A Drosophila connectome is a list of neurons in the Drosophila melanogaster (fruit fly) nervous system, and the chemical synapses between them. The fly's nervous system consists of the brain plus the ventral nerve cord, and both are known to differ considerably between male and female. [1] [2] Dense connectomes have been completed for the female adult brain, [3] the male nerve cord, [4] and the female larval stage. [5] The available connectomes show only chemical synapses - other forms of inter-neuron communication such as gap junctions or neuromodulators are not represented. Drosophila is the most complex creature with a connectome, which had only been previously obtained for three other simpler organisms, first C. elegans .[ citation needed ] The connectomes have been obtained by the methods of neural circuit reconstruction, which over the course of many years worked up through various subsets of the fly brain to the almost full connectomes that exist today.[ citation needed ]
Connectome research (connectomics) has a number of competing objectives. On the one hand, investigators prefer an organism small enough that the connectome can be obtained in a reasonable amount of time. This argues for a small creature. On the other hand, one of the main uses of a connectome is to relate structure and behavior, so an animal with a large behavioral repertoire is desirable. It's also very helpful to use an animal with a large existing community of experimentalists, and many available genetic tools. Drosophila meets all of these requirements:
The one fully-reconstructed adult female fruit fly brain contains about 128,000 neurons and roughly 50 million chemical synapses, and the single reconstructed male nerve cord has about 23,000 neurons and 70 million synapses. These numbers are not independent, since both the brain and the nerve cord contain portions of the several thousand ascending and descending neurons that run through the neck of the fly. The one female larval brain reconstructed contains roughly 3,000 neurons and 548 thousand chemical synapses. All of these numbers are known to vary between individuals. [8]
Drosophila connectomics started in 1991 with a description of the circuits of the lamina. [9] However the methods used were largely manual and further progress awaited more automated techniques.
In 2011, a high-level connectome, at the level of brain compartments and interconnecting tracts of neurons, for the full fly brain was published, [10] and is available online. [11] New techniques such as digital image processing began to be applied to detailed neural reconstruction. [12]
Reconstructions of larger regions soon followed, including a column of the medulla, [13] also in the visual system of the fruit fly, and the alpha lobe of the mushroom body. [14]
In 2017 a paper introduced an electron microscopy image stack of the whole adult female brain at synaptic resolution. The volume was available for sparse tracing of selected circuits. [15] [16]
In 2020, a dense connectome of half the central brain of Drosophila was released, [17] along with a web site that allows queries and exploration of this data. [18] The methods used in reconstruction and initial analysis of the 'hemibrain' connectome followed. [19]
In 2023, using the data from 2017 (above), the full brain connectome (for a female) was made available, containing roughly 5x10^7 chemical synapses between ~130,000 neurons. [3] A projectome, a map of projections between regions, can be derived from the connectome. In parallel, a consensus cell type atlas for the Drosophila brain was published, produced based on this 'FlyWire' connectome and the prior 'hemibrain'. [20] This resource includes 4,552 cell types: 3,094 as rigorous validations of those previously proposed in the hemibrain connectome; 1,458 new cell types, arising mostly from the fact that the FlyWire connectome spans the whole brain, whereas the hemibrain derives from a subvolume. Comparison of these distinct, adult Drosophila connectomes showed that cell type counts and strong connections were largely stable, but connection weights were surprisingly variable within and across animals.
In 2022, a group of scientists mapped the motor control circuits of the ventral nerve cord of a female fruit fly using electron microscopy. [21] In 2023, a dense reconstruction of the male fly ventral nerve chord was released. [22]
In 2023, Michael Winding et al. published a complete larval brain connectome. [23] [5] This connectome was mapped by annotating the previously collected electron microscopy volume. [24] They found that the larval brain was composed of 3,016 neurons and 548,000 synapses. 93% of brain neurons had a homolog in the opposite hemisphere. Of the synapses, 66.6% were axo-dendritic, 25.8% were axo-axonic, 5.8% were dendro-dendritic, and 1.8% were dendro-axonic.
To study the connectome, they treated it as a directed graph with the neurons forming nodes and the synapses forming the edges. Using this representation, Winding et al found that the larval brain neurons could be clustered into 93 different types, based on connectivity alone. These types aligned with the known neural groups including sensory neurons (visual, olfactory, gustatory, thermal, etc), descending neurons, and ascending neurons.
