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For other uses, see Brain (disambiguation).
The brain is the center of the nervous system in animals. All vertebrates have a brain, and most invertebrates have either a brain or a collection of ganglia. Some animals such as jellyfish and starfish do not have a centralized brain, and instead have a decentralized nervous system, while sponges lack either a brain or a nervous system. In vertebrates, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, balance, taste, and smell. Brains can be extremely complex. For example, the human brain contains roughly 100 billion neurons, linked with up to 10,000 synaptic connections each. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry signal pulses called action potentials to distant parts of the brain or body and target them to specific recipient cells. Charles Sherrington, a pioneering investigator of brain function, visualized the workings of the brain poetically:
From a philosophical point of view, it might be said that the most important function of the brain is to serve as the physical structure underlying the mind. From a biological point of view, though, the most important function is to generate behaviors that promote the welfare of an animal. Brains control behavior either by activating muscles, or by causing secretion of chemicals such as hormones. Not all behaviors require a brain. Even single-celled organisms may be capable of extracting information from the environment and acting in response to it.[2] Sponges, which lack a central nervous system, are capable of coordinated body contractions and even locomotion.[3] In vertebrates, the spinal cord by itself contains neural circuitry capable of generating reflex responses as well as simple motor patterns such as swimming or walking.[4] However, sophisticated control of behavior on the basis of complex sensory input requires the information-integrating capabilities of a centralized brain. In spite of rapid scientific progress, the way that brains work remains in many respects a mystery. The operations of individual neurons and synapses are now understood in considerable detail, but the way they cooperate in groups of millions has been very difficult to decipher. Methods of observation such as EEG recording and functional brain imaging tell us that brain operations are highly organized, but these methods do not have high enough resolution to reveal the activity of individual neurons. Thus, even the most fundamental principles of neural computation may to a large extent remain for future investigators to discover.[5] This article examines the brains of all types of animals, including humans, in a comparative way. Thus, it deals with the human brain to the extent that it shares properties with the brains of other species. For an account of features that only apply to humans, see the human brain article.
Structure of the brainGeneral anatomy
The human brain weighs about three pounds, or 1.5 kg.[6][7] In its natural state it is very soft, having approximately the consistency of pudding, although surrounded by leathery membranes. When alive it is pinkish on the outside, and mostly white on the inside, with subtle variations in color. The brains of other species have generally similar properties, but smaller sizes in relation to the body. The largest part of the human brain is the cerebral hemispheres, situated at the top and covered with a very convoluted "cortex".[8] Underneath the cerebrum lies the brainstem, appearing somewhat like a stalk on which the cerebrum is attached. At the back of the brain, beneath the cerebrum and behind the brainstem, is the cerebellum, a structure with a horizontally furrowed surface that makes it look different from anything else. These are the main structures visible from the outside, but a great deal more lies hidden below the surface. In other mammals, the same structures are present, but the cerebrum is not so large in relation to the brain as a whole. As a rule, the smaller the cerebrum, the less convoluted the cortex. The cortex of a rat or mouse is almost completely smooth. The cortex of a dolphin or whale, on the other hand, is substantially more convoluted than the cortex of a human. In vertebrates, the brain is surrounded by connective tissues called meninges, a system of membranes that separate the skull from the brain.[7] This three-layered covering is composed of (from the outside in) the dura mater ("hard mother"), arachnoid mater ("spidery mother"), and pia mater ("soft mother"). The arachnoid and pia are physically connected and thus often considered as a single layer, the pia-arachnoid. Below the arachnoid is the subarachnoid space which contains cerebrospinal fluid, a substance that protects the nervous system. Blood vessels enter the central nervous system through the perivascular space above the pia mater. The cells in the blood vessel walls are joined tightly, forming the blood-brain barrier which protects the brain from toxins that might enter through the blood. The brain is bathed in cerebrospinal fluid (CSF), which circulates in the narrow spaces between cells, and through cavities called ventricles. CSF is important both metabolically and mechanically: it provides neutrients to the brain, supports it, and cushions it against shocks. 
