Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may
make as many as several thousand synaptic connections with other cells.
[12]
When an action potential, traveling
along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The
neurotransmitter binds to receptor molecules in the membrane of the target cell.
[13]
Neurons often have extensive networks of dendrites, which receive synaptic connections. Shown is a pyramidal neuronfrom the hippocampus, stained for green fluorescent protein.
Synapses are the key functional elements of the brain.
[14]
The essential function of the brain is cell-to-cell
communication, and synapses are the points at which communication occurs. The human brain has been
estimated to contain approximately 100 trillion synapses;
[15]
even the brain of a fruit fly contains several million.
[16]
The functions of these synapses are very diverse: some are excitatory (excite the target cell); others are
inhibitory; others work by activating second messenger systems that change the internal chemistry of their target
cells in complex ways.
[14]
A large fraction of synapses are dynamically modifiable; that is, they are capable of
changing strength in a way that is controlled by the patterns of signals that pass through them. It is widely believed
that activity-dependent modification of synapses is the brain's primary mechanism for learning and memory.
[14]
Most of the space in the brain is taken up by axons, which are often bundled together in what are called nerve
fiber tracts. Many axons are wrapped in thick sheaths of a fatty substance called myelin, which serves to greatly
increase the speed of signal propagation. Myelin is white, so parts of the brain filled exclusively with nerve fibers
appear as light-colored white matter, in contrast to the darker-colored grey matter that marks areas with high
densities of neuron cell bodies.
[17]
[edit]The generic bilaterian nervous system
Nervous system of a generic bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front
Except for a few primitive types such as sponges (which have no nervous system
[18]
) and jellyfish (which have a
nervous system consisting of a diffuse nerve net
[18]
), all living animals are bilaterians, meaning animals with a
bilaterally symmetric body shape (that is, left and right sides that are approximate mirror images of each other).
[19]
All bilaterians are thought to have descended from a common ancestor that appeared early in
the Cambrian period, 550–600 million years ago, which had the shape of a simple tubeworm with a segmented
body.
[19]
At a schematic level, that basic worm-shape continues to be reflected in the body and nervous system
architecture of all modern bilaterians, including vertebrates.
[20]
The fundamental bilateral body form is a tube with a
hollow gut cavity running from the mouth to the anus, and a nerve cord with an enlargement (a ganglion) for each
body segment, with an especially large ganglion at the front, called the brain. The brain is small and simple in
some species, such as nematode worms; in other species, including vertebrates, it is the most complex organ in
the body.
[4]
Some types of worms, such as leeches, also have an enlarged ganglion at the back end of the nerve
cord, known as a "tail brain".
[21]
There are a few types of existing bilaterians that lack a recognizable brain, including echinoderms, tunicates, and a
group of primitive flatworms called Acoelomorpha. It has not been definitively established whether the existence of
these brainless species indicates that the earliest bilaterians lacked a brain, or whether their ancestors evolved in
a way that led to the disappearance of a previously existing brain structure.
[22]
[edit]Invertebrates
Fruit flies (Drosophila) have been extensively studied to gain insight into the role of genes in brain development.
This category includes arthropods, molluscs, and numerous types of worms. The diversity of invertebrate body
plans is matched by an equal diversity in brain structures.
[23]
Two groups of invertebrates have notably complex brains: arthropods (insects, crustaceans, arachnids, and
others), and cephalopods (octopuses, squids, and similar molluscs).
[24]
The brains of arthropods and cephalopods
arise from twin parallel nerve cords that extend through the body of the animal. Arthropods have a central brain
with three divisions and large optical lobes behind each eye for visual processing.
[24]
Cephalopods such as
the octopus and squid have the largest brains of any invertebrates.
[25]
There are several invertebrate species whose brains have been studied intensively because they have properties
that make them convenient for experimental work:
Fruit flies (Drosophila), because of the large array of techniques available for studying their genetics, have
been a natural subject for studying the role of genes in brain development.
[26]
In spite of the large evolutionary
distance between insects and mammals, many aspects of Drosophila neurogenetics have turned out to be
relevant to humans. The first biological clock genes, for example, were identified by
examining Drosophila mutants that showed disrupted daily activity cycles.
[27]
A search in the genomes of
vertebrates turned up a set of analogous genes, which were found to play similar roles in the mouse biological
clock—and therefore almost certainly in the human biological clock as well.
[28]
The nematode worm Caenorhabditis elegans, like Drosophila, has been studied largely because of its
importance in genetics.
[29]
In the early 1970s, Sydney Brenner chose it as a model systemfor studying the way
that genes control development. One of the advantages of working with this worm is that the body plan is very
stereotyped: the nervous system of the hermaphrodite morph contains exactly 302 neurons, always in the
same places, making identical synaptic connections in every worm.
