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Many insects use the pattern of polarized light in the sky for spatial orientation and navigation. We have investigated the polarization vision system in the desert locust. To create a common platform for anatomical studies on polarization vision pathways, Kurylas et al. (2008) have generated a three-dimensional (3D) standard brain from confocal microscopy image stacks of 10 male brains, using two different standardization methods, the Iterative Shape Averaging (ISA) procedure and the Virtual Insect Brain (VIB) protocol. Comparison of both standardization methods showed that the VIB standard is ideal for comparative volume analysis of neuropils, whereas the ISA standard is the method of choice to analyze the morphology and connectivity of neurons. The central complex is a key processing stage for polarization information in the locust brain. To investigate neuronal connections between diverse central-complex neurons, we generated a higher-resolution standard atlas of the central complex and surrounding areas, using the ISA method based on brain sections from 20 individual central complexes. To explore the usefulness of this atlas, two central-complex neurons, a polarization-sensitive columnar neuron (type CPU1a) and a tangential neuron that is activated during flight, the giant fan-shaped (GFS) neuron, were reconstructed 3D from brain sections. To examine whether the GFS neuron is a candidate to contribute to synaptic input to the CPU1a neuron, we registered both neurons into the standardized central complex. Visualization of both neurons revealed a potential connection of the CPU1a and GFS neurons in layer II of the upper division of the central body.
Migrating desert locusts, Schistocerca gregaria, are able to use the skylight polarization pattern for navigation. They detect polarized light with a specialized dorsal rim area in their compound eye. After multistage processing, polarization signals are transferred to the central complex, a midline-spanning brain area involved in locomotor control. Polarization-sensitive tangential neurons (TB-neurons) of the protocerebral bridge, a part of the central complex, give rise to a topographic arrangement of preferred polarization angles in the bridge, suggesting that the central complex acts as an internal sky compass. TB-neurons connect the protocerebral bridge with two adjacent brain areas, the posterior optic tubercles. To analyze the polarotopic organization of the central complex further, we investigated the number and morphologies of TB-neurons and the presence and colocalization of three neuroactive substances in these neurons. Triple immunostaining with antisera against Diploptera punctata allatostatin (Dip-AST), Manduca sexta allatotropin (Mas-AT), and serotonin (5HT) raised in the same host species revealed three spatially distinct TB-neuron clusters, each consisting of 10 neurons per hemisphere: cluster 1 and 3 showed Dip-AST/5HT immunostaining, whereas cluster 2 showed Dip-AST/Mas-AT immunostaining. Five subtypes of TB-neuron could be distinguished based on ramification patterns. Corresponding to ramification domains in the protocerebral bridge, the neurons invaded distinct but overlapping layers within the posterior optic tubercle. Similarly, neurons interconnecting the tubercles of the two hemispheres also targeted distinct layers of these neuropils. From these data we propose a neuronal circuit that may be suited to stabilize the internal sky compass in the central complex of the locust.
Myoinhibitory peptides (MIPs) are a family of insect W(X(6))Wamides with inhibitory effects on visceral muscles and juvenile hormone synthesis. Although MIPs are widely distributed within the nervous system, a detailed analysis of their distribution and function in insect brains is still missing. We analyzed the distribution of MIPs in the brain of the cockroach Leucophaea maderae. We focused on the accessory medulla (AMe), a small neuropil near the medulla that acts as the master circadian clock. Matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) and Nano-LC electrospray ionization (ESI) mass spectrometry revealed five Lem-MIPs in preparations of the AMe and corpora cardiaca. The complete sequences of two of these peptides were identified. Immunocytochemistry revealed wide distribution of MIP-related peptides in the cockroach brain. The superior median protocerebrum, parts of the central complex, and the tritocerebrum showed particularly dense immunostaining. In contrast, only a few local interneurons were stained in the antennal lobe and a few extrinsic neurons in the mushroom body, including a giant neuron innervating the calyces. The noduli of the AMe showed dense immunostaining, and neurons in all AMe cell groups except the anterior neurons were labeled. Pigment-dispersing factor- (PDF) and MIP immunostaining was colocalized in two neurons of the AMe. No colocalization of MIP- and PDF immunostaining was detected in the anterior optic commissure, but two small PDF-immunoreactive commissural fibers near the posterior optic commissure showed colocalized MIP immunostaining. The results suggest that several MIPs participate in different functional circuits of the circadian system and are involved in multiple brain circuits of the Madeira cockroach.
