41-60 of 184 Results


Deep Neural Networks in Computational Neuroscience  

Tim C. Kietzmann, Patrick McClure, and Nikolaus Kriegeskorte

The goal of computational neuroscience is to find mechanistic explanations of how the nervous system processes information to give rise to cognitive function and behavior. At the heart of the field are its models, that is, mathematical and computational descriptions of the system being studied, which map sensory stimuli to neural responses and/or neural to behavioral responses. These models range from simple to complex. Recently, deep neural networks (DNNs) have come to dominate several domains of artificial intelligence (AI). As the term “neural network” suggests, these models are inspired by biological brains. However, current DNNs neglect many details of biological neural networks. These simplifications contribute to their computational efficiency, enabling them to perform complex feats of intelligence, ranging from perceptual (e.g., visual object and auditory speech recognition) to cognitive tasks (e.g., machine translation), and on to motor control (e.g., playing computer games or controlling a robot arm). In addition to their ability to model complex intelligent behaviors, DNNs excel at predicting neural responses to novel sensory stimuli with accuracies well beyond any other currently available model type. DNNs can have millions of parameters, which are required to capture the domain knowledge needed for successful task performance. Contrary to the intuition that this renders them into impenetrable black boxes, the computational properties of the network units are the result of four directly manipulable elements: input statistics, network structure, functional objective, and learning algorithm. With full access to the activity and connectivity of all units, advanced visualization techniques, and analytic tools to map network representations to neural data, DNNs represent a powerful framework for building task-performing models and will drive substantial insights in computational neuroscience.


Developmental Neurobiology of Anxiety and Related Disorders  

Emily M. Cohodes and Dylan G. Gee

The majority of anxiety disorders emerge during childhood and adolescence, a developmental period characterized by dynamic changes in frontolimbic circuitry. Frontolimbic circuitry plays a key role in fear learning and has been a focus of recent efforts to understand the neurobiological correlates of anxiety disorders across development. Although less is known about the neurobiological underpinnings of anxiety disorders in youth than in adults, studies of pediatric anxiety have revealed alterations in both the structure and function of frontolimbic circuitry. The amygdala, prefrontal cortex (PFC), anterior cingulate cortex (ACC), and hippocampus contribute to fear conditioning and extinction, and interactions between these regions have been implicated in anxiety during development. Specifically, children and adolescents with anxiety disorders show altered amygdala volumes and exhibit heightened amygdala activation in response to neutral and fearful stimuli, with the magnitude of signal change in amygdala reactivity corresponding to the severity of symptomatology. Abnormalities in the PFC and ACC and their connections with the amygdala may reflect weakened top-down control or compensatory efforts to regulate heightened amygdala reactivity associated with anxiety. Taken together, alterations in frontolimbic connectivity are likely to play a central role in the etiology and maintenance of anxiety disorders. Future studies should aim to translate the emerging understanding of the neurobiological bases of pediatric anxiety disorders to optimize clinical interventions for youth.


Development of Lung Innervation  

Talita de Melo e Silva, Catherine Miriam Czeisler, and José Javier Otero

Breathing is essential for survival and is precisely regulated by the nervous system. From a neuroanatomical perspective, the respiratory tract is innervated by afferent and efferent autonomic nerves, which regulate aspects of airway function and ensure appropriate tissue oxygenation. The general concepts of how the peripheral nervous system (PNS) develops as it relates to lung function are reviewed. The vagus (cranial nerve X), a mixed motor and sensory nerve, supplies parasympathetic and sensory fibers to the airways. During development, preganglionic visceromotor efferent neurons of the cranial nerves arise in the hindbrain basal plate and later migrate dorsally through the neuroepithelium. The neural crest is a migratory and multipotent embryonic cell population that develops at the dorsal portion of the neural tube, which delaminates from the neuroepithelium to enter distinct pathways, forming various derivatives, among which include the peripheral nervous system. Neural crest cells emerging from the vagal region migrate into the ventral foregut and give rise to intrinsic ganglia in the respiratory tract that are innervated from the vagus and send out postganglionic fibers. The lung is innervated by sympathetic nerves derived from the upper thoracic and cervical ganglia. The sympathetic preganglionic neurons are derived from trunk neural crest cells that migrate, forming two chains of sympathetic ganglia referred to as the lateral vertebral sympathetic chains. Neural crest cells that migrate along defined pathways to generate sympathetic ganglia also derivate the dorsal root ganglia that send somatosensory afferent innervations to the respiratory tract.


