Human brain
The human brain is the main organ of the human nervous system. It is located in the head, protected by the skull. It has the same general structure as the brains of other mammals, but with a more developed cerebral cortex. Large animals such as whales and elephants have larger brains in absolute terms, but when measured using a measure of relative brain size, which compensates for body size, the quotient for the human brain is almost twice as large as that of a bottlenose dolphin, and three times as large as that of achimpanzee. Much of the size of the human brain comes from the cerebral cortex, especially the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought. The area of the cerebral cortex devoted to vision, the visual cortex, is also greatly enlarged in humans compared to other animals.
The human cerebral cortex is a thick layer of neural tissue that covers most of the brain. This layer is folded in a way that increases the amount of surface that can fit into the volume available. The pattern of folds is similar across individuals, although there are many small variations. The cortex is divided into four lobes – the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. (Some classification systems also include a limbic lobe and treat the insular cortex as a lobe.) Within each lobe are numerous cortical areas, each associated with a particular function, including vision, motor control, andlanguage. The left and right sides of the cortex are broadly similar in shape, and most cortical areas are replicated on both sides. Some areas, though, show strong lateralization, particularly areas that are involved in language. In most people, the left hemisphere is dominant for language, with the right hemisphere playing only a minor role. There are other functions, such asvisual-spatial ability, for which the right hemisphere is usually dominant.
Despite being protected by the thick bones of the skull, suspended incerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier, the human brain is susceptible to damage and disease. The most common forms of physical damage are closed head injuries such as a blow to the head, a stroke, or poisoning by a variety of chemicals which can act asneurotoxins, such as ethanol alcohol. Infection of the brain, though serious, is rare because of the biological barriers which protect it. The human brain is also susceptible to degenerative disorders, such as Parkinson's disease, andAlzheimer's disease, (mostly as the result of aging) and multiple sclerosis. A number of psychiatric conditions, such as schizophrenia and clinical depression, are thought to be associated with brain dysfunctions, although the nature of these is not well understood. The brain can also be the site of brain tumors and these can be benign or malignant.
There are some techniques for studying the brain that are used in other animals that are just not suitable for use in humans and vice versa. It is easier to obtain individual brain cells taken from other animals, for study. It is also possible to use invasive techniques in other animals such as inserting electrodes into the brain or disabling certains parts of the brain in order to examine the effects on behaviour – techniques that are not possible to be used in humans. However, only humans can respond to complex verbal instructions or be of use in the study of important brain functions such as language and other complex cognitive tasks, but studies from humans and from other animals, can be of mutual help. Medical imaging technologies such as functional neuroimaging and EEG recordings are important techniques in studying the brain. The complete functional understanding of the human brain is an ongoing challenge forneuroscience.
Structure
The adult human brain weighs on average about 1.3–1.4 kg (2.9–3.1 lb), or about 2% of total body weight, with a volume of around 1130 cubic centimetres (cm3) in women and 1260 cm3 in men, although there is substantial individual variation. Neurological differences between the sexes have not been shown to correlate in any simple way with IQ or other measures of cognitive performance.
The human brain is composed of neurons, glial cells, and blood vessels. The number of neurons, according to array tomography, has been shown to be on average about 86 billion in the adult male human brain with a roughly equal number of non-neuronal cells. Out of these, 16 billion (or 19% of all brain neurons) are located in the cerebral cortex (including subcortical white matter), 69 billion (or 80% of all brain neurons) are in the cerebellum, and fewer than 1% of all brain neurons are located in the rest of the brain.
The cerebral hemispheres (the cerebrum) form the largest part of the human brain and are situated above other brain structures. They are covered with a cortical layer (the cerebral cortex) which has a convoluted topography. Underneath the cerebrum lies the brainstem, resembling a stalk on which the cerebrum is attached. At the rear of the brain, beneath the cerebrum and behind the brainstem, is the cerebellum, a structure with a horizontally furrowed surface, the cerebellar cortex, that makes it look different from any other brain area. The same structures are present in other mammals, although they vary considerably in relative size. As a rule, the smaller the cerebrum, the less convoluted the cortex. The cortex of a rat or mouse is almost perfectly smooth. The cortex of a dolphin or whale, on the other hand, is more convoluted than the cortex of a human.
The living brain is very soft, having a consistency similar to soft gelatin or soft tofu. Although referred to as grey matter, the live cortex is pinkish-beige in color and slightly off-white in the interior.
General features
The human brain has many properties that are common to allvertebrate brains, including a basic division into three parts called theforebrain, midbrain, and hindbrain, with interconnected fluid-filledventricles, and a set of generic vertebrate brain structures including themedulla oblongata and pons of the brainstem, the cerebellum, optic tectum, thalamus, hypothalamus, basal ganglia, olfactory bulb, and many others.
