Neuronal wiring diagram of an adult brain

Of the 118,501 intrinsic neurons, 32,388 are fully contained in the central brain and 77,536 are fully contained in the optic lobes and ocellar ganglia (this number excludes the photoreceptors, which are sensory afferent neurons). The domination of the count by visual areas reflects the nature of Drosophila as a highly visual animal.

The optic lobes and ocellar ganglia also contain 8,053 neurons—the visual projection neurons (VPNs)—that project into the central brain. We provide a more detailed analysis of connections in the ocellar ganglion in Fig. 7. Many VPNs are columnar types that tile the visual field. VPNs target specific neuropils (for example, anterior optic tubercle (AOTU), posterior lateral protocerebrum (PLP) and posterior ventrolateral protocerebrum (PVLP)) or optic glomeruli in the central brain. The influence of VPNs can be very strong; 892 central neurons receive more than half their synapses from VPNs.

The hemibrain already characterized several VPN types along with their outputs in the central brain. Our whole-brain reconstruction reveals many other aspects of VPN connectivity, such as their inputs in the medulla, lobula and lobula plate. In addition to feedforward targeting of central neurons, VPNs make 20% of their synapses onto other VPNs and 21% onto optic lobe neurons. Ganguly et al. and Garner et al. further investigated the visual projections to the central complex and the mushroom body.

There are 524 neurons that project from the central brain to the optic lobes. We call these visual centrifugal neurons (VCNs). They are distinct from previously defined types of VCNs that are fully contained in the optic lobe and their functions are mostly unknown. VCNs are 15 times less numerous than VPNs. Nevertheless, half of all optic lobe neurons receive five or more synapses from VCNs, showing that much early visual processing incorporates feedback from the central brain. Centrifugal inputs to the retina are found in many vertebrate species, including humans.

Many VCNs arborize broadly in the optic lobe, appearing to cover the entire visual field. Some VCNs, however, cover only a subset of columns within a portion of the visual field. A few optic lobe neurons receive as many as 50% of their synapses from VCNs. These belong to the class of peptidergic neurons involved in circadian rhythms. Tm5c is a columnar type (necessary for the preference of Drosophila for UV over visible light), with more than 10% of its inputs coming from VCNs.

Neuron superclasses

The neuron classes introduced above are organized into a hierarchy, as explained in our companion paper. The three ‘flow’ classes (afferent, intrinsic and efferent) are divided into the nine superclasses (Fig. 2a). A simplified representation of the connectome as a graph in which nodes are superclasses is shown in Fig. 2b. Node sizes reflect neuron number and link widths indicate connection number. This is the first of several simplified representations of the connectome that we introduce here.