Vertical motion detectors and their synaptic relations in the third optic lobe of the fly

A comprehensive neuroanatomical survey of the Drosophila Lobula Plate Tangential Neurons with predictions for their optic flow sensitivity

Flying insects exhibit remarkable navigational abilities controlled by their compact nervous systems. Optic flow, the pattern of changes in the visual scene induced by locomotion, is a crucial sensory cue for robust self-motion estimation, especially during rapid flight. Neurons that respond to specific, large-field optic flow patterns have been studied for decades, primarily in large flies, such as houseflies, blowflies, and hover flies. The best-known optic-flow sensitive neurons are the large tangential cells of the dipteran lobula plate, whose visual-motion responses, and to a lesser extent, their morphology, have been explored using single-neuron neurophysiology. Most of these studies have focused on the large, Horizontal and Vertical System neurons, yet the lobula plate houses a much larger set of 'optic-flow' sensitive neurons, many of which have been challenging to unambiguously identify or to reliably target for functional studies. Here we report the comprehensive reconstruction and identification of the Lobula Plate Tangential Neurons in an Electron Microscopy (EM) volume of a whole Drosophila brain. This catalog of 58 LPT neurons (per brain hemisphere) contains many neurons that are described here for the first time and provides a basis for systematic investigation of the circuitry linking self-motion to locomotion control. Leveraging computational anatomy methods, we estimated the visual motion receptive fields of these neurons and compared their tuning to the visual consequence of body rotations and translational movements. We also matched these neurons, in most cases on a one-for-one basis, to stochastically labeled cells in genetic driver lines, to the mirror-symmetric neurons in the same EM brain volume, and to neurons in an additional EM data set. Using cell matches across data sets, we analyzed the integration of optic flow patterns by neurons downstream of the LPTs and find that most central brain neurons establish sharper selectivity for global optic flow patterns than their input neurons. Furthermore, we found that self-motion information extracted from optic flow is processed in distinct regions of the central brain, pointing to diverse foci for the generation of visual behaviors.

Neuronal circuits integrating visual motion information in Drosophila melanogaster

The detection of visual motion enables sophisticated animal navigation, and studies on flies have provided profound insights into the cellular and circuit bases of this neural computation. The fly's directionally selective T4 and T5 neurons encode ON and OFF motion, respectively. Their axons terminate in one of the four retinotopic layers in the lobula plate, where each layer encodes one of the four directions of motion. Although the input circuitry of the directionally selective neurons has been studied in detail, the synaptic connectivity of circuits integrating T4/T5 motion signals is largely unknown. Here, we report a 3D electron microscopy reconstruction, wherein we comprehensively identified T4/T5's synaptic partners in the lobula plate, revealing a diverse set of new cell types and attributing new connectivity patterns to the known cell types. Our reconstruction explains how the ON- and OFF-motion pathways converge. T4 and T5 cells that project to the same layer connect to common synaptic partners and comprise a core motif together with bilayer interneurons, detailing the circuit basis for computing motion opponency. We discovered pathways that likely encode new directions of motion by integrating vertical and horizontal motion signals from upstream T4/T5 neurons. Finally, we identify substantial projections into the lobula, extending the known motion pathways and suggesting that directionally selective signals shape feature detection there. The circuits we describe enrich the anatomical basis for experimental and computations analyses of motion vision and bring us closer to understanding complete sensory-motor pathways.

Full reconstruction of large lobula plate tangential cells in Drosophila from a 3D EM dataset

With the advent of neurogenetic methods, the neural basis of behavior is presently being analyzed in more and more detail. This is particularly true for visually driven behavior of Drosophila melanogaster where cell-specific driver lines exist that, depending on the combination with appropriate effector genes, allow for targeted recording, silencing and optogenetic stimulation of individual cell-types. Together with detailed connectomic data of large parts of the fly optic lobe, this has recently led to much progress in our understanding of the neural circuits underlying local motion detection. However, how such local information is combined by optic flow sensitive large-field neurons is still incompletely understood. Here, we aim to fill this gap by a dense reconstruction of lobula plate tangential cells of the fly lobula plate. These neurons collect input from many hundreds of local motion-sensing T4/T5 neurons and connect them to descending neurons or central brain areas. We confirm all basic features of HS and VS cells as published previously from light microscopy. In addition, we identified the dorsal and the ventral centrifugal horizontal, dCH and vCH cell, as well as three VSlike cells, including their distinct dendritic and axonal projection area.