The authors ordered these neuron types based on proximity to brain inputs vs brain outputs. Using this ordering, they could quantify the proportion of recurrent connections, as the set of connections going from neurons closer to outputs towards inputs. They found that 41% of all brain neurons formed a recurrent connection. The neuron types with the most recurrent connections were the dopaminergic neurons (57%), mushroom body feedback neurons (51%), mushroom body output neurons (45%), and convergence neurons (42%) (receiving input from mushroom body and lateral horn regions). These neurons, implicated in learning, memory, and action-selection, form a set of recurrent loops.
One of the main uses of the Drosophila connectome is to understand the neural circuits and other brain structure that gives rise to behavior. This area is under very active investigation. [25] [26] For example, the fruit fly connectome has been used to identify an area of the fruit fly brain that is involved in odor detection and tracking. Flies choose a direction in turbulent conditions by combining information about the direction of air flow and the movement of odor packets. Based on the fly connectome, processing must occur in the “fan-shaped body” where wind-sensing neurons and olfactory direction-sensing neurons cross. [27] [28]
A natural question is whether the connectome will allow simulation of the fly's behavior. However, the connectome alone is not sufficient. Additional information needed includes gap junction varieties and locations, identities of neurotransmitters, receptor types and locations, neuromodulators and hormones (with sources and receptors), the role of glial cells, time evolution rules for synapses, and more. [29] [30]
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for special senses such as vision, hearing and olfaction. Being the most specialized organ, it is responsible for receiving information from the sensory nervous system, processing those information and the coordination of motor control.
A neuron, neurone, or nerve cell is an excitable cell that fires electric signals called action potentials across a neural network in the nervous system. Neurons communicate with other cells via synapses, which are specialized connections that commonly use minute amounts of chemical neurotransmitters to pass the electric signal from the presynaptic neuron to the target cell through the synaptic gap.
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates, it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers, or axons, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor nerves (efferent), while those nerves that transmit information from the body to the CNS are called sensory nerves (afferent). The PNS is divided into two separate subsystems, the somatic and autonomic, nervous systems. The autonomic nervous system is further subdivided into the sympathetic, parasympathetic and enteric nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Nerves that exit from the brain are called cranial nerves while those exiting from the spinal cord are called spinal nerves.
A motor neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.
Drosophila melanogaster is a species of fly in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly", or "banana fly". In the wild, D. melanogaster are attracted to rotting fruit and fermenting beverages, and are often found in orchards, kitchens and pubs.
The development of the nervous system, or neural development (neurodevelopment), refers to the processes that generate, shape, and reshape the nervous system of animals, from the earliest stages of embryonic development to adulthood. The field of neural development draws on both neuroscience and developmental biology to describe and provide insight into the cellular and molecular mechanisms by which complex nervous systems develop, from nematodes and fruit flies to mammals.
Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defined nervous systems. Their neuroanatomy is therefore better understood. In vertebrates, the nervous system is segregated into the internal structure of the brain and spinal cord and the series of nerves that connect the CNS to the rest of the body. Breaking down and identifying specific parts of the nervous system has been crucial for figuring out how it operates. For example, much of what neuroscientists have learned comes from observing how damage or "lesions" to specific brain areas affects behavior or other neural functions.
Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system and the peripheral nervous system that do not produce electrical impulses. The neuroglia make up more than one half the volume of neural tissue in the human body. They maintain homeostasis, form myelin in the peripheral nervous system, and provide support and protection for neurons. In the central nervous system, glial cells include oligodendrocytes, astrocytes, ependymal cells and microglia, and in the peripheral nervous system they include Schwann cells and satellite cells.
In vertebrates, a neuroblast or primitive nerve cell is a postmitotic cell that does not divide further, and which will develop into a neuron after a migration phase. In invertebrates such as Drosophila, neuroblasts are neural progenitor cells which divide asymmetrically to produce a neuroblast, and a daughter cell of varying potency depending on the type of neuroblast. Vertebrate neuroblasts differentiate from radial glial cells and are committed to becoming neurons. Neural stem cells, which only divide symmetrically to produce more neural stem cells, transition gradually into radial glial cells. Radial glial cells, also called radial glial progenitor cells, divide asymmetrically to produce a neuroblast and another radial glial cell that will re-enter the cell cycle.
