Brain

From Wikipedia for FEVERv2
Jump to navigation Jump to search

This article is about the brains of all types of animals, including humans. Brain_sentence_0

For information specific to the human brain, see Human brain. Brain_sentence_1

For other uses, see Brain (disambiguation) and Brains (disambiguation). Brain_sentence_2

Not to be confused with Brane or Brian. Brain_sentence_3

Brain_table_infobox_0

BrainBrain_header_cell_0_0_0
IdentifiersBrain_header_cell_0_1_0
MeSHBrain_header_cell_0_2_0 Brain_cell_0_2_1
NeuroNamesBrain_header_cell_0_3_0 Brain_cell_0_3_1
TA98Brain_header_cell_0_4_0 Brain_cell_0_4_1
TA2Brain_header_cell_0_5_0 Brain_cell_0_5_1

A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. Brain_sentence_4

It is located in the head, usually close to the sensory organs for senses such as vision. Brain_sentence_5

It is the most complex organ in a vertebrate's body. Brain_sentence_6

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. Brain_sentence_7

Each neuron is connected by synapses to several thousand other neurons. Brain_sentence_8

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. Brain_sentence_9

Physiologically, brains exert centralized control over a body's other organs. Brain_sentence_10

They act on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. Brain_sentence_11

This centralized control allows rapid and coordinated responses to changes in the environment. Brain_sentence_12

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. Brain_sentence_13

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. Brain_sentence_14

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. Brain_sentence_15

This article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. Brain_sentence_16

It deals with the human brain insofar as it shares the properties of other brains. Brain_sentence_17

The ways in which the human brain differs from other brains are covered in the human brain article. Brain_sentence_18

Several topics that might be covered here are instead covered there because much more can be said about them in a human context. Brain_sentence_19

The most important is brain disease and the effects of brain damage, that are covered in the human brain article. Brain_sentence_20

Anatomy Brain_section_0

The shape and size of the brain varies greatly between species, and identifying common features is often difficult. Brain_sentence_21

Nevertheless, there are a number of principles of brain architecture that apply across a wide range of species. Brain_sentence_22

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. Brain_sentence_23

The simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. Brain_sentence_24

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. Brain_sentence_25

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. Brain_sentence_26

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. Brain_sentence_27

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. Brain_sentence_28

Cellular structure Brain_section_1

The brains of all species are composed primarily of two broad classes of cells: neurons and glial cells. Brain_sentence_29

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. Brain_sentence_30

Neurons, however, are usually considered the most important cells in the brain. Brain_sentence_31

The property that makes neurons unique is their ability to send signals to specific target cells over long distances. Brain_sentence_32

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. Brain_sentence_33

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. Brain_sentence_34

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. Brain_sentence_35

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. Brain_sentence_36

Axons transmit signals to other neurons by means of specialized junctions called synapses. Brain_sentence_37

A single axon may make as many as several thousand synaptic connections with other cells. Brain_sentence_38

When an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. Brain_sentence_39

The neurotransmitter binds to receptor molecules in the membrane of the target cell. Brain_sentence_40

Synapses are the key functional elements of the brain. Brain_sentence_41

The essential function of the brain is cell-to-cell communication, and synapses are the points at which communication occurs. Brain_sentence_42

The human brain has been estimated to contain approximately 100 trillion synapses; even the brain of a fruit fly contains several million. Brain_sentence_43

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. Brain_sentence_44

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. Brain_sentence_45

It is widely believed that activity-dependent modification of synapses is the brain's primary mechanism for learning and memory. Brain_sentence_46

Most of the space in the brain is taken up by axons, which are often bundled together in what are called nerve fiber tracts. Brain_sentence_47

A myelinated axon is wrapped in a fatty insulating sheath of myelin, which serves to greatly increase the speed of signal propagation. Brain_sentence_48

(There are also unmyelinated axons). Brain_sentence_49

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. Brain_sentence_50

Evolution Brain_section_2

Main article: Evolution of the brain Brain_sentence_51

Generic bilaterian nervous system Brain_section_3

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). Brain_sentence_52

All bilaterians are thought to have descended from a common ancestor that appeared early in the Cambrian period, 485-540 million years ago, and it has been hypothesized that this common ancestor had the shape of a simple tubeworm with a segmented body. Brain_sentence_53

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. Brain_sentence_54

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. Brain_sentence_55

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. Brain_sentence_56

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". Brain_sentence_57

There are a few types of existing bilaterians that lack a recognizable brain, including echinoderms and tunicates. Brain_sentence_58

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. Brain_sentence_59

Invertebrates Brain_section_4

This category includes tardigrades, arthropods, molluscs, and numerous types of worms. Brain_sentence_60

The diversity of invertebrate body plans is matched by an equal diversity in brain structures. Brain_sentence_61

Two groups of invertebrates have notably complex brains: arthropods (insects, crustaceans, arachnids, and others), and cephalopods (octopuses, squids, and similar molluscs). Brain_sentence_62

The brains of arthropods and cephalopods arise from twin parallel nerve cords that extend through the body of the animal. Brain_sentence_63

Arthropods have a central brain, the supraesophageal ganglion, with three divisions and large optical lobes behind each eye for visual processing. Brain_sentence_64

Cephalopods such as the octopus and squid have the largest brains of any invertebrates. Brain_sentence_65

There are several invertebrate species whose brains have been studied intensively because they have properties that make them convenient for experimental work: Brain_sentence_66

Brain_unordered_list_0

  • 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.Brain_item_0_0
  • 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.Brain_item_0_1
  • 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.Brain_item_0_2

Vertebrates Brain_section_5

The first vertebrates appeared over 500 million years ago (Mya), during the Cambrian period, and may have resembled the modern hagfish in form. Brain_sentence_67

Sharks appeared about 450 Mya, amphibians about 400 Mya, reptiles about 350 Mya, and mammals about 200 Mya. Each species has 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. Brain_sentence_68

All of these brains contain the same set of basic anatomical components, but many are rudimentary in the hagfish, whereas in mammals the foremost part (the telencephalon) is greatly elaborated and expanded. Brain_sentence_69

Brains are most simply compared in terms of their size. Brain_sentence_70

The relationship between brain size, body size and other variables has been studied across a wide range of vertebrate species. Brain_sentence_71

As a rule, brain size increases with body size, but not in a simple linear proportion. Brain_sentence_72

