Spinal motor neurons are arranged in columns with their identity determined by their longitudinal position, axonal projections and Hox gene expression [56]. There are several distinct columns stretching along rostrocaudal direction and each of the distinct columns innervates different targets. Hypaxial, medial (Lim3+) and lateral (Foxp1+) motor column neurons each targets body wall, axial and limb muscles respectively. Lateral motor column neurons can be further divided into Isl1+ medial and Lim1+ lateral divisions which innervate ventral and dorsal muscles. Within each column, there are pools of motor neurons innervating a single specific muscle. Those pools usually contain different types of motor neurons (fast-fatigable, fast-fatigue resistant or slow-twitch alpha motor neurons and gamma motor neurons) each with different firing patterns and morphology [57]. Certain diseases appear to affect specific subsets of motor neurons, e.g. pathological examinations of skeletal muscles of ALS patients show that fast fatigable muscle units are preferentially affected [58], highlighting the importance of generating the right iN cells.
