Difference between revisions of "Brain" - New World Encyclopedia

From New World Encyclopedia
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{{Claimed}}{{Contracted}}
 
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[[Image:Brain Mri nevit.svg|thumb|Representation of brain MRI]]
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[[Image:Brain.svg|thumb|Human brain]]
 
In animals, the '''brain''' or ''encephalon'' ([[Greek (language)|Greek]] for "in the head"), is the control center of the [[central nervous system]], responsible for [[behaviour]]. In most animals, the brain is located in the head, protected by the [[skull]] and close to the primary sensory apparatus of [[Visual perception|vision]], [[Hearing (sense)|hearing]], [[equilibrioception]], [[taste]], and [[olfaction]]. While all [[vertebrate]]s have a brain, most [[invertebrate]]s have either a centralized brain or collections of individual [[ganglion|ganglia]]. Primitive animals such as [[sponge]]s do not have a brain at all. 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 other [[neuron]]s.
 
In animals, the '''brain''' or ''encephalon'' ([[Greek (language)|Greek]] for "in the head"), is the control center of the [[central nervous system]], responsible for [[behaviour]]. In most animals, the brain is located in the head, protected by the [[skull]] and close to the primary sensory apparatus of [[Visual perception|vision]], [[Hearing (sense)|hearing]], [[equilibrioception]], [[taste]], and [[olfaction]]. While all [[vertebrate]]s have a brain, most [[invertebrate]]s have either a centralized brain or collections of individual [[ganglion|ganglia]]. Primitive animals such as [[sponge]]s do not have a brain at all. 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 other [[neuron]]s.
  
[[Image:Brain Mri nevit.svg|thumb|Representation of brain MRI]]
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The brain is the site of reason and intelligence, which include such components as [[cognition]], [[perception]], [[attention]], [[memory]] and [[emotion]]. The brain is also responsible for control of [[posture]] and [[motor control|movement]]s. It makes possible cognitive, [[motor learning|motor]] and other forms of [[learning]]. The brain can perform a variety of functions automatically, without the need for [[consciousness|conscious]] awareness, such as coordination of [[sensory system]]s (eg. [[sensory gating]] and [[multisensory integration]]), [[Animal locomotion|walking]], and [[homeostasis|homeostatic]] body functions such as [[heart rate]], [[blood]] pressure, fluid balance, and body temperature.
[[Image:Brain.svg|thumb|Human brain]]
 
  
Most brains exhibit a substantial distinction between the [[gray matter]] and [[white matter]]. Gray matter consists primarily of the [[cell (biology)|cell]] bodies of the neurons, while white matter is comprised mostly of the fibers ([[axon]]s) which connect neurons. The axons are surrounded by a [[fat]]ty [[Electrical insulation|insulating]] sheath called [[myelin]] ([[oligodendroglia]] cells), giving the white matter its distinctive color. The outer layer of the brain is gray matter called [[cerebral cortex]]. Deep in the brain, compartments of white matter ([[fasciculus|fasciculi]], fiber tracts), gray matter ([[nucleus (neuroanatomy)|nuclei]]) and spaces filled with [[cerebrospinal fluid]] ([[Ventricular system|ventricles]]) are found.
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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]]. There is also a branch of psychology that deals with the anatomy and physiology of the brain, known as [[biopsychology|biological psychology]]. This field of study focuses on each individual part of the brain and how it affects behavior.
  
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==The brain forms part of the nervous system==
 
The brain innervates the [[head]] through [[cranial nerve]]s, and it communicates with the [[spinal cord]], which innervates the body through [[spinal nerve]]s. Nervous fibers transmitting signals from the brain are called [[efferent]] fibers. The fibers transmitting signals to the brain are called [[afferent]] (or sensory) fibers. Nerves can be afferent, efferent or mixed (i.e., containing both types of fibers).   
 
The brain innervates the [[head]] through [[cranial nerve]]s, and it communicates with the [[spinal cord]], which innervates the body through [[spinal nerve]]s. Nervous fibers transmitting signals from the brain are called [[efferent]] fibers. The fibers transmitting signals to the brain are called [[afferent]] (or sensory) fibers. Nerves can be afferent, efferent or mixed (i.e., containing both types of fibers).   
  
