Human brain
From Wikipedia, the free encyclopedia
The human brain controls the central nervous system (CNS), by way of the cranial nerves and spinal cord, the peripheral nervous system (PNS) and regulates virtually all human activity.[1] Involuntary, or "lower," actions, such as heart rate, respiration, and digestion, are unconsciously governed by the brain,[1][2] specifically through the autonomic nervous system. Complex, or "higher," mental activity, such as thought, reason, and abstraction,[2] is consciously controlled.
The human brain is the most complex organ in the human body. It contains some one hundred billion neurons,[1][2] which are capable of electrical and chemical communication with tens of thousands of other nerve cells.[1][2] Nerve cells in turn rely on some quadrillion (1015) synaptic connections for their communications.
Anatomically, the brain can be divided into three parts: the forebrain, midbrain, and hindbrain;[3] the forebrain includes the several lobes of the cerebral cortex that control higher functions, while the mid- and hindbrain are more involved with unconscious, autonomic functions. During encephalization, human brain mass increased beyond that of other species relative to body mass. This process was especially pronounced in the neocortex, a section of the brain involved with language and consciousness. The neocortex accounts for about 76% of the mass of the human brain;[4] with a neocortex much larger than other animals, humans enjoy unique mental capacities despite having a neuroarchitecture similar to that of more primitive species. Basic systems that alert humans to stimuli, sense events in the environment, and maintain homeostasis are similar to those of basic vertebrates. Human consciousness is founded upon the extended capacity of the modern neocortex, as well as the greatly developed structures of the brain stem.
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[edit] Anatomy
The normal adult human brain typically weighs between 1 and 1.5 kg (3 lb) and has an average volume of 1.6 litres. The mature human brain consumes some 20-25% of the energy used by the body, while the developing brain of an infant consumes around 60%. Such heavy energy usage generates large quantities of heat, which must be continually removed to prevent brain damage. Both sexes have similar brain weight to body weight ratios,[5] though the differences in weight persist even when adjusted for relative body mass. Although no functional inference can be made from this statement, it does provide scientific proof that the weight for the brain of human males is larger and heavier than that of human females, usually by about 200 gramme (7 ounces). The human brain varies in many interesting ways between the sexes and may be an example of sexual dimorphism within the species (see also Sex and intelligence). It should also be noted that these differences in brain development and function may underlie the difference observed for weight and brain matter, and as such may not pertain directly to executive function since this faculty occurs by way of disparate neurophysiology for males and females.
The bulbous cerebral cortex is composed of convoluted grey matter internally supported by deep brain white matter. The two hemispheres of the brain are separated by a prominent central fissure and connect to each other at the corpus callosum. A well-developed cerebellum is found at the back of the brain. Brain stem structures are almost completely enveloped by the cerebellum and telencephalon, with the medulla oblongata projecting through the foramen magnum to merge with the spinal cord.
The blood supply to the brain involves the paired carotid arteries that enter the brain and communicate in the circle of Willis before branching out to their destinations. Further blood supply comes via the vertebral arteries. Blood drains from the brain through a network of sinuses that drain into the right and left internal jugular veins.
The brain is suspended in cerebrospinal fluid (CSF), which also fills spaces called ventricles inside it. The dense fluid protects the brain and spinal cord from shock; a brain that weighs 1,500 g in air weighs only 50 g when suspended in CSF (Livingston, 1965). Fluid movement within the brain is limited by the blood-brain barrier and the blood-cerebrospinal fluid barrier.
The brain is easily damaged by compression, so the fluid surrounding the central nervous system must be maintained at constant pressure. Humans are estimated to produce about 500 ml or more of cerebrospinal fluid each day, with only about 15 percent of the body's estimated 150 ml of CSF at any given time located in the ventricles of the brain. The remainder fills the subarachnoid space, which separates the soft tissues of the brain and spinal cord from the hard surrounding bones (skull and vertebrae). Elevated levels of CSF are associated with traumatic brain injury and hydrocephalus. Increased fluid pressure can result in permanent brain injury and death.
Anatomical restraints prevent the human brain from getting even bigger. At birth, an infant's skull is as large as it can be without imperiling the lives of mothers and infants during childbirth. The difficulty experienced by humans in giving birth is nearly unique in the animal kingdom, requiring the head of the emerging infant to be rotated as it passes through the birth canal. Female humans possess large pelvic openings to accommodate the birth of large-headed offspring, but at the cost of thinning of the pelvic bones. Bones too thin can no longer bear the mother's weight or the mechanical stresses of walking and running, and a compromised ability to flee dangers may prevent the female from coming of childbearing age.
