prevalent.

Arm Movements Induced by Electrical Microstimulation in the Superior Colliculus of the Macaque Monkey

“…Stimulation in the pontomedullary reticular formation (PMRF) evokes ipsilateral limb flexor and contralateral limb extensor muscle excitation sometimes accompanied by a reciprocal suppression of the antagonistic muscles. The PMRF is therefore not only involved in the control of locomotion (Matsuyama et al, 2004) and postural support during reaching movements (Schepens et al, 2008), but also in voluntary reaching (Buford and Davidson, 2004), and is thus in a position to relay reach related tectal output to arm muscles (Davidson and Buford, 2004). These findings and the short latencies presented in the present study could explain the presence of rapid arm muscle activity (Ͻ100 ms latency) that was time locked to the target appearance but not movement onset (Pruszynski et al, 2010).…”

Ipsilateral versus contralateral cortical motor projections to a shoulder adductor in chronic hemiparetic stroke: implications for the expression of arm synergies

“…There is also abnormal coupling between the shoulder abductors and elbow flexors (flexor synergy) in these stroke survivors. Stimulation of the reticular formation in monkeys has been shown to evoke a pattern of suppression/facilitation of distal and proximal arm muscles consistent with a possible role in the generation of the flexor synergy (Davidson and Buford 2004). Following a stroke, these pathways could be unmasked and produce the flexor and extensor synergies.…”

“…These pathways project bilaterally to motoneuron pools of axial and proximal limb muscles and exhibit extensive branching that can innervate neurons over many spinal segments, thus providing the infrastructure for obligatory co-activation of muscles acting at different joints. In particular, reticulospinal pathways may have potent direct input to ipsilateral motoneurons (Jankowska et al 2003), indirect input to contralateral motoneurons via interneurons and commissural connections, and can influence a variety of muscles in the proximal and distal limb (Davidson and Buford 2004). The fact that the onset latency of ipsilateral MEPs are significantly delayed (typically 5 ms for PMJ) relative to contralateral MEPs suggests these responses are likely mediated by a polysynaptic, possibly corticoreticulospinal, pathway (MacKinnon et al 2004;Ziemann 1999).…”

Absence of a proximal to distal gradient of motor deficits in the upper extremity early after stroke

“…If compensatory control was exerted by other descending motor pathways, e.g. reticulospinal tract, that exert control over more proximal limb musculature (Davidson and Buford, 2004), then a gradient of movement deficits might be more apparent. It is possible that our finding of similar movement deficits across upper extremity segments reflects lesions that disrupt similar proportions of proximal and distal inputs.…”