Difference between revisions of "Brain" - New World Encyclopedia

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[[Image:Brains.jpg|thumb|right|230px|Comparative brain sizes]]
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In [[animal]]s, the '''brain''', or ''encephalon'' ([[Greek (language)|Greek]] for "in the head"), is the control center of the [[central nervous system]]. In most animals, the brain is located in the head close to the primary sensory apparatus and the mouth. While all [[vertebrate]]s have a brain, [[invertebrate]]s have either a centralized brain or collections of individual [[ganglion|ganglia]]. Brains can be extremely complex.  For example, the [[human brain]] contains more than 100 billion [[neuron]]s, each linked to as many as 10,000 others<ref>{{cite web | last = Chudler | first = Eric H. | year = 2006 | url = http://staff.washington.edu/chudler/facts.html | title = Brain Facts and Figures | work = Neuroscience for Kids | accessdate = May 19 | accessyear = 2006}}</ref>.
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[[Image:Brain.svg|thumb|A drawing of the human brain.]]
  
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==History==
 
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{{main|History of the brain}}
 
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Early views on the function of the [[brain]] regarded it to be a form of “cranial stuffing” of sorts.  In Egypt, from the late [[Middle Kingdom]] onwards, in preparation for mummification, the brain was regularly removed, for it was the [[heart]] that was assumed to be the seat of intelligence.  According to [[Herodotus]], during the first step of mummification: ‘The most perfect practice is to extract as much of the brain as possible with an iron hook, and what the hook cannot reach is mixed with drugs.’  Over the next five-thousand years, this view came to be reversed; the brain is now known to be the seat of intelligence, although colloquial variations of the former remain as in “memorizing something by heart”.
 
  
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== Overview ==
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The '''brain''' is a centralized mass of [[nerve]] tissue enclosed within the [[cranium]] (skull) of [[vertebrate]]s; a related structure is also present in some [[invertebrate]]s. This [[organ (anatomy)|organ]] functions to process, store, and interpret information and to control the physiology and behavior of the body. In higher organisms, it is also the site of reason and intelligence, which include such components as [[cognition]], [[perception]], [[attention]], [[memory]] and [[emotion]].  
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The brain is not only important as the site of reason and intelligence, it is also the source of [[cognition]], [[emotion]], [[memory]], and [[motor learning|motor]], and other forms of [[learning]], and it controls and coordinates most [[sensory system]]s, [[Animal locomotion|movement]], [[behavior]], but it also controls [[homeostasis|homeostatic]] body functions such as [[heart rate]], [[blood]] pressure, fluid balance, and body temperature.  Some behaviors such as simple [[reflex action|reflexes]] and basic [[animal locomotion|locomotion]], can be executed under [[spinal cord]] control alone.
 
  
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Most brains exhibit a visible distinction between [[grey matter]] and [[white matter]]. Grey matter consists of the [[cell (biology)|cell]] bodies of the neurons, while white matter consists of the fibers ([[axon]]s) that connect neurons. The axons are surrounded by a [[fat]]ty [[Electrical insulation|insulating]] sheath called [[myelin]], giving the white matter its distinctive color. The outer, visible layers of the brain are the [[cerebral cortex|cortex]], and consist mainly of grey matter.
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Along with the [[spinal cord]], the brain forms part of the [[central nervous system]], a vast network of neurons that receives sensory signals from the [[peripheral nervous system]] and conveys information to the muscles and glands of the body. [[Neuron]]s, which generate [[action potential]]s that communicate information to other cells, constitute the essential class of brain cells. The [[human brain]], for example, contains more than 10 billion neurons, each linked to as many as 10,000 other neurons. The brain also contains various types of [[glial cell]]s, which support the function of neurons.
  
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The study of the brain is known as [[neuroscience]], a field of [[biology]] aimed at understanding the functions of the brain at every level, from the [[molecule|molecular]] up to the [[psychology|psychological]].
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[[Image:Brain Mri nevit.svg|thumb|right|Representation of a brain MRI, an imaging technique used to measure neural activity.]]
  
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==Mind and brain==
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The vertebrate brain has three main sections, commonly referred to as the [[hindbrain]], [[midbrain]], and [[forebrain]], each of which contains several structures. In general, as one moves from hindbrain to forebrain, one progresses from structures involved in more autonomic (or involuntary) functioning (like [[Animal locomotion|walking]] or maintaining constant [[heart rate]]) to those coordinating higher functioning (such as learning and memory).  
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{{portalpar|Mind and Brain}}
 
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A distinction is often made in the [[philosophy of mind]] between the [[mind]] and the brain, and there is some controversy as to their exact relationship, leading to the [[mind-body problem]]. The brain is defined as the physical, biological [[matter]] contained within the [[skull]], responsible for all electrochemical neuronal processes. The mind, however, is seen in terms of mental attributes, such as beliefs or desires.  Some suggest that the mind exists in some way independently of the brain, such as in a [[soul]] or [[epiphenomenalism|epiphenomenon]]. Others, such as [[strong AI]] theorists, say that the mind is directly analogous to [[computer software]] and the brain to [[computer hardware|hardware]].
 
  
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==Comparative anatomy==
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As we travel up the vertebrate phylogenetic scale from fish to mammals, the [[cerebrum]] (a structure within the forebrain) increases in size, complexity, and importance, dominating the nervous systems of mammals. For example, major damage to this area results in severe impairment or even coma in mammals, while a shark whose cerebrum has been removed can swim with relatively normal function. Nonetheless, animals have many structures in common, and progress in [[neuroscience]], a field of [[biology]] aimed at understanding the functions of the brain at every level, has come from research on the simpler nervous systems of invertebrates.
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[[Image:Mouse_brain.jpg|thumb|right|A mouse brain.]]
 
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Three groups of animals have notably complex brains: the [[arthropod]]s ([[insect]]s and [[crustacean]]s), the [[cephalopod]]s ([[octopus]]es, [[squid]]s, and similar [[mollusk]]s), and the [[craniata|craniates]] ([[vertebrate]]s)<ref name="butler">{{cite journal | last = Butler | first = Ann B. | title = Chordate Evolution and the Origin of Craniates: An Old Brain in a New Head | journal = The Anatomical Record | year = 2000 | volume = 261 | pages = 111–125 }}</ref>. The brain of arthropods and cephalopods arises 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.<ref name="butler"/>
 
  
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The brain of craniates develops from the [[anatomical terms of location|anterior]] section of a single dorsal [[neural tube|nerve cord]], which later becomes the [[spinal cord]]<ref>{{cite book | authorlink = Eric R. Kandel | last = Kandel | first = ER | coauthors = Schwartz JH, Jessell TM | title = [[Principles of Neural Science]] | edition = 4th ed. | publisher = McGraw-Hill | location = New York | year = 2000 | id = ISBN 0838577016 }}</ref>. In craniates, the brain is protected by the [[bone]]s of the [[skull]]. In vertebrates, increasing [[complexity]] in the [[cerebral cortex]] correlates with height on the [[phylogenetic tree|phylogenetic]] and [[evolutionary tree]]. Primitive vertebrates such as [[fish]], [[reptile]]s, and [[amphibian]]s have fewer than six layers of neurons in the outer layer of their brains. This cortical configuration is called the [[allocortex]] (or heterotypic cortex)<ref name="martin">{{cite book | last = Martin | first = John H. | title = Neuroanatomy: Text and Atlas | edition = Second Edition | publisher = McGraw-Hill | location = New York | year = 1996 | id = ISBN 007138183X }}</ref>.
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A central issue in [[philosophy]] and [[religion]] is how the brain relates to the [[mind]], that is the mind-body problem. (See [[Brain#The relation between mind and brain|The relation between mind and brain]] section below.)  
  
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More complex vertebrates such as [[mammal]]s have a six-layered [[neopallium|neocortex]] (or homotypic cortex, neopallium), in addition to having some parts of the brain that are allocortex.<ref name="martin"/> In mammals, increasing convolutions of the brain are characteristic of animals with more advanced brains. These convolutions provide a larger surface area for a greater number of neurons while keeping the volume of the brain compact enough to fit inside the skull. The folding allows more grey matter to fit into a smaller volume, similar to a really long slinky being able to fit into a tiny box when completely pushed together. The folds are called [[gyrus|gyri]], while the spaces between the folds are called [[Sulcus (anatomy)|sulci]].
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==Anatomy==
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===The brain is composed of two broad classes of cells===
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{{Neuron map|[[Neuron]]}}
  
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Although the general [[histology]] of the brain is similar from person to person, the structural anatomy can differ. Apart from the gross [[embryology|embryological]] divisions of the brain, the location of specific gyri and sulci, primary sensory regions, and other structures differs between species.
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The brain is composed of two broad classes of cells, [[neuron]]s and [[glia]], both of which contain several different cell types that perform specialized functions.  
  