At birth, the human skull is rather soft, and it deforms somewhat during its passage through the birth canal, then recovers its shape. This allows it to expand to make room for the brain, which continues to grow, at the same rate as that of an unborn fetus, for an additional year. In all other animals the growth rate of the brain slows significantly at birth.
[edit] Function
The human brain is the source of the conscious, cognitive mind. The mind is the set of cognitive processes related to perception, interpretation, imagination, memories, and crucially language (cf. Broca's area) of which a person may or may not be aware. Beyond cognitive functions, the brain regulates autonomic processes related to essential body functions such as respiration and heartbeat.
Extended neocortical capacity allows humans some control over emotional behavior, but neural pathways between emotive centers of the brain stem and cerebral motor control areas are shorter than those connecting complex cognitive areas in the neocortex with incoming sensory information from the brain stem. Powerful emotional pathways can modulate spontaneous emotive ｅxpression regardless of attempts at cerebral self-control. Emotive stability in humans is associated with planning, experience, and an environment that is both stable and stimulating.
The 19th century discovery of the primary motor cortex mapped to correspond with regions of the body led to popular belief that the brain was organized around a homunculus. A distorted figure drawn to represent the body's motor map in the prefrontal cortex was popularly recognized as the brain's homunculus, but function of the human brain is far more complex than this simple figure suggests. A similar, "sensory homunculus" can be drawn in the parietal lobe that parallels that in the frontal lobe.
The human brain appears to have no localized center of conscious control. The brain seems to derive consciousness from interaction among numerous systems within the brain. Executive functions rely on cerebral activities, especially those of the frontal lobes, but redundant and complementary processes within the brain result in a diffuse assignment of executive control that can be difficult to attribute to any single locale.
Midbrain functions include routing, selecting, mapping, and cataloging information, including information perceived from the environment and information that is remembered and processed throughout the cerebral cortex. Endocrine functions housed in the midbrain play a leading role in modulating arousal of the cortex and of autonomic systems.
Nerves from the brain stem complex where autonomic functions are modulated join nerves routing messages to and from the cerebrum in a bundle that passes through the spinal column to related parts of a body. Twelve pairs of cranial nerves, including some that innervate parts of the head, follow pathways from the medulla oblongata outside the spinal cord.
A definite description of the biological basis for consciousness so far eludes the best efforts of the current generation of researchers. But reasonable assumptions based on observable behaviors and on related internal responses have provided the basis for general classification of elements of consciousness and of likely neural regions associated with those elements. Researchers know people lose consciousness and regain it, they have identified partial losses of consciousness associated with particular neuropathologies and they know that certain conscious activities are impossible without particular neural structures.
[edit] Study of the brain
Although folklore would have it that about 90% of the human brain is dormant, this has proven scientifically unfounded. The fact that ~10% of neurons in the brain fire at any one time is a possible source of this misconception. (If a large percentage of the neurons were to fire at the same time, the result would be a Tonic-clonic seizure.)
Grey matter, the thin layer of cells covering the cerebrum, was believed by most scholars to be the primary center of cognitive and conscious processing. White matter, the mass of glial cells that support the cerebral grey matter, was assumed to primarily provide nourishment, physical support, and connective pathways for the more functional cells on the cerebral surface. But research fueled by the interest of Dr. Marian Diamond in the glial structure of Albert Einstein's brain led to a line of research that offered strong evidence that glial cells serve a computational role beyond merely transmitting processed signals between more functional parts of the brain. In 2004, Scientific American published an article suggesting scientists in the early 21st century are only beginning to study the "other half of the brain."
For many millennia the function of the brain was unknown. Ancient Egyptians threw the brain away prior to the process of mummification. Ancient thinkers such as Aristotle imagined that mental activity took place in the heart. Greek scholars assumed correctly that the brain serves a role in cooling the body, but incorrectly presumed the brain to function as a sort of radiator, rather than as a thermostat as is now understood. The Alexandrian biologists Herophilos and Erasistratus were among the first to conclude that the brain was the seat of intelligence. Galen's theory that the brain's ventricles were the sites of thought and emotion prevailed until the work of the Renaissance anatomist Vesalius.