The brain is the site of reason and intelligence, which include such components as [[cognition]], [[perception]], [[attention]], [[memory]] and [[emotion]]. The brain is also responsible for control of [[posture]] and [[motor control|movement]]s. It makes possible cognitive, [[motor learning|motor]] and other forms of [[learning]]. The brain can perform a variety of functions automatically, without the need for [[consciousness|conscious]] awareness, such as coordination of [[sensory system]]s (eg. [[sensory gating]] and [[multisensory integration]]), [[Animal locomotion|walking]], and [[homeostasis|homeostatic]] body functions such as [[heart rate]], [[blood]] pressure, fluid balance, and body temperature.
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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 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 (Jungueira et al, date). These highly specialized circuits make up systems which are the basis of [[perception]], different types of action, and higher cognitive function.
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===Components of the brain===
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{{Neuron map|[[Neuron]]}}
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Shortly after birth, neurons in mammalian brains cease cell division, at which time we have the greatest no. of neurons
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[[Neuron]]s are electrically active brain cells that process information, whereas [[Glia|Glial cells]] perform supporting function. In addition to being electrically active, neurons constantly synthesize neurotransmitters. Neurons modify their properties (guided by [[gene expression]]) under the influence of  their input signals. This [[plasticity]] underlies [[learning]] and [[adaptation]]. It is notable that some unused neuron pathways (constructions which have become physically isolated from other cells) may continue to exist long after the memory is absent from consciousness, possibly developing the subconscious.
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The neuron is the unit of function of the brain; the human brain consists of about 10 billion neurons
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p. 954: Information flows: afferent and efferent (further divided into voluntary and autonomic)
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Neurons are the cells that generate [[action potential]]s and convey information to other cells; these constitute the essential class of brain cells.
  
Many functions are controlled by coordinated activity of the brain and [[spinal cord]]. Moreover, 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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Blood-brain barrier constructed by glial cells called astrocytes; blood vessels throughout the body are permeable to many chems, some of which are toxic; barrier formed by surrounding the smallest, most permeable blood vessels in the brain; protection is crucial because, unlike other tissues of the body, it can’t recover from damage by generating new cells; not totally impermeable, however – fat-soluble substances like anesthetics and alcohol, have notable effects on the brain
  
The brain undergoes transitions from [[Awake|wakefulness]] to [[sleep]] (and subtypes of these states). These state transitions are crucially important for proper brain functioning. (For example, it is believed that sleep is important for knowledge consolidation, as the neurons appear to organize the day's stimuli during deep sleep by randomly firing off the most recently used neuron pathways; additionally, without sleep, normal subjects are observed to develop symptoms resembling mental illness, even auditory hallucinations). Every brain state is associated with characteristic [[brain waves]].
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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 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]].
  
[[Neuron]]s are electrically active brain cells that process information, whereas [[Glia|Glial cells]] perform supporting function. In addition to being electrically active, neurons constantly synthesize neurotransmitters. Neurons modify their properties (guided by [[gene expression]]) under the influence of  their input signals. This [[plasticity]] underlies [[learning]] and [[adaptation]]. It is notable that some unused neuron pathways (constructions which have become physically isolated from other cells) may continue to exist long after the memory is absent from consciousness, possibly developing the subconscious.
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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 (from the outside in) 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. 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.
  
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]]. There is also a branch of psychology that deals with the anatomy and physiology of the brain, known as [[biopsychology|biological psychology]]. This field of study focuses on each individual part of the brain and how it affects 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 [[Ventricular system|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 if unsupported by the CSF. The CSF allows the brain to float, easing the physical [[stress (physics)|stress]] caused by the brain’s mass.
  
 
==Comparative anatomy==
 
==Comparative anatomy==
 
[[Image:Mouse_brain.jpg|thumb|right|A mouse brain.]]
 
[[Image:Mouse_brain.jpg|thumb|right|A mouse brain.]]
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]]).<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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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).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.
  
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 0-8385-7701-6 }}</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 0-07-138183-X }}</ref>
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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]] (Kandel, 2000). 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) (Martin, 1996).  
  
More complex vertebrates such as [[mammal]]s have a six-layered [[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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More complex vertebrates such as [[mammal]]s have a six-layered [[neocortex]] (or homotypic cortex, neopallium), in addition to having some parts of the brain that are allocortex (Martin, 1996). 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]].
  
 
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.
 