The cortex is the part of the brain that most strongly distinguishes mammals from other vertebrates, primates from other mammals, and humans from other primates. In non-mammalian vertebrates, the surface of the cerebrum is lined with a comparatively simple layered structure called the pallium. In mammals, the pallium evolves into a complex 6-layered structure called neocortex. In primates, the neocortex is greatly enlarged in comparison to its size in non-primates, especially the part called the frontal lobes. In humans, this enlargement of the frontal lobes is taken to an extreme, and other parts of the cortex also become quite large and complex. Principles of brain architecture
The brain is the most complex biological structure known to us,[9] and comparing the brains of different species on the basis of overt appearance is often difficult. Nevertheless there are common principles of brain architecture that apply across a very wide range of species. These are revealed mainly by three approaches: evolution, development, and genetics. The evolutionary approach means comparing brain structures of different species, and using the principle that features found in all branches that descend from a given ancient form were probably present in the ancestor as well. The developmental approach means examining how the form of the brain changes during the progression from embyronic to adult stages. The genetic approach means analyzing gene expression in various parts of the brain across a range of species. Each approach complements and informs the other two. 
With the exception of a few primitive forms such as sponges and jellyfish, all of the animals on earth today are bilaterians, meaning animals with a bilaterally symmetric body shape (that is, left and right sides that are approximate mirror images of each other). Paleontologists believe that all bilaterians descend from a common ancestor that appeared early in the Cambrian period, 550-600 million years ago.[10] This ancestor had the shape of a simple tube worm with a segmented body, and at an abstract level, that worm-shape continues to be reflected in the body and nervous system plans of all modern bilaterians, including humans.[11] The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to 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". InvertebratesIn many invertebrates—insects, molluscs, worms of many types, etc.—the components of the nervous system, and their arrangement, differ so greatly from the vertebrate pattern that it is hard to make meaningful comparisons except on the basis of genetics. Two groups of invertebrates have notably complex brains: arthropods (insects, crustaceans, arachnids, and others), and cephalopods (octopuses, squids, and similar molluscs).[12] 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 with three divisions and large optical lobes behind each eye for visual processing.[12] Cephalopods have the largest brains of any invertebrates. The brain of the octopus in particular is highly developed, comparable in complexity to the brains of some vertebrates. There are a few invertebrates whose brains have been studied especially intensively. The large sea slug aplysia was chosen by Nobel Prize-winning neurophysiologist Eric Kandel, because of the simplicity and accessability of its nervous system, as a model for studying the cellular basis of learning and memory, and subjected to hundreds of experiments.[13]. The most thoroughly studied invertebrate brain by far, however, belongs to the fruit fly drosophila.[14] Because of the large array of techniques available for studying their genetics, fruit flies have been a natural subject for studying the role of genes in brain development. Remarkably, many aspects of drosophila neurogenetics have turned out to be relevant to humans. The first biological clock genes, for example, were identified by examining drosophila mutants that showed disrupted daily activity cycles.[15] A search in the genomes of vertebrates turned up 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.[16] Vertebrates
The first vertebrates appeared over 500 million years ago (Mya), during the Cambrian period, and may have somewhat resembled the modern hagfish in form. Sharks appeared about 450 Mya, amphibians about 400 Mya, reptiles about 350 Mya, and mammals about 200 Mya. It is dangerous to describe any modern species as more "primitive" than others, since all have an equally long evolutionary history, but the brains of modern hagfishes, lampreys, sharks, amphibians, reptiles, and mammals show a gradient of size and complexity that roughly follows the evolutionary sequence. All of these brains contain the same set of basic anatomical components, but many are rudimentary in hagfishes, whereas in mammals the foremost parts are greatly elaborated and expanded. 
All vertebrate brains share a common underlying form, which can most easily be appreciated by examining how they develop.[17] The first appearance of the nervous system is as a thin strip of tissue running along the back of the embryo. This strip thickens and then folds up to form a hollow tube. The front end of the tube develops into the brain. In its earliest form, the brain appears as three swellings, which eventually become the forebrain, midbrain, and hindbrain. In many classes of vertebrates these three parts remain similar in size in the adult, but in mammals the forebrain becomes much larger than the other parts, and the midbrain quite small. 
