While it is commonly known that mammals, after a certain young age, will never regenerate or increase the number of motor neurons that we have. Brain and other neurons regenerate for life, but motor neurons are a special type. However, the zebrafish is one animal that can regenerate new motor neurons throughout its life.
By blocking a signal protein called Notch 1, laboratory models suggest that production of motor neurons could be increased, and thus regeneration. Progenitor cells, more specifically, are what they are targeting. Mammals, including humans have these which can turn into motor neurons, however, as it stands, there's nothing to tell them to form motor neurons. With the blockage of Notch 1, researchers are hopeful that they can stimulate the growth of progenitor cells into motor neurons for regeneration of damaged neurons that have led to paralysis.
Work on some knowledge here: