Journal of Comparative Neurology

All insects studied to date possess a centrally located group of neuropils, known collectively as the central complex, that has been implicated in sensory integration and motor action selection. Among the functions prescribed to the central complex, none is perhaps as intriguing as its role in orientation and navigation. Neurobiological correlates of both current and desired headings have been described in insect CXs. Despite the diversity of arthropods, understanding of the CX as a navigational center originates entirely from terrestrial insects. Stomatopod crustaceans, commonly referred to as mantis shrimps, form an order of predatory marine crustaceans with intricate and diverse visual systems that maintain the distinction of being the only fully aquatic animal known to utilize the navigational strategy of path integration. They utilize idiothetic, celestial, and landmark cues to orient in the benthos. Here, we investigate the neuroanatomy of adult and developing mantis shrimp central complexes and associated neuropils to begin understanding this brain region in a sensorially and behaviorally complex crustacean.

FMRFamide is a neuropeptide that is widely distributed in invertebrates and known to be involved in many physiological functions. Previously we noted marked differences in expression of the fmrf gene in the stellate ganglion of Doryteuthis pealei* compared to the central nervous system. In this study we aimed to examen the brain systems of Doryteuthis pealei* for the presence and distribution of fmrf-expressing cells and fiber networks. Late squid embryos and hatchlings were examined by in situ hybridization and immunohistochemistry in whole mounts and tissue sections. All central lobes contained limited numbers of scattered neurons expressing fmrf, but the FMRFamide-containing fiber systems were abundant and extensive, mostly present in the neuropil of lobes. Main clusters of neurons were located in the magnocellular and chromatophore lobes of the posterior subesophageal mass (PSM), and in dorsal aspects of the basal lobe (BL). Dense FMRFamide-immunoreactive fibers were particularly seen in the optic lobe (OL), medial and posterior supraesophageal masses (MSM and SPM) often with a commissural organization. The data show that the central lobes of Doryteuthis pealei hatchlings have a matured FMRFamide system organized in a limited number of centers, but with widely distributed efferents. This suggests that FMRFamide neurons are already functionally engaged in the late embryo. The localization indicates that control of chromatophores and fin movement are amongst these functions.

Elephants have elaborate trunk skills and large, but poorly understood brains. Here we study trunk representations in elephant trigeminal nuclei, which form large protrusions on the ventral brainstem. These ventral brainstem protrusions have previously been referred to as inferior olive, but a delineation of the olivo-cerebellar tract reveals these (trigeminal) nuclei are not connected to the cerebellum via climbing fibers. In contrast, the olivo-cerebellar tract connects to a large dorsolateral nucleus with a serrated cellular architecture, the putative elephant inferior olive. Dense vascularization and intense cytochrome-oxidase reactivity distinguish several elongated trigeminal putative trunk modules, which repeat in the anterior-posterior direction. We focus on the most anterior and largest of these units, the putative nucleus principalis trunk module. Module neuron density is low and non-neural cells outnumber neurons by [~]108:1. Dendritic trees are elongated along the axis of axon bundles (myelin stripes) transversing the trunk module. Synchrotron X-ray-phase-contrast tomography suggests myelin-stripe-axons transverse the trunk module. We show a remarkable correspondence of trunk module myelin stripes and trunk folds. Myelin stripes show little relation to trigeminal neurons and stripe-axons appear to often go nowhere; we suggest that myelin stripes might serve to separate trunk-fold domains rather than to connect neurons. Myelin-stripes-to-folds mapping allowed to determine neural magnification factors, which changed from 1000:1 proximally to 5:1 in the trunk finger. Asian elephants have fewer ([~]640,000) trunk-module neurons than Africans ([~]740,000) and show enlarged representations of trunk parts involved in object wrapping. The elephant trigeminal trunk module is exquisitely organized into trunk-fold-related units.

The vomeronasal system (VNS) is critical for detecting pheromonal cues that modulate sociosexual behaviors. Despite its central role in chemical communication, our understanding of its anatomical and functional variability across mammals remains incomplete. This study provides the first detailed characterization of the VNS in the Iberian mole (Talpa occidentalis), a fossorial species endemic to the Iberian Peninsula. We performed a morphofunctional and neurochemical analysis of the vomeronasal organ (VNO) and the accessory olfactory bulb (AOB) using histology, immunohistochemistry, and lectin histochemistry. The VNO in T. occidentalis exhibited an unusual circular lumen lined by a uniform sensory epithelium, lacking the dual epithelial organization seen in most species. The vomeronasal cartilage was limited in extent and did not form the typical J-shaped structure. Importantly, no evidence of a vomeronasal pump was found, suggesting alternative mechanisms for semiochemical entry, likely facilitated by the organs anatomical position and continuous receptor distribution. Immunohistochemical analysis revealed strong expression of Gi2 and G{Upsilon}8 in sensory neurons, with weaker G0 expression, suggesting predominance of V1R-type signal transduction. The AOB, though small, exhibited clear lamination and specific marker localization (Gi2, OMP, CR, MAP2), indicating robust functional organization. Lectin binding revealed specific glycosylation patterns in the glomerular layer, with STL and LEA marking synaptic regions. These findings uncover unprecedented anatomical and molecular features in the VNS of T. occidentalis, positioning this species as a valuable model for studying vomeronasal diversity and evolution among Laurasiatherian mammals.

The octopus peduncle complex is an agglomeration of neural structures with remarkably diverse functional roles. The complex rests on the optic tract, between the optic lobe and the central brain, and comprises the peduncle lobe proper, the olfactory lobe, and the optic gland. The peduncle lobe regulates visuomotor behaviors, the optic glands control sexual maturation and maternal death, and the olfactory lobe is thought to receive input from the olfactory organ. Recent transcriptomic and metabolomic studies have identified candidate peptide and steroid ligands in the Octopus bimaculoides optic gland. With gene expression for these ligands and their biosynthetic enzymes, we show that optic gland neurochemistry extends beyond the traditional optic gland secretory tissue and into lobular territories. A key finding is that the classically defined olfactory lobe is itself a heterogenous territory and includes steroidogenic territories that overlap with cells expressing molluscan neuropeptides and the synthetic enzyme dopamine beta-hydroxylase.

Chemical communication is essential for mammalian survival from the earliest stages of life, yet most of what is known about the prenatal development of the olfactory and vomeronasal systems comes from laboratory rodents. These models, while invaluable, may not fully represent the developmental trajectories of wild species living under natural ecological pressures. Here we investigated the fetal development of the nasal chemosensory systems in the fossorial water vole (Arvicola scherman), a free-living arvicoline rodent with a highly subterranean lifestyle. We analyzed fetuses at embryonic days E17 and E21 (term) using classical histology, immunohistochemistry (markers: Gi2, Go, G{gamma}8, CB, CR, PGP 9.5, GAP-43, {beta}-tubulin, MAP2), and lectin histochemistry (UEA, LEA, SBA, STA, DBA). This combined approach enabled us to assess structural maturation, neuronal differentiation, and the temporal dynamics of glycoconjugate expression in the vomeronasal organ (VNO), olfactory epithelium (OE), and the main (MOB) and accessory olfactory bulbs (AOB). By E21, the MOB displayed a six-layered adult-like organization with well-defined glomeruli and interneuronal populations, whereas the AOB showed delayed morphological maturation but already exhibited selective molecular signatures in its nerve and superficial layers. Prenatally, the VNO underwent conspicuous structural differentiation, including stratification of the sensory epithelium, robust axonal fasciculation, and early development of vomeronasal glands. Immunohistochemical analysis revealed early expression of G-protein subunits and calcium-binding proteins, indicating premature pathway specification and interneuronal circuit formation. Lectin labeling provided additional insights: SBA emerged as a highly selective marker of the vomeronasal pathway; UEA highlighted early compartmentalization of vomeronasal projections; LEA showed a conserved, pan-chemosensory binding pattern across systems; and DBA, despite its lower specificity, revealed late-onset reactivity in postmitotic neurons. Together, these findings demonstrate that A. scherman exhibits a remarkably accelerated prenatal maturation of its chemosensory systems compared with laboratory rodents. This early functional readiness likely reflects adaptive pressures of a fossorial lifestyle, emphasizing the importance of incorporating wild species into developmental neurobiology to refine our understanding of mammalian chemosensory evolution.

To elucidate causality underlying the evolution of large brains in cetaceans, we examined the brains of 16 cetartiodactyl species for evidence of non-shivering thermogenesis. In comparison to the artiodactyl brain, the cetacean brain exhibits an expanded expression of uncoupling protein 1 (UCP1, UCPs being mitochondrial inner membrane proteins that dissipate the proton gradient to generate heat) in cortical neurons, localization of UCP4 within a substantial proportion of glia throughout the brain, and an increased density of noradrenergic axonal boutons (noradrenaline functioning to control concentrations of and activate UCPs). Thus, cetacean brains possess multiple characteristics indicative of intensified thermogenetic functionality that can be related to their current and historical obligatory aquatic niche. These findings necessitate reassessment of our concepts regarding the reasons for large brain evolution and associated functional capacities in cetaceans.

Octopus arms, notable for their complex anatomy and remarkable flexibility, have sparked significant interest within the neuroscience community. However, there remains a dearth of knowledge about the molecular and functional identities of various cell types in the arms nervous system. To address this gap, we used hybridization chain reaction (HCR) to identify distinct neuronal types in the arms of the pygmy octopus, Octopus bocki, including putative dopaminergic, octopaminergic, serotonergic, GABAergic, glutamatergic, cholinergic, and peptidergic neurons. We obtained high-resolution multiplexed fluorescent images at 0.28x0.28x1.0 M voxel size from 10 arm base and arm tip cross sections (each 50 M thick) and created three-dimensional reconstructions of the axial ganglia, illustrating the spatial distribution of multiple neuronal populations. Our analysis unveiled anatomically distinct and molecularly diverse scattered neurons, while also highlighting multiple populations of dense small excitatory neurons that appear uniformly distributed throughout the cortical layer. Our data provide new insights into how different types of neurons may contribute to the ability of an octopus to interact with its environment and execute complex tasks. In addition, our findings establish a benchmark for future studies, allowing pioneering exploration of octopus arm molecular neuroanatomy, and offering exciting new avenues in invertebrate neuroscience research.

The habenula (Hb), which contains medial and lateral subdivisions (MHb and LHb, respectively), has high intensity mu opioid binding and receptor (MOR) expression, yet the details of MOR localization across these regions remains debated. MHb and LHb participate in largely non-overlapping neural circuits, therefore accurately resolving MOR expression across MHb and LHb is critical for understanding how MOR ligands impact behaviors. Here we utilized in situ hybridization (ISH) and immunocytochemistry (ICC) to systematically map Oprm1 mRNA and MOR protein throughout the habenular complex. We studied both rat and mouse tissue to evaluate conserved expression across two common research species. Importantly, we found mRNA and protein in both the MHb and LHb in both. In rat, 39 {+/-} 3% (MHb) and 21 {+/-} 4% (LHb) of cells expressed Oprm1. These proportions were greater in mouse: 57 {+/-} 1% (MHb) and 32 {+/-} 4% (LHb). Within each species, Oprm1 labeling density per positive cell was greater in MHb compared to LHb (p < 0.0001 for rat and mouse). The highest intensity labeling was localized along the lateral edge of the MHb for both methods. ICC showed MOR localized to fibers and somata in MHb and LHb. In LHb, MOR labeling was most dense in intermediate sections along the anterior-posterior (AP) axis. In rats we also observed higher density labeling in dorsal LHb at intermediate AP levels and medial LHb more posteriorly. These results indicate that both MHb and LHb can contribute to MOR mediated actions through their respective circuits. Key PointsO_LIMu opioid receptor mRNA and protein is expressed in both the medial and lateral habenulae in rat and mouse. C_LIO_LIIn the medial habenula, most mu opioid receptor mRNA and protein was detected along its lateral border. C_LIO_LIAcross samples, Oprm1+ cells in the MHb contained more mRNA puncta per cell compared to lateral habenula cells. C_LI

The amygdala is a complex brain structure in the vertebrate telencephalon, essential for regulating social behaviors, emotions and (social) cognition. In contrast to the vast majority of neuron types described in the many nuclei of the mammalian amygdala, little is known about the neuronal diversity in non-mammals, making reconstruction of its evolution particularly difficult. Here, we characterize glutamatergic neuron types in the amygdala of the salamander Pleurodeles waltl. Our single-cell RNA sequencing data indicate the existence of at least ten distinct types and subtypes of glutamatergic neurons in the salamander amygdala. In situ hybridization for marker genes indicates that these neuron types are located in three major subdivisions: the lateral amygdala, the medial amygdala, and a newly-defined area demarcated by high expression of the transcription factor Sim1. The gene expression profiles of these neuron types suggest similarities with specific neuron types in the sauropsid and mammalian amygdala, and in particular the evolutionary conservation of Sim1-expressing amygdalar neurons in tetrapods. Taken together, our results reveal a surprising diversity of glutamatergic neuron types in the amygdala of salamanders, despite the anatomical simplicity of their brain.

Nitric oxide (NO) is a diffusible neuromodulator with roles in synaptic plasticity and memory flexibility, exerting its primary effects via the enzyme soluble guanylate cyclase (sGC). Despite its well-documented functions in mammals and insects, little is known about the neuroanatomical distribution and functional relevance of NO in birds, particularly in relation to dopaminergic systems. This study used histochemical and immunohistochemical techniques to map the distribution of NO-synthesizing neurons--identified by NADPH-diaphorase (NADPH-d) and nNOS activity--and their relation to sGC and tyrosine hydroxylase (TH)-positive dopaminergic pathways in the pigeon brain. We found extensive NADPH-d labeling throughout forebrain, midbrain, and hindbrain regions. Among TH-positive midbrain structures, the locus coeruleus exhibited high colocalization with nNOS, while moderate colocalization was seen in the ventral tegmental area substantia grisea centralis and substantia nigra. Notably, a significant proportion of sGC-positive neurons was targeted by TH and NADPH-d positive fibres in the pigeon NCL. Our findings support the potential for NO-dopamine interactions in avian species, reminiscent of memory-related mechanisms in Drosophila melanogaster, and contribute to an understanding of conserved pathways that may underlie flexible learning and memory processing during navigation or related tasks across vertebrates. This work also offers insight into comparative NADPH-d distribution among avian species, with implications for aging, spatial learning, and memory formation.

Striatal neurons play a central role in vertebrate action selection; however, their location in larval zebrafish is not well defined. We assayed for conserved striatal markers in the zebrafish subpallium using fluorescent in situ hybridization (FISH) and immunohistochemistry. Whole mount FISH revealed an inhibitory neuronal cluster rostral to the anterior commissure that expresses tac1, a gene encoding substance P. This molecular profile is shared by mammalian striatal direct pathway neurons. A second partially overlapping population of inhibitory neurons was identified that expresses penka, a gene encoding enkephalin. This molecular profile is shared by striatal indirect pathway neurons. Immunostaining for substance P and enkephalin confirmed the presence of these peptides in the subpallium. The tac1 and penka populations were both found to increase linearly across larval stages. Together, these findings support the existence of a striatal homologue in larval zebrafish that grows to match the development and increasing behavioural complexity of the organism.

The mammalian olfactory systems can be divided into several subsystems based on the anatomical location of their neuroreceptor cells and the family of receptors they express. The more in depth studied systems are the main olfactory system and the vomeronasal system, whose first integrative enters are the main and the accessory olfactory bulb, respectively. In addition, there is a range of olfactory subsystems which converge to the transition zone located between the main olfactory bulb and the accessory olfactory bulb., which has been termed as olfactory limbus (OL) and includes specialized glomeruli which receive uncanonical sensory afferences and interact with the MOB and AOB. Beyond the laboratory rodents, there is a lack of information regarding the olfactory subsystems of carnivores. We have focused on the specific study of the olfactory limbus of the fox, performing serial histological sections, general and specific histological stainings, including both double and simple immunohistochemical and lectin-histochemical labeling techniques. As a result, we have been able to determine that the OL of the fox shows an uncommon development with a high degree of development and complexity. This makes this species a novel mammalian model that could provide a wider understanding of non-canonical pathways involved in the processing of chemosensory cues.

In spinal cord and medulla, ependymal cells re organized in a monolayer forming the central canal (cc). In rodents, this region, also known as a stem cell niche, was shown to contain cerebrospinal fluid-contacting neurons (CSF-cNs). These neurons are GABAergic and because of their chemo- and mechanosensory properties they would represent a novel sensory system intrinsic to the central nervous system. In primates, little is known about these neurons and more generally about the region around the cc. Here, using immunohistochemical approaches, we investigated the organization of the cc region and CSF-cN properties in Macaca mulatta Rhesus monkey. In contrast to rodent, we observe along the whole medullo-spinal axis a large zone around the cc delimited by long radial ependymal fibers that is enriched with astrocytes and microglia but largely devoid of neuronal elements except for CSF-cNs. These primate CSF-cNs share with rodent CSF-cNs similar morphological and phenotypical features with a largely immature profile. Our data suggest that they extend their axons in the longitudinal axis to form fiber bundles close to the cc and we further show that CSF-cNs receive GABAergic and serotoninergic synaptic contacts on their soma and dendrite. Taken together our results reveal in Rh. monkey a specific organization of the region around the cc potentially forming a buffer zone between CSF and parenchyma where CSF-cNs would play a crucial role in the detection of CSF signals and their transmission to the central nervous system, a role that would need to be further investigated.

For decades we have known that the brain "drains" through the subarachnoid space following a route that crosses the cribriform plate to the nasal mucosa and cervical lymph nodes. Yet little is known about the potential role of the olfactory epithelia and associated lymphatic vasculature in the immune response. To better understand the immune response in the olfactory organs we used cell-specific fluorescent reporter lines in dissected, intact adult brains to visualize blood-lymphatic vasculature and neutrophils in the olfactory sensory system. Here we show that the extensive blood vasculature of the olfactory organs is associated with a lymphatic cell type resembling high endothelial venules (HEVs) of the lymph nodes in mammals and a second resembling Mural Lymphatic Endothelial Cells (muLECs) that extended from the brain to the peripheral olfactory epithelia. Surprisingly, the olfactory organs contained the only neutrophil populations observed in the brain. Damage to the olfactory epithelia resulted in a rapid increase of neutrophils within the olfactory organs as well as the appearance of neutrophils in the brain suggesting that neutrophils enter the brain in response to damage. Analysis of cell division during and after damage showed an increase in BrdU labeling in the olfactory epithelia and a subset of the neutrophils. Our results reveal a unique population of neutrophils in the olfactory organs that are associated with an extensive lymphatic vasculature suggesting a dual olfactory-immune function for this unique sensory system. HighlightsO_LIThe olfactory organ is the only region of the brain that contains resident neutrophils in the adult animal. C_LIO_LIDamage to olfactory sensory neurons triggers a rapid mobilization of neutrophils within the olfactory organ and in the central nervous system. C_LIO_LITwo types of lymphatic vasculature resembling Mural Lymphatic Endothelial Cells (muLEC) and High Endothelial Venules (HEV) are present in the olfactory sensory system. C_LIO_LILymphatic vasculature resembling Mural Lymphatic Endothelial Cells (muLEC) wrap the olfactory bulbs and extend across the cribriform plate to olfactory epithelia. C_LI

Evolutionary changes in catecholamine neurotransmitters such as dopamine and noradrenaline can lead to habitat specific behaviors. We used tyrosine hydroxylase, a conserved precursor to the biosynthesis of dopamine and noradrenaline, to compare catecholaminergic neurons in the brain of a species undergoing allopatric speciation. The teleost fish Astyanax mexicanus is extant in two readily available forms, an ancestral river dwelling form (surface) and various derived blind cave forms (cavefish). Adaptation to nutrient poor cave life without predation has led to marked differences in the behavior of this species. The cavefish has lost defensive responses, such as stimulus aversion, found in the ancestral surface fish and instead displays enhanced food seeking behaviors. This is reflected by an increase in catecholamine immunoreactivity in the cavefish brain in regions associated with non-visual sensory perception, motor control pathways, attention, and endocrine release. These neuroanatomical regions include the olfactory system, the basal telencephalon, the preoptic nuclei, the posterior tuberculum, caudal hypothalamus, and isthmus. These results indicate that the evolutionary shift from aversive defensive responses to attractive exploratory behaviors was driven by increases in the size and/or quantity of catecholaminergic neurons in the cavefish brain.

Although dopamine receptors (DARs) have been identified in various vertebrate neural circuits, their expression in the central auditory system remains poorly characterized. Reproductive-state changes in catecholamine innervation of central auditory nuclei in the plainfin midshipman fish (Porichthys notatus) highlight a unique, ethologically relevant role for catecholamines, including dopamine, in modulating auditory function to enhance reproductive success. Dopamines effects on these systems are mediated via its receptors; thus, the goal of the present study is to characterize excitatory dopamine D1Aa and inhibitory dopamine D2a receptor transcript expression throughout the central auditory system of the plainfin midshipman. Fluorescence in situ hybridization-immunohistochemistry (FISH-IHC) revealed robust D1Aa and D2a expression in forebrain auditory-recipient centers that receive catecholaminergic input: postcommissural and ventral nuclei of the ventral telencephalon, anterior tuberal nucleus, central posterior nucleus of the thalamus, and parvocellular preoptic nuclei. Large dopamine neurons in the periventricular posterior tuberculum, which are, in part, responsible for the reproductive-state changes in central and peripheral catecholamine innervation only express D2a, whereas large noradrenergic neurons in the locus coeruleus express both D1Aa and D2a. The midbrain torus semicircularis, periaqueductal gray, hindbrain octavolateralis efferent nucleus, and descending/secondary octaval nuclei also express both receptor types. D2a expression predominates over D1Aa, and we identify a subpopulation of cells throughout the auditory system that co-express both receptors. The robust distribution of inhibitory and excitatory dopamine receptor expression in the central auditory system, coupled with co-expression in a subset of cells, provides strong neuroanatomical evidence of dopamines complex role in modulating auditory sensitivity and processing.