Diagnosis and Treatment of Gambling Addiction  

Gemma Mestre-Bach and Marc N. Potenza

Gambling disorder (GD) is a relatively rare psychiatric concern that may carry substantial individual, familial, and societal harms. GD often presents complex challenges, with high prevalence in adolescents and young adults. GD often co-occurs with other psychiatric disorders, complicating treatment. GD has multiple biopsychosocial contributions, with genetic, environmental, and psychological factors implicated. Advances in neuroimaging and neurochemistry offer insights into the neurobiology of GD. GD diagnostic criteria have evolved, although identification often remains challenging given shame, stigma, ambivalence regarding treatment and limited screening. Because many people with GD do not receive treatment, identification (screening and treatment outreach) and therapeutic (behavioral, neuromodulatory, and pharmacological) approaches warrant increased consideration and development..


Drosophila Olfaction  

Quentin Gaudry and Jonathan Schenk

Olfactory systems are tasked with converting the chemical environment into electrical signals that the brain can use to optimize behaviors such as navigating towards resources, finding mates, or avoiding danger. Drosophila melanogaster has long served as a model system for several attributes of olfaction. Such features include sensory coding, development, and the attempt to link sensory perception to behavior. The strength of Drosophila as a model system for neurobiology lies in the myriad of genetic tools made available to the experimentalist, and equally importantly, the numerical reduction in cell numbers within the olfactory circuit. Modern techniques have recently made it possible to target nearly all cell types in the antennal lobe to directly monitor their physiological activity or to alter their expression of endogenous proteins or transgenes.


Drosophila Reward Circuits  

John S. Hernandez, Tariq M. Brown, and Karla R. Kaun

The ability to sense and respond to a rewarding stimulus is a key advantage for animals in their natural environment. The circuits that mediate these responses are complex, and it has been difficult to identify the fundamental principles of reward structure and function. However, the well-characterized brain anatomy and sophisticated neurogenetic tools in Drosophila melanogaster make the fly an ideal model to understand the mechanisms through which reward is encoded. Drosophila find food, water, intoxicating substances, and social acts rewarding. Basic monoaminergic neurotransmitters, including dopamine (DA), serotonin (5-HT), and octopamine (OA), play a central role in encoding these rewards. DA is central to sensing, encoding, responding, and predicting reward, whereas 5-HT and OA carry information about the environment that influences DA circuit activity. In contrast, slower-acting neuromodulators such as hormones and neuropeptides play a key role in both encoding the pleasurable stimulus and modulating how the internal environment of the fly influences reward sensation and seeking. Recurring circuit motifs for reward signaling identified in Drosophila suggest that these key principles will help elucidate understanding of how reward circuits function in all animals.


Echinoderm Nervous System  

Carlos A. Díaz-Balzac and José E. García-Arrarás

The nervous system of echinoderms has been studied for well over a century. Nonetheless, the information available is disparate, with in-depth descriptions for the nervous component of some groups or of particular organs while scant data is available for others. The best studied representatives to date are the nervous system of echinoid embryos and larva, and the adult holothurian nervous system. Although described sometimes inaccurately as a neural net, the echinoderm nervous system consists of well-defined neural structures. This is observed since early embryogenesis when activation of the anterior neuroectoderm gene regulatory networks initiate the formation of the embryonic nervous system. This system then undergoes expansion and differentiation to form the larval nervous system, which is centered on the ciliary bands. This “simpler” nervous system is then metamorphosed into the adult echinoderm nervous system. The adult echinoderm nervous system is composed of a central nervous system made up of a nerve ring connected to a series of radial nerve cords. Peripheral nerves extending from the radial nerve cords or nerve ring connect with the peripheral nervous system, located in other organs or effectors including the viscera, podia, body wall muscles, and connective tissue. Both the central and peripheral nervous systems are composed of complex and diverse subdivisions. These are mainly characterized by the expression of neurotransmitters, namely acetylcholine, catecholamines, histamine, amino acids, GABA, and neuropeptides. Other areas of interest include the amazing regenerative capabilities of echinoderms that have been shown to be able to regenerate their nervous system components; and the analysis of the echinoderm genome that has provided essential insights into the molecular basis of how echinoderms develop an adult pentaradial symmetry from bilaterally symmetric larvae and the role of the nervous system in this process.


