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The rough picture of cells involved in neural processing looks like this:

enter image description here

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?

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The quote from Quora has the correct answer; the distinction between neuronal and non-neuronal is an embryological rather than functional definition. Along those same lines, some of the other receptors you listed are considered sensory epithelia rather than neuronal. The retina is of neuronal origin, but hair cells in the cochlea are often not considered to be neuronal, despite the diagram you provided, nor are Merkel cells. The distinctions between epithelial and nervous tissue are often more difficult to identify because both derive from the same preceding embryonic tissues, whereas muscle is entirely separate.

In the end, these distinctions don't matter much unless you are interested in development or trivia (though I don't mean to discount the importance of development there, I just recommend that you don't worry too much about the distinctions if you are thinking only about final functions).

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  • $\begingroup$ But muscle cells cannot be entirely separate, because they react on synaptic input: at least in this respect they behave like neurons. $\endgroup$
    – Hans-Peter Stricker
    Commented Nov 5, 2017 at 0:06
  • $\begingroup$ What do you mean by "trivia"? $\endgroup$
    – Hans-Peter Stricker
    Commented Nov 5, 2017 at 0:07
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    $\begingroup$ They are from a completely separate embryonic tissue: mesoderm, not ectoderm. That means that if you draw a diagram of cell fates, muscles and neurons are not on the same "family tree." You'd have to go back to embryonic stem cells to find cells that could eventually become either mesoderm or ectoderm. "Reacts on synaptic input" is not a defining characteristic of neurons, they can be excited by other sources as well, and lots of other non-neuronal cells also show some excitable properties; immune cells, for example, and many others. $\endgroup$
    – Bryan Krause ♦
    Commented Nov 5, 2017 at 0:15
  • $\begingroup$ "Trivia" refers to typically obscure facts that are most useful as facts rather than for some other purpose. $\endgroup$
    – Bryan Krause ♦
    Commented Nov 5, 2017 at 0:17
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    $\begingroup$ Interesting thought, though the very important difference between convergent evolution, which is what you seem to be describing, is that cells with different fates do have access to the same genome and so can evolve together. I.e., a mesoderm-derived tissue and ectoderm-derived tissue can express some of the very same proteins; this of course isn't possible in convergent evolution. $\endgroup$
    – Bryan Krause ♦
    Commented Nov 5, 2017 at 0:31

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