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===Invertebrates===
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The neuron is the functional unit of the brain. Interconnected neurons form [[neural network]]s (or [[neural ensemble]]s). These networks are similar to man-made [[electrical circuit]]s in that they contain circuit elements (neurons) connected by biological wires (nerve fibers). These do not form simple one-to-one electrical circuits like many man-made circuits, however. Typically, neurons connect to at least one thousand other neurons (Jungueira and Carneiro 2003). These highly specialized circuits make up systems that are the basis of [[perception]], different types of action, and higher cognitive function.
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In insects, the brain has four parts, the optical lobes, the protocerebrum, the deutocerebrum, and the tritocerebrum. The optical lobes are behind each eye and process visual stimuli.<ref name="butler"/> The protocerebrum contains the [[mushroom bodies]], which respond to [[olfaction|smell]], and the central body complex. In some [[species]] such as [[bee]]s, the mushroom body receives input from the visual pathway as well. The deutocerebrum includes the [[antennal lobe]]s, which are similar to the mammalian [[olfactory bulb]], and the [[mechanosensor]]y [[neuropil]]s which receive information from [[touch]] receptors on the head and [[antenna (biology)|antennae]]. The antennal lobes of [[fly|flies]] and [[moth]]s are quite complex.
 
  
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In cephalopods, the brain has two regions: the supraesophageal mass and the subesophageal mass,<ref name="butler"/> separated by the [[esophagus]]. The supra- and subesophageal masses are connected to each other on either side of the esophagus by the basal lobes and the dorsal magnocellular lobes.<ref name="butler"/> The large optic lobes are sometimes not considered to be part of the brain, as they are anatomically separate and are joined to the brain by the optic stalks. However, the optic lobes perform much visual processing, and so functionally are part of the brain.
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In addition to neurons, the brain contains [[glial cell]]s in a roughly 10:1 proportion to neurons. Glial cells (''glia'' is the Greek term for “glue”) form a support system for neurons. They create the insulating myelin, provide structure to the neuronal network, manage waste, and clean up neurotransmitters.  
  
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===Vertebrates===
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The ''blood-brain barrier'' is constructed by a particular class of glial cell called ''astrocytes.'' Blood vessels throughout the body are permeable to many chemicals, some of which are toxic; thus, the astrocytes form a barrier to these chemicals by surrounding the smallest, most permeable blood vessels in the brain. Protection is crucial because, unlike other tissues of the body, the brain cannot recover from damage by generating new cells (in mammals, production of neurons by cell division ceases shortly after birth). However, the barrier is not totally impermeable: fat-soluble substances like anesthetics and alcohol have notable effects on the brain.
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[[Image:Brain-anatomy.jpg|right|thumb|350px|The lobes of the cerebral cortex include the [[frontal lobe|frontal]] (red), [[temporal lobe|temporal]] (green), [[occipital lobe|occipital]] (yellow), and [[parietal lobe]]s (orange). The [[cerebellum]] (blue) is not part of the telencephalon. In vertebrates a gross division into three major parts is used.]]
 
  
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The [[telencephalon]] (cerebrum) is the largest region of the mammalian brain. This is the structure that is most easily visible, and is what most people associate with the "brain". In humans, the fissures (sulci) and convolutions (gyri) give the brain a wrinkled appearance. In non-mammalian vertebrates with no cerebrum, the [[metencephalon]] is the highest center in the brain. Because humans walk upright, there is a flexure, or bend, in the brain between the [[brain stem]] and the cerebrum. Other vertebrates do not have this flexure, and so comparing the locations of certain brain structures between humans and other vertebrates can be confusing.
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The [[gray matter]] of the brain refers to a core area with many cell bodies ([[soma]]) of neurons, while [[white matter]] is made up of bundles of axons running up and down the cord, colored white because most axons are covered with a myelin sheath. The space between neurons is filled with dendrites as well as unmyelinated segments of axons; this area is referred to as the [[neuropil]].
  
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Behind (or in humans, below) the cerebrum is the cerebellum. The cerebellum is mainly involved in the control of movement <ref>{{cite book | authorlink = Eric R. Kandel | last = Kandel | first = ER | coauthors = Schwartz JH, Jessell TM | title = [[Principles of Neural Science]] | edition = 4th ed. | publisher = McGraw-Hill | location = New York | year = 2000 | id = ISBN 0838577016 }}</ref>, and is connected by thick white matter fibers (cerebellar peduncles) to the [[pons]].<ref name="martin"/> The cerebrum and the [[cerebellum]] each have two hemispheres. The telencephalic hemispheres are connected by the [[corpus callosum]], another large white matter tract. An outgrowth of the telencephalon called the [[olfactory bulb]] is a major structure in many animals, but in humans and other primates it is relatively small.
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===The brain is surrounded by connective tissue and bathed in fluid===
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In mammals, the brain is surrounded by [[connective tissue]]s called the [[meninges]], a system of membranes that separate the skull from the brain. This three-layered covering is composed of the [[dura mater]], [[arachnoid mater]], and [[pia mater]]. The arachnoid and pia are physically connected and thus often considered as a single layer, the pia-arachnoid. [[Blood vessel]]s enter the central nervous system through the perivascular space above the pia mater. The cells in the blood vessel walls are tightly joined, forming the blood-brain barrier described above.
  
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Vertebrate nervous systems are distinguished by [[encephalization]] and [[symmetry (biology)#Bilateral symmetry|bilateral symmetry]]. Encephalization refers to the tendency for more complex organisms to gain larger brains through evolutionary time. Larger vertebrates develop a complex, layered and interconnected neuronal circuitry. In modern species most closely related to the first vertebrates, brains are covered with gray matter that has a three-layer structure (allocortex). Their brains also contain deep brain nuclei and fiber tracts forming the white matter. Most regions of the human cerebral cortex have six layers of neurons (neocortex).<ref name="martin"/>
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The brain is bathed in [[cerebrospinal fluid]] (CSF), which circulates between layers of the meninges and through cavities in the brain called [[Ventricular system|ventricle]]s. This fluid is important both chemically (for [[metabolism]]) and mechanically (for shock-prevention). For example, the [[mass]] and [[density]] of the brain are such that it will begin to collapse under its own weight if unsupported by the CSF. The CSF allows the brain to float, easing the physical [[stress (physics)|stress]] caused by the brain’s mass (an adult human brain weighs approximately 1.5 [[kg]]).
  
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====Vertebrate brain regions====
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==Structure and function==
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(''See related article at [[List of regions in the human brain]]'')
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===The vertebrate brain is divided into three main regions===
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[[Image:EmbryonicBrain.png|thumb|right|300px|Diagram depicting the main subdivisions of the [[embryonic]] vertebrate brain. These regions will later differentiate into forebrain, midbrain and hindbrain structures.]]
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[[Image:EmbryonicBrain.png|thumb|right|250px|A diagram depicting the main subdivisions of the [[embryogenesis|embryonic]] vertebrate brain. These regions will later differentiate into forebrain, midbrain and hindbrain structures.]]
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According to the hierarchy based on embryonic and evolutionary development, [[chordate]] brains are composed of the three regions that later develop into five total divisions:
 
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*[[Rhombencephalon]] (hindbrain)
 
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**[[Myelencephalon]]
 
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**[[Metencephalon]]
 
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*[[Mesencephalon]] (midbrain)
 
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*[[Prosencephalon]] (forebrain)
 
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**[[Diencephalon]]
 
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**[[Telencephalon]]
 
  
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The brain can also be classified according to function, including divisions such as:
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Early in the development of all vertebrate embryos, a hollow neural tube forms three swellings at the head of the [[embryo]] that become the basic divisions of the brain: the hindbrain, midbrain, and forebrain. The rest of the tube develops into the spinal cord. Cranial and spinal nerves, which constitute the peripheral nervous system, sprout from the tube and continue to grow throughout embryonic development.
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*[[Limbic system]]
 
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*[[Sensory system]]s
 
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**[[Visual system]]
 
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**[[Olfactory system]]
 
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**[[Gustatory system]]
 
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**[[Auditory system]]
 
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**[[Somatosensory system]]
 
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*[[Muscle|Motor system]]
 
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*[[cerebral cortex|Associative areas]]
 
  
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====Humans====
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Each region, in turn, develops into several structures with diverse functions:
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{{main|human brain}}
 
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The structure of the human brain differs from that of other animals in several important ways. These differences allow for many abilities over and above those of other animals, such as advanced cognitive skills. Human encephalization is especially pronounced in the neocortex, the most complex part of the cerebral cortex. The proportion of the human brain that is devoted to the neocortex—especially to the [[prefrontal cortex]]—is larger than in all other [[animal]]s.
 
  
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Humans have unique neural capacities, but much of their brain structure is similar to that of other mammals. Basic systems that alert the nervous system to stimulus, that sense events in the environment, and monitor the condition of the body are similar to those of even non-mammalian vertebrates. The neural circuitry underlying human consciousness includes both the advanced neocortex and prototypical structures of the brainstem. The human brain also has a massive number of synaptic connections allowing for a great deal of [[parallel processing]].
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*Hindbrain (rhombencephalon):
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**The [[medulla]] and [[pons]] contain distinct groups of neurons involved in the control of physiological functions like breathing or basic motor patterns like swallowing.  
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**The [[cerebellum]] orchestrates and refines behavior patterns.
  
