Introduction to Human Brain
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. Involuntary, or "lower," actions, such as heart rate, respiration, and digestion, are unconsciously governed by the brain, specifically through the autonomic nervous system. Complex, or "higher," mental activity, such as thought, reason, and abstraction, is consciously controlled.
Anatomically, the brain can be divided into three parts: the forebrain, midbrain, and hindbrain; 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; 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.
Neurophysiology
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 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. The brain controls all movement from lifting a pencil to building a superstructure.
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 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. Visual perception generally is processed in the occipital lobe, whereas the primary auditory cortex resides in the temporal lobe. Midbrain functions include routing, selecting, mapping, and cataloguing 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 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.
Brain size
The balance of findings, indicate an average adult brain volume of 1130 cubic centimetres (cc) for women and 1260 cc for men. There is substantial variation however; a study of 46 adults aged 22-49 years and of mainly European descent, found an average brain volume of 1273.6cc for men, ranging from 1052.9 to 1498.5cc, and 1131.1cc for women, ranging from 974.9 to 1398.1cc. The right cerebral hemisphere is typically larger than the left, whereas the cerebellar hemispheres are typically of more similar size. There is evidence of an increase in average brain size over the course of the last centuries, estimated to have been around 0.5% per decade.
Overall, there is a background of similarity between adult brain volume measures of people of differing ages and sexes. Nevertheless, underlying structural asymmetries do exist. Males have been found to have on average greater cerebral, cerebellar and cerebral cortical lobar volumes, except possibly left parietal. The gender differences in size vary by more specific brain regions. Studies have tended to indicate that men have a relatively larger amygdala and hypothalamus, while women have a relatively larger caudate and hippocampus. When covaried for intracranial volume, height, and weight, the balance of studies indicates women have a higher percentage of gray matter, whereas men have a higher percentage of white matter and cerebrospinal fluid. There is high variability between individuals in these studies, however.
Adult twin studies have indicated high heritability estimates for overall brain size in adulthood (between 66% and 97%). The effect varies regionally within the brain, however, with high heritabilities of frontal lobe volumes (90-95%), moderate estimates in the hippocampus (40-69%), and environmental factors influencing several medial brain areas. In addition, lateral ventricle volume appears to be mainly explained by environmental factors, suggesting such factors also play a role in the surrounding brain tissue. Genes may cause the association between brain structure and cognitive functions, or the latter may influence the former during life. A number of candidate genes have been identified or suggested, but await replication.
A discovery in recent years is that the structure of the adult human brain changes when a new cognitive or motor skill, including vocabulary, is learned. Structural neuroplasticity (increased grey matter volume) has been demonstrated in adults after three months of training in a visual-motor skill, with the qualitative change (i.e. learning of a new task) appearing more critical for the brain to change its structure than continued training of an already-learned task. Such changes (e.g. revising for medical exams) have been shown to last for at least 3 months without further practicing; other examples include learning novel speech sounds, musical ability, navigation skills and learning to read mirror-reflected words.
Studies have tended to indicate (in terms of group averages) small to moderate correlations (averaging around 0.3 to 0.4) between brain volume and the narrow measure known as the IQ test. The most consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampus, and the cerebellum, but only account for a relatively small amount of variance in IQ, which itself only shows a partial relationship to the general concept of intelligence and real-world performance. In addition, brain volumes do not correlate strongly with other and more specific cognitive measures In men, IQ correlates more with gray matter volume in the frontal lobe and parietal lobe, whereas in women it correlates with gray matter volume in the frontal lobe and Broca's area, which is involved in language.
There is variation in child development in the size of different brain structures between individuals and genders. Total cerebral and grey matter volumes peak during the ages from 10–20 years (early in girls than boys), whereas white matter and ventricular volumes increase. There is a general pattern in neural development of childhood peaks followed by adolescent declines (e.g. synaptic pruning). Consistent with adult findings, average cerebral volume is approximately 10% larger in boys than girls. However, such differences should not be interpreted as imparting any sort of functional advantage or disadvantage; gross structural measures may not reflect functionally relevant factors such as neuronal connectivity and receptor density, and of note is the high variability of brain size even in narrowly defined groups, for example children at the same age may have as much as a 50% differences in total brain volume. Young girls have on average relative larger hippocampal volume, whereas the amygdala is larger in boys.
