Neuronal wiring diagram of an adult brain

A brain is defined as a structure of the nervous system that is co-localized with the sense organs in the head of an animal. Often left implicit in the definition is the idea of centralization—that most central nervous system (CNS) neurons are located in the brain. The idea involves a subtlety arising from the fact that neurons are spatially extended objects. If all of the synapses of a neuron are wholly contained in the brain, we say that the neuron is intrinsic to the brain. This contrasts with a neuron that straddles the brain and other CNS regions. The fraction of intrinsic neurons can be interpreted as the degree to which the CNS is centralized in the brain.

Of the 139,255 proofread neurons in FlyWire (Supplementary Video 1), 118,501 are intrinsic to the brain (Fig. 2a–c), which is defined as the central brain and optic lobes (Fig. 1a). Intrinsic neurons of the brain make up three-quarters of the adult fly nervous system and amount to 85% of brain neurons. Their predominance means that the brain communicates primarily with itself, and only secondarily with the outside world (Fig. 2b).

For comparison, intrinsic neurons of the larval fly brain make up one-quarter to one-third of its nervous system. Intrinsic neurons of the C. elegans brain make up 8–15% of its nervous system (Methods).