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The human brain is insensitive to pain. A headache comes from the muscles and nerves lining it, not the organ itself.
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*Midbrain (mesencephalon). All information conveyed between the higher brain and spinal cord must pass through the midbrain, which also contains structures involved in processing aspects of visual and auditory information.
  
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==Neurobiology==
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*Forebrain (prosencephalon):
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The brain is composed of two broad classes of cells, neurons and [[glia]] both of which contain several different cell types which perform different functions. Interconnected neurons form [[neural network]]s (or [[neural ensemble]]s). These networks are similar to man-made [[electrical circuit]]s in that they contain circuit elements (neurons) connected by biological wires (nerve fibers). These do not form simple one-to-one electrical circuits like many man-made circuits, however. Typically neurons connect to at least a thousand other neurons<ref>{{cite book | title = Basic Histology: Text and Atlas | edition = 10th ed. | first = L.C. | last = Junqueira | coauthors = J. Carneiro }} (Statistic from page 161)</ref>. These highly specialized circuits make up systems which are the basis of [[perception]], action, and higher cognitive function.
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**[[Diencephalon]] (central region). An upper structure (the [[thalamus]]) is the final relay station for sensory information going to the telencephalon. A lower structure called the [[hypothalamus]] regulates many physiological functions and biological drives.
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**The telencephalon (surrounding structures) consists of two cerebral hemispheres, also known as the [[cerebrum]]; in humans and other mammals, the cerebrum is by far the largest part of the brain, and it plays major roles in sensory perception, learning, memory, and conscious behavior.
  
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===Histology===
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Communication between the spinal cord and telencephalon travels through medulla, pons, midbrain, and diencephalons, structures that are collectively referred to as the ''brain stem.'' In general, more primitive and autonomic (involuntary or unconscious) functions are coordinated farther down this axis, while more complex and evolutionarily advanced functions are found higher on the brain stem.
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[[Image:Neuron.jpg|thumb|Diagram of basic features of a neuron.]]
 
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Neurons are the cells that generate action potentials and convey information to other cells; these constitute the essential class of brain cells.  
 
  
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In addition to neurons, the brain contains glial cells in a roughly 10:1 proportion to neurons. Glial cells ("glia" is Greek for “glue”) form a support system for neurons. They create the insulating myelin, provide structure to the neuronal network, manage waste, and clean up neurotransmitters. Most types of glia in the brain are present in the entire [[nervous system]]. Exceptions include the [[oligodendrocyte]]s which myelinate neural [[axon]]s (a role performed by [[Schwann cell]]s in the peripheral nervous system). The myelin in the oligodendrocytes insulates the axons of some neurons. [[White matter]] in the brain is myelinated neurons, while [[grey matter]] contains mostly cell [[soma (biology)|soma]], [[dendrite]]s, and unmyelinated portions of axons and glia. The space between neurons is filled with dendrites as well as unmyelinated segments of axons; this area is referred to as the [[neuropil]].
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===The regions of the brain may also be classified by function===
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In addition to the anatomical categories outlined above, another way of organizing our understanding of the nervous system is through functional divisions. Since the nervous system engages in parallel processing of information, any one anatomical structure may be involved in several functional subsystems:
  
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In mammals, the brain also contains [[connective tissue]] called the [[meninges]], a system of membranes that separate the skull from the brain. This three-layered covering is made of, from the outside in, [[dura mater]], [[arachnoid mater]], and [[pia mater]]. 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 vessel]]s 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 [[toxin]]s that might enter through the blood.
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*''Reticular system.'' A network of neuronal fibers that includes discrete groups of neurons (called ''nuclei''), distributed through a core of the medulla, pons, and midbrain. These specialized groups alert the forebrain to information coming from specific regions of the peripheral nervous system. Some nuclei are involved in controlling sleep and wakefulness.
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*''Limbic system.'' The evolutionarily primitive parts of the forebrain, which still have important functions in birds and mammals, but are completely covered by the more recent elaborations of the telencephalon called the [[neocortex]]. The limbic system is responsible for basic physiological drives, [[instinct]]s, and emotions, though pleasure and pain centers in the limbic system are believed to play roles in learning and physiological drives. The [[hippocampus]], one part of the limbic system, is necessary for the transfer of short-term memory to long-term memory in humans.
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*''Cerebrum.'' Regions of the brain that interact for consciousness and control of behavior.
  
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The brain is bathed in [[cerebrospinal fluid]] (CSF), which circulates between layers of the meninges and through cavities in the brain called [[ventricle]]s. It is important both chemically for [[metabolism]] and mechanically for shock-prevention. For example, the human brain weighs about 1-1.5 [[kilograms|kg]]. The [[mass]] and [[density]] of the brain are such that it will begin to collapse under its own weight. The CSF allows the brain to float, easing the [[stress (physics)|stress]] caused by the brain’s mass.
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The ''cerebral cortex'' is a sheet of gray matter that covers each cerebral hemisphere; it is ''convoluted'' (or folded) into ridges (called ''gyri'') and valleys (called ''sulci'') so that it fits into the skull. The ''corpus callosum'' is a white-matter tract that links the two hemispheres. Although the functions of some regions of the cerebral cortex are relatively easy to isolate and define, others are not: the latter areas fall under the general name of ''association cortex.''
  
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===Function===
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The cerebral cortex can be subdivided into four lobes:
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Vertebrate brains receive signals through nerves arriving from the sensors of the organism. These signals are then interpreted throughout the central nervous system reactions are formulated based upon reflex and learned experiences. A similarly extensive nerve network delivers signals from a brain to control muscles throughout the body. Anatomically, the majority of afferent and efferent nerves (with the exception of the [[cranial nerves]]) are connected to the spinal cord, which then transfers the signals to and from the brain.
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#The ''temporal lobe'' is an upper region involved in receiving and processing auditory information. Its association areas are involved in the recognition, identification, and naming of objects.
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#The ''occipital lobe'' receives and processes visual information. Its association areas are essential for making sense of the visual world and translating visual experience into language.
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#The ''parietal lobe'' contains the ''primary somatosensory cortex,'' which receives information through the thalamus about touch and pressure sensations. A major association function of the parietal lobe is attending to complex stimuli.
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#The ''frontal lobe'' contains a strip called the ''primary motor cortex.'' Its association functions are diverse and best described as having to do with planning.
  
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Sensory input is processed by the brain to recognize danger, find food, identify potential mates, and perform more sophisticated functions. [[Vision|Visual]], touch, and [[hearing (sense)|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]].
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==Comparative anatomy==
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[[Image:Mouse_brain.jpg|thumb|right|A mouse brain.]]
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Three groups of animals have notably complex brains: the [[arthropod]]s ([[insect]]s, [[crustacean]]s, [[arachnid]]s, and others); the [[cephalopod]]s ([[octopus]]es, [[squid]]s, and similar [[mollusk]]s); and the [[craniata|craniates]] ([[vertebrate]]s and [[hagfish]]) (Butler, 2000).  
  
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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.
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The brain of arthropods and cephalopods arises from twin parallel nerve cords that extend through the body of the animal, while that of craniates, as described above, develops from the [[anatomical terms of location|anterior]] section of a single dorsal [[neural tube|nerve cord]], which later becomes the [[spinal cord]] (Kandel et al. 2000).  
  
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Brains also produce a portion of the body's [[hormone]]s that can influence organs and glands elsewhere in a body—conversely, brains also react to hormones produced elsewhere in the body. In mammals, most of these hormones are released into the [[circulatory system]] by a structure called the [[pituitary gland]].
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===Invertebrates===
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The insect brain has four parts: the optical lobes, the protocerebrum, the deutocerebrum, and the tritocerebrum. The optical lobes, located behind each eye, process visual stimuli (Butler 2002). The protocerebrum contains the central body complex and the [[mushroom bodies]], which respond to [[olfaction|smell]]. In some [[species]] such as [[bee]]s, the mushroom body also receives input from the visual pathway. The deutocerebrum includes the [[antennal lobe]]s, which are similar to the mammalian [[olfactory bulb]], and the mechanosensory [[neuropil]]s, which receive information from [[Somatosensory system|touch]] receptors on the head and [[antenna (biology)|antennae]].  
  
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It is hypothesized that developed brains derive consciousness from the complex interactions between the numerous systems within the brain. Cognitive processing in mammals occurs in the cerebral cortex but relies on midbrain and [[limbic system|limbic]] functions as well. Among "younger" (in an evolutionary sense) vertebrates, advanced processing involves progressively rostral (forward) regions of the brain.
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In [[cephalopod]]s, the brain has two regions: the supraesophageal mass and the subesophageal mass, separated by the [[esophagus]] (Butler 2002). These two components are connected by the basal lobes and the dorsal magnocellular lobes. The large optic lobes are sometimes not considered to be part of the brain, as they are anatomically separate and are joined to the brain by the optic stalks. However, the optic lobes perform most visual processing, and so functionally are part of the brain.
  