Electrophysiology and Behavior of Cnidarian Nervous Systems  

Robert W. Meech

Although the Cnidaria have evolved a wide range of body forms matched with an equally varied neural anatomy, individual species exhibit common patterns of behavior. For example, in all species a key challenge for the nervous system is to transfer food from the peripherally mounted tentacles to the centrally located stomach. Foraging movements, necessary to maintain the food supply, must be accomplished in such a way as to avoid interference with the primary objective of getting prey into the mouth. Furthermore, the hunt for prey must be balanced by a measured response to “threat.” Different species respond to threat in markedly different ways, but in each case foraging is inhibited, just as it is during transmission of food. One hundred years ago, G. H. Parker questioned whether a centralized or a locally organized nervous system could best account for sea anemone behavior. Anatomical and electrophysiological studies now suggest that in most Cnidaria there is a degree of hierarchical control, with local reflexes coordinated by more condensed systems of neurons. This organization is highly developed in the nerve rings of hydrozoan medusae and takes the form of ganglion-like rhopalia in the Cubozoa. Even in hydrozoan polyps such as Hydra there are at least four separate neuronal systems. It is likely that the underlying mechanisms (containing both homologous and analogous elements) will be best revealed by a comparative approach that directly relates behavior with its molecular basis. Useful examples include comparisons between sea anemones with and without through-conducting systems; between hydra with and without oral rings; between medusae with and without coordinated escape swimming. Recent advances in transgenomic labeling have shown the way forward.


Enhancing the Regeneration of Neurons in the Central Nervous System  

Romain Cartoni, Frank Bradke, and Zhigang He

Injured axons fail to regenerate in the adult mammalian central nervous system, representing a major barrier for effective neural repair. Both extrinsic inhibitory environments and neuron-intrinsic mechanisms contribute to such regeneration failure. In the past decade, there has been an explosion in our understanding of neuronal injury responses and regeneration regulations. As a result, several strategies have been developed to promote axon regeneration with the potential of restoring functions after injury. This article will highlight these new developments, with an emphasis on cellular and molecular mechanisms from a neuron-centric perspective, and discuss the challenges to be addressed toward developing effective functional restoration strategies.


Evolution, Homology, Cell Classification, and Parallel Processing for Vision  

W. Martin Usrey and S. Murray Sherman

A first step in analyzing complex systems is a classification of component elements. This applies to retinal organization as well as to other circuit components in the visual system. There is great variety in the types of retinal ganglion cells and the targets of their axonal projections. Thus, a prerequisite to any deep understanding of the early visual system is developing a proper classification of its elements. How many distinct classes of retinal ganglion cells are there? Can the main classes be broken down into subclasses? What sort of functional correlates can be established for each class? Can homologous relationships between apparently similar classes between species be established? Can a common nomenclature based on homologous cell and circuit classes be developed?


Evolution of Neocortex for Sensory Processing  

Jon H. Kaas

The neocortex is a part of the forebrain of mammals that is an innovation of mammal-like “reptilian” synapsid ancestors of early mammals. This neocortex emerged from a small region of dorsal cortex that was present in earlier ancestors and is still found in the forebrain of present-day reptiles. Instead of the thick structure of six layers of cells (five layers) and fibers (one layer) of neocortex of mammals, the dorsal cortex was characterized by a single layer of pyramidal neurons and a scattering of small, largely inhibitory neurons. In reptiles, the dorsal cortex is dominated by visual inputs, with outputs that relate to behavior and memory. The thicker neocortex of six layers in early mammals was already divided into a number of functionally specialized zones called cortical areas that were predominantly sensory in function, while relating to important aspects of motor behavior via subcortical projections. These early sensorimotor areas became modified in various ways as different branches of the mammalian radiation evolved, and neocortex often increased in size and the number of cortical areas, likely by the process of specializations within areas that subdivided areas. At least some areas, perhaps most, subdivided in another way by evolving two or more alternating types of small regions of different functional specializations, now referred to as cortical modules or columns. The specializations within and across cortical areas included those in the sizes of neurons and the extents of their processes, the dendrites and axons, and thus connections with other neurons. As a result, the neocortex of present-day mammals varies greatly within and across phylogenetically related groups (clades), while retaining basic features of organization from early ancestral mammals. In a number of present-day (extant) mammals, brains are relatively small and have little neocortex, with few areas and little structural differentiation, thus resembling early mammals. Other small mammals with little neocortex have specialized some part via selective enlargement and structural modifications to promote certain sensory abilities. Other mammals have a neocortex that is moderately to greatly expanded, with more cortical areas directly related to sensory processing and cognition and memory. The human brain is extreme in this way by having more neocortex in proportion to the rest of the brain, more cortical neurons, and likely more cortical areas.