Golgi analysis of tangential neurons in the lobula plate ofDrosophila melanogaster

The lobula plate (LP), which is the third order optic neuropil of flies, houses wide-field neurons which are exquisitely sensitive to motion. Among Diptera, motion-sensitive neurons of larger flies have been studied at the anatomical and physiological levels. However, the neurons of Drosophila lobula plate are relatively less explored. As Drosophila permits a genetic analysis of neural functions, we have analysed the organization of lobula plate of Drosophila melanogaster. Neurons belonging to eight anatomical classes have been observed in the present study. Three neurons of the horizontal system (HS) have been visualized. The HS north (HSN) neuron, occupying the dorsal lobula plate is stunted in its geometry compared to that of larger flies. Associated with the HS neurons, thinner horizontal elements known as h-cells have also been visualized in the present study. Five of the six known neurons of the vertical system (VS) have been visualized. Three additional neurons in the proximal LP comparable in anatomy to VS system have been stained. We have termed them as additional VS (AVS)-like neurons. Three thinner tangential cells that are comparable to VS neurons, which are elements of twin vertical system (tvs); and two cells with wide dendritic fields comparable to CH neurons of Diptera have been also observed. Neurons comparable to VS cells but with 'tufted' dendrites have been stained. The HSN and VS1-VS2 neurons are dorsally stunted. This is possibly due to the shape of the compound eye of Drosophila which is reduced in the fronto-dorsal region as compared to larger flies.

Dendritic structure and receptive-field organization of optic flow processing interneurons in the fly

The third visual neuropil (lobula plate) of the blowfly Calliphora erythrocephala is a center for processing motion information. It contains, among others, 10 individually identifiable "vertical system" (VS) neurons responding to visual wide-field motions of arbitrary patterns. We demonstrate that each VS neuron is tuned to sense a particular aspect of optic flow that is generated during self-motion. Thus the VS neurons in the fly supply visual information for the control of head orientation, body posture, and flight steering. To reveal the functional organization of the receptive fields of the 10 VS neurons, we determined with a new method the distributions of local motion sensitivities and local preferred directions at 52 positions in the fly's visual field. Each neuron was identified by intracellular staining with Lucifer yellow and three-dimensional reconstructions from 10-micron serial sections. Thereby the receptive-field organization of each recorded neuron could be correlated with the location and extent of its dendritic arborization in the retinotopically organized neuropil of the lobula plate. The response fields of the VS neurons, i.e., the distributions of local preferred directions and local motion sensitivities, are not uniform but resemble rotatory optic flow fields that would be induced by the fly during rotations around various horizontal axes. Theoretical considerations and quantitative analyses of the data, which will be presented in a subsequent paper, show that VS neurons are highly specialized neural filters for optic flow processing and thus for the visual sensation of self-motions in the fly.

Two visual systems in one brain: neuropils serving the secondary eyes of the spider Cupiennius salei

Like other araneans, the wandering spider Cupiennius salei is equipped with one pair of principal eyes and three pairs of secondary eyes. Primary and secondary eyes serve two distinct sets of visual neuropils in the brain. This paper describes cellular organization in neuropils supplied by the secondary eyes, which individually send axons into three laminas resembling their namesakes serving insect superposition eyes. Secondary eye photoreceptors send axons to small-field projection neurons (L-cells) which extend from each lamina to supply three separate medullas. Each medulla is a vault of neuropil comprising only a few morphological types of neurons. These can be compared to a subset of retinotopic neurons in the medullas of calliphorid Diptera supplying giant motion-sensitive neurons in the lobula plate. In Cupiennius, neurons from secondary eye medullas converge at a single target neuropil called the "mushroom body." This region contains giant output neurons which, like their counterparts in the calliphorid lobula plate, lead to descending pathways that supply thoracic motor circuits. It is suggested that the cellular arrangements serving Cupiennius's secondary eyes are color independent pathways specialized for detecting horizontal motion. The present results do not support the classical view that the spider "mushroom body" is phylogenetically homologous or functionally analogous to its namesake in insects.