In general, smaller animals tend to have larger brains, measured as a fraction of body size. Brain_sentence_73

For mammals, the relationship between brain volume and body mass essentially follows a power law with an exponent of about 0.75. Brain_sentence_74

This formula describes the central tendency, but every family of mammals departs from it to some degree, in a way that reflects in part the complexity of their behavior. Brain_sentence_75

For example, primates have brains 5 to 10 times larger than the formula predicts. Brain_sentence_76

Predators tend to have larger brains than their prey, relative to body size. Brain_sentence_77

All vertebrate brains share a common underlying form, which appears most clearly during early stages of embryonic development. Brain_sentence_78

In its earliest form, the brain appears as three swellings at the front end of the neural tube; these swellings eventually become the forebrain, midbrain, and hindbrain (the prosencephalon, mesencephalon, and rhombencephalon, respectively). Brain_sentence_79

At the earliest stages of brain development, the three areas are roughly equal in size. Brain_sentence_80

In many classes of vertebrates, such as fish and amphibians, the three parts remain similar in size in the adult, but in mammals the forebrain becomes much larger than the other parts, and the midbrain becomes very small. Brain_sentence_81

The brains of vertebrates are made of very soft tissue. Brain_sentence_82

Living brain tissue is pinkish on the outside and mostly white on the inside, with subtle variations in color. Brain_sentence_83

Vertebrate brains are surrounded by a system of connective tissue membranes called meninges that separate the skull from the brain. Brain_sentence_84

Blood vessels enter the central nervous system through holes in the meningeal layers. Brain_sentence_85

The cells in the blood vessel walls are joined tightly to one another, forming the blood–brain barrier, which blocks the passage of many toxins and pathogens (though at the same time blocking antibodies and some drugs, thereby presenting special challenges in treatment of diseases of the brain). Brain_sentence_86

Neuroanatomists usually divide the vertebrate brain into six main regions: the telencephalon (cerebral hemispheres), diencephalon (thalamus and hypothalamus), mesencephalon (midbrain), cerebellum, pons, and medulla oblongata. Brain_sentence_87

Each of these areas has a complex internal structure. Brain_sentence_88

Some parts, such as the cerebral cortex and the cerebellar cortex, consist of layers that are folded or convoluted to fit within the available space. Brain_sentence_89

Other parts, such as the thalamus and hypothalamus, consist of clusters of many small nuclei. Brain_sentence_90

Thousands of distinguishable areas can be identified within the vertebrate brain based on fine distinctions of neural structure, chemistry, and connectivity. Brain_sentence_91

Although the same basic components are present in all vertebrate brains, some branches of vertebrate evolution have led to substantial distortions of brain geometry, especially in the forebrain area. Brain_sentence_92

The brain of a shark shows the basic components in a straightforward way, but in teleost fishes (the great majority of existing fish species), the forebrain has become "everted", like a sock turned inside out. Brain_sentence_93

In birds, there are also major changes in forebrain structure. Brain_sentence_94

These distortions can make it difficult to match brain components from one species with those of another species. Brain_sentence_95

Here is a list of some of the most important vertebrate brain components, along with a brief description of their functions as currently understood: Brain_sentence_96

See also: List of regions in the human brain Brain_sentence_97

Brain_unordered_list_1

  • The medulla, along with the spinal cord, contains many small nuclei involved in a wide variety of sensory and involuntary motor functions such as vomiting, heart rate and digestive processes.Brain_item_1_3
  • The pons lies in the brainstem directly above the medulla. Among other things, it contains nuclei that control often voluntary but simple acts such as sleep, respiration, swallowing, bladder function, equilibrium, eye movement, facial expressions, and posture.Brain_item_1_4
  • The hypothalamus is a small region at the base of the forebrain, whose complexity and importance belies its size. It is composed of numerous small nuclei, each with distinct connections and neurochemistry. The hypothalamus is engaged in additional involuntary or partially voluntary acts such as sleep and wake cycles, eating and drinking, and the release of some hormones.Brain_item_1_5
  • The thalamus is a collection of nuclei with diverse functions: some are involved in relaying information to and from the cerebral hemispheres, while others are involved in motivation. The subthalamic area (zona incerta) seems to contain action-generating systems for several types of "consummatory" behaviors such as eating, drinking, defecation, and copulation.Brain_item_1_6
  • The cerebellum modulates the outputs of other brain systems, whether motor related or thought related, to make them certain and precise. Removal of the cerebellum does not prevent an animal from doing anything in particular, but it makes actions hesitant and clumsy. This precision is not built-in, but learned by trial and error. The muscle coordination learned while riding a bicycle is an example of a type of neural plasticity that may take place largely within the cerebellum. 10% of the brain's total volume consists of the cerebellum and 50% of all neurons are held within its structure.Brain_item_1_7
  • The optic tectum allows actions to be directed toward points in space, most commonly in response to visual input. In mammals it is usually referred to as the superior colliculus, and its best-studied function is to direct eye movements. It also directs reaching movements and other object-directed actions. It receives strong visual inputs, but also inputs from other senses that are useful in directing actions, such as auditory input in owls and input from the thermosensitive pit organs in snakes. In some primitive fishes, such as lampreys, this region is the largest part of the brain. The superior colliculus is part of the midbrain.Brain_item_1_8
  • The pallium is a layer of gray matter that lies on the surface of the forebrain and is the most complex and most recent evolutionary development of the brain as an organ. In reptiles and mammals, it is called the cerebral cortex. Multiple functions involve the pallium, including smell and spatial memory. In mammals, where it becomes so large as to dominate the brain, it takes over functions from many other brain areas. In many mammals, the cerebral cortex consists of folded bulges called gyri that create deep furrows or fissures called sulci. The folds increase the surface area of the cortex and therefore increase the amount of gray matter and the amount of information that can be stored and processed.Brain_item_1_9
  • The hippocampus, strictly speaking, is found only in mammals. However, the area it derives from, the medial pallium, has counterparts in all vertebrates. There is evidence that this part of the brain is involved in complex events such as spatial memory and navigation in fishes, birds, reptiles, and mammals.Brain_item_1_10
  • The basal ganglia are a group of interconnected structures in the forebrain. The primary function of the basal ganglia appears to be action selection: they send inhibitory signals to all parts of the brain that can generate motor behaviors, and in the right circumstances can release the inhibition, so that the action-generating systems are able to execute their actions. Reward and punishment exert their most important neural effects by altering connections within the basal ganglia.Brain_item_1_11
  • The olfactory bulb is a special structure that processes olfactory sensory signals and sends its output to the olfactory part of the pallium. It is a major brain component in many vertebrates, but is greatly reduced in humans and other primates (whose senses are dominated by information acquired by sight rather than smell).Brain_item_1_12

