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In Kandel's Principles of Neural Science I found the following figure which shows the innervation of the organ of Corti:

enter image description here

From the legend to this figure (30-10, p. 602):

"The great majority of afferent axons end on inner hair cells, each of which constitutes the sole terminus for an average of 10 axons.[...] Efferent innervation of inner hair cells is sparse."

BUT: The depicted axons that "innervate afferently" hair cells seem to go into the wrong direction (downwards, not upwards). What do the (red) bulbs touching the hair cells represent. I guess synapses, but which one is the presynaptic and which one is the postsynaptic neuron? (I guess: The hair cell is the presynaptic neuron - it's a sensory neuron! But then the neurites emanating from it would be afferent dendrites of the ganglion cells, not axons. I'm lost. Might it be the case that transport along these neurites is active - like in axons, not passive like in dendrites? Or are they called axons because they are myelinated? Or is it because in bipolar cells it is not distinguished between axon and dendrite?)

Question: Why are these neurites called axons?

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  • $\begingroup$ "From the point of view of a neuron, its axon is efferent. From the point of view of the target neuron, the axon is afferent." Well, this is wrong. Afferent and efferent refer to the whole neuron, and indicates the orientation of the impulse (from the CNS or to the CNS, respectively). $\endgroup$
    – user24284
    Commented Nov 4, 2017 at 10:28
  • $\begingroup$ In which respect? See here: Efferent nerve fibers. Anyway, it doesn't answer my question: Why are these neurites called axons? $\endgroup$
    – Hans-Peter Stricker
    Commented Nov 4, 2017 at 10:30
  • $\begingroup$ It does answer. Your question is very clear: "What is an afferent axon?". If you made a different question in the body of your text I suggest you to pick just one. $\endgroup$
    – user24284
    Commented Nov 4, 2017 at 10:32
  • $\begingroup$ I'll change the title and remove the introductory remark. Thanks for the hint! $\endgroup$
    – Hans-Peter Stricker
    Commented Nov 4, 2017 at 10:33
  • $\begingroup$ @GerardoFurtado: Here I read that all neurons "have both afferent (incoming) and efferent (outgoing) connections". This conflicts a bit with what you say $\endgroup$
    – Hans-Peter Stricker
    Commented Nov 4, 2017 at 11:01

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I believe to understand now, thanks to @AliceD's answer to this question:

Why are Merkel cells innervated by an axon, and not a dendrite?

enter image description here

The axon seemingly emanating from the cell (hair or Merkel) is just by coincidence not a physiological part of the cell (hair or Merkel), but its functional axon. The tactile meniscus (in the case of the Merkel cell) plays the role of an "out-sourced" axon hillock of the cell (hair or Merkel) which possesses no axon hillock (and axon) on its own.

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  • $\begingroup$ Please be fair and tell me what's wrong with this picture/answer. $\endgroup$
    – Hans-Peter Stricker
    Commented Nov 4, 2017 at 22:56

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