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Brain

2007 Schools Wikipedia Selection. Related subjects: General Biology

   In animals, the brain, or encephalon ( Greek for "in the head"), is the
   control centre of the central nervous system. In most animals, the
   brain is located in the head, protected by the skull and close to the
   primary sensory apparatus of vision, hearing, taste and olfaction. In
   humans, it is an organ of thought. While all vertebrates have a brain,
   invertebrates have either a centralized brain or collections of
   individual ganglia. Brains can be extremely complex. For example, the
   human brain contains more than 100 billion neurons, each linked to as
   many as 10,000 others.
   A sketch of the human brain by artist Priyan Weerappuli, imposed upon
   the profile of Michaelangelo's David
   Enlarge
   A sketch of the human brain by artist Priyan Weerappuli, imposed upon
   the profile of Michaelangelo's David

Overview

   Enlarge

   Most brains exhibit a substantial distinction between grey matter and
   white matter. Gray matter consists of the cell bodies of the neurons,
   while white matter consists of the fibers ( axons) that connect
   neurons. The axons are surrounded by a fatty insulating sheath called
   myelin, giving the white matter its distinctive colour. The outer layer
   of the brain is gray matter called cerebral cortex. Deep in the brain,
   compartments of white matter ( fasciculi, fibre tracts), gray matter (
   nuclei) and spaces filled with cerebrospinal fluid ( ventricles) are
   found.

   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 signal 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 controls a wide variety of functions. It 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.

   Many functions are controlled by coordinated activity of the brain and
   spinal cord. Moreover, some behaviors such as simple reflexes and basic
   locomotion, can be executed under spinal cord control alone.

   The brain undergoes transitions from 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). Each brain state is associated
   with characteristic brain waves.

   Neurons are electrically active brain cells that process information,
   whereas Glial cells perform supporting function. Brain cell metabolism
   consumes considerable amounts of energy. In addition to being
   electrically active, neurons constantly synthesise neurotransmitters.
   Neurons modify their properties (guided by gene expression) under the
   influence of their input signals. This plasticity underlies learning
   and adaptation.

   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 assesses different parts of the
   body.

History

   Early human 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”.

   The first thoughts of the field of psychology actually came from
   ancient philosophers, including Aristotle. After philosophers have
   become more in tune with medical research over time, the idea of
   psychology had developed. From that point, different branches of
   psychology emerged with different individuals creating new theories.
   For example, Evolutionary Psychology was created with theories by
   Charles Darwin.

Mind and brain

   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 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. Some believe that the mind exists in some way independently of
   the brain, such as in a soul or epiphenomenon. Others, such as strong
   AI theorists, say that the mind is directly analogous to computer
   software and the brain to hardware.

Comparative anatomy

   A mouse brain.
   Enlarge
   A mouse brain.

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

   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. 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. 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, 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. 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

   The lobes of the cerebral cortex include the frontal (red), temporal
   (green), occipital (yellow), and parietal lobes (orange). The
   cerebellum (blue) is not part of the telencephalon. In vertebrates a
   gross division into three major parts is used.
   Enlarge
   The lobes of the cerebral cortex include the frontal (red), temporal
   (green), occipital (yellow), and parietal lobes (orange). The
   cerebellum (blue) is not part of the telencephalon. In vertebrates a
   gross division into three major parts is used.

   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 centre
   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.

   Behind (or in humans, below) the cerebrum is the cerebellum. The
   cerebellum is known to be involved in the control of movement, and is
   connected by thick white matter fibers (cerebellar peduncles) to the
   pons. 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.

   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
   fibre tracts forming the white matter. Most regions of the human
   cerebral cortex have six layers of neurons (neocortex).

Vertebrate brain regions

   (See related article at List of regions in the human brain)
   Diagram depicting the main subdivisions of the embryonic vertebrate
   brain. These regions will later differentiate into forebrain, midbrain
   and hindbrain structures.
   Enlarge
   Diagram depicting the main subdivisions of the embryonic vertebrate
   brain. These regions will later differentiate into forebrain, midbrain
   and hindbrain structures.

   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:
     * Rhombencephalon (hindbrain)
          + Myelencephalon
          + Metencephalon
     * Mesencephalon (midbrain)
     * Prosencephalon (forebrain)
          + Diencephalon
          + Telencephalon

   The brain can also be classified according to function, including
   divisions such as:
     * Limbic system
     * Sensory systems
          + Visual system
          + Olfactory system
          + Gustatory system
          + Auditory system
          + Somatosensory system
     * Motor system
     * Associative areas

Humans

   Enlarge

   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 animals.

   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.

Neurobiology

   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. These highly
   specialized circuits make up systems which are the basis of perception,
   different types of action, and higher cognitive function.

Histology

   Diagram of basic features of a neuron.
   Enlarge
   Diagram of basic features of a neuron.

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

   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. The CSF allows the brain to
   float, easing the physical stress caused by the brain’s mass.

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.
   Enlarge
   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, by 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 behaviour 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

Electrophysiology

   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

   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

   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 occurs in capillary
   beds in specific regions of the brain are thought to represent various
   neuronal activities. 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.

Behavioural

   Behavioural 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.

Anatomical

   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

   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 implants have been
   used to generate artificial hearing and (crude and experimental)
   artificial vision for deaf and blind people. Brain pacemakers are now
   commonly used to regulate brain activity in conditions such as
   Parkinson's disease.

Other matters

   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.

Brain as food

   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". 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 glands, with the latter form being the favorite food of
   French President Jacques Chirac. Similar delicacies from around the
   world include Mexican tacos de sesos made with cattle brain as well as
   squirrel brain in the US South. 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.. Indonesian cuisine specialty in Padang cuisine also served beef
   brain in a gravy coconut milk named gulai otak (beef brain curry).

   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. 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.

   Brain consumption can also result in contracting fatal transmissible
   spongiform encephalopathies such as Variant Creutzfeldt-Jakob disease
   and other prion diseases in humans and mad cow disease in cattle.
   Another prion disease called 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. Some archaeological evidence suggests that the mourning
   rituals of European Neanderthals also involved the consumption of the
   brain.

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