[30]
Brenner's team sliced worms into
thousands of ultrathin sections and photographed every section under an electron microscope, then visually
matched fibers from section to section, to map out every neuron and synapse in the entire body.
[31]
Nothing
approaching this level of detail is available for any other organism, and the information has been used to
enable a multitude of studies that would not have been possible without it.
[32]
The sea slug Aplysia was chosen by Nobel Prize-winning neurophysiologist Eric Kandel as a model for
studying the cellular basis of learning and memory, because of the simplicity and accessibility of its nervous
system, and it has been examined in hundreds of experiments.
[33]
[edit]Vertebrates
The brain of a shark
The first vertebrates appeared over 500 million years ago (Mya), during the Cambrian period, and may have
resembled the modern hagfish in form.
[34]
Sharks appeared about 450 Mya, amphibians about 400 Mya, reptiles
about 350 Mya, and mammals about 200 Mya. No modern species should be described as more "primitive" than
others, strictly speaking, since each has an equally long evolutionary history—but the brains of modern
hagfishes, lampreys, sharks, amphibians, reptiles, and mammals show a gradient of size and complexity that
roughly follows the evolutionary sequence. All of these brains contain the same set of basic anatomical
components, but many are rudimentary in the hagfish, whereas in mammals the foremost part (the telencephalon)
is greatly elaborated and expanded.
[35]
Brains are most simply compared in terms of their size. The relationship between brain size, body size and other
variables has been studied across a wide range of vertebrate species. As a rule, brain size increases with body
size, but not in a simple linear proportion. In general, smaller animals tend to have larger brains, measured as a
fraction of body size: the animal with the largest brain-size-to-body-size ratio is the hummingbird. For mammals,
the relationship between brain volume and body mass essentially follows a power law with an exponent of about
0.75.
[36]
This formula describes the central tendency, but every family of mammals departs from it to some degree,
in a way that reflects in part the complexity of their behavior. For example, primates have brains 5 to 10 times
larger than the formula predicts. Predators tend to have larger brains than their prey, relative to body size.
[37]
The main subdivisions of the embryonicvertebrate brain, which later differentiate into the forebrain, midbrain and hindbrain
All vertebrate brains share a common underlying form, which appears most clearly during early stages of
embryonic development. In its earliest form, the brain appears as three swellings at the front end of the neural
tube; these swellings eventually become the forebrain, midbrain, and hindbrain
(the prosencephalon,mesencephalon, and rhombencephalon, respectively). At the earliest stages of brain
development, the three areas are roughly equal in size. In many classes of vertebrates, such as fish and
amphibians, the three parts remain similar in size in the adult, but in mammals the forebrain becomes much larger
than the other parts, and the midbrain becomes very small.
[38]
The brains of vertebrates are made of very soft tissue.
[39]
Living brain tissue is pinkish on the outside and mostly
white on the inside, with subtle variations in color. Vertebrate brains are surrounded by a system of connective
tissue membranes called meninges that separate the skull from the brain. Blood vessels enter the central nervous
system through holes in the meningeal layers. The cells in the blood vessel walls are joined tightly to one another,
forming the so-called blood–brain barrier, which protects the brain from toxins that might enter through the
bloodstream.
[40]
Neuroanatomists usually divide the vertebrate brain into six main regions: the telencephalon (cerebral
hemispheres), diencephalon (thalamus and hypothalamus), mesencephalon (midbrain),cerebellum, pons,
and medulla oblongata. Each of these areas has a complex internal structure. Some parts, such as the cerebral
cortex and cerebellum, consist of layers that are folded or convoluted to fit within the available space. Other parts,
such as the thalamus and hypothalamus, consist of clusters of many small nuclei. Thousands of distinguishable
areas can be identified within the vertebrate brain based on fine distinctions of neural structure, chemistry, and
connectivity.
[39]
Although the same basic components are present in all vertebrate brains, some branches of vertebrate evolution
have led to substantial distortions of brain geometry, especially in the forebrain area. The brain of a shark shows
the basic components in a straightforward way, but in teleost fishes (the great majority of existing fish species), the
forebrain has become "everted", like a sock turned inside out. In birds, there are also major changes in forebrain
structure.
[41]
These distortions can make it difficult to match brain components from one species with those of
another species.
[42]
The main anatomical regions of the vertebrate brain, shown for shark and human. The same parts are present, but they differ greatly in size and shape.
Here is a list of some of the most important vertebrate brain components, along with a brief description of their
functions as currently understood:
The medulla, along with the spinal cord, contains many small nuclei involved in a wide variety of sensory and
motor functions.
