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Overview
Representation of brain MRI
Representation of brain MRI

Most brains exhibit a substantial distinction between the gray matter and white matter. Gray matter consists primarily of the cell bodies of the neurons, while white matter is comprised mostly of the fibers (axons) which connect neurons. The axons are surrounded by a fatty 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 (fasciculi, fiber tracts), gray matter (nuclei) and spaces filled with cerebrospinal fluid (ventricles) are found.

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, 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.

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 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.

[edit] 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.

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.
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:

The brain can also be classified according to function, including divisions such as:

Humans
Animation showing the human brain with the lobes highlighted
Animation showing the human brain with the lobes highlighted
Main article: 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 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

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

Histology
Neuron
Structure of a typical 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.

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.

Brain pathology
A human brain showing frontotemporal lobar degeneration causing frontotemporal dementia.
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.

Brain energy consumption

The neurons of the brain require a lot of energy. 75% of the blood sugar created by the liver is consumed by the brain.[citation needed] The brain also consumes 20% of the oxygen a human breathes.[citation needed] The energy consumption for the brain to simply survive is 0.1 calories per minute, while this value can be as high as 1.5 calories per minute during crossword puzzle-solving.[14] The demands of the brain limit its size in many species. Molossid bats and the Vespertilionid Nyctalus spp. have brains that have been reduced from the ancestral form to invest in wing-size for the sake of manoeuverability. This contrasts with fruit bats, which require more advanced neural structures and do not pursue their prey.[15]

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