![]() The Drosophila neuromuscular system is advantageous for several reasons. We decided to investigate this question in a relatively simple, genetically amenable motor system, that of the Drosophila embryo and larva. The task of understanding how the system is assembled would be greatly simplified if an underlying principle to the organisation of connectivity in a motor system could be demonstrated. Thus, it is not clear whether motor columns are simply a consequence of the process by which motor neurons are generated and specified or whether they actually reveal an underlying functional organisation in the motor system ( Landmesser 1978). However, motor pools (and columns) reflect the locations of motor neuron cell bodies, but not the regions of the spinal cord where their dendritic arbors receive synaptic connections. The organisation of motor pools is highly conserved among different species and, to a degree, reflects the distribution of the muscles that they supply ( Romanes 1951 Cruce 1974 Landmesser 1978). Motor neurons innervating the same muscles are clustered into pools, and motor pools are grouped into columns, each supplying a different muscle set ( Landmesser 1978 Tsuchida et al. In the vertebrate spinal cord, motor neurons are organised into pools and columns that form a neural correlate of the anatomy of the body musculature they innervate. Nonetheless, there are some regularities on the motor side. For motor systems, on the other hand, there appears to be no such simplifying anatomical correlate of function, at least insofar as the underlying patterns of neuronal connectivity are concerned. This straightforward anatomical outcome of the developmental process, which rather explicitly reflects the function of the neurons concerned, means that developmental observations and experiments can readily be interpreted in terms of axon growth and targeting within the orderly framework of the map. One of the reasons for this is that, in many sensory systems, in-growing sensory axons are marshalled to form a clear anatomical map of peripheral characteristics in the central nervous system (CNS) (for reviews, see Knudsen 2002 Keller and Vosshall 2003 McLaughlin et al. The way in which neural networks underlying locomotion are specified and assembled is less well understood than the development of other parts of the nervous system, particularly the sensory nervous system. This is an open-access article distributed under the terms of the Public Library of Science Open-Access License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Ĭompeting interests: The authors have declared that no conflicts of interest exist.ġ,1′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine perchlorate DiI,ġ,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate DO, Received: JAccepted: AugPublished: November 17, 2003Ĭopyright: © 2003 Landgraf et al. PLoS Biol 1(2):Īcademic Editor: Thomas M. Our results suggest that the cues that organise the myotopic map may be laid down early in development as the embryo subdivides into parasegmental units.Ĭitation: Landgraf M, Jeffrey V, Fujioka M, Jaynes JB, Bate M (2003) Embryonic Origins of a Motor System: Motor Dendrites Form a Myotopic Map in Drosophila. These findings will greatly simplify the task of understanding how a locomotor system is assembled. This is likely to be mirrored, at least in part, by endings of higher-order neurons from central pattern-generating circuits, which converge onto the motor neuron dendrites. The arrangement of motor neuron dendrites into a myotopic map represents a first layer of organisation in the motor system. ![]() It forms by an active process of dendritic growth independent of the presence of target muscles, proper differentiation of glial cells, or (in its initial partitioning) competitive interactions between adjacent dendritic domains. While muscles are segmental, the myotopic map is parasegmental in organisation. We find that dendritic arbors of motor neurons, rather than their cell bodies, are partitioned into domains to form a myotopic map, which represents centrally the distribution of body wall muscles peripherally. Using the neuromuscular system of the Drosophila embryo as a model and retrograde tracing and genetic methods, we have uncovered principles underlying the organisation of the motor system. The organisational principles of locomotor networks are less well understood than those of many sensory systems, where in-growing axon terminals form a central map of peripheral characteristics.
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