Neuroanatomists usually consider the brain to consist of six main regions: the telencephalon (cerebral hemispheres), diencephalon (thalamus and hypothalamus), mesencephalon (midbrain), cerebellum, pons, and medulla.[18] Some branches of vertebrate evolution have led to substantial changes in brain shape, especially in the forebrain. The brain of a shark shows the basic components in a straighforward way, but in teleost fishes (the great majority of modern species), the forebrain has become "everted", like a sock turned inside out. In birds, also, there are major changes in shape. One of the main structures in the avian forebrain, the dorsal ventricular ridge, was long thought to correspond to the basal ganglia of mammals, but is now thought to be more closely related to the neocortex. Mammals
The hindbrain and midbrain of mammals are generally similar to those of other vertebrates, but dramatic differences appear in the forebrain, which is not only greatly enlarged, but also altered in structure. In mammals, the surface of the cerebral hemispheres is mostly covered with 6-layered isocortex, more complex than the 3-layered pallium seen in most vertebrates. Also the hippocampus of mammals has a distinctive structure. Unfortunately, the evolutionary history of these mammalian features is difficult to work out. This is largely because of a "missing link" problem. The ancestors of mammals, called synapsids, split off from the ancestors of modern reptiles and birds about 350 million years ago. However, the most recent branching that has left living results within the mammals was the split between monotremes (the platypus and echidna), marsupials (opossum, kangaroo, etc.) and placentals (most living mammals), which took place about 120 million years ago. The brains of monotremes and marsupials are distinctive from those of placentals in some ways, but they have fully mammalian cortical and hippocampal structures. Thus, these structures must have evolved during the "dark ages" between 350 and 120 million years ago, a period for which we have no evidence except fossils—but fossils never preserve tissue as soft as brain. Primates, including humans
The primate brain contains the same structures as the brains of other mammals, but is considerably larger in proportion to body size. Most of the enlargement comes from a massive expansion of the cortex, focusing especially on the parts subserving vision and forethought. The visual processing network of primates is very complex, including at least 30 distinguishable areas, with a bewildering web of interconnections. Taking all of these together, visual processing makes use of about half of the brain. The other part of the brain that is greatly enlarged is the prefrontal cortex, whose functions are difficult to summarize succinctly, but relate to planning, working memory, motivation, attention, and executive control. Microscopic structure
The brain is composed of two broad classes of cells, neurons and glia. Neurons receive more attention, but glial cells actually outnumber them by about 10 to 1. Glia come in several types, which perform a number of critical functions, including structural support, metabolic support, insulation, and guidance of development. The property that makes neurons so important is that, unlike glia, they are capable of sending signals to each other over long distances. They send these signals by means of an axon, 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 extent of an axon can be extraordinary: to take an example, if a pyramidal cell of the neocortex were scaled up so that its cell body became the size of a human, its axon, equally scaled, would become a cable a few inches in diameter, extending as far as 10 miles. Counting all branches, the total axon length, thus scaled, could come to 100 miles or more. These axons transmit signals in the form of electrochemical pulses called action potentials, lasting less than a thousandth of a second and traveling along the axon at speeds of 0.1-10 meters per second. Some neurons emit action potentials constantly, at rates of 10-100 per second, usually in irregular temporal patterns; other neurons are quiet most of the time, but occasionally emit a burst of action potentials. Axons transmit signals to other neurons, or to non-neuronal cells, by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections. 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. Some types of neuronal receptors are excitatory, meaning that they increase the rate of action potentials in the target cell; other receptors are inhibitory, meaning that they decrease the rate of action potentials; others have complex modulatory effects on the target cell. Axons actually fill most of the space in the brain. Often large groups of them travel together in bundles called "nerve fiber tracts". In many cases, each axon is wrapped in a thick sheath of a fatty substance called myelin, which serves to greatly increase the speed of action potential propagation. Myelin is white in color, so parts of the brain filled exclusively with nerve fibers appear as "white matter", in contrast to the "gray matter" that marks areas where high densities of neuron cell bodies are located. DevelopmentThe brain does not simply grow; it develops in an intricately orchestrated sequence of steps.[19] Many neurons are created in special zones that contain stem cells, and then migrate through the tissue to reach their ultimate locations.[20] In the cortex, for example, the first stage of development is the formation of a "scaffold" by a special group of glial cells, called radial glia, which send fibers vertically across the cortex. New cortical neurons are created at the bottom of the cortex, and then "climb" along the radial fibers until they reach the layers they are destined to occupy in the adult. Once a neuron is in place, it begins to extend dendrites and an axon into the area around it.[21] Axons, because they commonly extend a great distance from the cell body and need to make contact with specific targets, grow in a particularly complex way. The tip of a growing axon consists of a blob of protoplasm called a "growth cone", studded with chemical receptors. These receptors sense the local environment, causing the growth cone to be attracted or repelled by various cellular elements, and thus to be pulled in a particular direction at each point along its path. The result of this pathfinding process is that the growth cone navigates through the brain until it reaches its destination area, where other chemical cues cause it to begin generating synapses. Taking the entire brain into account, many thousands of genes give rise to proteins that influence axonal pathfinding. The synaptic network that finally emerges is only partly determined by genes, though. In many parts of the brain, axons initially "overgrow", and then are "pruned" by mechanisms that depend on neural activity.[22] In the projection from the eye to the midbrain, for example, the structure in the adult contains a very precise mapping, connecting each point on the surface of the retina to a corresponding point in a midbrain layer. In the first stages of development, each axon from the retina is guided to the right general vicinity in the midbrain by chemical cues, but then branches very profusely and makes initial contact with a wide swath of midbrain neurons. The retina, before birth, contains special mechanisms that cause it to generate waves of activity that originate spontaneously at some point and then propagate slowly across the retinal layer.[23] These waves are useful because they cause neighboring neurons to be active at the same time: that is, they produce a neural activity pattern that contains information about the spatial arrangement of the neurons. This information is exploited in the midbrain by a mechanism that causes synapses to weaken, and eventually vanish, if activity in an axon is not followed by activity of the target cell. The result of this sophisticated process is a gradual tuning and tightening of the map, leaving it finally in its precise adult form. Similar things happen in other brain areas: an initial synaptic matrix is generated as a result of genetically determined chemical guidance, but then gradually refined by activity-dependent mechanisms, partly driven by internal dynamics, partly by external sensory inputs. In some cases, as with the retina-midbrain system, activity patterns depend on mechanism that operate only in the developing brain, and apparently exist solely for the purpose of guiding development. In humans, and many other mammals, new neurons are created mainly before birth. In humans, the infant brain actually contains substantially more neurons than the adult brain.[24] There are, however, a few areas where new neurons continue to be generated throughout life. The two areas for which this is well established are the olfactory bulb, which is involved in the sense of smell, and the dentate gyrus of the hippocampus, where there is evidence that the new neurons play a role in storing newly acquired memories. With these exceptions, however, the set of neurons that are present in early childhood is the set that are present for life. (Glial cells are different: as with most types of cells in the body, these are generated throughout the lifespan.) Although the pool of neurons is largely in place by birth, their axonal connections continue to develop for years afterward. In particular, in humans full myelination is not completed until the age of 5 or 6. Nature versus nurtureThere has long been debate about whether the qualities of mind, personality, and intelligence can mainly be attributed to heredity or to upbringing.[25] This is not just a philosophical question: it has great practical relevance to parents and educators. Although many details remain to be settled, neuroscience clearly shows that both factors are essential. Genes determine the general form of the brain, and genes determine how the brain reacts to experience. Experience, however, is required to refine the matrix of synaptic connections. In some respects it is mainly a matter of presence or absence of experience during critical periods of development.