As a mammalian brain, the human brain has special features that are common to all mammalian brains, most notably a six-layered cerebral cortex and a set of structures associated with it, including thehippocampus and amygdala. All vertebrates have a forebrain whose upper surface is covered with a layer of neural tissue called the pallium, but in all except mammals the pallium has a relatively simple three-layered cell structure. In mammals it has a much more complex six-layered cell structure, and is given a different name, the cerebral cortex. The hippocampus and amygdala also originate from the pallium, but are much more complex in mammals than in other vertebrates.
As a primate brain, the human brain has a much larger cerebral cortex, in proportion to body size, than most mammals, and a very highly developed visual system. The shape of the brain within the skull is also altered somewhat as a consequence of the upright position in which primates hold their heads.
As a hominid brain, the human brain is substantially enlarged even in comparison to the brain of a typical monkey. The sequence of evolution from Australopithecus (four million years ago) to Homo sapiens (modern man) was marked by a steady increase in brain size, particularly in the frontal lobes, which are associated with a variety of high-level cognitive functions.
Humans and other primates have some differences in gene sequence, and genes are differentially expressed in many brain regions. The functional differences between the human brain and the brains of other animals also arise from many gene–environment interactions.
Cerebral cortex
The dominant feature of the human brain is corticalization. The cerebral cortex in humans is so large that it overshadows every other part of the brain. A few subcortical structures show alterations reflecting this trend. The cerebellum, for example, has a medial zone connected mainly to subcortical motor areas, and a lateral zone connected primarily to the cortex. In humans the lateral zone takes up a much larger fraction of the cerebellum than in most other mammalian species. Corticalization is reflected in function as well as structure. In a rat, surgical removal of the entire cerebral cortex leaves an animal that is still capable of walking around and interacting with the environment. In a human, comparable cerebral cortex damage produces a permanent state of coma. The amount of association cortex, relative to the other two categories of sensory and motor, increases dramatically as one goes from simpler mammals, such as the rat and the cat, to more complex ones, such as the chimpanzee and the human. A gene present in the human genome but not in the chimpanzee : ArhGAP11B seems to play a major role in "corticalization. ArhGAP11B and human encephalisation The cerebral cortex is essentially a sheet of neural tissue, folded in a way that allows a large surface area to fit within the confines of the skull. When unfolded, each cerebral hemisphere has a total surface area of about 1.3 square feet (0.12 m2). Each cortical ridge is called a gyrus, and each groove or fissure separating one gyrus from another is called asulcus.
Cortical divisions
The cerebral cortex is nearly symmetrical with left and right hemispheres that are approximate mirror images of each other. Each hemisphere is conventionally divided into four "lobes", the frontal lobe,parietal lobe, occipital lobe, and temporal lobe. With one exception, this division into lobes does not derive from the structure of the cortex, though the lobes are named after the bones of the skull that overlie them, the frontal bone, parietal bone, temporal bone, and occipital bone. The borders between lobes lie beneath the sutures that link the skull bones together. The exception is the border between the frontal and parietal lobes, which lies behind the corresponding suture; instead it follows the anatomical boundary of the central sulcus, a deep fold in the brain's structure where the primary somatosensory cortex and primary motor cortex meet.
Because of the arbitrary way most of the borders between lobes are demarcated, they have little functional significance. With the exception of the occipital lobe, a small area that is entirely dedicated to vision, each of the lobes contains a variety of brain areas that have minimal functional relationship. The parietal lobe, for example, contains areas involved in somatosensation, hearing, language, attention, and spatial cognition. In spite of this heterogeneity, the division into lobes is convenient for reference. The main functions of the frontal lobe are to control attention, abstract thinking, behavior, problem solving tasks, and physical reactions and personality. The occipital lobe is the smallest lobe; its main functions are visual reception, visual-spatial processing, movement, and color recognition. The temporal lobe controls auditory and visual memories, language, and some hearing and speech.
Although there are enough variations in the shape and placement of gyri and sulci(cortical folds) to make every brain unique, most human brains show sufficiently consistent patterns of folding that allow them to be named. Many of the gyri and sulci are named according to the location on the lobes or other major folds on the cortex. These include:
- Superior, Middle, Inferior frontalgyrus: in reference to the frontal lobe
- Medial longitudinal fissure, which separates the left and right cerebral hemispheres
- Precentral and Postcentral sulcus: in reference to the central sulcus, which separates the frontal lobe from the parietal lobe
- Lateral sulcus, which divides the frontal lobe and parietal lobe above from the temporal lobe below
- Parieto-occipital sulcus, which separates the parietal lobes from theoccipital lobes, is seen to some small extent on the lateral surface of the hemisphere, but mainly on the medial surface.
- Trans-occipital sulcus: in reference to the occipital lobe
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