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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{{Cerebrum map|Human Brain|caption=<small>The lobes of the cerebral cortex include the frontal (blue), temporal (green), occipital (red), and parietal lobes (yellow). The cerebellum (not colored) is not part of the telencephalon. In vertebrates a gross division into three major parts is used.</small>}}
 
{{Cerebrum map|Human Brain|caption=<small>The lobes of the cerebral cortex include the frontal (blue), temporal (green), occipital (red), and parietal lobes (yellow). The cerebellum (not colored) is not part of the telencephalon. In vertebrates a gross division into three major parts is used.</small>}}
  
The [[telencephalon]] (cerebrum) is the largest region of the mammalian brain. This is the structure that is most easily visible in brain specimens, and is what most people associate with the "brain". In humans and several other animals, 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. Generally, comparing the locations of certain brain structures between humans and other vertebrates often reveals a number of differences.
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Vertebrate nervous systems are distinguished by [[symmetry (biology)#Bilateral symmetry|bilaterally symmetrical]] [[encephalization]]. 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).
  
Behind (or in humans, below) the cerebrum is the cerebellum. The cerebellum is known to be 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 0-8385-7701-6 }}</ref> and is connected by thick white matter fibers (cerebellar peduncles) to the [[pons]]<ref name="martin"/>. The cerebrum has two [[cerebral hemispheres]]. The [[cerebellum]] also has 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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[[Image:Brain animated color nevit.gif|thumb|Animation showing the human brain with the lobes highlighted]]
  
Vertebrate nervous systems are distinguished by [[symmetry (biology)#Bilateral symmetry|bilaterally symmetrical]] [[encephalization]]. 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 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 [[mammal]]s (indeed larger than in all animals, although only in mammals has the neocortex evolved to fulfill this kind of function).
  
==Anatomical organization of the vertebrate brain==
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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]].
[[Image:EmbryonicBrain.png|thumb|right|300px|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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==The vertebrate brain is divided into three main regions==
*[[Rhombencephalon]] (hindbrain)
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[[Image:EmbryonicBrain.png|thumb|left|300px|Diagram depicting the main subdivisions of the [[embryogenesis|embryonic]] vertebrate brain.  These regions will later differentiate into forebrain, midbrain and hindbrain structures.]]
**[[Myelencephalon]]
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**[[Metencephalon]]
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[[chordate]] brain Hollow tube of neural tissue early in devo of all vertebrate embryos 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 becomes the spinal cord; cranial and spinal nerves, which are the peripheral nervous system, sprout from the neural tube and grow throughout the embryo
*[[Mesencephalon]] (midbrain)
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*[[Prosencephalon]] (forebrain)
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Each region, in turn, develops into several structures:
**[[Diencephalon]]
 
**[[Telencephalon]]
 
  
==The regions of the brain can also be classified by function==
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*Hindbrain:
The brain can also be classified according to function, including divisions such as:
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**Medulla and pons contain distinct groups of neurons involved in the control of physiological functions like breathing or basic motor patterns like swallowing; all neuron info traveling between the spinal cord and higher brain must pass through the pons and medulla
*[[Limbic system]]
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**Cerebellum: orchestrates and refines behavior patterns
*[[Sensory system]]s
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**[[Visual system]]
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*Midbrain: all info between higher brain and spinal cord must pass through midbrain; also structures involved in processing aspects of visual and auditory info
**[[Olfactory system]]
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**[[Gustatory system]]
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*Forebrain:
**[[Auditory system]]
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**Diencephalon (central region): upper structure called thalamus (final relay station for sensory info going to telencephalon) and lower structure called hypothalamus (reg of many physio functions and bio drives)
**[[Somatosensory system]]
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**Telencephalon (surrounding structures): consists of two cerebral hemispheres, also known as the cerebrum; in humans, this is by far the largest part of the brain, and it plays major roles in sensory perception, learning, memory, and conscious behavior (as we go up the vertebrate phylogenetic scale from fish to mammals, the telencephalon increases in size, complexitiy, and importance, dominating the nervous systems of mammals; major damage to this area results in severe impairment or even coma, while in sharks, eg, after removal of telencephalon, it can swim almost normally)
*[[Muscle|Motor system]]
 
*[[cerebral cortex|Associative areas]]
 
  
==The human brain==
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Brain stem: communication between spinal cord and telencephalon travels through medulla, pons, midbrain, and diencephalons, structures collectively referred to as brain stem; in general, more primitive and autonomic functions are localized farther down this neural axis, while more complex and evolutionarily advanced functions are found higher on the axis.
[[Image:Brain animated color nevit.gif|thumb|Animation showing the human brain with the lobes highlighted]]
 