[26] In other respects, the quantity and quality of experience may be more relevant: for example, there is substantial evidence that animals raised in enriched environments have thicker cortices (indicating a higher density of synaptic connections) than animals whose levels of stimulation are restricted.[27] Functions of the brainVertebrate brains receive signals through nerves arriving from sensory systems. These signals are then processed throughout the central nervous system; reactions are formulated based upon reflex and learned experiences. A similarly extensive nerve network delivers signals from the brain to muscles throughout the body. Anatomically, the majority of afferent (incoming) and efferent (outgoing) nerves are connected to the spinal cord, which then transfers the signals to and from the brain. There are also, however, several cranial nerves that connect parts of the body directly to the brain. Sensory input is processed by the brain to recognize danger, find food, identify potential mates, and perform more sophisticated functions. Visual, touch, and auditory sensory pathways of vertebrates are routed to specific nuclei of the thalamus and then to regions of the cerebral cortex that are specific to each sensory system, the visual system, the auditory system, and the somatosensory system. Olfactory pathways are routed to the olfactory bulb, then to various parts of the olfactory system. Taste is routed through the brainstem and then to other portions of the gustatory system. To control movement the brain has several parallel systems of muscle control. The motor system controls voluntary muscle movement, aided by the motor cortex, cerebellum, and the basal ganglia. The system eventually projects to the spinal cord and then out to the muscle effectors. Nuclei in the brain stem control many involuntary muscle functions such as heart rate and breathing. In addition, many automatic acts (simple reflexes, locomotion) can be controlled by the spinal cord alone. Feedforward processing versus feedback processing
It is useful to distinguish between two ways of thinking about how the brain generates behavior. In the "feedforward mode", signals originating from sensory inputs are propagated through the brain until they ultimately reach motor output areas. In the "feedback mode", signals are generated within the brain by ongoing dynamic activity, and influence behaviors in ways that are not immediately caused by sensory inputs. As an example, consider the neural processing involved in two somewhat similar behaviors: first, an eye movement directed toward an object that has unexpectedly moved; second, an eye movement directed toward an object that has just entered our thoughts. In the first case, the neural processing sequence begins with photoreceptors in the retina, which send axons to the visual part of the thalamus, among other places. We can trace the resulting brain activation through a series of areas: the primary visual cortex, secondary visual cortex, motion-detecting visual cortex (area MT), frontal eye fields, superior colliculus, and ultimately the oculomotor nuclei of the brainstem, which are capable of directly activating the muscles that move the eyes. There are also a number of side-paths that modulate the reponse, but this is probably the primary circuit. In the second case, no clear beginning can be identified: instead, neural activity patterns circulating among several cortical areas, including the prefrontal cortex, parietal areas involved in attention, temporal areas involved in memory and object recognition, and occipital areas directly involved in vision, all combine at some moment to produce activation in an "executive" part of the prefrontal cortex. From this point, the sequence overlaps with the other: the prefrontal cortex activates the frontal eye fields, superior colliculus, etc. On the whole, neuroscientists understand feedforward processing considerably better than feedback processing. This is largely a result of experimental convenience: it is much easier to study a process if an experimenter has control over the event that triggers it. Nevertheless, both anatomical and functional considerations indicate that feedback signal flow is at least as important as feedforward flow. The great majority of connections in the brain, especially in the cerebral cortex, are reciprocal, and in many cases feedback connections are numerically dominant. In fact, neural connections that can be identified with feedforward signal processing pathways only make up a small fraction of the connections in the brain. Brain systemsThe brain can be divided into subsystems in a number of ways: anatomically (as described above), chemically, and functionally. Neurotransmitter systemsWith few exceptions, each neuron in the brain releases the same neurotransmitter, or set of neurotransmitters, at all of the synaptic connections it makes with other neurons.