early 17 SNa, focal plane 1

The SNa has a characteristic bifurcated morphology (not visible in this focal plane, which actually does not show the SNa itself but rather some of the muscles it innervates; the SNa can be seen out of focus under muscle 5). This bifurcation develops early, and SNa can usually be scored in late 16 embryos. The lateral (or posterior) branch of SNa innervates muscles 5 and 8, and the anterior branch innervates muscles 21-24. It is often difficult to see these synapses, however. What can usually be scored easily for SNa is the presence of the bifurcation and the length of the branches (whether the posterior branch has extended all the way to the anterior edge of the m cell, and whether the anterior branch reaches muscle 24). SNa is in a very deep focal plane relative to SNb; it is almost at the focal plane of the epidermal cells, which lie underneath the muscles.

To early 17 SNa focal plane 2

To early 17 brightfield

To diagram of muscles.

To 2nd diagram (muscles and motor axons).

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