This is an overview question. I wonder which principles of grouping neurons into higher units are there, and which principles I have overseen.
neurotransmitter systems: all neurons that release or react on a given (class or set of) neurotransmitters
functional systems: all neurons (and groups of neurons) that are involved in performing a given task or function
regions: all neurons in a given 3-dimensional volume of the brain, e.g. a voxel
nuclei etc: a group of nearby neurons which somehow belong to each other (morphologically or synaptically, possibly encapsulated by some "membrane")
clusters: a group of neurons (nearby or not) that are strongly interconnected by synapses
groups defined by other graph-theoretical measures
pathways: a somehow distinguished (e.g. shortest) synaptical route from neuron (group) A to neuron (group) B
To make my question specific and not too broad, let me just ask the following:
Which principles of grouping neurons is missing in the list above?
(There should be not so many.)
For each classified group of neurons some numbers will be interesting to know:
- How many neurons do they typically comprise?
- How many groups of the same kind are there?
- How strongly do they overlap?