[28] Thus, a neuron can be characterized by the neurotransmitters it releases. The two neurotransmitters that appear most frequently are glutamate (which is almost always excitatory), and GABA (which is almost always inhibitory). Neurons using these transmitters can be found in nearly every part of the brain. In fact, they combine numerically to make up more than 99% of the brain's entire pool of synapses. This does not mean that other neurotransmitters are unimportant, though. The great majority of psychoactive drugs exert their effects by altering neurotransmitter systems, and only a small proportion of them act directly on glutamatergic or GABAergic transmission. Drugs such as caffeine, nicotine, heroin, cocaine, Prozac, Thorazine, etc., etc. act on other neurotransmitters. Many of these other transmitters come from neurons that are localized in particular parts of the brain. Serotonin, for example—the primary target of antidepressant drugs and many dietary aids—comes exclusively from a small brainstem area called the Raphe nuclei. Norepinephrine, which is involved in arousal, comes exclusively from a nearby small area called the locus ceruleus. Histamine, as a neurotransmitter, comes from a tiny part of the hypothalamus called the tuberomammilary nucleus (histamine also has non-CNS functions, but the neurotransmitter function is what causes antihistamines to have sedative effects). Other neurotransmitters such as acetylcholine and dopamine have multiple sources in the brain, but are not as ubiquitously distributed as glutamate and GABA. Sensory systemsOne of the primary functions of a brain is to extract biologically relevant information from sensory inputs. Even in the human brain, sensory processes go well beyond the classical five senses of sight, sound, taste, touch, and smell: our brains are provided with information about temperature, balance, limb position, and the chemical composition of the bloodstream, among other things. All of these modalities are detected by specialized sensors that project signals into the brain. In non-humans, additional senses may be present, such as the infrared heat-sensors in the pit organs of snakes; or the "standard" senses may be used in nonstandard ways, as in the auditory "sonar" of bats. Every sensory system has idiosyncrasies, but here is a list of a few general principles, using the sense of hearing for examples:
All of these rules have exceptions, for example: (1) For the sense of touch (which is actually a set of at least half-a-dozen distinct mechanical senses), the sensory inputs terminate mainly in the spinal cord, on neurons that then project to the brainstem. (2) For the sense of smell, there is no relay in the thalamus; instead the signals go directly from the primary brain area—the olfactory bulb—to the cortex. Motor systemsMotor systems are areas of the brain that are more or less directly involved in producing body movements, that is, in activating muscles. With the exception of the muscles that control the eye, all of the "voluntary" muscles[29] in the body are directly innervated by motor neurons in the spinal cord, which therefore are the "final common path" for the movement-generating system. Spinal motor neurons are controlled both by neural circuits intrinsic to the spinal cord, and by inputs that descend from the brain. The intrinsic spinal circuits implement many reflex responses, and also contain pattern generators for rhythmic movements such as walking or swimming. The descending connections from the brain allow for more sophisticated control. The brain contains a number of areas that project directly to the spinal cord. At the lowest level are motor areas in the medulla and pons. At a higher level are areas in the midbrain, such as the red nucleus, which is responsible for coordinating movements of the arms and legs. At a higher level yet is the primary motor cortex, a strip of tissue located at the posterior edge of the frontal lobe. The primary motor cortex sends projections to the subcortical motor areas, but also sends a massive projection directly to the spinal cord, via the so-called pyramidal tract. This direct corticospinal projection allows for precise voluntary control of the fine details of movements. Other "secondary" motor-related brain areas do not project directly to the spinal cord, but instead act on the cortical or subcortical primary motor areas. Among the most important secondary areas are the premotor cortex, basal ganglia, and cerebellum:
In addition to all of the above, the brain and spinal cord contain extensive circuitry to control the autonomic nervous system, which works by secreting hormones and by modulating the "smooth" muscles of the gut. The autonomic nervous system affects heart rate, digestion, respiration rate, salivation, perspiration, urination, and sexual arousal—but most of its functions are not under direct voluntary control. Arousal systemsPerhaps the most obvious aspect of the behavior of any animal is the daily cycle between sleeping and waking. Arousal and alertness are also modulated on a finer time scale, though, by an extensive network of brain areas. A key component of the arousal system is the suprachiasmatic nucleus (SCN), a tiny part of the hypothalamus located directly above the point at which the optic nerves from the two