The modern study of the brain and its functions is known as neuroscience. Psychology is the scientific study of the mind and behavior. Neurophysiology is the study of normal healthy brain activity, while neurology and psychiatry are both medical approaches to the study of the mind and its disorders and pathology or mental illness respectively.
The brain is now thought to be the primary organ responsible for the phenomena of consciousness and thought. It also integrates and controls (together with the central nervous system) allostatic balance and autonomic functions in the body, regulates as well as directly producing many hormones, and performs processing, recognition, cognition and integration related to emotion. Studies of brain damage resulting from accidents led to the identification of specialized areas of the brain devoted to functions such as the processing of vision and audition.
Neuroimaging has allowed the function of the living brain to be studied in detail without damaging the brain. New imaging techniques allowed blood flow within the brain to be studied in detail during a wide range of psychological tests. Functional neuroimaging such as functional magnetic resonance imaging and positron emission tomography allows researchers to monitor activities of the brain as they occur (see also history of neuroimaging).
Molecular analysis of the brain has provided insight into some aspects of what the brain does as an organ, but not how it functions in higher-level processes. Further, the molecular and cell biological examination of brain pathology is hindered by the scarcity of appropriate samples for study, the (usual) inability to biopsy the brain from a living person suffering from a malady, and an incomplete description of the brain's microanatomy. With respect to the normal brain, comparative transcriptome analysis between the human and chimpanzee brain and between brain and liver (a common molecular baseline organ) has revealed specific and consistent differences in gene ｅxpression between human and chimpanzee brain and a general increase in the gene ｅxpression of many genes in humans as compared to chimpanzees. Furthermore, variations in gene ｅxpression in the cerebral cortex between individuals in either species is greater than between sub-regions of the cortex of a single individual.[6]
In addition to pathological and imaging studies, the study of computational networks, largely in computer science, provided another means through which to understand neural processes. A body of knowledge developed for the production of electronic, mathematical computation of systems provided a basis for researchers to develop and refine hypotheses about the computational function of biological neural networks. The study of neural networks now involves study of both biological and artificial neural networks.
A new discipline of cognitive science has started to fuse the results of these investigations with observations from psychology, philosophy, linguistics, and computer science as expressed in On Intelligence.
Recently the brain was used in bionics by several groups of researchers. In a particular example, a joint team of United States Navy researchers and Russian scientists from Nizhny Novgorod State University worked to develop an artificial analogue of olivocerebellar circuit, a part of the brain responsible for balance and limb movement. The researchers plan to use it to control Autonomous Underwater Vehicles.
[edit] Popular misconceptions
The following are some commonly held misconceptions of the mind and brain perpetuated through urban legends, mass media, and the promotion of dubious products to consumers (Sala, 1999). A number of practitioners of pseudoscience, New Age philosophies, and mystical or occult practices are known to use some of these ideas as a part of their belief systems (also see popular psychology).
- The human brain is firm and grey: The fresh/living brain is actually very soft, jelly-like, and deep red. It does not become firm and grey until it has been preserved with various chemicals/resins.
- Humans use only 10% or less of their brain: Even though many mysteries of brain function persist, every part of the brain has a known function.[7][8][9]
- This misconception most likely arose from a misunderstanding (or misrepresentation in an advertisement) of neurological research in the late 1800s or early 1900s when researchers either discovered that only about 10% of the neurons in the brain are firing at any given time or announced that they had only mapped the functions of 10% of the brain up to that time (accounts differ on this point).
- Another possible origin of the misconception is that only 10% of the cells in the brain are neurons; the rest are glial cells that, despite being involved in learning, do not function in the same way that neurons do.
- If all of a person's neurons began firing at once, that person would not become smarter, but would instead suffer a seizure. In fact, studies have shown that the brains of more intelligent people are less active than the brains of less intelligent people when working on the same problems.
- Some New Age proponents propagate this belief by asserting that the "unused" ninety percent of the human brain is capable of exhibiting psychic powers and can be trained to perform psychokinesis and extra-sensory perception.
- Mental abilities are separated into the left and right cerebral hemispheres: Some mental functions such as speech and language (cf. Broca's area, Wernicke's area) tend to be localized to specific areas in one hemisphere. If one hemisphere is damaged at a very early age however, these functions can often be recovered in part or even in full by the other hemisphere. Other abilities such as motor control, memory, and general reasoning are spread equally across the two hemispheres. See lateralization of brain function.
