Neuronal wiring diagram of an adult brain

Brain neurons that are not intrinsic can be divided into two categories, depending on the locations of their cell bodies. For afferent (sensory and ascending) neurons, the cell body is outside the brain, whereas for efferent (descending, motor and endocrine) neurons, the cell body is contained in the brain. It is generally accurate to think of an afferent neuron as a brain input, and an efferent neuron as a brain output. The relation to information flow is actually more subtle, however, as most fly neurites carry a mixture of presynapses and postsynapses on both dendrites and axons.

Our companion paper exhaustively identifies all afferent and efferent neurons contained in cross sections of nerves and the neck connective running between the brain and ventral nerve cord (VNC) (Fig. 2d). Almost 95% of these neurons were in the neck connective, antennal nerve and maxillary–labial nerve. Although afferents are truncated in our reconstruction, Schlegel et al. and other community members were able to determine the sensory organs corresponding to the 5,375 non-visual sensory neurons (Fig. 2e,f) on the basis of morphology and nerve assignments. Non-visual sensory neurons enter the brain through nerves (Fig. 2d) that mostly terminate in the antennal lobe or the SEZ (we define the SEZ as containing the following neuropils: saddle (SAD), gnathal ganglia (GNG), antennal mechanosensory and motor centre (AMMC) and prow (PRW) (neutropils definitions are provided in Extended Data Fig. 1)). The antennal lobe is the first relay centre for processing of olfactory information, and many of the olfactory receptor neuron (ORN) inputs to the antennal lobe were also reconstructed in the hemibrain. The SEZ receives more diverse inputs, including the projections of both mechanoreceptor and gustatory receptor neurons—these projections were not contained in the hemibrain. The nerves contained few efferent neurons, among which were head motor neurons (n = 106) or endocrine neurons (n = 80) (Fig. 2a–c). Many efferent neurons have branches in the SEZ, including most of the 106 motor neurons.

Visual afferents are by far the most numerous type of sensory input, and enter the brain directly rather than through nerves. There are photoreceptor axons projecting from the compound eyes (n = 11,118), ocelli (n = 273) and eyelets (n = 8, of which 4 have been identified).

The neurons traversing the neck connective were grouped into 1,303 efferent (descending) and 2,362 afferent (ascending) neurons (Fig. 2a–c). Cell-type annotations for many of these neurons are available, facilitating a matching of reconstructions from two separate electron microscopy datasets of a VNC and enabling circuits spanning the whole CNS (brain and VNC) to be at least schematically mapped.