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Hormones, incoming sensory information, and cognitive processing performed by the brain determine the brain state. Stimulus from any source can trigger a general arousal process that focuses cortical operations to processing of the new information. This focusing of cognition is known as [[attention]]. Cognitive priorities are constantly shifted by a variety of factors such as hunger, fatigue, belief, unfamiliar information, or threat. The simplest dichotomy related to the processing of threats is the [[fight-or-flight response]] mediated by the [[amygdala]] and other limbic structures.
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===Vertebrates===
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Vertebrate nervous systems are distinguished by [[symmetry (biology)#Bilateral symmetry|bilaterally symmetrical]] [[encephalization]], which refers to the tendency for more complex organisms to gain larger brains through evolutionary time. In vertebrates, increased size and [[complexity]] of the [[cerebral cortex]] correlate with height on the [[phylogenetic tree|phylogenetic]] and [[evolutionary tree]]. Primitive vertebrates such as [[fish]], [[reptile]]s, and [[amphibian]]s have fewer than six layers of neurons in the outer layer of their brains, a configuration called the [[allocortex]]. More complex vertebrates such as [[mammal]]s have a six-layered [[neocortex]], in addition to some parts of the brain that are allocortex (Martin, 1996).  
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===Brain pathology===
 
−
[[Image:Frontotemporal_degeneration.jpg|right|thumb|250px|A [[human brain]] showing [[frontotemporal lobar degeneration]] causing frontotemporal dementia.]]
 
−
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 and industrial accidents, is a leading cause of death in youth and middle age. In many cases, more damage is caused by resultant swelling ([[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 disease]]s, 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 illness]]es, such as [[clinical depression]], [[schizophrenia]], [[bipolar disorder]], and [[post-traumatic stress disorder]] are brain diseases that impact the personality and typically on other aspects of mental and somatic function. These disorders may be treated by [[psychiatry|psychiatric therapy]], [[medication|pharmaceutical]] intervention, or through a combination of treatments; therapeutic effectiveness varies significantly among individuals.
+
In mammals, increasing convolutions (folds) of the brain are also characteristic of animals with more advanced brains. These convolutions provide a larger surface area for a greater number of neurons, while keeping the volume of the brain compact enough to fit inside the skull.
  
−
Some infectious diseases affecting the brain are caused by [[virus|viral]] and [[bacteria]]l infection(s). 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 is linked to [[prion]]s. [[Kuru (disease)|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 substantiated in [[multiple sclerosis]], [[Parkinson's disease]], [[Lyme disease]], [[encephalopathy]], and [[encephalomyelitis]].
+
===The human brain is unique but shares many structures with other mammals===
 +
[[Image:Brain animated color nevit.gif|thumb|An animation of the human brain with the lobes highlighted.]]
 +
The structure of the human brain differs from that of other animals in several important ways, corresponding in function to more advanced cognitive skills. Human encephalization is especially pronounced in the neocortex, the most complex part of the cerebral cortex. The proportion of the human brain that is devoted to the neocortex—especially to the [[prefrontal cortex]]—is larger than in all other [[mammal]]s. The most dramatic increase in size of the cerebral cortex took place during the last several million years of human evolution; though [[elephant]]s, [[whale]]s, and [[porpoise]]s have larger brains in terms of mass, if we compare brain size to body size, humans and dolphins top the list, and humans have the largest ratio of brain size to body size.
  
−
Some brain disorders are [[congenital disorder|congenital]]. [[Tay-Sachs disease]], [[Fragile X syndrome]], and [[Down syndrome]] are all linked to [[gene]]tic and [[chromosome|chromosomal]] errors. Malfunctions in the embryonic [[neural development|development]] of the brain can be caused by genetic factors, by [[drug use]], and [[disease]] during a mother's [[pregnancy]].
+
Although humans have unique neural capacities, much of their brain structure is similar to that of other mammals. Basic systems that alert the nervous system to stimuli, sense events in the environment, and monitor the condition of the body are similar to those of even non-mammalian vertebrates. Thus, the neural circuitry underlying human consciousness includes both the advanced neocortex and prototypical structures of the brain stem.
  
−
Certain brain disorders are treated by brain surgeons (neurosurgeons) while others are treated by neurologists and psychiatrists.
+
==The brain functions as part of the nervous and endocrine systems==
 +
Vertebrate brains receive signals through nerves arriving from the sensors of the organism (known as ''afferent'' nerves). These signals are then processed throughout the central nervous system; reactions are formulated based upon reflex and learned experiences. A similarly extensive nerve network, composed of ''efferent nerves,'' delivers signals from the brain that control important muscles throughout the body. [[Anatomy|Anatomically]], the majority of afferent and efferent nerves (with the exception of the [[cranial nerves]]), are connected to the spinal cord, which then transfers signals to and from the brain.
  
−
==The study of the brain==
+
Sensory input is processed by the brain to recognize danger, find food, identify potential mates, and perform more sophisticated functions. The [[visual perception|visual]], touch, and [[hearing (sense)|auditory]] 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]]. Olfactory pathways are routed to the olfactory bulb, then to various parts of the [[olfactory system]]. [[Taste]] is routed through the brain stem and then to other portions of the [[gustatory system]].
−
===Fields of study===
 
−
[[Neuroscience]] seeks to understand the nervous system, including the brain, from a biological and [[computational neuroscience|computational]] perspective. [[Psychology]] seeks to understand behavior and the brain. The terms [[neurology]] and psychiatry usually refer to [[medicine|medical]] applications of neuroscience and psychology respectively. [[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 observation===
+
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.
−
{{main|neuroimaging}}
 
  
−
====Electrophysiology====
+
In addition to its role in the nervous system, the brain is also involved in the body's chemical communication system (the [[endocrine system]]). The [[pituitary gland]] in the brain produces a portion of the body's [[hormone]]s, which in turn can influence the production of hormones in other [[organ (anatomy)|organs]] and [[gland]]s; conversely, the brains reacts to certain hormones produced elsewhere in the body.
−
Each method for observing activity in the brain has its advantages and drawbacks. Electrophysiology allows scientists to record the electrical activity of individual neurons or groups of neurons.
 
  
−
====EEG====
+
Hormones, incoming sensory information, and cognitive processing performed by the brain determine the brain state. Stimulus from any source can trigger a general arousal process that focuses cortical operations to processing of the new information, a process known as [[attention]]. Cognitive priorities are constantly shifted by a variety of factors, such as hunger, fatigue, belief, unfamiliar information, or threat.  
−
By 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.
 
  
−
====fMRI and PET====
+
It is hypothesized that developed brains derive consciousness from the complex interactions between the numerous systems within the brain. Cognitive processing in mammals occurs in the cerebral cortex but relies on midbrain and [[limbic system|limbic]] functions as well. Among "younger" (in an evolutionary sense) vertebrates, advanced processing involves progressively rostral (forward) regions of the brain.
−
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. Similarly, a positron emission tomography (PET), is able to monitor [[glucose]] metabolism in different areas within the brain which can be correlated to the level of activity in that region.
 
  
−
====Behavioral====
+
==Brain pathology==
−
Behavioral 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.
+
[[Image:Frontotemporal_degeneration.jpg|right|thumb|250px|A [[human brain]] showing [[frontotemporal lobar degeneration]] causing frontotemporal dementia.]]
 +
Clinically, [[death]] is defined as an absence of brain activity as measured by EEG (a technique that gauges changes in electrical current from the cerebral cortex). Injuries to the brain tend to affect large areas of the organ, sometimes resulting in major deficits in intelligence, memory, and movement. Head trauma from, for example, vehicle and industrial accidents, is a leading cause of death in youth and middle age. In many cases, more damage is done by resultant swelling ([[edema]]) than by the impact itself. [[Stroke]], which results from blockage or rupturing of blood vessels in the brain, is another major cause of death from brain damage.
  
−
====Anatomical====
+
Other problems involving the brain can be more accurately classified as diseases rather than injuries. [[Neurodegenerative disease]]s, 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 declines in movement control, memory, and cognition. Currently, only the symptoms of these diseases can be treated. [[Mental illness]]es, such as [[clinical depression]], [[schizophrenia]], [[bipolar disorder]], and [[post-traumatic stress disorder]], are brain diseases that impact [[Wiktionary:personality|personality]] and often other aspects of mental and somatic function.  
−
[[post-mortem|Autopsy]] 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 methods====
+
Some infectious diseases affecting the brain are caused by [[virus]]es and [[bacteria]]. Infection of the meninges, the membrane that covers the brain, can lead to [[meningitis]]. [[Encephalitis]] is an acute [[inflammation]] of the [[brain]], commonly caused by a viral [[infection]]. An inflammation that includes both the brain and the [[spinal cord]] is called [[encephalomyelitis]]. [[Bovine spongiform encephalopathy]] (also known as mad cow disease), is deadly in [[cattle]] and humans; [[kuru (disease)|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 substantiated in [[multiple sclerosis]], [[Parkinson's disease]], [[Lyme disease]], and [[encephalopathy]].
−
Attempts have also been made to directly "read" the brain, which has been accomplished in a rudimentary manner through a brain-computer interface. Brain activity can be detected by implanted electrodes, raising the possibility of direct mind-computer interface. The reverse method has been successfully demonstrated: [[brain implant]]s have been used to generate artificial hearing and (crude and experimental) artificial vision for deaf and blind people. [[Brain pacemaker]]s are now commonly used to regulate brain activity in conditions such as Parkinson's disease.
 
