brain brain a brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. it is located in the head, usually close to the sensory organs for senses such as vision. it is the most complex organ in a vertebrate's body. in a human, the cerebral cortex contains approximately 14–16 billion neurons, and the estimated number of neurons in the cerebellum is 55–70 billion. each neuron is connected by synapses to several thousand other neurons. these neurons typically communicate with one another by means of long fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells. this article is about the brains of all types of animals, including humans. for information specific to the human brain, see human brain. for other uses, see brain (disambiguation) and brains (disambiguation). brainthe brain of a chimpanzeeidentifiersmeshd001921neuronames21ta98a14.1.03.001ta25415anatomical terminology physiologically, brains exert centralized control over a body's other organs. they act on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. this centralized control allows rapid and coordinated responses to changes in the environment. some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain. the operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. recent models in modern neuroscience treat the brain as a biological computer, very different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, and processes it in a variety of ways. this article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. it deals with the human brain insofar as it shares the properties of other brains. the ways in which the human brain differs from other brains are covered in the human brain article. several topics that might be covered here are instead covered there because much more can be said about them in a human context. the most important that are covered in the human brain article are brain disease and the effects of brain damage. anatomy cross section of the olfactory bulb of a rat, stained in two different ways at the same time: one stain shows neuron cell bodies, the other shows receptors for the neurotransmitter gaba. the shape and size of the brain varies greatly between species, and identifying common features is often difficult. nevertheless, there are a number of principles of brain architecture that apply across a wide range of species. some aspects of brain structure are common to almost the entire range of animal species; others distinguish "advanced" brains from more primitive ones, or distinguish vertebrates from invertebrates. the simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, and then sliced apart for examination of the interior. visually, the interior of the brain consists of areas of so-called grey matter, with a dark color, separated by areas of white matter, with a lighter color. further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations. it is also possible to examine the microstructure of brain tissue using a microscope, and to trace the pattern of connections from one brain area to another. cellular structure neurons generate electrical signals that travel along their axons. when a pulse of electricity reaches a junction called a synapse, it causes a neurotransmitter chemical to be released, which binds to receptors on other cells and thereby alters their electrical activity. the brains of all species are composed primarily of two broad classes of cells: neurons and glial cells. glial cells (also known as glia or neuroglia) come in several types, and perform a number of critical functions, including structural support, metabolic support, insulation, and guidance of development. neurons, however, are usually considered the most important cells in the brain. the property that makes neurons unique is their ability to send signals to specific target cells over long distances. they send these signals by means of an axon, which is a thin protoplasmic fiber that extends from the cell body and projects, usually with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body. the length of an axon can be extraordinary: for example, if a pyramidal cell (an excitatory neuron) of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon, equally magnified, would become a cable a few centimeters in diameter, extending more than a kilometer. these axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. some neurons emit action potentials constantly, at rates of 10–100 per second, usually in irregular patterns; other neurons are quiet most of the time, but occasionally emit a burst of action potentials. axons transmit signals to other neurons by means of specialized junctions called synapses. a single axon may make as many as several thousand synaptic connections with other cells. when an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. the neurotransmitter binds to receptor molecules in the membrane of the target cell. synapses are the key functional elements of the brain. the essential function of the brain is cell-to-cell communication, and synapses are the points at which communication occurs. the human brain has been estimated to contain approximately 100 trillion synapses; even the brain of a fruit fly contains several million. the functions of these synapses are very diverse: some are excitatory (exciting the target cell); others are inhibitory; others work by activating second messenger systems that change the internal chemistry of their target cells in complex ways. a large number of synapses are dynamically modifiable; that is, they are capable of changing strength in a way that is controlled by the patterns of signals that pass through them. it is widely believed that activity-dependent modification of synapses is the brain's primary mechanism for learning and memory. most of the space in the brain is taken up by