Mammals Brain_section_6

The most obvious difference between the brains of mammals and other vertebrates is in terms of size. Brain_sentence_98

On average, a mammal has a brain roughly twice as large as that of a bird of the same body size, and ten times as large as that of a reptile of the same body size. Brain_sentence_99

Size, however, is not the only difference: there are also substantial differences in shape. Brain_sentence_100

The hindbrain and midbrain of mammals are generally similar to those of other vertebrates, but dramatic differences appear in the forebrain, which is greatly enlarged and also altered in structure. Brain_sentence_101

The cerebral cortex is the part of the brain that most strongly distinguishes mammals. Brain_sentence_102

In non-mammalian vertebrates, the surface of the cerebrum is lined with a comparatively simple three-layered structure called the pallium. Brain_sentence_103

In mammals, the pallium evolves into a complex six-layered structure called neocortex or isocortex. Brain_sentence_104

Several areas at the edge of the neocortex, including the hippocampus and amygdala, are also much more extensively developed in mammals than in other vertebrates. Brain_sentence_105

The elaboration of the cerebral cortex carries with it changes to other brain areas. Brain_sentence_106

The superior colliculus, which plays a major role in visual control of behavior in most vertebrates, shrinks to a small size in mammals, and many of its functions are taken over by visual areas of the cerebral cortex. Brain_sentence_107

The cerebellum of mammals contains a large portion (the neocerebellum) dedicated to supporting the cerebral cortex, which has no counterpart in other vertebrates. Brain_sentence_108

Primates Brain_section_7

Brain_table_general_1

Encephalization QuotientBrain_table_caption_1
SpeciesBrain_header_cell_1_0_0 EQBrain_header_cell_1_0_1
HumanBrain_cell_1_1_0 7.4–7.8Brain_cell_1_1_1
Common chimpanzeeBrain_cell_1_2_0 2.2–2.5Brain_cell_1_2_1
Rhesus monkeyBrain_cell_1_3_0 2.1Brain_cell_1_3_1
Bottlenose dolphinBrain_cell_1_4_0 4.14Brain_cell_1_4_1
ElephantBrain_cell_1_5_0 1.13–2.36Brain_cell_1_5_1
DogBrain_cell_1_6_0 1.2Brain_cell_1_6_1
HorseBrain_cell_1_7_0 0.9Brain_cell_1_7_1
RatBrain_cell_1_8_0 0.4Brain_cell_1_8_1

See also: Human brain Brain_sentence_109

The brains of humans and other primates contain the same structures as the brains of other mammals, but are generally larger in proportion to body size. Brain_sentence_110

The encephalization quotient (EQ) is used to compare brain sizes across species. Brain_sentence_111

It takes into account the nonlinearity of the brain-to-body relationship. Brain_sentence_112

Humans have an average EQ in the 7-to-8 range, while most other primates have an EQ in the 2-to-3 range. Brain_sentence_113

Dolphins have values higher than those of primates other than humans, but nearly all other mammals have EQ values that are substantially lower. Brain_sentence_114

Most of the enlargement of the primate brain comes from a massive expansion of the cerebral cortex, especially the prefrontal cortex and the parts of the cortex involved in vision. Brain_sentence_115

The visual processing network of primates includes at least 30 distinguishable brain areas, with a complex web of interconnections. Brain_sentence_116

It has been estimated that visual processing areas occupy more than half of the total surface of the primate neocortex. Brain_sentence_117

The prefrontal cortex carries out functions that include planning, working memory, motivation, attention, and executive control. Brain_sentence_118

It takes up a much larger proportion of the brain for primates than for other species, and an especially large fraction of the human brain. Brain_sentence_119

Development Brain_section_8

Main article: Neural development Brain_sentence_120

The brain develops in an intricately orchestrated sequence of stages. Brain_sentence_121

It changes in shape from a simple swelling at the front of the nerve cord in the earliest embryonic stages, to a complex array of areas and connections. Brain_sentence_122

Neurons are created in special zones that contain stem cells, and then migrate through the tissue to reach their ultimate locations. Brain_sentence_123

Once neurons have positioned themselves, their axons sprout and navigate through the brain, branching and extending as they go, until the tips reach their targets and form synaptic connections. Brain_sentence_124

In a number of parts of the nervous system, neurons and synapses are produced in excessive numbers during the early stages, and then the unneeded ones are pruned away. Brain_sentence_125

For vertebrates, the early stages of neural development are similar across all species. Brain_sentence_126

As the embryo transforms from a round blob of cells into a wormlike structure, a narrow strip of ectoderm running along the midline of the back is induced to become the neural plate, the precursor of the nervous system. Brain_sentence_127

The neural plate folds inward to form the neural groove, and then the lips that line the groove merge to enclose the neural tube, a hollow cord of cells with a fluid-filled ventricle at the center. Brain_sentence_128

At the front end, the ventricles and cord swell to form three vesicles that are the precursors of the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). Brain_sentence_129

At the next stage, the forebrain splits into two vesicles called the telencephalon (which will contain the cerebral cortex, basal ganglia, and related structures) and the diencephalon (which will contain the thalamus and hypothalamus). Brain_sentence_130

At about the same time, the hindbrain splits into the metencephalon (which will contain the cerebellum and pons) and the myelencephalon (which will contain the medulla oblongata). Brain_sentence_131

Each of these areas contains proliferative zones where neurons and glial cells are generated; the resulting cells then migrate, sometimes for long distances, to their final positions. Brain_sentence_132

Once a neuron is in place, it extends dendrites and an axon into the area around it. Brain_sentence_133

Axons, because they commonly extend a great distance from the cell body and need to reach specific targets, grow in a particularly complex way. Brain_sentence_134

The tip of a growing axon consists of a blob of protoplasm called a growth cone, studded with chemical receptors. Brain_sentence_135

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. Brain_sentence_136

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. Brain_sentence_137

Considering the entire brain, thousands of genes create products that influence axonal pathfinding. Brain_sentence_138