  
−
===Other matters===
+
Other brain disorders are [[congenital disorder|congenital]]. [[Tay-Sachs disease]], [[Fragile X syndrome]], and [[Down syndrome]] are all linked to [[gene]]tic and [[chromosome|chromosomal]] errors. Malfunctions in the embryonic [[neural development|development]] of the brain can be caused by genetic factors, [[drug use]], and [[disease]] during a mother's [[pregnancy]].
−
[[computer science|Computer scientists]] have produced simulated neural networks loosely based on the structure of neuron connections in the brain. [[Artificial intelligence]] seeks to replicate brain function—although not necessarily brain mechanisms—but as yet has been met with limited success.
 
  
−
Creating [[algorithm]]s 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 [[mathematics|mathematical]] tools of [[chaos theory]] and [[dynamical system]]s. Current research has also focused on recreating the neural structure of the brain with the aim of producing human-like cognition and artificial intelligence.
+
==Study of the brain==
 +
===Fields of study===
 +
[[Neuroscience]] seeks to understand the nervous system, including the brain, from a biological and [[computational neuroscience|computational]] perspective, while [[psychology]] seeks to understand the brain's relation to behavior. The terms [[neurology]] and psychiatry usually refer to [[medicine|medical]] applications of neuroscience and psychology respectively. [[Cognitive science]] is an interdisciplinary endeavor that attempts to unify neuroscience and psychology with other fields that concern themselves with the brain, such as [[computer science]] and [[philosophy]].
  
−
==Brain as food==
+
One controversial endeavor involving the study of the brain has involved efforts to create [[artificial intelligence]], which seeks to replicate brain function—although not necessarily brain mechanisms. [[Computer science|Computer scientists]] have produced simulated neural networks loosely based on the structure of neural connections in the brain. Creating [[algorithm]]s 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 [[mathematics|mathematical]] tools of [[chaos theory]] and [[dynamical system]]s. Current research has also focused on recreating the neural structure of the brain with the aim of producing human-like cognition and artificial intelligence.
−
[[Image:Porkbrain.jpg|right|thumb|250px|Pork brain, ready to be cooked]]
 
−
Like most other internal organs, the brain can serve as nourishment. For example, in the [[southern United States]] canned [[pork]] brain in [[gravy]] can be purchased for consumption as food. This form of brain is often fried with [[scrambled eggs]] to produce the famous "Eggs n' Brains".<ref>{{cite web | author = Lukas, Paul | title = Inconspicuous Consumption: Mulling Brains | work = New York magazine | url = http://www.bozosoft.com/mike/meat/brains-article.html | accessdate = 14 October | accessyear = 2005 }}</ref> The brain of animals also features in [[French cuisine]] such as in the dish ''[tête de veau]'', or ''head of calf''. Although it might consist only of the outer meat of the skull and [[jaw]], the full meal includes the brain, [[tongue]], and [[gland]]s, with the latter form being the favorite food of [[President of the French Republic|French President]] [[Jacques Chirac]].<ref>{{cite web | url = http://cavelife.net/columns/2002columns/2002Mar13.html | author = Glover, William | title = Tales from the Loir: Tête de Veau | work = Cave Life in France | accessdate = 14 October | accessyear = 2005 }}</ref> Similar delicacies from around the world include [[Mexico|Mexican]] ''[[taco]]s de sesos'' made with cattle brain as well as [[squirrel]] brain in the US South.<ref>{{cite web | url = http://www.weird-food.com/weird-food-mammal.html | work = Weird-Food.com | title = Weird Foods: Mammal | accessdate = 14 October | accessyear = 2005 }}</ref> The Anyang tribe of [[Cameroon]] practiced a tradition in which a new [[tribal chief]] would consume the brain of a hunted [[gorilla]] while another senior member of the [[tribe]] would eat the heart.<ref>{{cite web | url = http://www.berggorilla.de/english/gjournal/texte/18culture.html | author = Meder, Angela | title = Gorillas in African Culture and Medicine | work = Gorilla Journal | accessdate = 14 October | accessyear = 2005 }}</ref>
 
  
−
Consuming the brain and other nerve tissue of animals is not without risks. The first problem is that the brain is made up of 60% fat due to the myelin (which itself is 70% fat) insulating the axons of neurons and glia.<ref>{{cite web | url = http://www.autisminfo.com/dorfman.htm | title = Nutritional Summary: Notes Taken From a Recent Autism Society Meeting | author = Dorfman, Kelly | work = Diet and Autism | accessdate = 14 October | accessyear = 2005 }}</ref> As an example, a 140 g can of "pork brains in milk gravy", a single serving, contains 3500 milligrams of [[cholesterol]], 1170% of our recommended daily intake.<ref>{{cite web | url = http://thewvsr.com/porkbrains.htm | title = Pork Brains in Milk Gravy | accessdate = 14 October | accessyear = 2005 }}</ref>
+
===Methods of observation===
 +
[[Image:Pnsagittal.jpg|thumb|250px|A scan of the brain using fMRI.]]
 +
Each method for observing activity in the brain has its advantages and drawbacks:
  
−
Brain consumption can also result in contracting fatal [[Transmissible spongiform encephalopathy|transmissible spongiform encephalopathies]] such as Variant [[Creutzfeldt-Jakob disease]] and other [[prion]] diseases in humans and [[Bovine spongiform encephalopathy|mad cow disease]] in cattle.<ref>{{cite journal | last = Collinge | first = John | date = 2001 | title = Prion diseases of humans and animals: their causes and molecular basis | journal = Annual Review of Neuroscience | volume = 24 | pages = 519–50 | url = http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11283320 | id = PMID 11283320 }}</ref>. Another prion disease called [[Kuru (disease)|kuru]] has been traced to a funerary ritual among the [[Fore]] people of [[Papua New Guinea]] in which those close to the dead would eat the brain of the deceased to create a sense of [[immortality]].<ref>{{cite journal | last = Collins | first = S | coauthors = McLean CA, Masters CL | date = 2001 | title = Gerstmann-Straussler-Scheinker syndrome,fatal familial insomnia, and kuru: a review of these less common human transmissible spongiform encephalopathies | journal = Journal of Clinical Neuroscience | volume = 8 | issue = 5 | ages = 387–97 | url = http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11535002 | id = PMID 11535002 }}</ref> Some [[archaeology|archaeological]] evidence suggests that the mourning rituals of [[Europe]]an [[Neanderthal]]s also involved the consumption of the brain.<ref>{{cite book | url = http://search.barnesandnoble.com/booksearch/isbninquiry.asp?ean=9781582432533&displayonly=CHP | title = The Aztec Treasure House | last = Connell | first = Evan S. | publisher = Counterpoint Press | year = 2001 | id = ISBN 1582431620 }}</ref>
+
*''Electrophysiology'' allows scientists to record the electrical activity of individual neurons or groups of neurons. 
 +
*By placing electrodes on the scalp, one can record the summed electrical activity of the cortex through 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.
 +
*Apart 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 has much better spatial resolution. Its main advantage over fMRI is a direct relationship between neural activation and measurement.
 +
*''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 distinguish between inhibitory or excitatory activity. fMRI is a noninvasive, indirect method for measuring neural activity. 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 that can be correlated to the level of activity in that region.
 +
*''Behavioral 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.
  
−
It is not only humans who eat the brains of other animals. The two species of [[chimpanzee]], though generally [[vegetarian]], are known to eat the brains of [[monkey]]s to obtain fat in their diet.<ref>{{cite web|url=http://www.positivehealth.com/Reviews/books/horrobin68.htm|title=Review of ''The Madness of Adam and Eve: How Schizophrenia Shaped Humanity''|author=Clarke, Bella|work=Human Given magazine|date=14 October|year=2005}}</ref>
+
==The relation between mind and brain==
 +
A distinction is not often made in the [[philosophy of mind]] between the [[mind]] and the brain, and there is some controversy as to their exact relationship, leading to the [[mind-body problem]]. The mind-body problem concerns the explanation of the relationship, if any, that obtains between minds, or mental processes, and bodily states or processes.
  