eyes cross. The SCN contains the body's central biological clock. Neurons there show activity levels that rise and fall with a period of about 24 hours: these activity fluctuations are driven by rhythmic changes in expression of a set of "clock genes". The SCN continues to keep time even if it is excised from the brain and placed in a dish of warm nutrient solution, but it ordinarily recieves input from the optic nerves that allow daily light-dark cycles to calibrate the clock. The SCN projects to a set of areas in the hypothalamus, brainstem, and midbrain that are involved in implementing sleep-wake cycles. An important component of the system is the so-called reticular formation, a group of neuron-clusters scattered diffusely through the core of the lower brain. Reticular neurons send signals to the thalamus, which in turn sends activity-level-controlling signals to every part of the cortex. Damage to the reticular formation can produce a permanent state of coma. Sleep involves great changes in brain activity. Until the 1950s it was generally believed that the brain essentially shuts off during sleep, but this is now known to be far from true: activity continues, but the pattern becomes very different. In fact, there are two types of sleep, slow wave sleep (non-dreaming) and REM sleep (dreaming), each with its own distinct brain activity pattern. During slow wave sleep, activity in the cortex takes the form of large synchronized waves, where in the waking state it is noisy and desynchronized. Levels of the neurotransmitters norepinephrine and serotonin drop during slow wave sleep, and fall almost to zero during REM sleep; levels of acetylcholine show the reverse pattern. The brain and behaviorThe brain and the mindThe mind-body problem is one of the central problems in the history of philosophy. The brain is the physical and biological matter contained within the skull, responsible for electrochemical neuronal processes while the mind consists of mental attributes, like beliefs, desires, and perceptions. There are scientifically demonstrable correlations between mental events and neuronal events; the philosophical question is whether these phenomena are identical, at least partially distinct, or related in some unknown way. There are three major philosophies of mind: dualism, materialism, and idealism. Dualism holds that the mind exists independently of the brain;[30] materialism holds that mental phenomena are identical to neuronal phenomena;[31] and idealism holds that only mental substances and phenomena exist.[31] Pathology
Clinically, death is defined as an absence of brain activity as measured by EEG. Injuries to the brain tend to affect large areas of the organ, sometimes causing major deficits in intelligence, memory, and movement. Head trauma caused, for example, by vehicle or industrial accidents, is a leading cause of death in youth and middle age. In many cases, more damage is caused by resultant edema than by the impact itself. Stroke, caused by the blockage or rupturing of blood vessels in the brain, is another major cause of death from brain damage. Other problems in the brain can be more accurately classified as diseases rather than injuries. Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, motor neurone disease, and Huntington's disease are caused by the gradual death of individual neurons, leading to decrements in movement control, memory, and cognition. Currently only the symptoms of these diseases can be treated. Mental disorders, such as clinical depression, schizophrenia, bipolar disorder and post-traumatic stress disorder may involve particular patterns of neuropsychological functioning related to various aspects of mental and somatic function. These disorders may be treated by psychotherapy, psychiatric medication or social intervention and personal recovery work; the underlying issues and associated prognosis vary significantly between individuals. Some infectious diseases affecting the brain are caused by viruses and bacteria. Infection of the meninges, the membrane that covers the brain, can lead to meningitis. Bovine spongiform encephalopathy (also known as mad cow disease), is deadly in cattle and humans and is linked to prions. Kuru is a similar prion-borne degenerative brain disease affecting humans. Both are linked to the ingestion of neural tissue, and may explain the tendency in some species to avoid cannibalism. Viral or bacterial causes have been reported in multiple sclerosis and Parkinson's disease, and are established causes of encephalopathy, and encephalomyelitis. Many brain disorders are congenital. Tay-Sachs disease, Fragile X syndrome, and Down syndrome are all linked to genetic and chromosomal errors. Many other syndromes, such as the intrinsic circadian rhythm disorders, are suspected to be congenital as well. Malfunctions in the embryonic development of the brain can be caused by genetic factors, drug use, nutritional deficiencies, and infectious diseases during pregnancy. Certain brain disorders are treated by brain neurosurgeons while others are treated by neurologists and psychiatrists. 
