Neuroplasticity in the spinal cord associated with transcranial direct current stimulation in health and disease: a...

Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique widely used to modulate cortical excitability, yet its influence on spinal mechanisms remains less understood. This review synthesizes current evidence on the effects of tDCS on human spinal circuitry in both health and disease. Immediate and transient modulations of the H-reflex, a scientific analog of the stretch reflex, have been observed, suggesting short-lived alterations in spinal excitability. However, an important body of the literature also did not find any modulation of this marker. Complementary approaches, based on H-reflex conditioning paradigms, investigated in more detail specific spinal circuits that might be especially sensitive to tDCS. Such studies indicated that anodal tDCS targeted specific spinal circuits, including reciprocal inhibition, recurrent inhibition, and propriospinal pathways, with polarity- and pathway-specific effects, but not presynaptic inhibition. The effectiveness of tDCS depends on multiple factors, including polarity, montage, stimulation site, task specificity, and individual responsiveness. While variability across studies remains a major challenge, converging evidence supports the capacity of tDCS to engage spinal networks. The literature highlights that tDCS does not result in a general arousal of all spinal circuits but rather target specific structures. Future work should refine protocols, integrate multimodal strategies, and personalize interventions to fully harness spinal neuromodulation for rehabilitation and performance enhancement.