The Functional Organization of Vertebrate Retinal Circuits for Vision  

Tom Baden, Timm Schubert, Philipp Berens, and Thomas Euler

Visual processing begins in the retina—a thin, multilayered neuronal tissue lining the back of the vertebrate eye. The retina does not merely read out the constant stream of photons impinging on its dense array of photoreceptor cells. Instead it performs a first, extensive analysis of the visual scene, while constantly adapting its sensitivity range to the input statistics, such as the brightness or contrast distribution. The functional organization of the retina abides to several key organizational principles. These include overlapping and repeating instances of both divergence and convergence, constant and dynamic range-adjustments, and (perhaps most importantly) decomposition of image information into parallel channels. This is often referred to as “parallel processing.” To support this, the retina features a large diversity of neurons organized in functionally overlapping microcircuits that typically uniformly sample the retinal surface in a regular mosaic. Ultimately, each circuit drives spike trains in the retina’s output neurons, the retinal ganglion cells. Their axons form the optic nerve to convey multiple, distinctive, and often already heavily processed views of the world to higher visual centers in the brain. From an experimental point of view, the retina is a neuroscientist’s dream. While part of the central nervous system, the retina is largely self-contained, and depending on the species, it receives little feedback from downstream stages. This means that the tissue can be disconnected from the rest of the brain and studied in a dish for many hours without losing its functional integrity, all while retaining excellent experimental control over the exclusive natural network input: the visual stimulus. Once removed from the eyecup, the retina can be flattened, thus its neurons are easily accessed optically or using visually guided electrodes. Retinal tiling means that function studied at any one place can usually be considered representative for the entire tissue. At the same time, species-dependent specializations offer the opportunity to study circuits adapted to different visual tasks: for example, in case of our fovea, high-acuity vision. Taken together, today the retina is amongst the best understood complex neuronal tissues of the vertebrate brain.


Functional Specialization Across the Visual Cortex  

Erez Freud, Tzvi Ganel, and Galia Avidan

Vision is the most important sensory modality for humans, serving a range of fundamental daily behaviors from recognizing objects, people, places, and actions to navigation and visually guided interactions with objects and other individuals. One of the most prominent accounts of cortical functional specialization implies that the visual cortex is segregated into two pathways. The ventral pathway originates from the early visual cortex in the occipital lobe and projects to the inferior surface of the temporal cortex, and it mediates vision for perception. The dorsal pathway extends from the occipital lobe to the posterior portion of the parietal cortex, and it mediates vision for action. This key characterization of the visual system is supported by classic neuropsychological, behavioral, and neuroimaging evidence. Recent research offers new insights on the developmental trajectory of this dissociation as well as evidence for interactions between the two pathways. Importantly, an emerging hypothesis points to the existence of a third visual pathway located on the lateral surface of the ventral pathway and its potential roles in action recognition and social cognition.


Gastropod Feeding Systems: Evolution of Neural Circuits that Generate Diverse Behaviors  

Paul Benjamin and Michael Crossley

It is conceptually reasonable to explore how the evolution of behavior involves changes in neural circuitry. Progress in determining this evolutionary relationship has been limited in neuroscience because of difficulties in identifying individual neurons that contribute to the evolutionary development of behaviors across species. However, the results from the feeding systems of gastropod mollusks provide evidence for this concept of co-evolution because the evolution of different types of feeding behaviors in this diverse group of mollusks is mirrored by species-specific changes in neural circuitry. The evolution of feeding behaviors involves changes in the motor actions that allow diverse food items to be acquired and ingested. The evolution in neural control accompanies this variation in food and the associated changes in flexibility of feeding behaviors. This is present in components of the feeding network that are involved in decision making, rhythm generation, and behavioral switching but is absent in background mechanisms that are conserved across species, such as those controlling arousal state. These findings show how evolutionary changes, even at the single neuron level, closely reflect the details of behavioral evolution.


