The human brain is the main organ of the human
central 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 a chimpanzee, though the quotient for a treeshrew's brain is
larger than that of a human's.[3] 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 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 or other trauma, 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; 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.
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] The adult 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 the cerebellum.[5][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.
Comparative anatomy Edit
Human brain viewed through a mid-line
incision
The human brain has many properties that
are common to all vertebrate brains, including a basic division into three parts
called the forebrain, midbrain, and hindbrain, with interconnected fluid-filled
ventricles, and a set of generic vertebrate brain structures including the
medulla 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 associated structures, including the
hippocampus and amygdala. The upper surface of the forebrain of other
vertebrates is covered in a layer of neural tissue called the pallium. The
pallium is a relatively simple three-layered cell structure. The hippocampus
and the amygdala originate from the pallium but in mammals they are much more
complex.
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.[11]
The neuroimmune system of the brain is structurally
distinct from the peripheral immune system which protects the rest of the body;