The synaptic network that finally emerges is only partly determined by genes, though. Brain_sentence_139

In many parts of the brain, axons initially "overgrow", and then are "pruned" by mechanisms that depend on neural activity. Brain_sentence_140

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. Brain_sentence_141

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. Brain_sentence_142

The retina, before birth, contains special mechanisms that cause it to generate waves of activity that originate spontaneously at a random point and then propagate slowly across the retinal layer. Brain_sentence_143

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. Brain_sentence_144

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. Brain_sentence_145

The result of this sophisticated process is a gradual tuning and tightening of the map, leaving it finally in its precise adult form. Brain_sentence_146

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. Brain_sentence_147

In some cases, as with the retina-midbrain system, activity patterns depend on mechanisms that operate only in the developing brain, and apparently exist solely to guide development. Brain_sentence_148

In humans and many other mammals, new neurons are created mainly before birth, and the infant brain contains substantially more neurons than the adult brain. Brain_sentence_149

There are, however, a few areas where new neurons continue to be generated throughout life. Brain_sentence_150

The two areas for which adult neurogenesis 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. Brain_sentence_151

With these exceptions, however, the set of neurons that is present in early childhood is the set that is present for life. Brain_sentence_152

Glial cells are different: as with most types of cells in the body, they are generated throughout the lifespan. Brain_sentence_153

There has long been debate about whether the qualities of mind, personality, and intelligence can be attributed to heredity or to upbringing—this is the nature and nurture controversy. Brain_sentence_154

Although many details remain to be settled, neuroscience research has clearly shown that both factors are important. Brain_sentence_155

Genes determine the general form of the brain, and genes determine how the brain reacts to experience. Brain_sentence_156

Experience, however, is required to refine the matrix of synaptic connections, which in its developed form contains far more information than the genome does. Brain_sentence_157

In some respects, all that matters is the presence or absence of experience during critical periods of development. Brain_sentence_158

In other respects, the quantity and quality of experience are important; for example, there is substantial evidence that animals raised in enriched environments have thicker cerebral cortices, indicating a higher density of synaptic connections, than animals whose levels of stimulation are restricted. Brain_sentence_159

Physiology Brain_section_9

The functions of the brain depend on the ability of neurons to transmit electrochemical signals to other cells, and their ability to respond appropriately to electrochemical signals received from other cells. Brain_sentence_160

The electrical properties of neurons are controlled by a wide variety of biochemical and metabolic processes, most notably the interactions between neurotransmitters and receptors that take place at synapses. Brain_sentence_161

Neurotransmitters and receptors Brain_section_10

Neurotransmitters are chemicals that are released at synapses when the local membrane is depolarised and Ca enters into the cell, typically when an action potential arrives at the synapse – neurotransmitters attach themselves to receptor molecules on the membrane of the synapse's target cell (or cells), and thereby alter the electrical or chemical properties of the receptor molecules. Brain_sentence_162

With few exceptions, each neuron in the brain releases the same chemical neurotransmitter, or combination of neurotransmitters, at all the synaptic connections it makes with other neurons; this rule is known as Dale's principle. Brain_sentence_163

Thus, a neuron can be characterized by the neurotransmitters that it releases. Brain_sentence_164

The great majority of psychoactive drugs exert their effects by altering specific neurotransmitter systems. Brain_sentence_165

This applies to drugs such as cannabinoids, nicotine, heroin, cocaine, alcohol, fluoxetine, chlorpromazine, and many others. Brain_sentence_166

The two neurotransmitters that are most widely found in the vertebrate brain are glutamate, which almost always exerts excitatory effects on target neurons, and gamma-aminobutyric acid (GABA), which is almost always inhibitory. Brain_sentence_167

Neurons using these transmitters can be found in nearly every part of the brain. Brain_sentence_168

Because of their ubiquity, drugs that act on glutamate or GABA tend to have broad and powerful effects. Brain_sentence_169

Some general anesthetics act by reducing the effects of glutamate; most tranquilizers exert their sedative effects by enhancing the effects of GABA. Brain_sentence_170

There are dozens of other chemical neurotransmitters that are used in more limited areas of the brain, often areas dedicated to a particular function. Brain_sentence_171

Serotonin, for example—the primary target of many antidepressant drugs and many dietary aids—comes exclusively from a small brainstem area called the raphe nuclei. Brain_sentence_172

Norepinephrine, which is involved in arousal, comes exclusively from a nearby small area called the locus coeruleus. Brain_sentence_173

Other neurotransmitters such as acetylcholine and dopamine have multiple sources in the brain but are not as ubiquitously distributed as glutamate and GABA. Brain_sentence_174

Electrical activity Brain_section_11

As a side effect of the electrochemical processes used by neurons for signaling, brain tissue generates electric fields when it is active. Brain_sentence_175

When large numbers of neurons show synchronized activity, the electric fields that they generate can be large enough to detect outside the skull, using electroencephalography (EEG) or magnetoencephalography (MEG). Brain_sentence_176

EEG recordings, along with recordings made from electrodes implanted inside the brains of animals such as rats, show that the brain of a living animal is constantly active, even during sleep. Brain_sentence_177

Each part of the brain shows a mixture of rhythmic and nonrhythmic activity, which may vary according to behavioral state. Brain_sentence_178

In mammals, the cerebral cortex tends to show large slow delta waves during sleep, faster alpha waves when the animal is awake but inattentive, and chaotic-looking irregular activity when the animal is actively engaged in a task, called beta and gamma waves. Brain_sentence_179

During an epileptic seizure, the brain's inhibitory control mechanisms fail to function and electrical activity rises to pathological levels, producing EEG traces that show large wave and spike patterns not seen in a healthy brain. Brain_sentence_180

Relating these population-level patterns to the computational functions of individual neurons is a major focus of current research in neurophysiology. Brain_sentence_181

Metabolism Brain_section_12

All vertebrates have a blood–brain barrier that allows metabolism inside the brain to operate differently from metabolism in other parts of the body. Brain_sentence_182

Glial cells play a major role in brain metabolism by controlling the chemical composition of the fluid that surrounds neurons, including levels of ions and nutrients. Brain_sentence_183

Brain tissue consumes a large amount of energy in proportion to its volume, so large brains place severe metabolic demands on animals. Brain_sentence_184

The need to limit body weight in order, for example, to fly, has apparently led to selection for a reduction of brain size in some species, such as bats. Brain_sentence_185