{{main|human brain}}
 
  
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 [[mammal]]s (indeed larger than in all animals, although only in mammals has the neocortex evolved to fulfill this kind of function).
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==The regions of the brain can also be classified by function==
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Now that we’ve looked at anatomical divisions, another way of organizing our understanding of the nervous system is through functional divisions; keep in mind that ns engages in parallel processing of information; any one anatomical structure may be involved in several functional subsystems:
  
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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*Spinal cord
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*Reticular system: network of neuronal fibers that includes discrete groups of neurons (called a nucleus), distributed through core of medulla, pons, and midbrain; alerts the forebrain; some nuclei involved in controlling sleep and wakefulness
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*Limbic system (the evolutionarily primitive parts of the forebrain, which still have imp functions in birds and mammals, but they are completely covered by the more recent elaborations of the telencephalon called the neocortex); 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; hippocampus, one part of the ls, is necessary in humans for the transfer of short-term to long-term memory
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*Cerebrum: regions interact for consciousness and control of behavior [what is diff between cerebrum and cerebral cortex?]
  
==Neurobiology==
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cerebral cortex, sheet of gray matter that covers each cerebral hemisphere; convoluted or folded into ridges (called gyri) and valleys (called sulci) so that it fits into the skull; under the cc is white matter
The brain is composed of two broad classes of cells, [[neuron]]s 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]], different types of action, and higher cognitive function.
 
  
===Components of the brain===
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corpus callosum: white-matter tract that connects the hemispheres
{{Neuron map|[[Neuron]]}}
 
Neurons are the cells that generate [[action potential]]s and convey information to other cells; these constitute the essential class of brain cells.
 
  
In addition to neurons, the brain contains [[glial cell]]s 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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function of some regions of the cc are easier to define, others not so much; latter areas fall under the general name of association cortex
  
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 (from the outside in) 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. 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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subdivisions of cc:
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#temporal lobe: upper region involved in receiving and processing auditory info; its association areas are involved in recognition, identification, and naming of objects
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#occipital lobe: receive and process visual info; association areas are essential for making sense of the visual world and translating visual exp into language
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#parietal lobe: central sulcus (a deep valley that separates parietal and frontal lobes); the strip just behind the cs is the primary somatosensory cortex, which receives info through the thalamus about touch and pressure sensations; a major association function of the pc is attending to complex stimuli
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#frontal lobe: strip just in front of the cs is called the primary motor cortex; association functions are diverse and best described as having to do with planning
  
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. 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 if unsupported by the CSF. The CSF allows the brain to float, easing the physical [[stress (physics)|stress]] caused by the brain’s mass.
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most dramatic increase in size of the cc took place during the last several million yrs of human evo; 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; humans have the largest ratio of brain size to body size, and they have the most highly devo’s cerebral cortex
  
===Function===
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==Function==
 
Vertebrate brains receive signals through nerves arriving from the sensors of the organism. These signals are then processed throughout the central nervous system; reactions are formulated based upon reflex and learned experiences. A similarly extensive nerve network delivers signals from a brain to 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 the signals to and from the brain.
 
Vertebrate brains receive signals through nerves arriving from the sensors of the organism. These signals are then processed throughout the central nervous system; reactions are formulated based upon reflex and learned experiences. A similarly extensive nerve network delivers signals from a brain to 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 the signals to and from the brain.
  
Line 100: Line 112:
 
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.
 
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.
  
===Brain pathology===
+
==Brain pathology==
 
[[Image:Frontotemporal_degeneration.jpg|right|thumb|250px|A [[human brain]] showing [[frontotemporal lobar degeneration]] causing frontotemporal dementia.]]
 
[[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.
 
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.
Line 136: Line 148:
 
==Mind and brain==
 
==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 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. Only some adhere to [[metaphysics|metaphysically]] [[dualistic]] approaches in which the mind exists independently of the brain in some way, such as a [[soul]] or [[epiphenomenalism|epiphenomenon]] or [[strong emergence|emergent]] phenomenon. Other dualisms maintain that the mind is a distinct ''[[physics|physical]]'' phenomenon, such as [[electromagnetism|electromagnetic field]], or a [[quantum mind|quantum effect]]. [[Materialistic]] options include beliefs that mentality is [[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 minds do not exist at all, and mentalistic language will be replaced by neurological terminology.
 