Gastropod Learning and Memory (Aplysia, Hermissenda, Lymnaea, and Others)  

Alexis Bédécarrats and Romuald Nargeot

Euopisthobranchia (Aplysia), Nudipleura (Tritonia, Hermissenda, Pleurobranchaea), and Panpulmonata (Lymnaea, Helix, Limax) gastropod mollusks exhibit a variety of reflex, rhythmic, and motivated behaviors that can be modified by elementary forms of learning and memory. The relative simplicity of their nervous systems and behavioral repertoires has allowed the uncovering of processes of neuronal and synaptic plasticity underlying non-associative learning, such as habituation, sensitization, and different forms of associative learning, such as classical and operant conditioning. Decades of work on these simpler and accessible animal systems have almost uniquely yielded an understanding into the mechanistic basis of learning and memory spanning behavior, neuronal circuitry, and molecules. Given the conservative nature of evolutionary processes, the mechanisms deciphered have also provided valuable insights into the neural basis of learning and memory in other metazoans, including higher vertebrates.


General Principles for Sensory Coding  

Tatyana O. Sharpee

Sensory systems exist to provide an organism with information about the state of the environment that can be used to guide future actions and decisions. Remarkably, two conceptually simple yet general theorems from information theory can be used to evaluate the performance of any sensory system. One theorem states that there is a minimal amount of energy that an organism has to spend in order to capture a given amount of information about the environment. The second theorem states that the maximum rate with which the organism can acquire resources from the environment, relative to its competitors, is limited by the information this organism collects about the environment, also relative to its competitors. These two theorems provide a scaffold for formulating and testing general principles of sensory coding but leave unanswered many important practical questions of implementation in neural circuits. These implementation questions have guided thinking in entire subfields of sensory neuroscience, and include: What features in the sensory environment should be measured? Given that we make decisions on a variety of time scales, how should one solve trade-offs between making simpler measurements to guide minimal decisions vs. more elaborate sensory systems that have to overcome multiple delays between sensation and action. Once we agree on the types of features that are important to represent, how should they be represented? How should resources be allocated between different stages of processing, and where is the impact of noise most damaging? Finally, one should consider trade-offs between implementing a fixed strategy vs. an adaptive scheme that readjusts resources based on current needs. Where adaptation is considered, under what conditions does it become optimal to switch strategies? Research over the past 60 years has provided answers to almost all of these questions but primarily in early sensory systems. Joining these answers into a comprehensive framework is a challenge that will help us understand who we are and how we can make better use of limited natural resources.


Genetics and Evolution of Color Vision in Primates  

Gerald H. Jacobs

Color is a central feature of human perceptual experience where it functions as a critical component in the detection, identification, evaluation, placement, and appreciation of objects in the visual world. Its role is significantly enhanced by the fact that humans evolved a dimension of color vision beyond that available to most other mammals. Many fellow primates followed a similar path and in recent years the basic mechanisms that support color vision—the opsin genes, photopigments, cone signals, and central processing—have been the subjects of hundreds of investigations. Because of the tight linkage between opsin gene structure and the spectral sensitivity of cone photopigments, it is possible to trace pathways along which color vision may have evolved in primates. In turn, such information allows the development of hypotheses about the nature of color vision and its utility in nonhuman primates. These hypotheses are being critically evaluated in field studies where primates solve visual problems in the presence of the full panoply of photic cues. The intent of this research is to determine which aspects of these cues are critically linked to color vision and how their presence facilitates, impedes, or fails to influence the solutions. These investigations are challenging undertakings and the emerging literature is replete with contradictory conclusions. But steady progress is being made and it appears that (a) some of the original ideas about there being a restricted number of tasks for which color vision might be optimally utilized by nonhuman primates (e. g., fruit harvest) were too simplistic and (b) depending on circumstances that can include both features of proximate visual stimuli (spectral cues, luminance cues, size cues, motion cues, overall light levels) and situational variables (social cues, developmental status, species-specific traits) the utilization of color vision by nonhuman primates is apt to be complex and varied.


