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In the excerpt below from Morphology of Invertebrate Neurons and Synapses

For the better part of a century, conclusions were made largely by matching the depth relations between the terminals of input neurons and the dendrites of their presumed targets; this practice became transferred to arthropods and cephalopods. The practice may be true for particular terminals or dendrites, and in vertebrates, but it has been long gainsaid by many examples in invertebrates. In the particular case of Drosophila, now known from dense reconstructions, EM denies any such simple dichotomy in important details, with many terminals having postsynaptic sites and many dendrites that are presynaptic as well as postsynaptic (Takemura et al., 2008).

I'm having trouble understanding what the acronym "EM" stands for, the author seemed to throw it in without prior definition (my guess is it's some neuroscience jargon). I also don't understand what the author means by "depth relation". Could someone explain to me the two terms assuming a college intro level background in neuroscience?

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EM is Electron Microscopy; the definition is found earlier in the passage in the normal form where the whole term is given followed by the abbreviation in quotes; you should look for this in the future.

...suited to connectomic approaches using electron microscopy (EM; Lacalli, 2009).

I'm not super familiar with invertebrate morphology, but from context in terms of "depth relation" they are talking about inferring directions of connectivity based on their locations in EM sections. If you find axon terminals from one cell type and dendrites from another at the same depth, you can infer they are likely connected. The passage explains that this doesn't work well in invertebrates that do not have a axon -> dendrite connectivity pattern. Dendrites synapse on other dendrites, and you can't easily tell what the direction of connectivity is from EM alone.

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