Fig. 7.

Parameters of microcircuitry in mammalian neocortex and Drosophila brain. A-C: Representative EM sections of mouse neocortex (A), Drosophila larval brain (B) and Drosophila adult brain (C) shown at the same scale. Red dots in A and B indicate profiles of individual sectioned neurites. D: Frequency distribution of neurites with different diameters in mammalian cortex and Drosophila brain. Indicated are also the range of diameters that correspond to different neuropile elements (yellow/orange: dendrites; green/blue: axons). E: Comparison of several core parameters in mouse and Drosophila neuropile. In both systems, terminal axons are varicose neurites which form presynaptic sites on their varicosities. Diameters of varicosities are in the range of 0.5 to 1.5µm (average in mouse: 0.7µm; in Drosophila: 0.9µm); the thin segments of terminal axons have an average diameter of approximately 0.33µm. The diameter of synapses (presynaptic sites) is also quite similar in both systems (0.32µm in mammalian cortex, 0.25µm in fly brain. Also the overall cable length of terminal axons per volume unit is comparable: 4,100µm per 1000µm3 in mouse, and between 1500 and 4,000µm in different micro-volumes of fly brain. The major difference between mouse and fly neuropile lies in the size and branching density of dendrites. In Drosophila, dendrites are very thin (average diameter: 0.13µm) and densely branched; in mammalian brain, dendrites are thick (average diameter: 0.9µm; see even thicker examples of dendrites in panel A) and branches are much further apart. This is also reflected in the dendritic cable length which is 450mm per 1000µm3 in mouse and more than 10fold higher in Drosophila. Lower branch density as well as the absence of polyadic synapses in mouse cortex neuropile also results in a considerably less dense connectivity, schematically shown in F. Shown for Drosophila is the dense overlapping regulon motif, in which the large majority of neurite segments encountered in any micro-volume of 100µm3 or more is engaged. In a mammalian cortical micro-volume of that size, dendrite segments are unbranched; the only type of connectivity is convergence, whereby multiple terminal axons converge on a dendritic segment that happens to be within their range.

Bar: 1µm