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In Kandel's "Principles of Neural Sciences" in the chapter about the anatomical organization of the brain one reads (p. 323, 4th ed.):
"Although a variety of [relay] neurons are involved at each stage in information processing, these neurons generally fall into two functional classes: principal (or projection) neurons and local interneurons. The axons of principal neurons convey information to the next stage in the system. Interneurons may receive inputs from the same sources as the principal cells, but they contact only local cells involved in the same processing stage [(relay)]."
This means: All neurons of a "relay" (= functional unit) receive external input, but only some give external output. I wonder if there are examples of functional units (nuclei, cortical areas, etc.) with significant numbers of
- interneurons receiving input only from local cells
- principal neurons receiving input only from local cells
If there were, there would also be distinguished input neurons, receiving external input but not giving external output.