Human brain
The human cerebral cortex is a thick layer of neural tissue that covers the two cerebral hemispheres that make up most of the brain. This layer is folded in a way that increases the amount of surface area that can fit into the volume available. The pattern of folds is similar across individuals but shows 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, and language. The left and right hemispheres 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 as visual-spatial ability, for which the right hemisphere is usually dominant.
Despite being protected by the thick bones of the skull, suspended in cerebrospinal 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 number of chemicals that can act as neurotoxins, such as alcohol. Infection of the brain, though serious, is rare because of the protective blood-to brain and blood-to cerebral fluid barriers. The human brain is also susceptible to degenerative disorders, such as Parkinson's disease, forms of dementia including Alzheimer'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 not suitable for use in humans and vice versa.[citation needed] 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 for neuroscience.
Details
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Identifiers
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Cerebrum[1]
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ἐγκέφαλος (enképhalos)[2]
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General features
The adult human brain weighs on average about 1.3–1.5 kg (2.9–3.3 lb), or about 2% of total body weight,[4][5] with a volume of around 1130 cm3 in women and 1260 cm3 in men, although there is substantial individual variation.[6] Neurological differences between the sexes have not been shown to correlate in any simple way with IQ or other measures of cognitive performance.[7]
The human brain is composed of neurons, glial cells, and blood vessels. The number of neurons is estimated at roughly 100 billion.[8] According to Azevedo et al. (2009), the adult male human brain is estimated to contain 86±8 billion neurons, with a roughly equal number (85±10 billion) 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 thecerebellum, and fewer than 1% of all brain neurons are located in the rest of the brain.[9]
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.[10] 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 gel-like consistency similar to soft tofu. Although referred to as grey matter, the live cortex is pinkish-beige in color and slightly off-white in the interior.
Metabolism
The brain consumes up to twenty percent of the energy used by the human body, more than any other organ. Brain metabolism normally relies upon blood glucose as an energy source, but during times of low glucose (such as fasting, exercise, or limited carbohydrate intake), the brain will use ketone bodies for fuel with a smaller need for glucose. The brain can also utilize lactate during exercise.[28] Long-chain fatty acids cannot cross the blood–brain barrier, but the liver can break these down to produce ketones. However, the medium-chain fatty acids octanoic and heptanoic acids can cross the barrier and be used by the brain.[29][30][31] The brain stores glucose in the form of glycogen, albeit in significantly smaller amounts than that found in the liver or skeletal muscle.[32]
The brain consumes up to twenty percent of the energy used by the human body, more than any other organ. Brain metabolism normally relies upon blood glucose as an energy source, but during times of low glucose (such as fasting, exercise, or limited carbohydrate intake), the brain will use ketone bodies for fuel with a smaller need for glucose. The brain can also utilize lactate during exercise.[28] Long-chain fatty acids cannot cross the blood–brain barrier, but the liver can break these down to produce ketones. However, the medium-chain fatty acids octanoic and heptanoic acids can cross the barrier and be used by the brain.[29][30][31] The brain stores glucose in the form of glycogen, albeit in significantly smaller amounts than that found in the liver or skeletal muscle.[32]
Although the human brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucoseutilization.[33] The need to limit body weight has led to selection for a reduction of brain size in some species, such as bats, who need to be able to fly.[34] The brain mostly uses glucose for energy, and deprivation of glucose, as can happen in hypoglycemia, can result in loss of consciousness. The energy consumption of the brain does not vary greatly over time, but active regions of the cortex consume somewhat more energy than inactive regions: this fact forms the basis for the functional brain imaging methods PET and fMRI.[35] These are nuclear medicine, functional imaging techniques which produce a three-dimensional image of metabolic activity.
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