Most of the brain's energy consumption goes into sustaining the electric charge (membrane potential) of neurons. Brain_sentence_186

Most vertebrate species devote between 2% and 8% of basal metabolism to the brain. Brain_sentence_187

In primates, however, the percentage is much higher—in humans it rises to 20–25%. Brain_sentence_188

The energy consumption of the brain does not vary greatly over time, but active regions of the cerebral cortex consume somewhat more energy than inactive regions; this forms the basis for the functional brain imaging methods of PET, fMRI, and NIRS. Brain_sentence_189

The brain typically gets most of its energy from oxygen-dependent metabolism of glucose (i.e., blood sugar), but ketones provide a major alternative source, together with contributions from medium chain fatty acids (caprylic and heptanoic acids), lactate, acetate, and possibly amino acids. Brain_sentence_190

Function Brain_section_13

Information from the sense organs is collected in the brain. Brain_sentence_191

There it is used to determine what actions the organism is to take. Brain_sentence_192

The brain processes the raw data to extract information about the structure of the environment. Brain_sentence_193

Next it combines the processed information with information about the current needs of the animal and with memory of past circumstances. Brain_sentence_194

Finally, on the basis of the results, it generates motor response patterns. Brain_sentence_195

These signal-processing tasks require intricate interplay between a variety of functional subsystems. Brain_sentence_196

The function of the brain is to provide coherent control over the actions of an animal. Brain_sentence_197

A centralized brain allows groups of muscles to be co-activated in complex patterns; it also allows stimuli impinging on one part of the body to evoke responses in other parts, and it can prevent different parts of the body from acting at cross-purposes to each other. Brain_sentence_198

Perception Brain_section_14

The human brain is provided with information about light, sound, the chemical composition of the atmosphere, temperature, the position of the body in space (proprioception), the chemical composition of the bloodstream, and more. Brain_sentence_199

In other animals additional senses are present, such as the infrared heat-sense of snakes, the magnetic field sense of some birds, or the electric field sense mainly seen in aquatic animals. Brain_sentence_200

Each sensory system begins with specialized receptor cells, such as photoreceptor cells in the retina of the eye, or vibration-sensitive hair cells in the cochlea of the ear. Brain_sentence_201

The axons of sensory receptor cells travel into the spinal cord or brain, where they transmit their signals to a first-order sensory nucleus dedicated to one specific sensory modality. Brain_sentence_202

This primary sensory nucleus sends information to higher-order sensory areas that are dedicated to the same modality. Brain_sentence_203

Eventually, via a way-station in the thalamus, the signals are sent to the cerebral cortex, where they are processed to extract the relevant features, and integrated with signals coming from other sensory systems. Brain_sentence_204

Motor control Brain_section_15

Motor systems are areas of the brain that are involved in initiating body movements, that is, in activating muscles. Brain_sentence_205

Except for the muscles that control the eye, which are driven by nuclei in the midbrain, all the voluntary muscles in the body are directly innervated by motor neurons in the spinal cord and hindbrain. Brain_sentence_206

Spinal motor neurons are controlled both by neural circuits intrinsic to the spinal cord, and by inputs that descend from the brain. Brain_sentence_207

The intrinsic spinal circuits implement many reflex responses, and contain pattern generators for rhythmic movements such as walking or swimming. Brain_sentence_208

The descending connections from the brain allow for more sophisticated control. Brain_sentence_209

The brain contains several motor areas that project directly to the spinal cord. Brain_sentence_210

At the lowest level are motor areas in the medulla and pons, which control stereotyped movements such as walking, breathing, or swallowing. Brain_sentence_211

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. Brain_sentence_212

At a higher level yet is the primary motor cortex, a strip of tissue located at the posterior edge of the frontal lobe. Brain_sentence_213

The primary motor cortex sends projections to the subcortical motor areas, but also sends a massive projection directly to the spinal cord, through the pyramidal tract. Brain_sentence_214

This direct corticospinal projection allows for precise voluntary control of the fine details of movements. Brain_sentence_215

Other motor-related brain areas exert secondary effects by projecting to the primary motor areas. Brain_sentence_216

Among the most important secondary areas are the premotor cortex, supplementary motor area, basal ganglia, and cerebellum. Brain_sentence_217

In addition to all of the above, the brain and spinal cord contain extensive circuitry to control the autonomic nervous system which controls the movement of the smooth muscle of the body. Brain_sentence_218

Brain_table_general_2

Major areas involved in controlling movementBrain_table_caption_2
AreaBrain_header_cell_2_0_0 LocationBrain_header_cell_2_0_1 FunctionBrain_header_cell_2_0_2
Ventral hornBrain_header_cell_2_1_0 Spinal cordBrain_cell_2_1_1 Contains motor neurons that directly activate musclesBrain_cell_2_1_2
Oculomotor nucleiBrain_header_cell_2_2_0 MidbrainBrain_cell_2_2_1 Contains motor neurons that directly activate the eye musclesBrain_cell_2_2_2
CerebellumBrain_header_cell_2_3_0 HindbrainBrain_cell_2_3_1 Calibrates precision and timing of movementsBrain_cell_2_3_2
Basal gangliaBrain_header_cell_2_4_0 ForebrainBrain_cell_2_4_1 Action selection on the basis of motivationBrain_cell_2_4_2
Motor cortexBrain_header_cell_2_5_0 Frontal lobeBrain_cell_2_5_1 Direct cortical activation of spinal motor circuitsBrain_cell_2_5_2
Premotor cortexBrain_header_cell_2_6_0 Frontal lobeBrain_cell_2_6_1 Groups elementary movements into coordinated patternsBrain_cell_2_6_2
Supplementary motor areaBrain_header_cell_2_7_0 Frontal lobeBrain_cell_2_7_1 Sequences movements into temporal patternsBrain_cell_2_7_2
Prefrontal cortexBrain_header_cell_2_8_0 Frontal lobeBrain_cell_2_8_1 Planning and other executive functionsBrain_cell_2_8_2

Sleep Brain_section_16

Main article: Sleep Brain_sentence_219

See also: Arousal Brain_sentence_220

Many animals alternate between sleeping and waking in a daily cycle. Brain_sentence_221

Arousal and alertness are also modulated on a finer time scale by a network of brain areas. Brain_sentence_222

A key component of the sleep 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. Brain_sentence_223