axons, which are often bundled together in what are called nerve fiber tracts. a myelinated axon is wrapped in a fatty insulating sheath of myelin, which serves to greatly increase the speed of signal propagation. (there are also unmyelinated axons). myelin is white, making parts of the brain filled exclusively with nerve fibers appear as light-colored white matter, in contrast to the darker-colored grey matter that marks areas with high densities of neuron cell bodies. generic bilaterian nervous system nervous system of a generic bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front except for a few primitive organisms such as sponges (which have no nervous system) and cnidarians (which have a nervous system consisting of a diffuse nerve net), all living multicellular animals are bilaterians, meaning animals with a bilaterally symmetric body shape (that is, left and right sides that are approximate mirror images of each other). all bilaterians are thought to have descended from a common ancestor that appeared late in the cryogenian period, 700–650 million years ago, and it has been hypothesized that this common ancestor had the shape of a simple tubeworm with a segmented body. at a schematic level, that basic worm-shape continues to be reflected in the body and nervous system architecture of all modern bilaterians, including vertebrates. the fundamental bilateral body form is a tube with a hollow gut cavity running from the mouth to the anus, and a nerve cord with an enlargement (a ganglion) for each body segment, with an especially large ganglion at the front, called the brain. the brain is small and simple in some species, such as nematode worms; in other species, including vertebrates, it is the most complex organ in the body. some types of worms, such as leeches, also have an enlarged ganglion at the back end of the nerve cord, known as a "tail brain". there are a few types of existing bilaterians that lack a recognizable brain, including echinoderms and tunicates. it has not been definitively established whether the existence of these brainless species indicates that the earliest bilaterians lacked a brain, or whether their ancestors evolved in a way that led to the disappearance of a previously existing brain structure. invertebrates fruit flies (drosophila) have been extensively studied to gain insight into the role of genes in brain development. this category includes tardigrades, arthropods, molluscs, and numerous types of worms. the diversity of invertebrate body plans is matched by an equal diversity in brain structures. two groups of invertebrates have notably complex brains: arthropods (insects, crustaceans, arachnids, and others), and cephalopods (octopuses, squids, and similar molluscs). the brains of arthropods and cephalopods arise from twin parallel nerve cords that extend through the body of the animal. arthropods have a central brain, the supraesophageal ganglion, with three divisions and large optical lobes behind each eye for visual processing. cephalopods such as the octopus and squid have the largest brains of any invertebrates. there are several invertebrate species whose brains have been studied intensively because they have properties that make them convenient for experimental work: fruit flies (drosophila), because of the large array of techniques available for studying their genetics, have been a natural subject for studying the role of genes in brain development. in spite of the large evolutionary distance between insects and mammals, many aspects of drosophila neurogenetics have been shown to be relevant to humans. the first biological clock genes, for example, were identified by examining drosophila mutants that showed disrupted daily activity cycles. a search in the genomes of vertebrates revealed a set of analogous genes, which were found to play similar roles in the mouse biological clock—and therefore almost certainly in the human biological clock as well. studies done on drosophila, also show that most neuropil regions of the brain are continuously reorganized throughout life in response to specific living conditions. the nematode worm caenorhabditis elegans, like drosophila, has been studied largely because of its importance in genetics. in the early 1970s, sydney brenner chose it as a model organism for studying the way that genes control development. one of the advantages of working with this worm is that the body plan is very stereotyped: the nervous system of the hermaphrodite contains exactly 302 neurons, always in the same places, making identical synaptic connections in every worm. brenner's team sliced worms into thousands of ultrathin sections and photographed each one under an electron microscope, then visually matched fibers from section to section, to map out every neuron and synapse in the entire body. the complete neuronal wiring diagram of c.elegans – its connectome was achieved. nothing approaching this level of detail is available for any other organism, and the information gained has enabled a multitude of studies that would otherwise have not been possible. the sea slug aplysia californica was chosen by nobel prize-winning neurophysiologist eric kandel as a model for studying the cellular basis of learning and memory, because of the simplicity and accessibility of its nervous system, and it has been examined in hundreds of experiments. vertebrates the brain of a shark the first vertebrates appeared over 500 million years ago (mya), during the cambrian period, and may have resembled the modern hagfish in form. jawed fish appeared by 445 mya, amphibians by 350 mya, reptiles by 310 mya and mammals by 200 mya (approximately). each species has an equally long evolutionary history, but the brains of modern hagfishes, lampreys, sharks, amphibians, reptiles, and mammals