Distribution and structure of identified tonic and phasic synapses between L-neurones in the locust ocellar tract

The ultrastructures and distributions of the discrete anatomical synapses which constitute two distinct types of output connections made by individual ocellar L-neurons, L1-3, are described. Outputs to neurones L4-5 are excitatory and transmit tonically, whereas reciprocal connections among the three L1-3 neurones are inhibitory and incapable of transmission for longer than a few milliseconds. The tonically transmitting synapses are located in the lateral ocellar tract and are made between the axons of L1-3, which do not receive inputs, and short branches of L4-5, which make no outputs. Each excitatory connection is composed of a few hundred discrete anatomical synapses, each characterised by a bar-shaped presynaptic density which is 0.15-1.5 microns in length and associated with a large number of round synaptic vesicles. Two postsynaptic profiles are apposed to each presynaptic density. Associated with tonic synapses are abundant invaginations of the presynaptic membrane. Synapses of the reciprocal, inhibitory, phasic connections occur in the protocerebral arbors of L1-3, among numerous output synapses of these neurones. Each phasic connection is composed of a few tens of discrete anatomical synapses. Each bar-shaped presynaptic density is associated with two postsynaptic profiles, and is 0.1-1.0 microns long. Compared with the tonic, excitatory connection, there are fewer vesicles and fewer invaginations of the presynaptic membrane associated with each synapse.

Structure of a tonically transmitting synapse between identified interneurone in the locust brain

The ultrastructure and distribution of synapses between large second-order and third-order neurones of a locust ocellus are described. Pairs of neurones, shown by physiological tests to be connected by a tonically transmitting, excitatory connection, have been injected with hexamminecobaltic ions, which allows their profiles to be recognised with electron microscopy. The synapses are made in the lateral ocellar tract, between the axons of some of the second-order neurones and a network of fine processes from the third-order neurone. A physiological connection is composed of several thousand discrete anatomical synapses. Each anatomical synapse is composed of a presynaptic density associated with a cloud of round, electron-lucent vesicles overlying an intercellular cleft. Parallel with the presynaptic density is a row of omega-shaped invaginations of the presynaptic plasmalemma. The synapses are diadic and are close to synapses from small, unidentified neuronal profiles onto the third-order neurone.

Regional specialization in synaptic input and output in an identified local nonspiking interneuron of the crayfish revealed by light and electron microscopy

Lateral inhibition of several mechanosensory interneurons in the crayfish terminal ganglion is mediated by a pair of identified local nonspiking interneurons called local directionally selective (LDS) interneurons. The ultrastructure and synaptic distribution of these interneurons were investigated by using intracellular labeling with horseradish peroxidase for electron microscopy. The LDS interneuron has bilateral, asymmetric arborizations. They are connected by a thick transverse neurite on which no synapses are made. The neurites on the side ipsilateral to the cell body are smooth and receive almost entirely input synapses. On the other hand, the contralateral neurites have prominent numerous swellings or varicosities from which many short fine spines arise, with intermingled input and output synapses. Both synapses, however, are not always associated with varicosities and spines. Presynaptic neurites of the LDS interneuron contain only round agranular synaptic vesicles in the vicinity of synaptic specializations. These morphological findings correlate well with the physiology of this interneuron: it is functionally polarized to receive input on one side (ipsilateral to the cell body) of the ganglion and convey it to the other side.

The morphology, ultrastructure, and distribution of synapses on an intersegmental interneuron of the locust

The structure and synaptic relationships of an intersegmental interneurone in the mesothoracic ganglion of the locust were studied by intracellular labelling for light and electron microscopy. Before labelling, the interneurone was characterized physiologically. It is depolarized and produces spikes when either of the two forewings are elevated, and by sensory inputs from the tarsi of the middle legs. The gross morphology of the interneurone is quite distinctive. Its cell body is on the ventral surface of the ganglion near the midline and its primary neurite lies in the T-tract. Its axon performs a characteristic lateral loop around the lateral dorsal tract before entering an anterior connective and ascending at least as far as the prothoracic ganglion. Within the mesothoracic ganglion, the interneurone gives rise to a profusion of branches. Ultrastructural examination and reconstruction of three regions of the interneurone from serial sections shows that input and output synapses are present in approximately equal numbers, and often are close to each other. There is no evidence for separate input and output regions. Output synapses are characterized by the presence of a presynaptic bar and a population of 500-2,000 round, electron-lucent vesicles. Most of the estimated 10(5)-10(6) synapses occur on the smaller diameter processes or on spines. The spines are of uniform diameter along their lengths, and are pre- or postsynaptic or both, but the postsynaptic spines are of smaller diameter than the others.