A neural circuit is a population of neurons interconnected by synapses to carry out a specific function when activated. Multiple neural circuits interconnect with one another to form large scale brain networks.
Brain mapping is a set of neuroscience techniques predicated on the mapping of (biological) quantities or properties onto spatial representations of the brain resulting in maps.
The ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, integrating sensory input and locomotor output. Because arthropods have an open circulatory system, decapitated insects can still walk, groom, and mate — illustrating that the circuitry of the ventral nerve cord is sufficient to perform complex motor programs without brain input.
Neuromorphology is the study of nervous system form, shape, and structure. The study involves looking at a particular part of the nervous system from a molecular and cellular level and connecting it to a physiological and anatomical point of view. The field also explores the communications and interactions within and between each specialized section of the nervous system. Morphology is distinct from morphogenesis. Morphology is the study of the shape and structure of biological organisms, while morphogenesis is the study of the biological development of the shape and structure of organisms. Therefore, neuromorphology focuses on the specifics of the structure of the nervous system and not the process by which the structure was developed. Neuromorphology and morphogenesis, while two different entities, are nonetheless closely linked.
Brainbow is a process by which individual neurons in the brain can be distinguished from neighboring neurons using fluorescent proteins. By randomly expressing different ratios of red, green, and blue derivatives of green fluorescent protein in individual neurons, it is possible to flag each neuron with a distinctive color. This process has been a major contribution to the field of neural connectomics.
A connectome is a comprehensive map of neural connections in the brain, and may be thought of as its "wiring diagram". An organism's nervous system is made up of neurons which communicate through synapses. A connectome is constructed by tracing the neuron in a nervous system and mapping where neurons are connected through synapses.
Connectomics is the production and study of connectomes: comprehensive maps of connections within an organism's nervous system. More generally, it can be thought of as the study of neuronal wiring diagrams with a focus on how structural connectivity, individual synapses, cellular morphology, and cellular ultrastructure contribute to the make up of a network. The nervous system is a network made of billions of connections and these connections are responsible for our thoughts, emotions, actions, memories, function and dysfunction. Therefore, the study of connectomics aims to advance our understanding of mental health and cognition by understanding how cells in the nervous system are connected and communicate. Because these structures are extremely complex, methods within this field use a high-throughput application of functional and structural neural imaging, most commonly magnetic resonance imaging (MRI), electron microscopy, and histological techniques in order to increase the speed, efficiency, and resolution of these nervous system maps. To date, tens of large scale datasets have been collected spanning the nervous system including the various areas of cortex, cerebellum, the retina, the peripheral nervous system and neuromuscular junctions.
In the field of computational neuroscience, brain simulation is the concept of creating a functioning computer model of a brain or part of a brain. Brain simulation projects intend to contribute to a complete understanding of the brain, and eventually also assist the process of treating and diagnosing brain diseases. Simulations utilize mathematical models of biological neurons, such as the hodgkin-huxley model, to simulate the behavior of neurons, or other cells within the brain.
Neural circuit reconstruction is the reconstruction of the detailed circuitry of the nervous system of an animal. It is sometimes called EM reconstruction since the main method used is the electron microscope (EM). This field is a close relative of reverse engineering of human-made devices, and is part of the field of connectomics, which in turn is a sub-field of neuroanatomy.
Gregory Stephen Xavier Edward Jefferis is a British neuroscientist known for his work on the circuit basis of olfactory perception in the vinegar fly, Drosophila melanogaster. He is a tenured Programme Leader at the MRC Laboratory of Molecular Biology in Cambridge (UK) and associated with the Department of Zoology at the University of Cambridge.
A descending neuron is a neuron that conveys signals from the brain to neural circuits in the spinal cord (vertebrates) or ventral nerve cord (invertebrates). As the sole conduits of information between the brain and the body, descending neurons play a key role in behavior. Their activity can initiate, maintain, modulate, and terminate behaviors such as locomotion. Because the number of descending neurons is several orders of magnitude smaller than the number of neurons in either the brain or spinal cord/ventral nerve cord, this class of cells represents a critical bottleneck in the flow of information from sensory systems to motor circuits.
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