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 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. Only some adhere to [[metaphysics|metaphysically]] [[dualistic]] approaches in which the mind exists independently of the brain in some way, such as a [[soul]] or [[epiphenomenalism|epiphenomenon]] or [[strong emergence|emergent]] phenomenon. Other dualisms maintain that the mind is a distinct ''[[physics|physical]]'' phenomenon, such as [[electromagnetism|electromagnetic field]], or a [[quantum mind|quantum effect]]. [[Materialistic]] options include beliefs that mentality is [[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 minds do not exist at all, and mentalistic language will be replaced by neurological terminology.
 
 
 
 
  
 
==References==
 
==References==
 
*Bear, M.F., Connors, B.W. and M.A. Paradiso. 2001. ''Neuroscience: Exploring the Brain.'' Baltimore: Lippincott. ISBN 0781739446
 
*Bear, M.F., Connors, B.W. and M.A. Paradiso. 2001. ''Neuroscience: Exploring the Brain.'' Baltimore: Lippincott. ISBN 0781739446
*Purves
+
*Butler, A. B. 2002. Chordate Evolution and the Origin of Craniates: An Old Brain in a New Head. ''The Anatomical Record'' 261:111–25.
 +
*Kandel, E.R., Schwartz, J.H. and T.M. Jessell. 2000. ''Principles of Neural Science,'' 4th ed. New York: McGraw-Hill ISBN 0-8385-7701-6
 +
*Martin, J.H. 1996. ''Neuroanatomy: Text and Atlas'', 2nd ed. New York: McGraw-Hill. ISBN 0-07-138183-X
 +
*Purves, W., D. Sadava, G. Orians, and C. Heller. 2004. ''Life: The Science of Biology,'' 7th edition. Sunderland, MA: Sinauer. ISBN 0716766728
  
 
==Further reading==
 
==Further reading==
*{{cite book|author=Junqueira, L.C., and J. Carneiro|title=Basic Histology: Text and Atlas, Tenth Edition|publisher=Lange Medical Books McGraw-Hill|year=2003|id=ISBN 0-07-121565-4}}
+
*Junqueira, L.C. and J. Carneiro. 2003. ''Basic Histology: Text and Atlas'', 10th edition. New York: Lange Medical Books, McGraw-Hill. ISBN 0-07-121565-4
* Kinseher Richard, Geborgen in Liebe und Licht - Gemeinsame Ursache von Intuition, Déjà-vu-, Schutzengel-, und Nahtod-Erlebnissen, BoD, 2006, ISBN:3-8334-51963, German language: (A new theory: A LIVE-scan of the episodic memory, can be observed during near-death-experiences. The stored experiences are then judged by the topical intellect.)
+
*Sala, S.D., ed. 1999. ''Mind myths: Exploring popular assumptions about the mind and brain.'' New York: J. Wiley & Sons. ISBN 0-471-98303-9
*{{cite book|author=Sala, Sergio Della, editor.|title=Mind myths: Exploring popular assumptions about the mind and brain|publisher=J. Wiley & Sons, New York|year=1999|id=ISBN 0-471-98303-9}}
+
*Vander, A., Sherman, J. and D. Luciano. 2001. ''Human Physiology: The Mechanisms of Body Function.'' New York: McGraw-Hill. ISBN 0-07-118088-5
*{{cite book|author=Vander, A., J. Sherman, D. Luciano|title=Human Physiology: The Mechanisms of Body Function|publisher=McGraw Hill Higher Education|year=2001|id=ISBN 0-07-118088-5}}
 
  
 
==External links==
 
==External links==

Revision as of 16:45, 2 September 2007

Representation of brain MRI
Human brain

In animals, the brain or encephalon (Greek for "in the head"), is the control center of the central nervous system, responsible for behaviour. In most animals, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, equilibrioception, taste, and olfaction. While all vertebrates have a brain, most invertebrates have either a centralized brain or collections of individual ganglia. Primitive animals such as sponges do not have a brain at all. Brains can be extremely complex. For example, the human brain contains more than 100 billion neurons, each linked to as many as 10,000 other neurons.

The brain is the site of reason and intelligence, which include such components as cognition, perception, attention, memory and emotion. The brain is also responsible for control of posture and movements. It makes possible cognitive, motor and other forms of learning. The brain can perform a variety of functions automatically, without the need for conscious awareness, such as coordination of sensory systems (eg. sensory gating and multisensory integration), walking, and homeostatic body functions such as heart rate, blood pressure, fluid balance, and body temperature.

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 molecular up to the psychological. There is also a branch of psychology that deals with the anatomy and physiology of the brain, known as biological psychology. This field of study focuses on each individual part of the brain and how it affects behavior.