Significant dynamic changes in brain structure take place through adulthood and ageing, with substantial variation between individuals. In later decades, men show greater volume loss in whole brain volume and in the frontal lobes, and temporal lobes, whereas in women there is increased volume loss in the hippocampus and parietal lobes. Men show a steeper decline in global grey matter volume, although in both sexes it varies by region with some areas exhibiting little or no age effect. Overall white matter volume does not appear to decline with age, although there is variation between brain regions.
Some anatomical trends are correlated in our evolutionary path with brain size; basicranium becomes more flexed with increasing brain size relative to basicranial length Due to human migration spacial differentiation of brain size is more diffused than earlier in time. In paleolithic past largest cranial capacity, in vivo filed by brains, is observed in Neanderthal specimens. An average Neandertahl had bigger brains than worldwide average person today. Microcefalin common allele was introgested, as dated by -D lineage mutational variance around 35 kya. Neanderthal population is major candidate among ancestral genetic pools where this gene evolved to be spread and shared by majority of Humans.
Study of the brain
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. The research by Dr. Marian Diamond offered strong evidence that glial cells serve a computational role beyond merely transmitting processed signals between more functional parts of the brain.
For many millennia the function of the brain was unknown. Ancient thinkers such as Aristotle imagined that mental activity took place in the heart. Greek scholars assumed that the brain serves a role in cooling the body, incorrectly believing it to function as a sort of radiator. 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 seen as 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.
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.
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.
Neurolinguistics
In human beings, it is the left hemisphere that usually contains the specialized language areas. While this holds true for 97% of right-handed people, about 19% of left-handed people have their language areas in the right hemisphere and as many as 68% of them have some language abilities in both the left and the right hemisphere. The two hemispheres are thought to contribute to the processing and understanding of language: the left hemisphere processes the linguistic meaning of prosody, while the right hemisphere processes the emotions conveyed by prosody. Studies of children have provided some fascinating information: If a child has damage to the left hemisphere, the child may develop language in the right hemisphere instead. The younger the child, the better the recovery. So, although the "natural" tendency is for language to develop on the left, our brains are capable of adapting to difficult circumstances, if the damage occurs early enough.
The first language area within the left hemisphere to be discovered is called Broca's Area, after Paul Broca.It seems to be more generally involved in the ability to deal with grammar itself, at least the more complex aspects of grammar.
Pathology
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 or industrial accidents, is a leading cause of death in youth and middle age. In many cases, more damage is caused by resultant 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 disorders, such as clinical depression, schizophrenia, bipolar disorder and post-traumatic stress disorder may involve particular patterns of neuropsychological functioning related to various aspects of mental and somatic function. These disorders may be treated by psychotherapy, psychiatric medication or social intervention and personal recovery work; the underlying issues and associated prognosis vary significantly between 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 reported in multiple sclerosis and Parkinson's disease, and are established causes of encephalopathy, and encephalomyelitis.
Many brain disorders are congenital. Tay-Sachs disease, Fragile X syndrome, and Down syndrome are all linked to genetic and chromosomal errors. Many other syndromes, such as the intrinsic circadian rhythm disorders, are suspected to be congenital as well. Malfunctions in the embryonic development of the brain can be caused by genetic factors, drug use, nutritional deficiencies, and infectious diseases during pregnancy.
Certain brain disorders are treated by brain neurosurgeons while others are treated by neurologists and psychiatrists.
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 transistor, and the specialized 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 to the equivalent would be the idea of a logical process, but the nature and existence of such entities are subjects of philosophical debate.
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. 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, 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.The human brain is able to interpret and solve complex problems that are not formalized using its powers of pattern recognition and interpretation, whereas the computer with current software and current hardware is only able to solve formalized problems due to more limited pattern recognition capability. A human can understand context in an arbitrary text, something even the most powerful and best software is not able to discern.
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.
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