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In the Wikipedia article on nuclei I read:

A nucleus is one of the two most common forms of nerve cell organization, the other being layered structures.

Is there a specific and plausible reason, why the thalamic reticular nucleus is called a nucleus, even though it is - anatomically - a layer?

Has the TRN once been a somehow spherical nucleus and evolved into a two-dimensional layer? Or does the TRN play a functional role that normally nuclei play, but not layers?

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The thalamic reticular nucleus (TRN) is a thin layer of GABAergic cells adjacent to the thalamus (Fig. 1). Because of its strategic location between the thalamus and cortex, the TRN is often suggested to be important to attention and regulation of information flow between the thalamus and cortex (Lam & Sherman, 2011).

A nucleus is defined as a

[p]ortion of tissue that are compact accumulations of neurons having roughly similar connections and functions

Reticular is defined as

A fine network or net-like structure.

So reticular nucleus translates itself as a net-like accumulation of neurons.

In contrast to the wikipedia meaning, therefore, I interpret nucleus as more of a cluster of functional organizational units rather than a morphological characteristic of that cluster per se.

TRN
Fig. 1. Thalamic reticular nucleus. source: Dana Foundation

Reference
- Lam & Sherman, J Neurosci (2011); 31(18): 6791-9

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  • $\begingroup$ Thanks for the clarification. But this makes things terminologically difficult: applying - maybe unreflectedly - anatomical terms to functional concepts. $\endgroup$
    – Hans-Peter Stricker
    Commented Aug 21, 2017 at 12:13
  • $\begingroup$ @HansStricker happy to help. I have attempted to further clarify the answer (last sentence). Also note these kind of questions are welcome over at CogSci too. Up to you. $\endgroup$
    – AliceD ♦
    Commented Aug 21, 2017 at 12:17
  • $\begingroup$ My question stated otherwise: In which respect is TRN a "portion of tissue that is a compact accumulation of neurons having roughly similar connections (what to?)"? And don't have the neurons of a layer have roughly similar functions? $\endgroup$
    – Hans-Peter Stricker
    Commented Aug 21, 2017 at 13:09
  • $\begingroup$ @HansStricker This is a good question (I promise I don't hate you, I just don't like some of your questions). The distinction that the Wikipedia article makes is between "layered structures" and "nuclei", not between "layers" and "nuclei". Thalamus is not a layered structure; the cells are clustered into nuclei, not into laminae like cortex or cerebellum. There isn't any structure consisting of just one layer that is thought of as "layered", you really need at least 2 layers to be layered. TRN projects inhibitory connections into the rest of thalamus. $\endgroup$
    – Bryan Krause ♦
    Commented Aug 21, 2017 at 16:03
  • $\begingroup$ Also you should note that in general, most anatomical terms for the brain far predate knowledge about their functions. If you want to really dissect out (pun intended) the reasons for the names of everything you are risking some level of insanity, unless the curiosity interests you. Some nuclei are named by color, some by what they happen to be under even if they are completely unrelated, some are named from an almost imperceptible similarity to sea creatures. $\endgroup$
    – Bryan Krause ♦
    Commented Aug 21, 2017 at 16:06
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Obviously, I didn't read the Wikipedia article on nuclei carefully enough. Anyway, it was great to get some learned opinions. In the Wikipedia article on reads:

The term "nucleus" is in some cases used rather loosely, to mean simply an identifiably distinct group of neurons, even if they are spread over an extended area. The reticular nucleus of the thalamus, for example, is a thin layer of inhibitory neurons that surrounds the thalamus.

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