The central complex is a group of midline-crossing neuropils in the insect brain involved in head direction coding, sky compass navigation, and spatial visual memory. To compare the neuroarchitecture and neurochemistry of the central complex in insects that differ in locomotion, ways of orientation, time of activity (diurnal, nocturnal), and evolutionary history, we studied the distribution of γ-aminobutyric acid (GABA) immunostaining in the central complex of 29 species, ranging from Zygentoma to Diptera. In all species, the lower division of the central body was densely innervated by GABA-immunoreactive tangential neurons. These neurons had additional arborizations in the bulb, a distinct region of synaptic complexes in the lateral complex, and somata in a cell cluster mediodorsally to the antennal lobe. Differences in the appearance of GABA immunostaining in the lower division of the central body corresponded to differences in neuropil architecture, such as transformation of the lower division into a toroid in certain Diptera and Heteroptera. In nearly all species two additional systems of tangential neuron of the upper division of the central body were GABA-immunoreactive. One of these systems diffusely invaded a superior layer, while the second system showed fan-like projections in an inferior layer. Sparse immunostaining in the protocerebral bridge was detected in cockroaches, a cricket, and two hemipteran species. The data show that three systems of GABA-immunoreactive tangential neurons of the central body are highly conserved and suggest that the layered organization of the upper division of the central body is, likewise, largely maintained from basal to advanced species.
The central complex of the insect brain comprises a group of neuropils involved in spatial orientation and memory. In fruit flies it mediates place learning based on visual landmarks and houses neurons that encode the orientation for goal-directed locomotion, based on landmarks and self-motion cues for angular path-integration. In desert locusts, the central complex holds a compass-like representation of head directions, based on the polarization pattern of skylight. Through intracellular recordings from immobilized locusts, we investigated whether sky compass neurons of the central complex also represent the position or any salient feature of possible landmarks, in analogy to the observations in flies. Neurons showed strongest responses to the novel appearance of a small moving square, but we found no evidence for a topographic representation of object positions. Responses to an individual square were independent of direction of motion and trajectory, but showed rapid adaptation to successive stimulation, unaffected by changing the direction of motion. Responses reappeared, however, if the moving object changed its trajectory or if it suddenly reversed moving direction against the movement of similar objects that make up a coherent background-flow as induced by ego-motion. Response amplitudes co-varied with the precedent state of dynamic background activity, a phenomenon that has been related to attention-dependent saliency coding in neurons of the mammalian primary visual cortex. The data show that neurons of the central complex of the locust brain are visually bimodal, signaling sky compass direction and the novelty character of moving objects. These response properties might serve to attune compass-aided locomotor control to unexpected events in the environment. The difference to data obtained in fruit flies might relate to differences in the lifestyle of landmark learners (fly) and compass navigators (locust), point to the existence of parallel networks for the two orientation strategies, or reflect differences in experimental conditions.
The central complex is a group of modular neuropils in the insect brain with a key role in visual memory, spatial orientation, and motor control. In desert locusts the neurochemical organization of the central complex has been investigated in detail, including the distribution of dopamine-, serotonin-, and histamine-immunoreactive neurons. In the present study we identified neurons immunoreactive with antisera against octopamine, tyramine, and the enzymes required for their synthesis, tyrosine decarboxylase (TDC) and tyramine β-hydroxylase (TBH). Octopamine- and tyramine immunostaining in the central complex differed strikingly. In each brain hemisphere tyramine immunostaining was found in four neurons innervating the noduli, 12-15 tangential neurons of the protocerebral bridge, and about 17 neurons that supplied the anterior lip region and parts of the central body. In contrast, octopamine immunostaining was present in two bilateral pairs of ascending fibers innervating the upper division of the central body and a single pair of neurons with somata near the esophageal foramen that gave rise to arborizations in the protocerebral bridge. Immunostaining for TDC, the enzyme converting tyrosine to tyramine, combined the patterns seen with the tyramine- and octopamine antisera. Immunostaining for TBH, the enzyme converting tyramine to octopamine, in contrast, was strikingly similar to octopamine immunolabeling. We conclude that tyramine and octopamine act as neurotransmitters/modulators in distinct sets of neurons of the locust central complex with TBH likely being the rate-limiting enzyme for octopamine synthesis in a small subpopulation of TDC-containing neurons.
