Convergence of Neuronal Assemblies at Optic Foci

A neuronal assembly is an arrangement composed of many neurons that all show approximately the same morphological features. Examples are arrays of retinotopic neurons in the visual system; small projection neuron dendrites distributed amongst antennal lobe glomeruli; and ensembles of dendrites spanning the protocerebral bridge.

Usually, neurons within an assembly send their axons to a common target in the brain. In the case of retinotopic neurons in the lobula complex (the 3rd and 4th visual neuropils), axons from each assembly terminate in a structure generically known as an "optic focus," or "optic glomerulus," because of their similarity to antennal lobe glomeruli. One of these optic glomeruli is the anteriorly placed anterior optic tubercle (ant op tu).

The dendrites of each descending neuron clasp and invade the body of one or more optic glomeruli, just as do the dendrites of an antennal lobe projection neuron clasp and invade an olfactory glomerulus. Dendrites of descending neurons are characteristically associated with each optic glomerulus and their axons carry information from these to thoracic and abdominal ganglia. Although a major input to descending neurons is from the visual system, intracellular recordings have demonstrated that most descending neurons are multimodal: they respond to combinations of sensory stimuli from different sensory systems.

The images below provide examples of neuronal assemblies in the lobula (Fig. 1A) and their target dendrites (Figs. 2A, 3A). The axons from many assemblies distribute to discrete optic foci, as in Fig. 2B. The convergence of axons from one assembly onto a descending neuron's dendrites is shown in Fig. 3B.

This pair of figures compares an isomorphic "neuronal assembly" of columnar neurons in the lobula of the fly C. erythrocephala (Fig. 1A) with a single Golgi-impregnated neuron of this cell type (Fig. 1B). Note that the axons from the assembly converge to a common terminal site.

Descending neurons have complex arrangements of dendrites organized into discrete zones. In this example (Fig. 2A), the medial dendrites (left) are situated within ventral body and antennal mechanosensory neuropils. The lateral dendrites (right) form a tight cluster of processes that are associated with a termination site of axons from a neuronal assembly in the lobula. Each neuronal assembly in the lobula sends its axons to a discrete termination area ("optic focus" or "optic glomerulus") . Fig. 2B illustrates several such glomeruli, each of which would receive dendrites from descending neurons or from interneurons that connect a glomerulus to another brain area.

Dye uptake by descending neurons (Fig. 3A) and by neuronal assemblies in the lobula (3B) illustrate the characteristic relationship between the lateral dendrites of descending neurons and an optic glomerulus (red) of lobula output terminals. This arrangement is reminiscent of that seen between olfactory receptor axons and the dendrites of antennal lobe projection neurons in an olfactory glomerulus.

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