The brain forms part of the nervous system

The brain innervates the head through cranial nerves, and it communicates with the spinal cord, which innervates the body through spinal nerves. Nervous fibers transmitting signals from the brain are called efferent fibers. The fibers transmitting signals to the brain are called afferent (or sensory) fibers. Nerves can be afferent, efferent or mixed (i.e., containing both types of fibers).

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 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 a thousand other neurons (Jungueira et al, date). These highly specialized circuits make up systems which are the basis of perception, different types of action, and higher cognitive function.

Components of the brain

Neuron
Neuron-no labels.png
Nucleus
Node of
Ranvier
Axon Terminal
Schwann cell
Myelin sheath
Structure of a typical neuron

Shortly after birth, neurons in mammalian brains cease cell division, at which time we have the greatest no. of neurons Neurons are electrically active brain cells that process information, whereas Glial cells perform supporting function. In addition to being electrically active, neurons constantly synthesize neurotransmitters. Neurons modify their properties (guided by gene expression) under the influence of their input signals. This plasticity underlies learning and adaptation. It is notable that some unused neuron pathways (constructions which have become physically isolated from other cells) may continue to exist long after the memory is absent from consciousness, possibly developing the subconscious.

The neuron is the unit of function of the brain; the human brain consists of about 10 billion neurons

p. 954: Information flows: afferent and efferent (further divided into voluntary and autonomic)

Neurons are the cells that generate action potentials and convey information to other cells; these constitute the essential class of brain cells.

Blood-brain barrier constructed by glial cells called astrocytes; blood vessels throughout the body are permeable to many chems, some of which are toxic; barrier formed by surrounding the smallest, most permeable blood vessels in the brain; protection is crucial because, unlike other tissues of the body, it can’t recover from damage by generating new cells; not totally impermeable, however – fat-soluble substances like anesthetics and alcohol, have notable effects on the brain

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 oligodendrocytes which myelinate neural axons (a role performed by Schwann cells 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, dendrites, 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.

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 (from the outside in) 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. Below the arachnoid is the subarachnoid space which contains cerebrospinal fluid, a substance that protects the nervous system. Blood vessels enter the central nervous system through the perivascular space above the pia mater. The cells in the blood vessel walls are joined tightly, forming the blood-brain barrier which protects the brain from toxins that might enter through the blood.

The brain is bathed in cerebrospinal fluid (CSF), which circulates between layers of the meninges and through cavities in the brain called ventricles. It is important both chemically for metabolism and mechanically for shock-prevention. For example, the human brain weighs about 1-1.5 kg. 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.

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. Arthropods have a central brain with three divisions and large optical lobes behind each eye for visual processing.

The brain of craniates develops from the anterior section of a single dorsal nerve cord, which later becomes the spinal cord (Kandel, 2000). In craniates, the brain is protected by the bones of the skull. In vertebrates, increasing complexity in the cerebral cortex correlates 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. This cortical configuration is called the allocortex (or heterotypic cortex) (Martin, 1996).

More complex vertebrates such as mammals have a six-layered neocortex (or homotypic cortex, neopallium), in addition to having some parts of the brain that are allocortex (Martin, 1996). 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 gyri, while the spaces between the folds are called sulci.

Although the general histology of the brain is similar from person to person, the structural anatomy can differ. Apart from the gross embryological divisions of the brain, the location of specific gyri and sulci, primary sensory regions, and other structures differs between species.

Invertebrates

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.[1] The protocerebrum contains the mushroom bodies, which respond to smell, and the central body complex. In some species such as bees, the mushroom body receives input from the visual pathway as well. 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. The antennal lobes of flies and moths are quite complex.

In cephalopods, the brain has two regions: the supraesophageal mass and the subesophageal mass,[1] 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.[1] 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.

Vertebrates

Human Brain
Lobes of the brain NL.svg
Frontal lobe
Temporal lobe
Parietal lobe
Occipital lobe
The lobes of the cerebral cortex include the frontal (blue), temporal (green), occipital (red), and parietal lobes (yellow). The cerebellum (not colored) is not part of the telencephalon. In vertebrates a gross division into three major parts is used.


Vertebrate nervous systems are distinguished by bilaterally symmetrical encephalization. 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).

Animation showing the human brain with the lobes highlighted

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 mammals (indeed larger than in all animals, although only in mammals has the neocortex evolved to fulfill this kind of function).

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.