[43]
The pons lies in the brainstem directly above the medulla. Among other things, it contains nuclei that control
sleep, respiration, swallowing, bladder function, equilibrium, eye movement, facial expressions, and posture.
[44]
The hypothalamus is a small region at the base of the forebrain, whose complexity and importance belies its
size. It is composed of numerous small nuclei, each with distinct connections and neurochemistry. The
hypothalamus regulates sleep and wake cycles, eating and drinking, hormone release, and many other critical
biological functions.
[45]
The thalamus is another collection of nuclei with diverse functions. Some are involved in relaying information
to and from the cerebral hemispheres. Others are involved in motivation. The subthalamic area (zona incerta)
seems to contain action-generating systems for several types of "consummatory" behaviors, including eating,
drinking, defecation, and copulation.
[46]
The cerebellum modulates the outputs of other brain systems to make them precise. Removal of the
cerebellum does not prevent an animal from doing anything in particular, but it makes actions hesitant and
clumsy. This precision is not built-in, but learned by trial and error. Learning how to ride a bicycle is an
example of a type of neural plasticity that may take place largely within the cerebellum.
[47]
The optic tectum allows actions to be directed toward points in space, most commonly in response to visual
input. In mammals it is usually referred to as the superior colliculus, and its best-studied function is to direct
eye movements. It also directs reaching movements and other object-directed actions. It receives strong
visual inputs, but also inputs from other senses that are useful in directing actions, such as auditory input in
owls and input from the thermosensitive pit organs in snakes. In some fishes, such as lampreys, this region is
the largest part of the brain.
[48]
The pallium is a layer of gray matter that lies on the surface of the forebrain. In reptiles and mammals, it is
called the cerebral cortex. Multiple functions involve the pallium, including olfaction andspatial memory. In
mammals, where it becomes so large as to dominate the brain, it takes over functions from many other brain
areas. In many mammals, the cerebral cortex consists of folded bulges called gyri that create deep furrows or
fissures called sulci. The folds increase the surface area of the cortex and therefore increase the amount of
gray matter and the amount of information that can be processed.
[49]
The hippocampus, strictly speaking, is found only in mammals. However, the area it derives from, the medial
pallium, has counterparts in all vertebrates. There is evidence that this part of the brain is involved in spatial
memory and navigation in fishes, birds, reptiles, and mammals.
[50]
The basal ganglia are a group of interconnected structures in the forebrain. The primary function of the basal
ganglia appears to be action selection: they send inhibitory signals to all parts of the brain that can generate
motor behaviors, and in the right circumstances can release the inhibition, so that the action-generating
systems are able to execute their actions. Reward and punishment exert their most important neural effects by
altering connections within the basal ganglia.
[51]
The olfactory bulb is a special structure that processes olfactory sensory signals and sends its output to the
olfactory part of the pallium. It is a major brain component in many vertebrates, but is greatly reduced in
primates.
[52]
[edit]Mammals
The most obvious difference between the brains of mammals and other vertebrates is in terms of size. On
average, a mammal has a brain roughly twice as large as that of a bird of the same body size, and ten times as
large as that of a reptile of the same body size.
[53]
Size, however, is not the only difference: there are also substantial differences in shape. The hindbrain and
midbrain of mammals are generally similar to those of other vertebrates, but dramatic differences appear in the
forebrain, which is greatly enlarged and also altered in structure.
[54]
The cerebral cortex is the part of the brain that
most strongly distinguishes mammals. In non-mammalian vertebrates, the surface of the cerebrum is lined with a
comparatively simple three-layered structure called the pallium. In mammals, the pallium evolves into a complex
six-layered structure called neocortex or isocortex.
[55]
Several areas at the edge of the neocortex, including
the hippocampus and amygdala, are also much more extensively developed in mammals than in other
vertebrates.
[54]
The elaboration of the cerebral cortex carries with it changes to other brain areas. The superior colliculus, which
plays a major role in visual control of behavior in most vertebrates, shrinks to a small size in mammals, and many
of its functions are taken over by visual areas of the cerebral cortex.
[53]
The cerebellum of
mammals contains a large portion (the neocerebellum) dedicated to supporting the
cerebral cortex, which has no counterpart in other vertebrates.
[56]
[edit]Primates
See also: Human brain
The brains of humans and other primates contain the same structures as the brains of
other mammals, but are generally larger in proportion to body size.
[60]
The most widely
accepted way of comparing brain sizes across species is the so-called encephalization
quotient (EQ), which takes into account the nonlinearity of the brain-to-body relationship.
