1. A method of generating populations of motor neuron progenitor cells from neural stem cells, comprising the steps of (a) culturing neural stem cells in a culture medium comprising: a Wnt signaling pathway agonist, wherein the Wnt signaling pathway agonist is a GSK3 inhibitor selected from the group consisting of CHIR99021 and 6-bromo-iridium-3′-oxime, and wherein the Wnt signaling pathway agonist is used at a concentration of about 1 μM to 3 μM, a bone morphogenetic protein (BMP) signaling pathway inhibitor selected from the group consisting of DMH-1, Dorsomorphin, and LDN-193189, wherein the concentration of the BMP signaling pathway inhibitor is about 1 μM to 5 μM, an inhibitor of activin-nodal signaling, wherein the inhibitor of activin-nodal signaling is SB431542 in an amount of about 1 μM to 5 μM, retinoic acid in an amount of about 0.1 μM to 1 μM, and purmorphamine in an amount of about 0.1 μM to about 1 μM, (b) detecting within the cultured cells of step (a) a population positive for Olig2 and negative for Nkx2.2, wherein a population of at least 85% pure Olig2±/Nkx2.2− motor neuron progenitor cells is obtained.