The praying mantis is an insect which relies on vision for capturing prey, avoiding being eaten and for spatial orientation. It is well known for its ability to use stereopsis for estimating the distance of objects. The neuronal substrate mediating visually driven behaviors, however, is not very well investigated. To provide a basis for future functional studies, we analyzed the anatomical organization of visual neuropils in the brain of the praying mantis Hierodula membranacea and provide supporting evidence from a second species, Rhombodera basalis, with particular focus on the lobula complex (LOX). Neuropils were three-dimensionally reconstructed from synapsin-immunostained whole mount brains. The neuropil organization and the pattern of γ-aminobutyric acid immunostaining of the medulla and LOX were compared between the praying mantis and two related polyneopteran species, the Madeira cockroach and the desert locust. The investigated visual neuropils of the praying mantis are highly structured. Unlike in most insects the LOX of the praying mantis consists of five nested neuropils with at least one neuropil not present in the cockroach or locust. Overall, the mantis LOX is more similar to the LOX of the locust than the more closely related cockroach suggesting that the sensory ecology plays a stronger role than the phylogenetic distance of the three species in structuring this center of visual information processing.
Despite the importance of the insect nervous system for functional and developmental neuroscience, descriptions of insect brains have suffered from a lack of uniform nomenclature. Ambiguous definitions of brain regions and fiber bundles have contributed to the variation of names used to describe the same structure. The lack of clearly determined neuropil boundaries has made it difficult to document precise locations of neuronal projections for connectomics study. To address such issues, a consortium of neurobiologists studying arthropod brains, the Insect Brain Name Working Group, has established the present hierarchical nomenclature system, using the brain of Drosophila melanogaster as the reference framework, while taking the brains of other taxa into careful consideration for maximum consistency and expandability. The following summarizes the consortium's nomenclature system and highlights examples of existing ambiguities and remedies for them. This nomenclature is intended to serve as a standard of reference for the study of the brain of Drosophila and other insects.
Invertebrate tachykinin-related peptides (TKRPs) comprise a group of signaling molecules having sequence similarities to mammalian tachykinins. A growing body of evidence has demonstrated the presence of TKRPs in the central nervous system of insects. In this investigation, we used an antiserum against locustatachykinin-II to reveal the distribution pattern of these peptides in the brain of the moth Heliothis virescens. Immunolabeling was found throughout the brain of the heliothine moth. Most of the roughly 500 locustatachykinin-II immunoreactive cell bodies, that is, ca. 400, were located in the protocerebrum, whereas the rest was distributed in the deutocerebrum, tritocerebrum, and the gnathal ganglion. Abundant immunoreactive processes were located in the same regions. Labeled processes in the protocerebrum were especially localized in optic lobe, central body, lateral accessory lobe, superior protocerebrum, and lateral protocerebrum, while those in the deutocerebrum were present exclusively in the antennal lobe. In addition to brain interneurons, four pairs of median neurosecretory cells in the pars intercerebralis with terminal processes in the corpora cardiaca and aorta wall were immunostained. No sexual dimorphism in immunoreactivity was found. Comparing the data obtained here with findings from other insect species reveals considerable differences, suggesting species-specific roles of tachykinin-related peptides in insects.
The lateral accessory lobes (LALs) are prominent integration centers in the insect brain. In the desert locust Schistocerca gregaria, they are connected with the anterior optic tubercles (AOTus), with the central complex, and with the ventral nerve cord. Two subcompartments of the LALs, the lateral triangle and the median olive, are easily recognized by their prominent granular texture. Both areas are part of the polarization vision pathway in the locust brain; they receive input from projection neurons of the AOTu and are the site of presumed dendritic arborizations of tangential neurons of the lower division of the central body. Both types of neuron are sensitive to polarized light and most likely play a role in sky compass navigation of the locust. We show here that neurons from the AOTu and tangential neurons of the central body form large microglomerular contacts in the median olive and lateral triangle. Presynaptic elements from the AOTu end in small numbers of large cup-shaped terminals. These cups enclose many small gamma-aminobutyric acid (GABA)-immunoreactive (-ir) profiles from tangential neurons of the lower division of the central body. Each cup-shaped profile makes numerous (>150) dyadic output synapses with the small postsynaptic GABA-ir profiles. No synaptic connections were found between the small core profiles. The microglomerular organization of the median olive and lateral triangle is unlike that of any other synaptic microglomeruli reported for the insect brain. It might provide precise spike timing information possibly used to extract spatial information by comparison of binocular inputs in the central complex.
