High-intensity transcranial magnetic stimulation reveals differential cortical contributions to prepared responses

Abstract

Corticospinal output pathways have typically been considered to be the primary driver for voluntary movements of the hand/forearm; however, more recently, reticulospinal drive has also been implicated in the production of these movements. Although both pathways may play a role, the reticulospinal tract is thought to have stronger connections to flexor muscles than to extensors. Similarly, movements involuntarily triggered via a startling acoustic stimulus (SAS) are believed to receive greater reticular input than voluntary movements. To investigate a differential role of reticulospinal drive depending on movement type or acoustic stimulus, corticospinal drive was transiently interrupted using high-intensity transcranial magnetic stimulation (TMS) applied during the reaction time (RT) interval. This TMS-induced suppression of cortical drive leads to RT delays that can be used to assess cortical contributions to movement. Participants completed targeted flexion and extension movements of the wrist in a simple RT paradigm in response to a control auditory go signal or SAS. Occasionally, suprathreshold TMS was applied over the motor cortical representation for the prime mover. Results revealed that TMS significantly increased RT in all conditions. There was a significantly longer TMS-induced RT delay seen in extension movements than in flexion movements and a greater RT delay in movements initiated in response to control stimuli compared with SAS. These results suggest that the contribution of reticulospinal drive is larger for wrist flexion than for extension. Similarly, movements triggered involuntarily by an SAS appear to involve greater reticulospinal drive, and relatively less corticospinal drive, than those that are voluntarily initiated. NEW & NOTEWORTHY Through the use of the transcranial magnetic stimulation-induced silent period, we provide novel evidence for a greater contribution of reticulospinal drive, and a relative decrease in corticospinal drive, to movements involuntarily triggered by a startle compared with voluntary movements. These results also provide support for the notion that both cortical and reticular structures are involved in the neural pathway underlying startle-triggered movements. Furthermore, our results indicate greater reticulospinal contribution to wrist flexion than extension movements.

Changes in corticospinal excitability during the preparation phase of ballistic and ramp contractions

“…This agrees with the observation that M1 exhibits substantial preparatory activity (Riehle & Requin, 1989) whereas the spinal cord only exhibits modest changes (Fetz et al, 2002;Prut & Fetz, 1999). It does not exclude that other pathways, such as the reticulospinal pathway (Smith et al, 2019;Fisher et al, 2012), may also be involved. The present work was however not designed to investigate this possibility.…”

“…2012; Smith et al . 2019), may also be involved. The present work was, however, not designed to investigate this possibility.…”

A Virtual Reality Muscle–Computer Interface for Neurorehabilitation in Chronic Stroke: A Pilot Study

“…This may relate to the fact that the trained task was designed to produce voluntary control over the extensors (not the flexors), and/or because wrist extensors may receive a greater proportion of monosynaptic corticospinal projections compared with the wrist flexor muscles [78,79]. Because changes in reticulospinal pathways may ultimately cause pathological synergistic muscle activation after stroke [80][81][82], it may be that there is a greater contribution of reticulospinal drive to wrist flexion compared with extension movements [83]. Accordingly, if one aim of our training task was to reduce unintended flexor activation, then the reinforced neural activity might have been a reduction of brainstem or reticulospinal output to the flexors.…”

Extensors respond faster than flexors for the MCP joints even under the loss of the corticospinal system

“…Use of the reticulospinal tract results in increased flexor activity, by increasing the resting potential of the motoneurons up to the threshold [21]. The reticulospinal tract is even know to suppress extensor activity [19], [20]. This might be why the net torque is generated greater during flexion in comparison to during extension.…”

“…Alternative pathways to the damaged CST includes the ipsilateral corticospinal tract [17], vestibulospinal tract [18] and reticulospinal tract [16]. In particular, use of the reticulospinal tract facilitates flexors while suppressing extensors [19], [20]. This pathway excites the resting potential of the motoneurons closer to their thresholds via interneuronal excitation [21].…”