−
==See also==
+
The brain is defined as the physical and biological [[matter]] contained within the [[skull]], responsible for all electrochemical neuronal processes. The mind, however, is seen in terms of mental attributes, such as beliefs or desires. There is a concept, tracing back at least to [[Plato]], [[Aristotle]], and the Sankhya and Yoga schools of [[Hinduism|Hindu]] philosophy, that "mental" phenomena are, in some respects, "non-physical" (distinct from the body). Only some adhere to [[metaphysics|metaphysically]] [[dualistic]] approaches in which the mind exists independently of the brain in some way, such as a [[soul]], [[epiphenomenalism|epiphenomenon]], or [[strong emergence|emergent]] phenomenon. Other dualisms maintain that the mind is a distinct ''[[physics|physical]]'' phenomenon, such as an [[electromagnetism|electromagnetic field]] or a [[quantum mind|quantum effect]]. Some envision a physical mind that mirrors the physical body, guiding its [[instinct|instinctual]] activities and development, while adding the concept for humans of a spiritual mind that mirrors a [[human body#The human body in religious and philosophical context|spiritual body]] and including aspects like philosphical and religious thought. Some [[materialist]]s argue that mentality is equivalent to [[behaviorism|behavior]] or [[functionalism|function]] or, in the case of [[computationalism|computationalists]] and [[strong AI]] theorists, [[computer software]] (with the brain playing the role of [[computer hardware|hardware]]). [[Idealism]], the belief that all is mind, still has some adherents. At the other extreme, [[eliminative materialism|eliminative materialist]]s believe that minds do not exist at all, and that mentalistic language will be replaced by neurological terminology.
−
*[[Nervous system]]
 
−
*[[Central nervous system]]
 
−
*[[Neuroscience]]
 
−
*[[Neurology]]
 
−
*[[A/S ratio]]
 
−
*[[Brain damage]]
 
−
*[[Brain-computer interface]]
 
−
*[[Human brain]]
 
−
*[[List of regions in the human brain|Regions in the human brain]]
 
−
*[[Traumatic brain injury]]
 
  
−
==Further reading==
+
==References==
−
    * Junqueira, L.C., and J. Carneiro (2003). Basic Histology: Text and Atlas, Tenth Edition. Lange Medical Books McGraw-Hill. ISBN 0071215654.
+
*Bear, M. F., B. W. Connors, and M. A. Paradiso. 2001. ''Neuroscience: Exploring the Brain.'' Baltimore: Lippincott. ISBN 0781739446.
−
    * Sala, Sergio Della, editor. (1999). Mind myths: Exploring popular assumptions about the mind and brain. J. Wiley & Sons, New York. ISBN 0471983039.
+
*Butler, A. B. 2002. Chordate evolution and the origin of craniates: An old brain in a new head. ''The Anatomical Record'' 261: 111–25.
−
    * Vander, A., J. Sherman, D. Luciano (2001). Human Physiology: The Mechanisms of Body Function. McGraw Hill Higher Education. ISBN 0071180885.
+
*Junqueira, L. C., and J. Carneiro. 2003. ''Basic Histology: Text and Atlas,'' 10th edition. New York: Lange Medical Books, McGraw-Hill. ISBN 0071215654.
 +
*Kandel, E. R., J. H. Schwartz, and T. M. Jessell. 2000. ''Principles of Neural Science,'' 4th ed. New York: McGraw-Hill ISBN 0838577016.
 +
*Martin, J. H. 1996. ''Neuroanatomy: Text and Atlas,'' 2nd ed. New York: McGraw-Hill. ISBN 007138183X.
 +
*Purves, W., D. Sadava, G. Orians, and C. Heller. 2004. ''Life: The Science of Biology,'' 7th edition. Sunderland, MA: Sinauer. ISBN 0716766728.
 +
*Sala, S. D., ed. 1999. ''Mind Myths: Exploring Popular Assumptions About the Mind and Brain.'' New York: J. Wiley & Sons. ISBN 0471983039.
 +
*Vander, A., J. Sherman, and D. Luciano. 2001. ''Human Physiology: The Mechanisms of Body Function.'' New York: McGraw-Hill. ISBN 0071180885.
  
 +
==External links==
 +
All links retrieved March 2, 2013.
 +
* Hawkins, Jeff. 2003. [http://www.ted.com/talks/view/id/125 Brain science is about to fundamentally change computing]. Monterey, CA: Ted Talks.
 +
* [http://science.howstuffworks.com/life/inside-the-mind/human-brain/brain.htm How Your Brain Works] at HowStuffWorks.
 +
* [http://brainmuseum.org/ Comparative Mammalian Brain Collection].
 +
* [http://braininfo.rprc.washington.edu BrainInfo for Neuroanatomy].
 +
* [http://www.brainmaps.org/ BrainMaps.org], interactive high-resolution digital brain atlas based on scanned images of serial sections of both primate and non-primate brains.
  
−
===References===
+
{{nervous system}}
  
−
  1. Chudler, Eric H. (2006). Brain Facts and Figures. Neuroscience for Kids. Retrieved on May 19, 2006.
 
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  2. a b c d e Butler, Ann B. (2000). "Chordate Evolution and the Origin of Craniates: An Old Brain in a New Head". The Anatomical Record 261: 111–125.
 
−
  3. Kandel, ER; Schwartz JH, Jessell TM (2000). Principles of Neural Science, 4th ed., New York: McGraw-Hill. ISBN 0838577016.
 
−
  4. a b c d Martin, John H. (1996). Neuroanatomy: Text and Atlas, Second Edition, New York: McGraw-Hill. ISBN 007138183X.
 
−
  5. Kandel, ER; Schwartz JH, Jessell TM (2000). Principles of Neural Science, 4th ed., New York: McGraw-Hill. ISBN 0838577016.
 
−
  6. Junqueira, L.C.; J. Carneiro. Basic Histology: Text and Atlas, 10th ed.. (Statistic from page 161)
 
−
  7. Lukas, Paul. Inconspicuous Consumption: Mulling Brains. New York magazine. Retrieved on 14 October 2005.
 
−
  8. Glover, William. Tales from the Loir: Tête de Veau. Cave Life in France. Retrieved on 14 October 2005.
 
−
  9. Weird Foods: Mammal. Weird-Food.com. Retrieved on 14 October 2005.
 
−
  10. Meder, Angela. Gorillas in African Culture and Medicine. Gorilla Journal. Retrieved on 14 October 2005.
 
−
  11. Dorfman, Kelly. Nutritional Summary: Notes Taken From a Recent Autism Society Meeting. Diet and Autism. Retrieved on 14 October 2005.
 
−
  12. Pork Brains in Milk Gravy. Retrieved on 14 October 2005.
 
−
  13. Collinge, John (2001). "Prion diseases of humans and animals: their causes and molecular basis". Annual Review of Neuroscience 24: 519–50. PMID 11283320.
 
−
  14. Collins, S, McLean CA, Masters CL (2001). "Gerstmann-Straussler-Scheinker syndrome,fatal familial insomnia, and kuru: a review of these less common human transmissible spongiform encephalopathies". Journal of Clinical Neuroscience 8 (5). PMID 11535002.
 
−
  15. Connell, Evan S. (2001). The Aztec Treasure House. Counterpoint Press. ISBN 1582431620.
 
−
  16. Clarke, Bella (14 October). Review of The Madness of Adam and Eve: How Schizophrenia Shaped Humanity. Human Given magazine.
 
−
 
−
==External links==
 
−
* [http://www.stanford.edu/group/hopes/basics/braintut/ab0.html Brain Tutorial]
 
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* [http://brainmuseum.org/ Comparative Mammalian Brain Collection]
 
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* [http://www.rmcybernetics.com/science/cybernetics/ai_vision_perception_brain.htm RMCybernetics - The Brain and Artificial Intelligence]
 
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* [http://braininfo.rprc.washington.edu BrainInfo for Neuroanatomy]
 
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* [http://faculty.washington.edu/chudler/neurok.html Neuroscience for kids]
 
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* [http://www.newscientist.com/channel/being-human/brain Everything you wanted to know about the brain] — Provided by ''[[New Scientist]]''.
 
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* [http://purl.net/net/neurowiki neuroscience wiki]
 
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* [http://www.brainmaps.org/ BrainMaps.org], interactive high-resolution digital brain atlas based on scanned images of serial sections of both primate and non-primate brains
 
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* [http://www.scienceclarified.com/Bi-Ca/Brain.html | An orderly article about the brain]
 
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{{nervous system}}
 
  
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{{credit|61484229}}
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{{credit|Brain|144583362}}
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]
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[[Category:Anatomy and physiology]]

Revision as of 16:33, 2 March 2013

A drawing of the human brain.


The brain is a centralized mass of nerve tissue enclosed within the cranium (skull) of vertebrates; a related structure is also present in some invertebrates. This organ functions to process, store, and interpret information and to control the physiology and behavior of the body. In higher organisms, it is also the site of reason and intelligence, which include such components as cognition, perception, attention, memory and emotion.

Along with the spinal cord, the brain forms part of the central nervous system, a vast network of neurons that receives sensory signals from the peripheral nervous system and conveys information to the muscles and glands of the body. Neurons, which generate action potentials that communicate information to other cells, constitute the essential class of brain cells. The human brain, for example, contains more than 10 billion neurons, each linked to as many as 10,000 other neurons. The brain also contains various types of glial cells, which support the function of neurons.

Representation of a brain MRI, an imaging technique used to measure neural activity.

