Neuronal Architecture of Flexor and Extensor Muscle Co-activation During Elbow Flexion Revealed by PRV Tracing

The regulatory mechanisms underlying the co-contraction of the biceps and triceps brachii muscles during elbow flexion remain unclear. This study aimed to elucidate the structural connections between nerve cells responsible for this co-activation. We employed the modified Sihler's staining technique to visualize the nerve-dense regions within the biceps and triceps brachii muscles. Additionally, a retrograde tracer (pseudorabies virus) expressing enhanced green and red fluorescent proteins was injected into these regions to map the structural relationships of nerve cells in the spinal cord, red nucleus of the midbrain, and the M1 region of the cerebral cortex. The nerve-dense regions in the biceps and triceps brachii muscles were located at 74.75 % and 61.90 % of muscle length, respectively. Three days post-injection, we observed single-labeled motor neurons innervating the biceps (green) and triceps (red), as well as double-labeled interneurons in the anterior horn of the spinal cord. Six days after injection, single- and double-labeled neurons were identified in the red nucleus and the M1 cortex. The presence of double-labeled neurons in both the spinal cord and brain regions provides evidence for the neural basis of co-activation in flexor and extensor muscles during elbow flexion. This study advances our understanding of the neural mechanisms controlling muscle co-contraction.