Brain energy consumptionAlthough the brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization.[32] The demands of the brain limit its size in some species, such as bats.[33] The brain mostly utilizes glucose for energy, and deprivation of glucose, as can happen in hypoglycemia, can result in loss of consciousness. The energy consumption of the brain does not vary greatly over time, but active regions of the cortex consume somewhat more energy than inactive regions: this fact forms the basis for the functional brain imaging methods PET and fMRI.[34] How the brain is studiedFields of studyNeuroscience seeks to understand the nervous system, including the brain, from a biological and computational perspective. Psychology seeks to understand behavior and the brain. Neurology refers to the medical applications of neuroscience. The brain is also one of the most important organs studied in psychiatry, the branch of medicine which exists to study, prevent, and treat mental disorders.[35][36][37] Cognitive science seeks to unify neuroscience and psychology with other fields that concern themselves with the brain, such as computer science (artificial intelligence and similar fields) and philosophy. Methods of observationEach method for observing activity in the brain has its advantages and drawbacks. ElectrophysiologyElectrophysiology allows scientists to record the electrical activity of individual neurons or groups of neurons. EEGBy placing electrodes on the scalp one can record the summed electrical activity of the cortex in a technique known as electroencephalography (EEG). EEG measures the mass changes in electrical current from the cerebral cortex, but can only detect changes over large areas of the brain with very little sub-cortical activity. MEGApart from measuring the electric field around the skull it is possible to measure the magnetic field directly in a technique known as magnetoencephalography (MEG). This technique has the same temporal resolution as EEG but much better spatial resolution, although admittedly not as good as fMRI. The main advantage over fMRI is a direct relationship between neural activation and measurement. fMRI and PET
Functional magnetic resonance imaging (fMRI) measures changes in blood flow in the brain, but the activity of neurons is not directly measured, nor can it be distinguished whether this activity is inhibitory or excitatory. fMRI is a noninvasive, indirect method for measuring neural activity that is based on BOLD; Blood Oxygen Level Dependent changes. The changes in blood flow that occur in capillary beds in specific regions of the brain are thought to represent various neuronal activities (metabolism of synaptic reuptake). Similarly, a positron emission tomography (PET), is able to monitor glucose and oxygen metabolism as well as neurotransmitter activity in different areas within the brain which can be correlated to the level of activity in that region. BehavioralBehavioral tests can measure symptoms of disease and mental performance, but can only provide indirect measurements of brain function and may not be practical in all animals. In humans however, a neurological exam can be done to determine the location of any trauma, lesion, or tumor within the brain, brain stem, or spinal cord. AnatomicalAutopsy analysis of the brain allows for the study of anatomy and protein expression patterns, but is only possible after the human or animal is dead. Magnetic resonance imaging (MRI) can be used to study the anatomy of a living creature and is widely used in both research and medicine. Other studiesComputer scientists have produced simulated "artificial neural networks" loosely based on the structure of neuron connections in the brain. Some artificial intelligence research seeks to replicate brain function—although not necessarily brain mechanisms—but as yet has been met with only limited success. Creating algorithms to mimic a biological brain is very difficult because the brain is not a static arrangement of circuits, but a network of vastly interconnected neurons that are constantly changing their connectivity and sensitivity. More recent work in both neuroscience and artificial intelligence models the brain using the mathematical tools of chaos theory and dynamical systems. Current research has also focused on recreating the neural structure of the brain with the aim of producing human-like cognition and artificial intelligence. History of understanding of the brainEarly views were divided as to whether the seat of the soul lies in the brain or heart. On one hand, it was impossible to miss the fact that awareness feels like it is localized in the head, and that blows to the head can cause unconsciousness much more easily than blows to the chest, and that shaking the head causes dizziness. On the other hand, the brain to a superficial examination seems inert, whereas the heart is constantly beating. Cessation of the heartbeat means death; strong emotions produce changes in the heartbeat; and emotional distress often produces a sensation of pain in the region of the heart ("heartache"). Aristotle favored the heart, and thought that the function of the brain is merely to cool the blood. Hippocrates, the "father of medicine", thought differently. In his account of epilepsy he wrote:
The famous Roman physician Galen also advocated the importance of the brain, and theorized in some depth about how it might work. Even after physicians and philosophers had accepted the primacy of the brain, though, the idea of the heart as seat of intelligence continued to survive in popular idioms, such as "learning something by heart".[39] Galen did a masterful job of tracing out the anatomical relationships between brain, nerves, and muscles, demonstrating that all muscles in the body are connected to the brain via a branching network of nerves. He postulated that nerves activate muscles mechanically, by carrying a mysterious substance he called pneumata psychikon, usually translated as "animal spirits". His ideas were widely known during the Middle Ages, but not much further progress came until the Renaissance, when detailed anatomical study resumed, combined with the theoretical speculations of Descartes and his followers. Descartes, like Galen, thought of the nervous system in hydraulic terms. He believed that the highest cognitive functions—language in particular—are carried out by a non-physical res cogitans, but that the majority of behaviors of humans and animals could be explained mechanically. The first real progress toward a modern understanding of nervous function, though, came from the investigations of Luigi Galvani, who discovered that a shock of static electricity applied to an exposed nerve of a dead frog could cause its leg to contract. See alsoNotes
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