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Fig. 2. Cellular strategy of motor axon guidance in the ISNb pathway.
(A) Left schematic shows the ISNb motor axon projection patterns through the ventrolateral muscle fibers. Right schematic is a cross-sectional view of the stereotyped ISNb nerve pathway. Arrows indicate each defasciculation choice point (CP). (B) One of the most important mechanisms underlying motor axon pathfinding is selective fasciculation and defasciculation at specific choice points. At least seven ISNb motor neurons, which are born and lie at different positions in the VNC, extend their axons to form a fascicule with the ISN before exiting the VNC (left). After they recognize the first choice point (CP1), the ISNb axons defasciculate from the ISN. When the defasciculated ISNb nerve subsequently reaches the CP2, two axons selectively segregate from the main ISNb bundle (arrowhead in brown), and innervate both muscles 6 and 7 (middle). The main ISNb bundle grows dorsally to arrive at the CP3, which is located between muscles 6 and 13, thereby causing further selective defasciculation of three axons, and their innervation to muscles 13, 14, and 30 (right). Finally, at the CP4, the remaining two axons defasciculate, and extend in the opposite direction, to form synaptic connections with muscle 12 (shown in panel A).
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