Principles of grouping neurons 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
independent sets: of a multipartite subgraph of the brain network
groups defined by other graph-theoretical measures
pathways: a somehow distinguished (e.g. shortest) synaptical route from neuron (group) A to neuron (group) B
cortical layers
cortical columns


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?
In anatomy/physiology, there are 2 major categorizations of the nervous system:


Central nervous system (brain + spinal cord) and peripheral nervous system (all other neural structures)
Somatic nervous system, which controls skeletal muscles and skin, and and autonomic n. system, which controls internal organs, blood vessels and glands and is further divided to sympathetic and parasympathetic system. Both the somatic and autonomic system have motor neurons that control muscles and sensory neurons that convey sensory signals. 




Image source: Wikipedia, creative commons license

Neurons can be further categorized by neurotransmitters involved (dopamine system, acetylcholine system, etc.), and by various anatomical and functional subsystems.

Examples of anatomical and functional structures formed by neurons:


Cortical areas (functional areas of the brain cortex) (Lumen)
Main parts of the brain: cerebrum, cerebellum and brainstem, some of which can be further divided into hemispheres, lobes and folds
Deep structures of the brain: thalamus, hypotalamus, hypocampus, amygdala, corpus callosum, limbic system, pituitary gland, pineal gland, and structures collectivelly called basal ganglia, which include the caudate, putamen, and globus pallidus in the cerebrum, the substantia nigra in the midbrain, and the subthalamic nucleus in the diencephalon (Mayfield Clinic). 
Areas containing either neuron cell bodies or axons: gray matter (cell bodies) and white matter (axons) of the brain and spinal cord (Indiana.edu)
Histological areas of the brain: layers and columns (Wikipedia)
Nuclei of the cranial nerves (Wikipedia)
Centers (cardiovascular center, reflex center for cough, vomiting, etc.) in medulla oblongata (Wikipedia)
Corticospinal tracts (Wikipedia)
Pathways - the term is usually used for cranial nerves: visual pathway, auditory pathway, etc.
Ganglia - anatomical structures containing neuron body cells in the peripheral nervous system (Wikipedia)
Nerve plexuses - branching networks of intersecting nerves (Wikipedia)
Reflex arcs (Wikipedia)