We describe a pericapillary organ in the rat forebrain and cerebellar cortex. It consists of series of tripartite synapses enveloped by astrocytic endfeet linked to the capillary wall by synaptic extensions. Reciprocal specializations of the pericyte-capillary blood vessel with such specialized synapses suggests a mechanoreceptor role. In Golgi impregnated and 3D reconstructions of cerebral cortex and thalamus, series of tripartite synapses appear sequentially ordered in a tributary dendrite paralleled by synaptic outgrowths termed here golf club-like extensions apposed to a longitudinal crest from the capillary basal lamina. To facilitate identification of principal cell dendrites and arriving axons to these putative mechanosensory structures, we utilized the cerebellar cortex since it has a well known organization and observed that afferent fibers and interneurons display interactions with the capillary wall. Afferent mossy fiber rosettes and ascending granule cell axons and dendrites define pericapillary on passage interactions surrounded by endfeet. The ability of such structures to modulate synaptic transmission is supported by the presence of mRNA of the mechanosensitive channel Piezo 1 in the mossy fiber rosettes, pyramidal isocortical and thalamic neurons. This suggests that ascending impulses to the cerebellar and cortical targets are presynaptically modulated by the reciprocal interaction with the mechanosensory pericapillary organ.

Retrosplenial area 29e, which was a cortical region described mostly in earlier rodent literature, is often included in the dorsal presubiculum (PrSd) or postsubiculum (PoS) in modern literature and commonly used brain atlases. Recent anatomical and molecular studies have revealed that retrosplenial area 29e belongs to the superficial layers of area prostriata, which in primates is found to be important in fast analysis of quickly moving objects in far peripheral visual field. As in primates, the prostriata in rodents adjoins area 29 (granular retrosplenial area), area 30 (agranular retrosplenial area), medial visual cortex, PrSd-PoS, parasubiculum (PaS) and postrhinal cortex (PoR). The present study aims to reveal the chemo-architecture of the prostriata versus PrSd-PoS or PaS by means of a systematic survey of gene expression patterns in adult and developing mouse brains. First, we find many genes that display differential expression across the prostriata, PrSd-PoS and PaS and that show obvious laminar expression patterns. Second, we reveal subsets of genes that selectively express in the dorsal or ventral parts of the prostriata, suggesting the existence of at least two subdivisions. Third, we detect some genes that shows differential expression in the prostriata of postnatal mouse brains from adjoining regions, thus enabling identification of the developing area prostriata. Fourth, gene expression difference of the prostriata from the medial visual cortex and PoR is also observed. Finally, molecular and connectional features of the prostriata in rodents and non-human primates are discussed and compared.

The evolutionary origin of nervous systems in animals remains elusive and is largely hidden from the fossil record. Ctenophores, one of the earliest-branching animals possessing neurons, are instrumental to our understanding of nervous system origin, and a few rare ctenophore fossils preserve traces of nervous tissue as carbonaceous remains. Cambrian ctenophores appear to exhibit a more diverse neuroanatomy than that of modern species, suggesting secondary loss in extant ctenophores. However, much remains unknown about the origin and ontogeny giving rise to the structural organization of modern ctenophore nervous systems. Here, by investigating the neural anatomy of the ctenophore Mnemiopsis leidyi during development, we identified a ladder-like nerve net (LNN) beneath the comb rows that converges into condensed neurites and connects to the aboral organ. Examination of carbon-rich areas of Ctenorhabdotus capulus, an extinct ctenophore from the Burgess Shale, reveals a pattern similar to that of M. leidyi, consistent with a shared neural organization. Furthermore, M. leidyi exhibits a condensed comb nerve, resembling the longitudinal nerve preserved in the Cambrian ctenophore Fasciculus vesanus and the giant axon of extant Euplokamis dunlapae. Our study reveals conserved evolutionary constraints shaping nervous system architectures linked to locomotory organs and indicates that the different modes of nervous system organization observed in Cambrian ctenophores are variably retained in modern species.