Recent studies on Drosophila melanogaster and other insects have revealed important insights into the functions and evolution of neuropeptide signaling. In contrast, in- and output connections of insect peptidergic circuits are largely unexplored. Existing morphological descriptions typically do not determine the exact spatial location of peptidergic axonal pathways and arborizations within the neuropil, and do not identify peptidergic in- and output compartments. Such information is however fundamental to screen for possible peptidergic network connections, a prerequisite to understand how the CNS controls the activity of peptidergic neurons at the synaptic level. We provide a precise 3D morphological description of peptidergic neurons in the thoracic and abdominal neuromeres of the Drosophila larva based on fasciclin-2 (Fas2) immunopositive tracts as landmarks. Comparing the Fas2 "coordinates" of projections of sensory or other neurons with those of peptidergic neurons, it is possible to identify candidate in- and output connections of specific peptidergic systems. These connections can subsequently be more rigorously tested. By immunolabeling and GAL4-directed expression of marker proteins, we analyzed the projections and compartmentalization of neurons expressing 12 different peptide genes, encoding approximately 75% of the neuropeptides chemically identified within the Drosophila CNS. Results are assembled into standardized plates which provide a guide to identify candidate afferent or target neurons with overlapping projections. In general, we found that putative dendritic compartments of peptidergic neurons are concentrated around the median Fas2 tracts and the terminal plexus. Putative peptide release sites in the ventral nerve cord were also more laterally situated. Our results suggest that i) peptidergic neurons in the Drosophila ventral nerve cord have separated in- and output compartments in specific areas, and ii) volume transmission is a prevailing way of peptidergic communication within the CNS. The data can further be useful to identify colocalized transmitters and receptors, and develop peptidergic neurons as new landmarks.
Nitric oxide (NO) is a gaseous messenger molecule formed during conversion of L: -arginine into L: -citrulline by the enzyme NO synthase (NOS), which belongs to a group of NADPH diaphorases. Because of its gaseous diffusion properties, NO differs from classical neurotransmitters in that it is not restricted to synaptic terminals. In target cells, NO activates soluble guanylyl cyclase leading to an increase in cGMP levels. In insects, this NO/cGMP-signalling pathway is involved in development, memory formation and processing of visual, olfactory and mechanosensory information. We have analysed the distribution of putative NO donor and target cells in the central complex, a brain area involved in sky-compass orientation, of the locust Schistocerca gregaria by immunostaining for L: -citrulline and cGMP. Six types of citrulline-immunostained neurons have been identified including a bilateral pair of hitherto undescribed neurons that connect the lateral accessory lobes with areas anterior to the medial lobes of the mushroom bodies. Three-dimensional reconstructions have revealed the connectivity pattern of a set of 18 immunostained pontine neurons of the central body. All these neurons appear to be a subset of previously mapped NADPH-diaphorase-positive neurons of the central complex. At least three types of central-complex neurons show cGMP immunostaining including a system of novel columnar neurons connecting the upper division of the central body and the lateral triangle of the lateral accessory lobe. Our results provide the morphological basis for further studies of the function of the labelled neurons and new insights into NO/cGMP signalling.
Butterflies have sophisticated color vision. While the spectral organization of the compound eye has been well characterized in the Japanese yellow swallowtail butterfly, Papilio xuthus, neural mechanisms underlying its color vision are largely unexplored. Towards a better understanding of signal processing in the visual system of P. xuthus, we used immunocytochemical techniques to analyze the distribution of transmitter candidates, namely, histamine, serotonin, tyramine and γ-aminobutyric acid (GABA). Photoreceptor terminals in the lamina and medulla exhibited histamine immunoreactivity as demonstrated in other insects. The anti-histamine antiserum also labeled a few large medulla neurons. Medulla intrinsic neurons and centrifugal neurons projecting to the lamina showed serotonin immunoreactivity. Tyramine immunostaining was detected in a subset of large monopolar cells (LMCs) in the lamina, transmedullary neurons projecting to the lobula plate, and cell bodies surrounding the first optic chiasma. An anti-GABA antiserum labeled a subset of LMCs and populations of columnar and tangential neurons surrounding the medulla. Each of the four antisera also labeled a few centrifugal neurons that innervate the lobula complex from the central brain, suggesting that they have neuromodulatory roles. A distinctive feature we found in this study is the possibility that tyramine and GABA act as transmitters in LMCs of P. xuthus, which has not been reported in any other insects so far.