[57]
Humans have an average EQ in the 7-to-8 range, while most other primates have an EQ in the 2-to-3 range.
Dolphins have values higher than those of primates other than humans,
[58]
but nearly all other mammals have EQ
values that are substantially lower.
Most of the enlargement of the primate brain comes from a massive expansion of the cerebral cortex, especially
the prefrontal cortex and the parts of the cortex involved in vision.
[61]
The visual processing network of primates
includes at least 30 distinguishable brain areas, with a complex web of interconnections. It has been estimated
that visual processing areas occupy more than half of the total surface of the primate neocortex.
[62]
The prefrontal
cortex carries out functions that include planning, working memory, motivation, attention, and executive control. It
takes up a much larger proportion of the brain for primates than for other species, and an especially large fraction
of the human brain.
[63]
[edit]Physiology
The functions of the brain depend on the ability of neurons to transmit electrochemical signals to other cells, and
their ability to respond appropriately to electrochemical signals received from other cells. The electrical
properties of neurons are controlled by a wide variety of biochemical and metabolic processes, most notably the
interactions between neurotransmitters and receptors that take place at synapses.
[13]
[edit]Neurotransmitters and receptors
Neurotransmitters are chemicals that are released at synapses when an action potential activates them—
neurotransmitters attach themselves to receptor molecules on the membrane of the synapse's target cell, and
thereby alter the electrical or chemical properties of the receptor molecules. With few exceptions, each neuron in
the brain releases the same chemical neurotransmitter, or combination of neurotransmitters, at all the synaptic
connections it makes with other neurons; this rule is known as Dale's principle.
[64]
Thus, a neuron can be
characterized by the neurotransmitters that it releases. The great majority of psychoactive drugs exert their effects
by altering specific neurotransmitter systems. This applies to drugs such
as marijuana, nicotine, heroin, cocaine, alcohol, fluoxetine, chlorpromazine, and many others.
[65]
The two neurotransmitters that are used most widely in the vertebrate brain are glutamate, which almost always
exerts excitatory effects on target neurons, and gamma-aminobutyric acid (GABA), which is almost always
inhibitory. Neurons using these transmitters can be found in nearly every part of the brain.
[66]
Because of their
ubiquity, drugs that act on glutamate or GABA tend to have broad and powerful effects. Some general anesthetics
Encephalization Quotient
Species EQ
[57]
Human 7.4-7.8
Chimpanzee 2.2-2.5
Rhesus monkey 2.1
Bottlenose dolphin 4.14
[58]
Elephant 1.13-2.36
[59]
Dog 1.2
Horse 0.9
Rat 0.4
act by reducing the effects of glutamate; most tranquilizers exert their sedative effects by enhancing the effects of
GABA.
[67]
There are dozens of other chemical neurotransmitters that are used in more limited areas of the brain, often areas
dedicated to a particular function. Serotonin, for example—the primary target of antidepressant drugs and many
dietary aids—comes exclusively from a small brainstem area called the Raphe nuclei.
[68]
Norepinephrine, which is
involved in arousal, comes exclusively from a nearby small area called the locus coeruleus.
[69]
Other
neurotransmitters such asacetylcholine and dopamine have multiple sources in the brain, but are not as
ubiquitously distributed as glutamate and GABA.
[70]
[edit]Electrical activity
Brain electrical activity recorded from a human patient during an epileptic seizure
As a side effect of the electrochemical processes used by neurons for signaling, brain tissue generates electric
fields when it is active. When large numbers of neurons show synchronized activity, the electric fields that they
generate can be large enough to detect outside the skull, using electroencephalography (EEG).
[71]
EEG
recordings, along with recordings made from electrodes implanted inside the brains of animals such as rats, show
that the brain of a living animal is constantly active, even during sleep.
[72]
Each part of the brain shows a mixture of
rhythmic and nonrhythmic activity, which may vary according to behavioral state. In mammals, the cerebral cortex
tends to show large slow delta waves during sleep, faster alpha waves when the animal is awake but inattentive,
and chaotic-looking irregular activity when the animal is actively engaged in a task. During an epileptic seizure, the
brain's inhibitory control mechanisms fail to function and electrical activity rises to pathological levels, producing
EEG traces that show large wave and spike patterns not seen in a healthy brain. Relating these population-level
patterns to the computational functions of individual neurons is a major focus of current research in
neurophysiology.
[72]
[edit]Metabolism
All vertebrates have a blood-brain barrier that allows metabolism inside the brain to operate differently from
metabolism in other parts of the body. Glial cells play a major role in brain metabolism, by controlling the chemical
composition of the fluid that surrounds neurons, including levels of ions and nutrients.
