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In the Wikipedia article on nuclei one reads:

"The neurons in one nucleus usually have roughly similar connections and functions."

I read this as "a nucleus usually is roughly homogenous", i.e. contains a small number of neuron types, distributed roughly equally and with a roughly homogenous connectivity pattern.

But it is also stated:

"A nucleus may itself have a complex internal structure, with multiple types of neurons arranged in clumps (subnuclei) or layers."

What I would like to know:

  1. Is my reading correct?

  2. Are there really more nuclei of the more simple (homogeneous) type than of the more complex type?

  3. Which are prototypical examples of both of the types (more simple and more complex)?

  4. Which are the greatest nuclei of "the" simple type (with respect to volume or number of neurons, taking "simple" with a grain of salt)?

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  1. I think you are reading too much into that statement. The proper reading of that statement is to add on the implied "..compared to other nuclei." That is, what defines a nucleus, in most cases, is some shared similarity in connectivity. That does not mean homogeneity.

I also think your reading of the second statement is incorrect; it does not suggest that there are "simple and complex" nuclei as two distinct categories, rather it is indicating that "complexity" varies, and gives a couple examples; essentially, it is exactly saying that you should not assume homogeneity.

  1. I don't think it makes much sense to think in terms of "number" of nuclei, and anyways, nuclei can be somewhat arbitrary. For example, take the auditory thalamus: the medial geniculate nucleus (MGN). The MGN is typically divided into three parts: the dorsal, ventral, and medial divisions. Someone chose to call them all the medial geniculate nucleus, based on gross appearance, but with more functional understanding we can classify each part separately. Should this be one nucleus or three? It really doesn't matter, it's just semantics and terminology to get people talking about the same structure. In one context, it makes sense to talk about the whole MGN, in other cases it makes sense to differentiate between MGv, MGm, and MGd. You could call them subnuclei, or nuclei, or whatever you want. Most people choose to just be consistent with the terminology used in their field for historical reasons because it doesn't matter.

  2. I don't really think it makes sense to think of in terms of prototypes; nuclei are different from each other.

  3. Again, you'd have to first define simple and complex. I don't think there is meaningful dichotomous distinction.

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  • $\begingroup$ I am aware that there is no strict dichotomy between "simple" and "complex" but a continuum. Maybe I should have pointed this out. $\endgroup$ – Hans-Peter Stricker Aug 28 '17 at 16:16
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    $\begingroup$ Okay - your questions #2 and #4 don't work if that is true. #3 maybe but I think it's still a problematic; the simple/complex continuum is also not one-dimensional (i.e., there are different potential types/definitions for complexity). $\endgroup$ – Bryan Krause Aug 28 '17 at 16:29
  • $\begingroup$ You are right! But why do you say "the simple/complex continuum is also not one-dimensional"? Isn't this at the very heart of the problem? Given this "problem": How could one ever approach it? $\endgroup$ – Hans-Peter Stricker Aug 28 '17 at 16:35
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    $\begingroup$ Exactly - in my opinion it's an ill-posed problem. So, the issue isn't trying to approach the problem, but deciding not to. I think it's more helpful to appreciate and embrace the diversity of nuclei; the same approach is important in biology in general. It's much harder to reduce biological concepts to strict rules the way other fields sometimes strive for. If you pick any two nuclei, and claim one is simpler than the other, I can almost guarantee someone can make an argument for the inverse based on a different definition of complexity. $\endgroup$ – Bryan Krause Aug 28 '17 at 16:42
  • $\begingroup$ Nevertheless, there is possibly some truth in the dichotomy (as in most dichotomies). And why throw the baby out with the bathwater? $\endgroup$ – Hans-Peter Stricker Aug 28 '17 at 16:48

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