The vertebrate brain has three main sections, commonly referred to as the hindbrain, midbrain, and forebrain, each of which contains several structures. In general, as one moves from hindbrain to forebrain, one progresses from structures involved in more autonomic (or involuntary) functioning (like walking or maintaining constant heart rate) to those coordinating higher functioning (such as learning and memory).

As we travel up the vertebrate phylogenetic scale from fish to mammals, the cerebrum (a structure within the forebrain) increases in size, complexity, and importance, dominating the nervous systems of mammals. For example, major damage to this area results in severe impairment or even coma in mammals, while a shark whose cerebrum has been removed can swim with relatively normal function. Nonetheless, animals have many structures in common, and progress in neuroscience, a field of biology aimed at understanding the functions of the brain at every level, has come from research on the simpler nervous systems of invertebrates.

A central issue in philosophy and religion is how the brain relates to the mind, that is the mind-body problem. (See The relation between mind and brain section below.)

Anatomy

The brain is composed of two broad classes of cells

Neuron
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Nucleus
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Axon Terminal
Schwann cell
Myelin sheath
Structure of a typical neuron

The brain is composed of two broad classes of cells, neurons and glia, both of which contain several different cell types that perform specialized functions.

The neuron is the functional unit of the brain. Interconnected neurons form neural networks (or neural ensembles). These networks are similar to man-made electrical circuits in that they contain circuit elements (neurons) connected by biological wires (nerve fibers). These do not form simple one-to-one electrical circuits like many man-made circuits, however. Typically, neurons connect to at least one thousand other neurons (Jungueira and Carneiro 2003). These highly specialized circuits make up systems that are the basis of perception, different types of action, and higher cognitive function.

In addition to neurons, the brain contains glial cells in a roughly 10:1 proportion to neurons. Glial cells (glia is the Greek term for “glue”) form a support system for neurons. They create the insulating myelin, provide structure to the neuronal network, manage waste, and clean up neurotransmitters.

The blood-brain barrier is constructed by a particular class of glial cell called astrocytes. Blood vessels throughout the body are permeable to many chemicals, some of which are toxic; thus, the astrocytes form a barrier to these chemicals by surrounding the smallest, most permeable blood vessels in the brain. Protection is crucial because, unlike other tissues of the body, the brain cannot recover from damage by generating new cells (in mammals, production of neurons by cell division ceases shortly after birth). However, the barrier is not totally impermeable: fat-soluble substances like anesthetics and alcohol have notable effects on the brain.

The gray matter of the brain refers to a core area with many cell bodies (soma) of neurons, while white matter is made up of bundles of axons running up and down the cord, colored white because most axons are covered with a myelin sheath. The space between neurons is filled with dendrites as well as unmyelinated segments of axons; this area is referred to as the neuropil.

The brain is surrounded by connective tissue and bathed in fluid

In mammals, the brain is surrounded by connective tissues called the meninges, a system of membranes that separate the skull from the brain. This three-layered covering is composed of the dura mater, arachnoid mater, and pia mater. The arachnoid and pia are physically connected and thus often considered as a single layer, the pia-arachnoid. Blood vessels enter the central nervous system through the perivascular space above the pia mater. The cells in the blood vessel walls are tightly joined, forming the blood-brain barrier described above.

The brain is bathed in cerebrospinal fluid (CSF), which circulates between layers of the meninges and through cavities in the brain called ventricles. This fluid is important both chemically (for metabolism) and mechanically (for shock-prevention). For example, the mass and density of the brain are such that it will begin to collapse under its own weight if unsupported by the CSF. The CSF allows the brain to float, easing the physical stress caused by the brain’s mass (an adult human brain weighs approximately 1.5 kg).

Structure and function

The vertebrate brain is divided into three main regions

A diagram depicting the main subdivisions of the embryonic vertebrate brain. These regions will later differentiate into forebrain, midbrain and hindbrain structures.

Early in the development of all vertebrate embryos, a hollow neural tube forms three swellings at the head of the embryo that become the basic divisions of the brain: the hindbrain, midbrain, and forebrain. The rest of the tube develops into the spinal cord. Cranial and spinal nerves, which constitute the peripheral nervous system, sprout from the tube and continue to grow throughout embryonic development.

Each region, in turn, develops into several structures with diverse functions:

  • Hindbrain (rhombencephalon):
    • The medulla and pons contain distinct groups of neurons involved in the control of physiological functions like breathing or basic motor patterns like swallowing.
    • The cerebellum orchestrates and refines behavior patterns.
  • Midbrain (mesencephalon). All information conveyed between the higher brain and spinal cord must pass through the midbrain, which also contains structures involved in processing aspects of visual and auditory information.
  • Forebrain (prosencephalon):
    • Diencephalon (central region). An upper structure (the thalamus) is the final relay station for sensory information going to the telencephalon. A lower structure called the hypothalamus regulates many physiological functions and biological drives.
    • The telencephalon (surrounding structures) consists of two cerebral hemispheres, also known as the cerebrum; in humans and other mammals, the cerebrum is by far the largest part of the brain, and it plays major roles in sensory perception, learning, memory, and conscious behavior.

Communication between the spinal cord and telencephalon travels through medulla, pons, midbrain, and diencephalons, structures that are collectively referred to as the brain stem. In general, more primitive and autonomic (involuntary or unconscious) functions are coordinated farther down this axis, while more complex and evolutionarily advanced functions are found higher on the brain stem.

The regions of the brain may also be classified by function

In addition to the anatomical categories outlined above, another way of organizing our understanding of the nervous system is through functional divisions. Since the nervous system engages in parallel processing of information, any one anatomical structure may be involved in several functional subsystems:

  • Reticular system. A network of neuronal fibers that includes discrete groups of neurons (called nuclei), distributed through a core of the medulla, pons, and midbrain. These specialized groups alert the forebrain to information coming from specific regions of the peripheral nervous system. Some nuclei are involved in controlling sleep and wakefulness.
  • Limbic system. The evolutionarily primitive parts of the forebrain, which still have important functions in birds and mammals, but are completely covered by the more recent elaborations of the telencephalon called the neocortex. The limbic system is responsible for basic physiological drives, instincts, and emotions, though pleasure and pain centers in the limbic system are believed to play roles in learning and physiological drives. The hippocampus, one part of the limbic system, is necessary for the transfer of short-term memory to long-term memory in humans.
  • Cerebrum. Regions of the brain that interact for consciousness and control of behavior.

The cerebral cortex is a sheet of gray matter that covers each cerebral hemisphere; it is convoluted (or folded) into ridges (called gyri) and valleys (called sulci) so that it fits into the skull. The corpus callosum is a white-matter tract that links the two hemispheres. Although the functions of some regions of the cerebral cortex are relatively easy to isolate and define, others are not: the latter areas fall under the general name of association cortex.

The cerebral cortex can be subdivided into four lobes:

  1. The temporal lobe is an upper region involved in receiving and processing auditory information. Its association areas are involved in the recognition, identification, and naming of objects.
  2. The occipital lobe receives and processes visual information. Its association areas are essential for making sense of the visual world and translating visual experience into language.
  3. The parietal lobe contains the primary somatosensory cortex, which receives information through the thalamus about touch and pressure sensations. A major association function of the parietal lobe is attending to complex stimuli.
  4. The frontal lobe contains a strip called the primary motor cortex. Its association functions are diverse and best described as having to do with planning.

Comparative anatomy

A mouse brain.

Three groups of animals have notably complex brains: the arthropods (insects, crustaceans, arachnids, and others); the cephalopods (octopuses, squids, and similar mollusks); and the craniates (vertebrates and hagfish) (Butler, 2000).

The brain of arthropods and cephalopods arises from twin parallel nerve cords that extend through the body of the animal, while that of craniates, as described above, develops from the anterior section of a single dorsal nerve cord, which later becomes the spinal cord (Kandel et al. 2000).

Invertebrates

The insect brain has four parts: the optical lobes, the protocerebrum, the deutocerebrum, and the tritocerebrum. The optical lobes, located behind each eye, process visual stimuli (Butler 2002). The protocerebrum contains the central body complex and the mushroom bodies, which respond to smell. In some species such as bees, the mushroom body also receives input from the visual pathway. The deutocerebrum includes the antennal lobes, which are similar to the mammalian olfactory bulb, and the mechanosensory neuropils, which receive information from touch receptors on the head and antennae.

In cephalopods, the brain has two regions: the supraesophageal mass and the subesophageal mass, separated by the esophagus (Butler 2002). These two components are connected by the basal lobes and the dorsal magnocellular lobes. The large optic lobes are sometimes not considered to be part of the brain, as they are anatomically separate and are joined to the brain by the optic stalks. However, the optic lobes perform most visual processing, and so functionally are part of the brain.

