Effect of Motor Task on Cortex Brainstem Modulation: Preliminary Results

Reticulospinal Tracts (RSTs) have divergent connections to multiple spinal segments that innervate many upper extremity muscles. Therefore, increased RST engagement can often lead to muscle coactivation across multiple limb joints. The RST originates from the reticular formation (RF) and receives projections from the cortex. This provides the anatomical basis for cortex-brainstem modulation. Currently, we know little about how cortex modulates the RF to control RST engagement during motor preparation for various motor tasks, such as tasks involving proximal and distal upper limb joint coordination vs. a purely distal task. We hypothesize that since a simultaneous arm lifting and hand opening task (LIFTOPEN) requires more selective muscle recruitment than a hand opening task (OPEN), the cortex will suppress the RF to reduce the RST engagement at distal muscles during LIFTOPEN. To test this hypothesis, we investigated the startReact response in thirteen able-bodied participants performing the OPEN and LIFTOPEN tasks in response to a startling and non-startling acoustic stimulation. Our results showed that activation of distal muscles was significantly decreased, and the startle response was delayed in LIFTOPEN compared to OPEN. Both results suggest that the cortex suppressed RF and reduced the RST engagement in LIFTOPEN compared to OPEN.Clinical Relevance— Our results provide foundational knowledge of the task-specific nature of cortex-brainstem modulation. This scientific finding provides a base to compare how a unilateral brain injury may affect this cortex-brainstem modulation.