Genetics of C. elegans Behavior  

Denise S. Walker, Yee Lian Chew, and William R. Schafer

The nematode Caenorhabditis elegans is among the most intensely studied animals in modern experimental biology. Its amenability to classical and molecular genetics, compact nervous system, and transparency to optogenetic recording and manipulation have led to its being widely used to investigate how individual gene products act in the context of neuronal circuits to generate behavior. C. elegans was the first animal neuronal connectome to be characterized at the level of individual neurons and synapses, and the wiring organization shows significant parallels with the micro- and macro-level structures of more complex brains. It uses a wide array of sensory cues (mechanical, gustatory, olfactory, and thermal), with impressive precision, to determine an appropriate behavioral response, in the form of changes in locomotion and other motor outputs. The small number of neurons (302) and their highly stereotyped morphology have enabled the interrogation of the precise function of individual neurons, in terms of both the genes that they express and the behaviors in which they function. Approximately one third are sensory neurons, located around the mouth and in the tail, as well as along the length of the body. A complex network of interneurons connects these to the motor neurons, the majority of which output to the body wall muscles, arranged along the body, determining the changes in direction and speed of locomotion required for behaviors such as taxiing up gradients of food-related cues or avoiding noxious cues. The ability to combine information about the precise function of individual neurons with circuit knowledge, genetics, and optogenetic approaches has been critical in advancing the molecular and circuit-level understanding of the mechanisms underlying C. elegans’s behavior. In common with that of other animals, C. elegans’s behavior is subject to change in response to prior experience, and this same complement of experimental approaches has provided significant insight into how plasticity and changes in behavioral states are achieved.


Gut Dysbiosis and Recovery of Function After Spinal Cord Injury  

Kristina A. Kigerl and Phillip G. Popovich

Spinal cord injury (SCI) disrupts the autonomic nervous system (ANS) and impairs communication with organ systems throughout the body, resulting in chronic multi-organ pathology and dysfunction. This dysautonomia contributes to the pronounced immunosuppression and gastrointestinal dysfunction seen after SCI. All of these factors likely contribute to the development of gut dysbiosis after SCI—an imbalance in the composition of the gut microbiota that can impact the development and progression of numerous pathological conditions, including SCI. The gut microbiota are the community of microbes (bacteria, viruses, fungi) that live in the GI tract and are critical for nutrient absorption, digestion, and immune system development. These microbes also communicate with the CNS through modulation of the immune system, production of neuroactive metabolites and neurotransmitters, and activation of the vagus nerve. After SCI, gut dysbiosis develops and persists for more than one year from the time of injury. In experimental models of SCI, gut dysbiosis is correlated with changes in inflammation and functional recovery. Moreover, probiotic treatment can improve locomotor recovery and immune function in the gut-associated lymphoid tissue (GALT). Since different types of bacteria produce different metabolites with unique physiological and pathological effects throughout the body, it may be possible to predict the prevalence or severity of post-injury immune dysfunction and other related comorbidities (e.g., metabolic disease, fatigue, anxiety) using microbiome sequencing data. As research identifies microbial-derived small molecules and the genes responsible for their production, it is likely that it will become feasible to manipulate these molecules to affect human biology and disease.


Hemichordate Nervous System  

Norio Miyamoto and Hiroshi Wada

Hemichordates are marine invertebrates consisting of two distinct groups: the solitary enteropneusts and the colonial pterobranchs. Hemichordates are phylogenetically a sister group to echinoderm composing Ambulacraria. The adult morphology of hemichordates shares some features with chordates. For that reason, hemichordates have been considered key organisms to understand the evolution of deuterostomes and the origin of the chordate body plan. The nervous system of hemichordates is also important in the discussion of the origin of centralized nervous systems. However, unlike other deuterostomes, such as echinoderms and chordates, information on the nervous system of hemichordates is limited. Recent improvements in the accessibility of embryos, development of functional tools, and genomic resources from several model organisms have provided essential information on the nervous system organization and neurogenesis in hemichordates. The comparison of the nervous system between hemichordates and other bilaterians helps to elucidate the origin of the chordate central nervous system. Extant hemichordates are divided into two groups: enteropneusts and pterobranchs. The nervous system of adult enteropneusts consists of nerve cords and the basiepidermal nerve net. The two nerve cords run along the dorsal and ventral midlines. The dorsal nerve cord forms a tubular structure in the collar region. The two nerve cords are connected through the prebranchial nerve ring. The larval nervous system of enteropneusts develops along the ciliary band and there is a ganglion at the anterior end of the body called the apical ganglion. A pair of pigmented eyespots is situated at the lateral side of the apical ganglion. The adult nervous system of pterobranchs is basiepidermal and there are several condensations of plexuses. The most prominent one is the brain, located at the base of the tentaculated arms. From the brain, small fibers radiate and enter tentaculated arms to form a tentacle nerve in each. There is a basiepidermal nerve cord in the ventral midline of the trunk.