The SCN contains the body's central biological clock. Brain_sentence_224

Neurons there show activity levels that rise and fall with a period of about 24 hours, circadian rhythms: these activity fluctuations are driven by rhythmic changes in expression of a set of "clock genes". Brain_sentence_225

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 receives input from the optic nerves, through the retinohypothalamic tract (RHT), that allows daily light-dark cycles to calibrate the clock. Brain_sentence_226

The SCN projects to a set of areas in the hypothalamus, brainstem, and midbrain that are involved in implementing sleep-wake cycles. Brain_sentence_227

An important component of the system is the reticular formation, a group of neuron-clusters scattered diffusely through the core of the lower brain. Brain_sentence_228

Reticular neurons send signals to the thalamus, which in turn sends activity-level-controlling signals to every part of the cortex. Brain_sentence_229

Damage to the reticular formation can produce a permanent state of coma. Brain_sentence_230

Sleep involves great changes in brain activity. Brain_sentence_231

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 patterns become very different. Brain_sentence_232

There are two types of sleep: REM sleep (with dreaming) and NREM (non-REM, usually without dreaming) sleep, which repeat in slightly varying patterns throughout a sleep episode. Brain_sentence_233

Three broad types of distinct brain activity patterns can be measured: REM, light NREM and deep NREM. Brain_sentence_234

During deep NREM sleep, also called slow wave sleep, activity in the cortex takes the form of large synchronized waves, whereas in the waking state it is noisy and desynchronized. Brain_sentence_235

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. Brain_sentence_236

Homeostasis Brain_section_17

For any animal, survival requires maintaining a variety of parameters of bodily state within a limited range of variation: these include temperature, water content, salt concentration in the bloodstream, blood glucose levels, blood oxygen level, and others. Brain_sentence_237

The ability of an animal to regulate the internal environment of its body—the milieu intérieur, as the pioneering physiologist Claude Bernard called it—is known as homeostasis (Greek for "standing still"). Brain_sentence_238

Maintaining homeostasis is a crucial function of the brain. Brain_sentence_239

The basic principle that underlies homeostasis is negative feedback: any time a parameter diverges from its set-point, sensors generate an error signal that evokes a response that causes the parameter to shift back toward its optimum value. Brain_sentence_240

(This principle is widely used in engineering, for example in the control of temperature using a thermostat.) Brain_sentence_241

In vertebrates, the part of the brain that plays the greatest role is the hypothalamus, a small region at the base of the forebrain whose size does not reflect its complexity or the importance of its function. Brain_sentence_242

The hypothalamus is a collection of small nuclei, most of which are involved in basic biological functions. Brain_sentence_243

Some of these functions relate to arousal or to social interactions such as sexuality, aggression, or maternal behaviors; but many of them relate to homeostasis. Brain_sentence_244

Several hypothalamic nuclei receive input from sensors located in the lining of blood vessels, conveying information about temperature, sodium level, glucose level, blood oxygen level, and other parameters. Brain_sentence_245

These hypothalamic nuclei send output signals to motor areas that can generate actions to rectify deficiencies. Brain_sentence_246

Some of the outputs also go to the pituitary gland, a tiny gland attached to the brain directly underneath the hypothalamus. Brain_sentence_247

The pituitary gland secretes hormones into the bloodstream, where they circulate throughout the body and induce changes in cellular activity. Brain_sentence_248

Motivation Brain_section_18

The individual animals need to express survival-promoting behaviors, such as seeking food, water, shelter, and a mate. Brain_sentence_249

The motivational system in the brain monitors the current state of satisfaction of these goals, and activates behaviors to meet any needs that arise. Brain_sentence_250

The motivational system works largely by a reward–punishment mechanism. Brain_sentence_251

When a particular behavior is followed by favorable consequences, the reward mechanism in the brain is activated, which induces structural changes inside the brain that cause the same behavior to be repeated later, whenever a similar situation arises. Brain_sentence_252

Conversely, when a behavior is followed by unfavorable consequences, the brain's punishment mechanism is activated, inducing structural changes that cause the behavior to be suppressed when similar situations arise in the future. Brain_sentence_253

Most organisms studied to date utilize a reward–punishment mechanism: for instance, worms and insects can alter their behavior to seek food sources or to avoid dangers. Brain_sentence_254

In vertebrates, the reward-punishment system is implemented by a specific set of brain structures, at the heart of which lie the basal ganglia, a set of interconnected areas at the base of the forebrain. Brain_sentence_255

The basal ganglia are the central site at which decisions are made: the basal ganglia exert a sustained inhibitory control over most of the motor systems in the brain; when this inhibition is released, a motor system is permitted to execute the action it is programmed to carry out. Brain_sentence_256

Rewards and punishments function by altering the relationship between the inputs that the basal ganglia receive and the decision-signals that are emitted. Brain_sentence_257

The reward mechanism is better understood than the punishment mechanism, because its role in drug abuse has caused it to be studied very intensively. Brain_sentence_258

Research has shown that the neurotransmitter dopamine plays a central role: addictive drugs such as cocaine, amphetamine, and nicotine either cause dopamine levels to rise or cause the effects of dopamine inside the brain to be enhanced. Brain_sentence_259

Learning and memory Brain_section_19

Almost all animals are capable of modifying their behavior as a result of experience—even the most primitive types of worms. Brain_sentence_260

Because behavior is driven by brain activity, changes in behavior must somehow correspond to changes inside the brain. Brain_sentence_261

Already in the late 19th century theorists like Santiago Ramón y Cajal argued that the most plausible explanation is that learning and memory are expressed as changes in the synaptic connections between neurons. Brain_sentence_262

Until 1970, however, experimental evidence to support the synaptic plasticity hypothesis was lacking. Brain_sentence_263

In 1971 Tim Bliss and Terje Lømo published a paper on a phenomenon now called long-term potentiation: the paper showed clear evidence of activity-induced synaptic changes that lasted for at least several days. Brain_sentence_264

Since then technical advances have made these sorts of experiments much easier to carry out, and thousands of studies have been made that have clarified the mechanism of synaptic change, and uncovered other types of activity-driven synaptic change in a variety of brain areas, including the cerebral cortex, hippocampus, basal ganglia, and cerebellum. Brain_sentence_265

Brain-derived neurotrophic factor (BDNF) and physical activity appear to play a beneficial role in the process. Brain_sentence_266