[73]
Brain tissue consumes a large amount of energy in proportion to its volume, so large brains place severe
metabolic demands on animals. The need to limit body weight in order, for example, to fly, has apparently led to
selection for a reduction of brain size in some species, such as bats.
[74]
Most of the brain's energy consumption
goes into sustaining the electric charge (membrane potential) of neurons.
[73]
Most vertebrate species devote
between 2% and 8% of basal metabolism to the brain. In primates, however, the fraction is much higher—in
humans it rises to 20–25%.
[75]
The energy consumption of the brain does not vary greatly over time, but active
regions of the cerebral cortex consume somewhat more energy than inactive regions; this forms the basis for the
functional brain imaging methods PET and fMRI.
[76]
In humans and many other species, the brain gets most of its
energy from oxygen-dependent metabolism of glucose (i.e., blood sugar).
[73]
In some species, though, alternative
sources of energy may be used, including lactate, ketones, amino acids, glycogen, and possibly lipids.
[77]
[edit]Functions
From an evolutionary-biological perspective, the function of the brain is to provide coherent control over the
actions of an animal. A centralized brain allows groups of muscles to be co-activated in complex patterns; it also
allows stimuli impinging on one part of the body to evoke responses in other parts, and it can prevent different
parts of the body from acting at cross-purposes to each other.
[78]
To generate purposeful and unified action, the brain first brings information from sense organs together at a
central location. It then processes this raw data to extract information about the structure of the environment. Next
it combines the processed sensory information with information about the current needs of an animal and with
memory of past circumstances. Finally, on the basis of the results, it generates motor response patterns that are
suited to maximize the welfare of the animal. These signal-processing tasks require intricate interplay between a
variety of functional subsystems.
[78]
[edit]Information processing
The invention of electronic computers in the 1940s, along with the development of mathematical information
theory, led to a realization that brains can potentially be understood as information processing systems. This
concept formed the basis of the field of cybernetics, and eventually gave rise to the field now known
as computational neuroscience.
[79]
The earliest attempts at cybernetics were somewhat crude in that they treated
the brain as essentially a digital computer in disguise, as for example in John von Neumann's 1958 book, The
Computer and the Brain.
[80]
Over the years, though, accumulating information about the electrical responses of
brain cells recorded from behaving animals has steadily moved theoretical concepts in the direction of increasing
realism.
[79]
Model of a neural circuit in thecerebellum, as proposed by James S. Albus
The essence of the information processing approach is to try to understand brain function in terms of information
flow and implementation of algorithms.
[79]
One of the most influential early contributions was a 1959 paper
titled What the frog's eye tells the frog's brain: the paper examined the visual responses of neurons in
the retina and optic tectum of frogs, and came to the conclusion that some neurons in the tectum of the frog are
wired to combine elementary responses in a way that makes them function as "bug perceivers".
[81]
A few years
later David Hubel and Torsten Wieseldiscovered cells in the primary visual cortex of monkeys that become active
when sharp edges move across specific points in the field of view—a discovery that eventually brought them a
Nobel Prize.
[82]
Followup studies in higher-order visual areas found cells that detect binocular disparity, color,
movement, and aspects of shape, with areas located at increasing distances from the primary visual cortex
showing increasingly complex responses.
[83]
Other investigations of brain areas unrelated to vision have revealed
cells with a wide variety of response correlates, some related to memory, some to abstract types of cognition such
as space.
[84]
Theorists have worked to understand these response patterns by constructing mathematical models of neurons
and neural networks, which can be simulated using computers.
[79]
Some useful models are abstract, focusing on
the conceptual structure of neural algorithms rather than the details of how they are implemented in the brain;
other models attempt to incorporate data about the biophysical properties of real neurons.
[85]
No model on any
level is yet considered to be a fully valid description of brain function, though. The essential difficulty is that
sophisticated computation by neural networks requires distributed processing in which hundreds or thousands of
neurons work cooperatively—current methods of brain activity recording are only capable of isolating action
potentials from a few dozen neurons at a time.
[86]
[edit]Perception
Diagram of signal processing in theauditory system
One of the primary functions of a brain is to extract biologically relevant information from sensory inputs. The
human brain is provided with information about light, sound, the chemical composition of the atmosphere,
temperature, head orientation, limb position, the chemical composition of the bloodstream, and more. In other
animals additional senses may be present, such as the infrared heat-sense of snakes, the magnetic field sense of
some birds, or the electric field sense of some types of fish. Moreover, other animals may develop existing sensory
systems in new ways, such as the adaptation by bats of the auditory sense into a form of sonar. One way or
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