- Learning can be achieved more powerfully through subliminal techniques: Technically, information that is entirely subliminal cannot be perceived at all. The extent to which subliminal techniques can influence learning depends largely on what level of perception the techniques affect.
- Hypnosis can lead to perfect recall of details: Not only is this not entirely true, an incompetent or deceptive hypnotist can actually implant (deliberately or unintentionally by leading questions) false memories of events that never occurred.[10][11][12] This is because memory is not stored as "facts", but as impressions and emotions which can be reinterpreted as people mature or change.
- New neurons cannot be created, they only die as one ages. In fact, new neurons can grow within the mature adult brain; this process is known as neurogenesis. Regardless of neuron growth or death, brain function and capabilities can be learned and developed throughout life.
- The brain cells do not duplicate at all after it has developed (around late childhood). Although most of the brain does not grow, the cells in the hypothalamus are still growing.
[edit] Brain enhancement
Various methods have been proposed to improve the cognitive performance of the human brain including pharmacological methods (nootropic drugs), electric stimulation (direct current polarization) and surgery. More advanced methods of brain enhancement may be possible in the future, perhaps including direct brain-computer interfaces. These proposed enhancements are a major focus of Transhumanism.
[edit] Comparison of the brain and a computer
Much interest has been focused on comparing the brain with computers. A variety of obvious analogies exist: for example, individual neurons can be compared with a microchip, and the specialised parts of the brain can be compared with graphics cards and other system components. However, such comparisons are fraught with difficulties. Perhaps the most fundamental difference between brains and computers is that today's computers operate by performing often sequential instructions from an input program, while no clear analogy of a program appears in human brains. The closest equivalent would be the idea of a logical process, but the nature and existence of such entities are subjects of philosophical debate. Given Turing's model of computation, the Turing machine, this may be a functional, not fundamental, distinction. However, Maass and Markram have recently argued that "in contrast to Turing machines, generic computations by neural circuits are not digital, and are not carried out on static inputs, but rather on functions of time" (the Turing machine computes computable functions). Ultimately, computers were not designed to be models of the brain, though subjects like neural networks attempt to abstract the behavior of the brain in a way that can be simulated computationally.
In addition to the technical differences, other key differences exist. The brain is massively parallel and interwoven, whereas programming of this kind is extremely difficult for computer software writers (most parallel systems run semi-independently, for example each working on a small separate 'chunk' of a problem). The human brain is also mediated by chemicals and analog processes, many of which are only understood at a basic level and others of which may not yet have been discovered, so that a full description is not yet available in science. Finally, and perhaps most significantly, the human brain appears hard-wired with certain abilities, such as the ability to learn language (cf. Broca's area), to interact with experience and unchosen emotions, and usually develops within a culture. This is different from a computer in that a computer needs software to perform many of its functions beyond its basic computational capabilities.
Nevertheless, there have been numerous attempts to quantify differences in capability between the human brain and computers. According to Hans Moravec, by extrapolating from known capabilities of the retina to process image inputs, a brain has a processing capacity of 100 trillion instructions per second, and is likely to be surpassed by computers by 2030. [13]
The computational power of the human brain is difficult to ascertain, as the human brain is not easily paralleled to the binary number processing of today's computers. For instance, multiplying two large numbers can be accomplished in a fraction of a second with a typical calculator or desktop computer, while the average human may require a pen-and-paper approach to keep track of each stage of the calculation over a period of five or more seconds. Yet, while the human brain is calculating a math problem in an attentive state, it is subconsciously processing data from millions of nerve cells that handle the visual input of the paper and surrounding area, the aural input from both ears, and the sensory input of millions of cells throughout the body. The brain is regulating the heartbeat, monitoring oxygen levels, hunger and thirst requirements, breathing patterns and hundreds of other essential factors throughout the body. It is simultaneously comparing data from the eyes and the sensory cells in the arms and hands to keep track of the position of the pen and paper as the calculation is being performed. It quickly traverses a vast, interconnected network of cells for relevant information on how to solve the problem it is presented, what symbols to write and what their functions are, as it graphs their shape and communicates to the hand how to make accurate and controlled strokes to draw recognizable shapes and numbers onto a page. It is important to note, however, that individual neurons do compute in binary.
[edit] See also
- Neuroanatomy
- Cephalic disorders, concerning defects of the head, especially the brain
- Holonomic brain theory
- History of the brain
- Regions in the human brain
- The Memory-Prediction Framework
- Neuroanthropology
- Lateralization of brain function
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