Neuroscientists currently distinguish several types of learning and memory that are implemented by the brain in distinct ways: Brain_sentence_267

Brain_unordered_list_2

  • Working memory is the ability of the brain to maintain a temporary representation of information about the task that an animal is currently engaged in. This sort of dynamic memory is thought to be mediated by the formation of cell assemblies—groups of activated neurons that maintain their activity by constantly stimulating one another.Brain_item_2_13
  • Episodic memory is the ability to remember the details of specific events. This sort of memory can last for a lifetime. Much evidence implicates the hippocampus in playing a crucial role: people with severe damage to the hippocampus sometimes show amnesia, that is, inability to form new long-lasting episodic memories.Brain_item_2_14
  • Semantic memory is the ability to learn facts and relationships. This sort of memory is probably stored largely in the cerebral cortex, mediated by changes in connections between cells that represent specific types of information.Brain_item_2_15
  • Instrumental learning is the ability for rewards and punishments to modify behavior. It is implemented by a network of brain areas centered on the basal ganglia.Brain_item_2_16
  • Motor learning is the ability to refine patterns of body movement by practicing, or more generally by repetition. A number of brain areas are involved, including the premotor cortex, basal ganglia, and especially the cerebellum, which functions as a large memory bank for microadjustments of the parameters of movement.Brain_item_2_17

Research Brain_section_20

Main article: Neuroscience Brain_sentence_268

"Brain research" redirects here. Brain_sentence_269

For the scientific journal, see Brain Research. Brain_sentence_270

The field of neuroscience encompasses all approaches that seek to understand the brain and the rest of the nervous system. Brain_sentence_271

Psychology seeks to understand mind and behavior, and neurology is the medical discipline that diagnoses and treats diseases of the nervous system. Brain_sentence_272

The brain is also the most important organ studied in psychiatry, the branch of medicine that works to study, prevent, and treat mental disorders. Brain_sentence_273

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. Brain_sentence_274

The oldest method of studying the brain is anatomical, and until the middle of the 20th century, much of the progress in neuroscience came from the development of better cell stains and better microscopes. Brain_sentence_275

Neuroanatomists study the large-scale structure of the brain as well as the microscopic structure of neurons and their components, especially synapses. Brain_sentence_276

Among other tools, they employ a plethora of stains that reveal neural structure, chemistry, and connectivity. Brain_sentence_277

In recent years, the development of immunostaining techniques has allowed investigation of neurons that express specific sets of genes. Brain_sentence_278

Also, functional neuroanatomy uses medical imaging techniques to correlate variations in human brain structure with differences in cognition or behavior. Brain_sentence_279

Neurophysiologists study the chemical, pharmacological, and electrical properties of the brain: their primary tools are drugs and recording devices. Brain_sentence_280

Thousands of experimentally developed drugs affect the nervous system, some in highly specific ways. Brain_sentence_281

Recordings of brain activity can be made using electrodes, either glued to the scalp as in EEG studies, or implanted inside the brains of animals for extracellular recordings, which can detect action potentials generated by individual neurons. Brain_sentence_282

Because the brain does not contain pain receptors, it is possible using these techniques to record brain activity from animals that are awake and behaving without causing distress. Brain_sentence_283

The same techniques have occasionally been used to study brain activity in human patients suffering from intractable epilepsy, in cases where there was a medical necessity to implant electrodes to localize the brain area responsible for epileptic seizures. Brain_sentence_284

Functional imaging techniques such as fMRI are also used to study brain activity; these techniques have mainly been used with human subjects, because they require a conscious subject to remain motionless for long periods of time, but they have the great advantage of being noninvasive. Brain_sentence_285

Another approach to brain function is to examine the consequences of damage to specific brain areas. Brain_sentence_286

Even though it is protected by the skull and meninges, surrounded by cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier, the delicate nature of the brain makes it vulnerable to numerous diseases and several types of damage. Brain_sentence_287

In humans, the effects of strokes and other types of brain damage have been a key source of information about brain function. Brain_sentence_288

Because there is no ability to experimentally control the nature of the damage, however, this information is often difficult to interpret. Brain_sentence_289

In animal studies, most commonly involving rats, it is possible to use electrodes or locally injected chemicals to produce precise patterns of damage and then examine the consequences for behavior. Brain_sentence_290

Computational neuroscience encompasses two approaches: first, the use of computers to study the brain; second, the study of how brains perform computation. Brain_sentence_291

On one hand, it is possible to write a computer program to simulate the operation of a group of neurons by making use of systems of equations that describe their electrochemical activity; such simulations are known as biologically realistic neural networks. Brain_sentence_292

On the other hand, it is possible to study algorithms for neural computation by simulating, or mathematically analyzing, the operations of simplified "units" that have some of the properties of neurons but abstract out much of their biological complexity. Brain_sentence_293

The computational functions of the brain are studied both by computer scientists and neuroscientists. Brain_sentence_294

Computational neurogenetic modeling is concerned with the study and development of dynamic neuronal models for modeling brain functions with respect to genes and dynamic interactions between genes. Brain_sentence_295

Recent years have seen increasing applications of genetic and genomic techniques to the study of the brain and a focus on the roles of neurotrophic factors and physical activity in neuroplasticity. Brain_sentence_296

The most common subjects are mice, because of the availability of technical tools. Brain_sentence_297

It is now possible with relative ease to "knock out" or mutate a wide variety of genes, and then examine the effects on brain function. Brain_sentence_298

More sophisticated approaches are also being used: for example, using Cre-Lox recombination it is possible to activate or deactivate genes in specific parts of the brain, at specific times. Brain_sentence_299

History Brain_section_21

See also: History of neuroscience Brain_sentence_300

The oldest brain to have been discovered was in Armenia in the Areni-1 cave complex. Brain_sentence_301

The brain, estimated to be over 5,000 years old, was found in the skull of a 12 to 14-year-old girl. Brain_sentence_302

Although the brains were shriveled, they were well preserved due to the climate found inside the cave. Brain_sentence_303

Early philosophers were divided as to whether the seat of the soul lies in the brain or heart. Brain_sentence_304

Aristotle favored the heart, and thought that the function of the brain was merely to cool the blood. Brain_sentence_305

Democritus, the inventor of the atomic theory of matter, argued for a three-part soul, with intellect in the head, emotion in the heart, and lust near the liver. Brain_sentence_306

