*4.2.1 Spinal cord stimulation*

Spinal cord stimulation (SCS) involves the application of electrical stimulus to generate muscle contractions that allows for functional limb use. SCS has been successfully applied to improve ambulatory ability in patients with incomplete SCI [31]. Moreover, a small human study has shown that SCS combined with rehabilitation provides functional recovery of voluntary lower extremity movement in the chronic phase of SCI [53].

## **4.3 Cell therapies**

Stem cell-based regenerative therapy has many roles in SCI recovery, including modulating the inflammatory response, providing trophic support, and regenerating axons into lost neural circuits [54]. Early research in this field used embryonic stem cells (ESCs), however, ethical concerns led to the introduction of other types of stem cell populations. In fact, adult tissue-derived stem cells, specifically bone marrow-derived cells, have emerged as a leading transplantable cell type for many CNS disorders [55]. Mesenchymal stem cells, olfactory ensheathing cells, Schwann cells, neural stem cells, and oligodendrocyte progenitor cells have been evaluated in phase I/II clinical trials of SCI and have shown promising results [31].