The vertebrate brain is divided into three main regions

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

chordate brain Hollow tube of neural tissue early in devo of all vertebrate embryos 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 becomes the spinal cord; cranial and spinal nerves, which are the peripheral nervous system, sprout from the neural tube and grow throughout the embryo

Each region, in turn, develops into several structures:

  • Hindbrain:
    • Medulla and pons contain distinct groups of neurons involved in the control of physiological functions like breathing or basic motor patterns like swallowing; all neuron info traveling between the spinal cord and higher brain must pass through the pons and medulla
    • Cerebellum: orchestrates and refines behavior patterns
  • Midbrain: all info between higher brain and spinal cord must pass through midbrain; also structures involved in processing aspects of visual and auditory info
  • Forebrain:
    • Diencephalon (central region): upper structure called thalamus (final relay station for sensory info going to telencephalon) and lower structure called hypothalamus (reg of many physio functions and bio drives)
    • Telencephalon (surrounding structures): consists of two cerebral hemispheres, also known as the cerebrum; in humans, this is by far the largest part of the brain, and it plays major roles in sensory perception, learning, memory, and conscious behavior (as we go up the vertebrate phylogenetic scale from fish to mammals, the telencephalon increases in size, complexitiy, and importance, dominating the nervous systems of mammals; major damage to this area results in severe impairment or even coma, while in sharks, eg, after removal of telencephalon, it can swim almost normally)

Brain stem: communication between spinal cord and telencephalon travels through medulla, pons, midbrain, and diencephalons, structures collectively referred to as brain stem; in general, more primitive and autonomic functions are localized farther down this neural axis, while more complex and evolutionarily advanced functions are found higher on the axis.

The regions of the brain can also be classified by function

Now that we’ve looked at anatomical divisions, another way of organizing our understanding of the nervous system is through functional divisions; keep in mind that ns engages in parallel processing of information; any one anatomical structure may be involved in several functional subsystems:

  • Spinal cord
  • Reticular system: network of neuronal fibers that includes discrete groups of neurons (called a nucleus), distributed through core of medulla, pons, and midbrain; alerts the forebrain; some nuclei involved in controlling sleep and wakefulness
  • Limbic system (the evolutionarily primitive parts of the forebrain, which still have imp functions in birds and mammals, but they are completely covered by the more recent elaborations of the telencephalon called the neocortex); 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; hippocampus, one part of the ls, is necessary in humans for the transfer of short-term to long-term memory
  • Cerebrum: regions interact for consciousness and control of behavior [what is diff between cerebrum and cerebral cortex?]

cerebral cortex, sheet of gray matter that covers each cerebral hemisphere; convoluted or folded into ridges (called gyri) and valleys (called sulci) so that it fits into the skull; under the cc is white matter

corpus callosum: white-matter tract that connects the hemispheres

function of some regions of the cc are easier to define, others not so much; latter areas fall under the general name of association cortex

subdivisions of cc:

  1. temporal lobe: upper region involved in receiving and processing auditory info; its association areas are involved in recognition, identification, and naming of objects
  2. occipital lobe: receive and process visual info; association areas are essential for making sense of the visual world and translating visual exp into language
  3. parietal lobe: central sulcus (a deep valley that separates parietal and frontal lobes); the strip just behind the cs is the primary somatosensory cortex, which receives info through the thalamus about touch and pressure sensations; a major association function of the pc is attending to complex stimuli
  4. frontal lobe: strip just in front of the cs is called the primary motor cortex; association functions are diverse and best described as having to do with planning

most dramatic increase in size of the cc took place during the last several million yrs of human evo; 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; humans have the largest ratio of brain size to body size, and they have the most highly devo’s cerebral cortex

Function

Vertebrate brains receive signals through nerves arriving from the sensors of the organism. These signals are then processed throughout the central nervous system; reactions are formulated based upon reflex and learned experiences. A similarly extensive nerve network delivers signals from a brain to 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 the 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. Visual, touch, and auditory sensory pathways of vertebrates are routed to specific nuclei of the thalamus and then to regions of the cerebral cortex that are specific to each sensory system. The visual system, the auditory system, and the somatosensory system. Olfactory pathways are routed to the olfactory bulb, then to various parts of the olfactory system. Taste is routed through the brainstem and then to other portions of the gustatory system.