While the ability of honeybees to navigate relying on sky-compass information has been investigated in a large number of behavioral studies, the underlying neuronal system has so far received less attention. The sky-compass pathway has recently been described from its input region, the dorsal rim area (DRA) of the compound eye, to the anterior optic tubercle (AOTU). The aim of this study is to reveal the connection from the AOTU to the central complex (CX). For this purpose, we investigated the anatomy of large microglomerular synaptic complexes in the medial and lateral bulbs (MBUs/LBUs) of the lateral complex (LX). The synaptic complexes are formed by tubercle-lateral accessory lobe neuron 1 (TuLAL1) neurons of the AOTU and GABAergic tangential neurons of the central body's (CB) lower division (TL neurons). Both TuLAL1 and TL neurons strongly resemble neurons forming these complexes in other insect species. We further investigated the ultrastructure of these synaptic complexes using transmission electron microscopy. We found that single large presynaptic terminals of TuLAL1 neurons enclose many small profiles (SPs) of TL neurons. The synaptic connections between these neurons are established by two types of synapses: divergent dyads and divergent tetrads. Our data support the assumption that these complexes are a highly conserved feature in the insect brain and play an important role in reliable signal transmission within the sky-compass pathway.
The central complex is a group of highly interconnected neuropils in the insect brain. It is involved in the control of spatial orientation, based on external compass cues and various internal needs. The functional and neurochemical organization of the central complex has been studied in detail in the desert locust Schistocerca gregaria. In addition to classical neurotransmitters, immunocytochemistry has provided evidence for a major contribution of neuropeptides to neural signaling within the central complex. To complement these data, we have identified all orcokinin-immunoreactive neurons in the locust central complex and associated brain areas. About 50 bilateral pairs of neurons innervating all substructures of the central complex exhibit orcokinin immunoreactivity. Among these were about 20 columnar neurons, 33 bilateral pairs of tangential neurons of the central body, and seven pairs of tangential neurons of the protocerebral bridge. In silico transcript analysis suggests the presence of eight different orcokinin-A type peptides in the desert locust. Double label experiments showed that all orcokinin-immunostained tangential neurons of the lateral accessory lobe cluster were also immunoreactive for GABA and the GABA-synthesizing enzyme glutamic acid decarboxylase. Two types of tangential neurons of the upper division of the central body were, furthermore, also labeled with an antiserum against Dip-allatostatin I. No colocalization was found with serotonin immunostaining. The data provide additional insights into the neurochemical organization of the locust central complex and suggest that orcokinin-peptides of the orcokinin-A gene act as neuroactive substances at all stages of signal processing in this brain area.
The central complex is a group of neuropils in the center of the insect brain. It consists of four major subunits: the upper and lower divisions of the central body (CBU, CBL), the protocerebral bridge (PB), and the paired noduli. A distinctive feature of the central complex is a modular architecture characterized by rows of 16 columns, intersected in the central body by stacks of layers. Evidence from locusts suggests that the central complex plays a major role in sky compass orientation. To understand signal processing in this brain area further, we have analyzed the morphologies of columnar neurons of the central complex of the locust Schistocerca gregaria. Intracellular dye fills revealed 21 types of columnar neurons that connect columns of different subunits, three types of pontine neurons linking pairs of columns within the CBU, and one amacrine cell. Most neurons appeared to be part of isomorphic sets with cell type-specific heterolateral projection patterns. Evaluation of arborization areas and neuron polarity suggests that these neurons are either intrinsic to the central complex or provide output to the lateral accessory lobes (LALs) or anterior lip region. No direct connections were found between the CBU and CBL. Instead, neurons of either subdivision were connected with the PB, but projected to non-overlapping regions in the LALs and to different layers of the noduli. This study provides novel insights into the functional organization of the central complex, especially with respect to its likely role in right-left signal matching and decision making.
