The SNb develops late, and phenotypes where alterations are observed within the ventrolateral muscle (VLM) field are best viewed at early stage 17. At late stage 16 the SNb still has a growth cone-like morphology, and often has not established its most distal branch at the 12/13 cleft. The SNb contains the axons of RP1, 3, 4, and 5. RP5 innervates muscle 12, RP1 and 4 muscle 13, and RP3 muscles 6 and 7. The SNb has a characteristic morphology at early 17, with a linear synaptic branch at the muscle 6/7 cleft, a more "blobby" synapse at the proximal edge of muscle 13 (we call this the 13/30 synapse), and a linear synapse at 12/13. In this focal plane muscles 6 and 7, which are the most interior, are in focus. The final segment of the SNb to the 12/13 synapse is also in focus, because the SNb navigates on the internal face of muscle 13. Proximal to muscle 13 the SNb is underneath (external to) the VLMs. The tn and m cell are in focus here. The ISN and SNa are much deeper and cannot be seen. To early 17 SNb focal plane 2 To early 17 brightfield To diagram of muscles. To diagram of muscles. To 2nd diagram (muscles and motor axons). Back to primer table of contents

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