” George Romanes, October 2010 The wiring of circuits in the vertebrate central nervous system (CNS) typically adheres to a structural

blueprint that directs neurons at particular locations to form orderly connections with their synaptic targets. The existence of neural order has long been evident in mature and developing nervous systems—from the earliest functional studies of cortical mapping to the illustrations of every developing sulcus and synapse that Cajal deigned to describe. Defining exactly how elemental features of neuronal organization influence circuit wiring poses a significant challenge, however. We do not yet have any real insight into why some regions of the CNS arrange their resident neurons in laminar lattices,

and others in nuclear niches. Nor is the impact of neuronal settling position GDC-0199 in vivo on the intricacies of see more circuit assembly well understood. The urge to unravel the tight anatomical fabric tying neuronal architecture to connectivity has prompted several large-scale anatomical reconstruction projects (Lu et al., 2009 and Helmstaedter et al., 2011). These “-omics” efforts invite reflection on prior analyses of the organization of CNS neurons, gleaned through more traditional methods, and what they can tell us about principles of circuit assembly. Many of the early attempts to explore the topographic link between brain organization and behavior focused on the neural control of movement. With graphic simplicity, classical depictions of “homuncular” motor maps emphasized the linear contiguity of motor cortical areas that control muscles involved in hip, knee, ankle, or foot movements (Woolsey et al., 1952). More recent analyses have charted a topographic arrangement of motor cortical areas that is considerably more complex and less contiguous (Hatanaka et al., 2001, Aflalo and Graziano, 2006 and Rathelot and Strick, 2006). Yet it remains true that primary motor cortex maps onto limb positional

coordinates in an orderly and predictable ADP ribosylation factor manner. As with cortical areas, the spinal motor neurons that innervate an individual limb muscle are not scattered willy-nilly in the ventral horn, but are clustered into spatially coherent “pools” that occupy stereotypic locations within the entire field of limb-innervating motor neurons (reviewed in McHanwell and Biscoe, 1981). But there is also a higher-order, and less-well-appreciated, topographic design to spinal motor maps. The set of motor pools that innervates muscles exerting synergistic functions at a particular hindlimb joint are themselves grouped together, forming minicolumns or columels that run along the rostrocaudal axis of the lumbar spinal cord (Romanes, 1964).