To control movement the brain has several parallel systems of muscle control. The motor system controls voluntary muscle movement, aided by the motor cortex, cerebellum, and the basal ganglia. The system eventually projects to the spinal cord and then out to the muscle effectors. Nuclei in the brain stem control many involuntary muscle functions such as heart rate and breathing. In addition, many automatic acts (simple reflexes, locomotion) can be controlled by the spinal cord alone.

Brains also produce a portion of the body's hormones that can influence organs and glands elsewhere in a body—conversely, brains also react to hormones produced elsewhere in the body. In mammals, the hormones that regulate hormone production throughout the body are produced in the brain by the structure called the pituitary gland.

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.

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.

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. 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 diseases, such as Alzheimer's disease, Parkinson's disease, motor neurone disease, and Huntington's disease are caused by the gradual death of individual neurons, leading to decrements in movement control, memory, and cognition. Currently only the symptoms of these diseases can be treated. Mental illnesses, such as clinical depression, schizophrenia, bipolar disorder, and post-traumatic stress disorder are brain diseases that impact personality and, typically, other aspects of mental and somatic function. These disorders may be treated by psychiatric therapy, pharmaceutical intervention, or through a combination of treatments; therapeutic effectiveness varies significantly among individuals.

Some infectious diseases affecting the brain are caused by viruses and bacteria. Infection of the meninges, the membrane that covers the brain, can lead to meningitis. Bovine spongiform encephalopathy (also known as mad cow disease), is deadly in cattle and humans and is linked to prions. Kuru is a similar prion-borne degenerative brain disease affecting humans. Both are linked to the ingestion of neural tissue, and may explain the tendency in some species to avoid cannibalism. Viral or bacterial causes have been substantiated in multiple sclerosis, Parkinson's disease, Lyme disease, encephalopathy, and encephalomyelitis.

Some 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.

Certain brain disorders are treated by brain surgeons (neurosurgeons) while others are treated by neurologists and psychiatrists.

Study of the brain

Fields of study

Neuroscience seeks to understand the nervous system, including the brain, from a biological and computational perspective. Psychology seeks to understand behavior and the brain. The terms neurology and psychiatry usually refer to 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

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 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.

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 much better spatial resolution, although admittedly not as good as fMRI. The main advantage over fMRI is a direct relationship between neural activation and measurement.

A scan of the brain using fMRI

Functional magnetic resonance imaging (fMRI) measures changes in blood flow in the brain, but the activity of neurons is not directly measured, nor can it be distinguished whether this activity is inhibitory or excitatory. fMRI is a noninvasive, indirect method for measuring neural activity that is based on BOLD; Blood Oxygen Level Dependent changes. The changes in blood flow that occur in capillary beds in specific regions of the brain are thought to represent various neuronal activities (metabolism of synaptic reuptake). Similarly, a positron emission tomography (PET), is able to monitor glucose and oxygen metabolism as well as neurotransmitter activity in different areas within the brain which can be correlated to the level of activity in that region.

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.

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 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.

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 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. Only some adhere to metaphysically dualistic approaches in which the mind exists independently of the brain in some way, such as a soul or epiphenomenon or emergent phenomenon. Other dualisms maintain that the mind is a distinct physical phenomenon, such as electromagnetic field, or a quantum effect. Materialistic options include beliefs that mentality is 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 minds do not exist at all, and mentalistic language will be replaced by neurological terminology.

References
ISBN links support NWE through referral fees

  • Bear, M.F., Connors, B.W. 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.
  • Kandel, E.R., Schwartz, J.H. and T.M. Jessell. 2000. Principles of Neural Science, 4th ed. New York: McGraw-Hill ISBN 0-8385-7701-6
  • Martin, J.H. 1996. Neuroanatomy: Text and Atlas, 2nd ed. New York: McGraw-Hill. ISBN 0-07-138183-X
  • Purves, W., D. Sadava, G. Orians, and C. Heller. 2004. Life: The Science of Biology, 7th edition. Sunderland, MA: Sinauer. ISBN 0716766728

Further reading

  • Junqueira, L.C. and J. Carneiro. 2003. Basic Histology: Text and Atlas, 10th edition. New York: Lange Medical Books, McGraw-Hill. ISBN 0-07-121565-4
  • Sala, S.D., ed. 1999. Mind myths: Exploring popular assumptions about the mind and brain. New York: J. Wiley & Sons. ISBN 0-471-98303-9
  • Vander, A., Sherman, J. and D. Luciano. 2001. Human Physiology: The Mechanisms of Body Function. New York: McGraw-Hill. ISBN 0-07-118088-5

External links

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