The unknown author of On the Sacred Disease, a medical treatise in the Hippocratic Corpus, came down unequivocally in favor of the brain, writing: Brain_sentence_307

The Roman physician Galen also argued for the importance of the brain, and theorized in some depth about how it might work. Brain_sentence_308

Galen traced out the anatomical relationships among brain, nerves, and muscles, demonstrating that all muscles in the body are connected to the brain through a branching network of nerves. Brain_sentence_309

He postulated that nerves activate muscles mechanically by carrying a mysterious substance he called pneumata psychikon, usually translated as "animal spirits". Brain_sentence_310

Galen's 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 René Descartes and those who followed him. Brain_sentence_311

Descartes, like Galen, thought of the nervous system in hydraulic terms. Brain_sentence_312

He believed that the highest cognitive functions are carried out by a non-physical res cogitans, but that the majority of behaviors of humans, and all behaviors of animals, could be explained mechanistically. Brain_sentence_313

The first real progress toward a modern understanding of nervous function, though, came from the investigations of Luigi Galvani (1737–1798), who discovered that a shock of static electricity applied to an exposed nerve of a dead frog could cause its leg to contract. Brain_sentence_314

Since that time, each major advance in understanding has followed more or less directly from the development of a new technique of investigation. Brain_sentence_315

Until the early years of the 20th century, the most important advances were derived from new methods for staining cells. Brain_sentence_316

Particularly critical was the invention of the Golgi stain, which (when correctly used) stains only a small fraction of neurons, but stains them in their entirety, including cell body, dendrites, and axon. Brain_sentence_317

Without such a stain, brain tissue under a microscope appears as an impenetrable tangle of protoplasmic fibers, in which it is impossible to determine any structure. Brain_sentence_318

In the hands of Camillo Golgi, and especially of the Spanish neuroanatomist Santiago Ramón y Cajal, the new stain revealed hundreds of distinct types of neurons, each with its own unique dendritic structure and pattern of connectivity. Brain_sentence_319

In the first half of the 20th century, advances in electronics enabled investigation of the electrical properties of nerve cells, culminating in work by Alan Hodgkin, Andrew Huxley, and others on the biophysics of the action potential, and the work of Bernard Katz and others on the electrochemistry of the synapse. Brain_sentence_320

These studies complemented the anatomical picture with a conception of the brain as a dynamic entity. Brain_sentence_321

Reflecting the new understanding, in 1942 Charles Sherrington visualized the workings of the brain waking from sleep: Brain_sentence_322

The invention of electronic computers in the 1940s, along with the development of mathematical information theory, led to a realization that brains can potentially be understood as information processing systems. Brain_sentence_323

This concept formed the basis of the field of cybernetics, and eventually gave rise to the field now known as computational neuroscience. Brain_sentence_324

The earliest attempts at cybernetics were somewhat crude in that they treated the brain as essentially a digital computer in disguise, as for example in John von Neumann's 1958 book, The Computer and the Brain. Brain_sentence_325

Over the years, though, accumulating information about the electrical responses of brain cells recorded from behaving animals has steadily moved theoretical concepts in the direction of increasing realism. Brain_sentence_326

One of the most influential early contributions was a 1959 paper titled What the frog's eye tells the frog's brain: the paper examined the visual responses of neurons in the retina and optic tectum of frogs, and came to the conclusion that some neurons in the tectum of the frog are wired to combine elementary responses in a way that makes them function as "bug perceivers". Brain_sentence_327

A few years later David Hubel and Torsten Wiesel discovered cells in the primary visual cortex of monkeys that become active when sharp edges move across specific points in the field of view—a discovery for which they won a Nobel Prize. Brain_sentence_328

Follow-up studies in higher-order visual areas found cells that detect binocular disparity, color, movement, and aspects of shape, with areas located at increasing distances from the primary visual cortex showing increasingly complex responses. Brain_sentence_329

Other investigations of brain areas unrelated to vision have revealed cells with a wide variety of response correlates, some related to memory, some to abstract types of cognition such as space. Brain_sentence_330

Theorists have worked to understand these response patterns by constructing mathematical models of neurons and neural networks, which can be simulated using computers. Brain_sentence_331

Some useful models are abstract, focusing on the conceptual structure of neural algorithms rather than the details of how they are implemented in the brain; other models attempt to incorporate data about the biophysical properties of real neurons. Brain_sentence_332

No model on any level is yet considered to be a fully valid description of brain function, though. Brain_sentence_333

The essential difficulty is that sophisticated computation by neural networks requires distributed processing in which hundreds or thousands of neurons work cooperatively—current methods of brain activity recording are only capable of isolating action potentials from a few dozen neurons at a time. Brain_sentence_334

Furthermore, even single neurons appear to be complex and capable of performing computations. Brain_sentence_335

So, brain models that don't reflect this are too abstract to be representative of brain operation; models that do try to capture this are very computationally expensive and arguably intractable with present computational resources. Brain_sentence_336

However, the Human Brain Project is trying to build a realistic, detailed computational model of the entire human brain. Brain_sentence_337

The wisdom of this approach has been publicly contested, with high-profile scientists on both sides of the argument. Brain_sentence_338

In the second half of the 20th century, developments in chemistry, electron microscopy, genetics, computer science, functional brain imaging, and other fields progressively opened new windows into brain structure and function. Brain_sentence_339

In the United States, the 1990s were officially designated as the "Decade of the Brain" to commemorate advances made in brain research, and to promote funding for such research. Brain_sentence_340

In the 21st century, these trends have continued, and several new approaches have come into prominence, including multielectrode recording, which allows the activity of many brain cells to be recorded all at the same time; genetic engineering, which allows molecular components of the brain to be altered experimentally; genomics, which allows variations in brain structure to be correlated with variations in DNA properties and neuroimaging. Brain_sentence_341

Other uses Brain_section_22

As food Brain_section_23

Animal brains are used as food in numerous cuisines. Brain_sentence_342

In rituals Brain_section_24

Some archaeological evidence suggests that the mourning rituals of European Neanderthals also involved the consumption of the brain. Brain_sentence_343

The Fore people of Papua New Guinea are known to eat human brains. Brain_sentence_344

In funerary rituals, those close to the dead would eat the brain of the deceased to create a sense of immortality. Brain_sentence_345

A prion disease called kuru has been traced to this. Brain_sentence_346

See also Brain_section_25

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Brain.