The cockroach Leucophaea maderae is an established model in circadian rhythm research. Its circadian clock is located in the accessory medulla of the brain. Pigment-dispersing factor-immunoreactive (PDF-ir) neurons of the accessory medulla act as circadian pacemakers controlling locomotor activity rhythms. To characterize the neuronal network of the circadian system in L. maderae, the PDF-ir neurons were implemented into a standardized three-dimensional atlas of the cockroach brain. Serial confocal images from 20 wholemount brains were used for the construction of the atlas comprising 21 neuropils. Two different standardization protocols were employed: the iterative shape averaging (ISA) procedure using an affine transformation followed by iterative non-rigid registrations, and the virtual insect brain (VIB) protocol employing local non-rigid transformations after global and local rigid transformations. Quantitative analysis of the 20 brains revealed that volumes of the accessory medulla are directly correlated with the volumes of the medulla, the protocerebral bridge, and the upper division of the central body, suggesting functional connections among these neuropils. For a standardized reconstruction of the circadian pacemaker network, the ISA protocol was used to register PDF-ir neurons in the standard cockroach brain. The registration revealed that two PDF-ir arborization areas in the brain are highly interconnected with other PDF-ir projection sites and appear to be contacted both by fibers in the posterior and the anterior optic commissures. The distances between PDF-ir branching areas show specific numerical relationships that might be physiologically relevant for temporal encoding.
Modular midline neuropils, termed arcuate body (Chelicerata, Onychophora) or central body (Myriapoda, Crustacea, Insecta), are a prominent feature of the arthropod brain. In insects and crayfish, the central body is connected to a second midline-spanning neuropil, the protocerebral bridge. Both structures are collectively termed central complex. While some investigators have assumed that central and arcuate bodies are homologous, others have questioned this view. Stimulated by recent evidence for a role of the central complex in polarization vision and object recognition, the architectures of midline neuropils and their associations with the visual system were compared across panarthropods. In chelicerates and onychophorans, second-order neuropils subserving the median eyes are associated with the arcuate body. The central complex of decapods and insects, instead, receives indirect input from the lateral (compound) eye visual system, and connections with median eye (ocellar) projections are present. Together with other characters these data are consistent with a common origin of arcuate bodies and central complexes from an ancestral modular midline neuropil but, depending on the choice of characters, the protocerebral bridge or the central body shows closer affinity with the arcuate body. A possible common role of midline neuropils in azimuth-dependent sensory and motor tasks is discussed.
Mass migration of desert locusts is a common phenomenon in North Africa and the Middle East but how these insects navigate is still poorly understood. Laboratory studies suggest that locusts are able to exploit the sky polarization pattern as a navigational cue. Like other insects locusts detect polarized light through a specialized dorsal rim area (DRA) of the eye. Polarization signals are transmitted through the optic lobe to the anterior optic tubercle (AOTu) and, finally, to the central complex in the brain. Whereas neurons of the AOTu integrate sky polarization and chromatic cues in a daytime dependent manner, the central complex holds a topographic representation of azimuthal directions suggesting a role as an internal sky compass. To understand further the integration of sky compass cues we studied polarization-sensitive (POL) neurons in the medulla that may be intercalated between DRA photoreceptors and AOTu neurons. Five types of POL-neuron were characterized and four of these in multiple recordings. All neurons had wide arborizations in medulla layer 4 and most, additionally, in the dorsal rim area of the medulla and in the accessory medulla, the presumed circadian clock. The neurons showed type-specific orientational tuning to zenithal polarized light and azimuth tuning to unpolarized green and UV light spots. In contrast to neurons of the AOTu, we found no evidence for color opponency and daytime dependent adjustment of sky compass signals. Therefore, medulla layer 4 is a distinct stage in the integration of sky compass signals that precedes the time-compensated integration of celestial cues in the AOTu.
The central complex (CX) comprises a group of midline neuropils in the insect brain, consisting of the protocerebral bridge (PB), the upper (CBU) and lower division (CBL) of the central body and a pair of globular noduli. It receives prominent input from the visual system and plays a major role in spatial orientation of the animals. Vertical slices and horizontal layers of the CX are formed by columnar, tangential, and pontine neurons. While pontine and columnar neurons have been analyzed in detail, especially in the fruit fly and desert locust, understanding of the organization of tangential cells is still rudimentary. As a basis for future functional studies, we have studied the morphologies of tangential neurons of the CX of the desert locust Schistocerca gregaria. Intracellular dye injections revealed 43 different types of tangential neuron, 8 of the PB, 5 of the CBL, 24 of the CBU, 2 of the noduli, and 4 innervating multiple substructures. Cell bodies of these neurons were located in 11 different clusters in the cell body rind. Judging from the presence of fine versus beaded terminals, the vast majority of these neurons provide input into the CX, especially from the lateral complex (LX), the superior protocerebrum, the posterior slope, and other surrounding brain areas, but not directly from the mushroom bodies. Connections are largely subunit- and partly layer-specific. No direct connections were found between the CBU and the CBL. Instead, both subdivisions are connected in parallel with the PB and distinct layers of the noduli.
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