Vertebrates

Vertebrate nervous systems are distinguished by bilaterally symmetrical encephalization, which refers to the tendency for more complex organisms to gain larger brains through evolutionary time. In vertebrates, increased size and complexity of the cerebral cortex correlate with height on the phylogenetic and evolutionary tree. Primitive vertebrates such as fish, reptiles, and amphibians have fewer than six layers of neurons in the outer layer of their brains, a configuration called the allocortex. More complex vertebrates such as mammals have a six-layered neocortex, in addition to some parts of the brain that are allocortex (Martin, 1996).

In mammals, increasing convolutions (folds) of the brain are also characteristic of animals with more advanced brains. These convolutions provide a larger surface area for a greater number of neurons, while keeping the volume of the brain compact enough to fit inside the skull.

The human brain is unique but shares many structures with other mammals

An animation of the human brain with the lobes highlighted.

The structure of the human brain differs from that of other animals in several important ways, corresponding in function to more advanced cognitive skills. Human encephalization is especially pronounced in the neocortex, the most complex part of the cerebral cortex. The proportion of the human brain that is devoted to the neocortex—especially to the prefrontal cortex—is larger than in all other mammals. The most dramatic increase in size of the cerebral cortex took place during the last several million years of human evolution; though elephants, whales, and porpoises have larger brains in terms of mass, if we compare brain size to body size, humans and dolphins top the list, and humans have the largest ratio of brain size to body size.

Although humans have unique neural capacities, much of their brain structure is similar to that of other mammals. Basic systems that alert the nervous system to stimuli, sense events in the environment, and monitor the condition of the body are similar to those of even non-mammalian vertebrates. Thus, the neural circuitry underlying human consciousness includes both the advanced neocortex and prototypical structures of the brain stem.

The brain functions as part of the nervous and endocrine systems

Vertebrate brains receive signals through nerves arriving from the sensors of the organism (known as afferent nerves). These signals are then processed throughout the central nervous system; reactions are formulated based upon reflex and learned experiences. A similarly extensive nerve network, composed of efferent nerves, delivers signals from the brain that control important muscles throughout the body. Anatomically, the majority of afferent and efferent nerves (with the exception of the cranial nerves), are connected to the spinal cord, which then transfers signals to and from the brain.

Sensory input is processed by the brain to recognize danger, find food, identify potential mates, and perform more sophisticated functions. The visual, touch, and auditory 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. Olfactory pathways are routed to the olfactory bulb, then to various parts of the olfactory system. Taste is routed through the brain stem 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.

In addition to its role in the nervous system, the brain is also involved in the body's chemical communication system (the endocrine system). The pituitary gland in the brain produces a portion of the body's hormones, which in turn can influence the production of hormones in other organs and glands; conversely, the brains reacts to certain hormones produced elsewhere in the body.

Hormones, incoming sensory information, and cognitive processing performed by the brain determine the brain state. Stimulus from any source can trigger a general arousal process that focuses cortical operations to processing of the new information, a process known as attention. Cognitive priorities are constantly shifted by a variety of factors, such as hunger, fatigue, belief, unfamiliar information, or threat.

It is hypothesized that developed brains derive consciousness from the complex interactions between the numerous systems within the brain. Cognitive processing in mammals occurs in the cerebral cortex but relies on midbrain and limbic functions as well. Among "younger" (in an evolutionary sense) vertebrates, advanced processing involves progressively rostral (forward) regions of the brain.

Brain pathology

A human brain showing frontotemporal lobar degeneration causing frontotemporal dementia.

Clinically, death is defined as an absence of brain activity as measured by EEG (a technique that gauges changes in electrical current from the cerebral cortex). Injuries to the brain tend to affect large areas of the organ, sometimes resulting in major deficits in intelligence, memory, and movement. Head trauma from, for example, vehicle and industrial accidents, is a leading cause of death in youth and middle age. In many cases, more damage is done by resultant swelling (edema) than by the impact itself. Stroke, which results from blockage or rupturing of blood vessels in the brain, is another major cause of death from brain damage.

Other problems involving 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 declines in movement control, memory, and cognition. Currently, only the symptoms of these diseases can be treated. Mental illnesses, such as clinical depression, schizophrenia, bipolar disorder, and post-traumatic stress disorder, are brain diseases that impact personality and often other aspects of mental and somatic function.

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. Encephalitis is an acute inflammation of the brain, commonly caused by a viral infection. An inflammation that includes both the brain and the spinal cord is called encephalomyelitis. Bovine spongiform encephalopathy (also known as mad cow disease), is deadly in cattle and humans; 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 substantiated in multiple sclerosis, Parkinson's disease, Lyme disease, and encephalopathy.

Other brain disorders are congenital. Tay-Sachs disease, Fragile X syndrome, and Down syndrome are all linked to genetic and chromosomal errors. Malfunctions in the embryonic development of the brain can be caused by genetic factors, drug use, and disease during a mother's pregnancy.

Study of the brain

Fields of study

Neuroscience seeks to understand the nervous system, including the brain, from a biological and computational perspective, while psychology seeks to understand the brain's relation to behavior. The terms neurology and psychiatry usually refer to medical applications of neuroscience and psychology respectively. Cognitive science is an interdisciplinary endeavor that attempts to unify neuroscience and psychology with other fields that concern themselves with the brain, such as computer science and philosophy.

One controversial endeavor involving the study of the brain has involved efforts to create artificial intelligence, which seeks to replicate brain function—although not necessarily brain mechanisms. Computer scientists have produced simulated neural networks loosely based on the structure of neural connections in the brain. 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.

Methods of observation

A scan of the brain using fMRI.

Each method for observing activity in the brain has its advantages and drawbacks:

  • Electrophysiology allows scientists to record the electrical activity of individual neurons or groups of neurons.
  • By placing electrodes on the scalp, one can record the summed electrical activity of the cortex through 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.
  • Apart 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 has much better spatial resolution. Its main advantage over fMRI is a direct relationship between neural activation and measurement.
  • 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 distinguish between inhibitory or excitatory activity. fMRI is a noninvasive, indirect method for measuring neural activity. 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 that can be correlated to the level of activity in that region.
  • Behavioral 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.

The relation between mind and brain

A distinction is not often made in the philosophy of mind between the mind and the brain, and there is some controversy as to their exact relationship, leading to the mind-body problem. The mind-body problem concerns the explanation of the relationship, if any, that obtains between minds, or mental processes, and bodily states or processes.

The brain is defined as the physical and biological matter contained within the skull, responsible for all electrochemical neuronal processes. The mind, however, is seen in terms of mental attributes, such as beliefs or desires. There is a concept, tracing back at least to Plato, Aristotle, and the Sankhya and Yoga schools of Hindu philosophy, that "mental" phenomena are, in some respects, "non-physical" (distinct from the body). Only some adhere to metaphysically dualistic approaches in which the mind exists independently of the brain in some way, such as a soul, epiphenomenon, or emergent phenomenon. Other dualisms maintain that the mind is a distinct physical phenomenon, such as an electromagnetic field or a quantum effect. Some envision a physical mind that mirrors the physical body, guiding its instinctual activities and development, while adding the concept for humans of a spiritual mind that mirrors a spiritual body and including aspects like philosphical and religious thought. Some materialists argue that mentality is equivalent to behavior or function or, in the case of computationalists and strong AI theorists, computer software (with the brain playing the role of hardware). Idealism, the belief that all is mind, still has some adherents. At the other extreme, eliminative materialists believe that minds do not exist at all, and that mentalistic language will be replaced by neurological terminology.

References
ISBN links support NWE through referral fees

  • Bear, M. F., B. W. Connors, and M. A. Paradiso. 2001. Neuroscience: Exploring the Brain. Baltimore: Lippincott. ISBN 0781739446.
  • Butler, A. B. 2002. Chordate evolution and the origin of craniates: An old brain in a new head. The Anatomical Record 261: 111–25.
  • Junqueira, L. C., and J. Carneiro. 2003. Basic Histology: Text and Atlas, 10th edition. New York: Lange Medical Books, McGraw-Hill. ISBN 0071215654.
  • Kandel, E. R., J. H. Schwartz, and T. M. Jessell. 2000. Principles of Neural Science, 4th ed. New York: McGraw-Hill ISBN 0838577016.
  • Martin, J. H. 1996. Neuroanatomy: Text and Atlas, 2nd ed. New York: McGraw-Hill. ISBN 007138183X.
  • Purves, W., D. Sadava, G. Orians, and C. Heller. 2004. Life: The Science of Biology, 7th edition. Sunderland, MA: Sinauer. ISBN 0716766728.
  • Sala, S. D., ed. 1999. Mind Myths: Exploring Popular Assumptions About the Mind and Brain. New York: J. Wiley & Sons. ISBN 0471983039.
  • Vander, A., J. Sherman, and D. Luciano. 2001. Human Physiology: The Mechanisms of Body Function. New York: McGraw-Hill. ISBN 0071180885.

External links

All links retrieved March 2, 2013.

Nervous system
v·d·e
Brain | Spinal cord | Nerve cord | Central nervous system | Peripheral nervous system | Somatic nervous system | Autonomic nervous system | Sympathetic nervous system | Parasympathetic nervous system | Neuron | Axon | Soma (biology) | Dendrite | Hindbrain


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