The rough picture of cells involved in neural processing looks like this:
I wonder, why primary sensory (receptor) cells (like rods, cones, hair cells, Merkel cells) are consensually considered neurons (accidently having no synaptic input, but synaptic output), while muscle cells are not (even though they have synaptic input, but accidently no synaptic output).
The last cells that are unequivocally neurons are the motor neurons. Curiously, the first cells that are unequivocally neurons are mostly called "ganglion cells", not "ganglion neurons".
Is it all just a manner of speaking/matter of parlance and words? I believe not.
I received an interesting answer from Quora:
"Muscle cells are derived from mesoderm, sensory cells (like all neurons) are derived from ectoderm."
But is this all that is to say? What’s the “deeper” source of this asymmetry? In the beginning there was only one type of cell, which was both sensory and muscle?