Cerebral cortex
Overview
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum
Telencephalon
The cerebrum or telencephalon, together with the diencephalon, constitutes the forebrain. The cerebrum is the most anterior region of the vertebrate central nervous system. Telencephalon refers to the embryonic structure, from which the mature cerebrum develops...

 of the mammalian brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

. It plays a key role in memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

, attention
Attention
Attention is the cognitive process of paying attention to one aspect of the environment while ignoring others. Attention is one of the most intensely studied topics within psychology and cognitive neuroscience....

, perceptual awareness
Awareness
Awareness is the state or ability to perceive, to feel, or to be conscious of events, objects or sensory patterns. In this level of consciousness, sense data can be confirmed by an observer without necessarily implying understanding. More broadly, it is the state or quality of being aware of...

, thought
Thought
"Thought" generally refers to any mental or intellectual activity involving an individual's subjective consciousness. It can refer either to the act of thinking or the resulting ideas or arrangements of ideas. Similar concepts include cognition, sentience, consciousness, and imagination...

, language
Language
Language may refer either to the specifically human capacity for acquiring and using complex systems of communication, or to a specific instance of such a system of complex communication...

, and consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

. It is constituted of up to six horizontal layers, each of which has a different composition in terms of neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

s and connectivity. The human cerebral cortex is 2–4 mm (0.08–0.16 inches) thick.

In preserved brains, it has a gray color, hence the name "gray matter
Gray Matter
"Gray Matter" is a short story by Stephen King, first published in the October 1973 issue of Cavalier magazine, and later collected in King's 1978 collection Night Shift. It is set in the same area as King's novel Dreamcatcher.-Setting:...

".
Encyclopedia
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum
Telencephalon
The cerebrum or telencephalon, together with the diencephalon, constitutes the forebrain. The cerebrum is the most anterior region of the vertebrate central nervous system. Telencephalon refers to the embryonic structure, from which the mature cerebrum develops...

 of the mammalian brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

. It plays a key role in memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

, attention
Attention
Attention is the cognitive process of paying attention to one aspect of the environment while ignoring others. Attention is one of the most intensely studied topics within psychology and cognitive neuroscience....

, perceptual awareness
Awareness
Awareness is the state or ability to perceive, to feel, or to be conscious of events, objects or sensory patterns. In this level of consciousness, sense data can be confirmed by an observer without necessarily implying understanding. More broadly, it is the state or quality of being aware of...

, thought
Thought
"Thought" generally refers to any mental or intellectual activity involving an individual's subjective consciousness. It can refer either to the act of thinking or the resulting ideas or arrangements of ideas. Similar concepts include cognition, sentience, consciousness, and imagination...

, language
Language
Language may refer either to the specifically human capacity for acquiring and using complex systems of communication, or to a specific instance of such a system of complex communication...

, and consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

. It is constituted of up to six horizontal layers, each of which has a different composition in terms of neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

s and connectivity. The human cerebral cortex is 2–4 mm (0.08–0.16 inches) thick.

In preserved brains, it has a gray color, hence the name "gray matter
Gray Matter
"Gray Matter" is a short story by Stephen King, first published in the October 1973 issue of Cavalier magazine, and later collected in King's 1978 collection Night Shift. It is set in the same area as King's novel Dreamcatcher.-Setting:...

". In contrast to gray matter that is formed from neurons and their unmyelinated
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...

 fibers, the white matter below them is formed predominantly by myelinated axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

s interconnecting neurons in different regions of the cerebral cortex with each other and neurons in other parts of the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

.

The surface of the cerebral cortex is folded in large mammals, such that more than two-thirds of it in the human brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

 is buried in the grooves, called "sulci
Sulcus (neuroanatomy)
In neuroanatomy, a sulcus is a depression or fissure in the surface of the brain.It surrounds the gyri, creating the characteristic appearance of the brain in humans and other large mammals....

". The phylogenetically most recent part of the cerebral cortex, the neocortex
Neocortex
The neocortex , also called the neopallium and isocortex , is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI...

 (also called isocortex), is differentiated into six horizontal layers; the more ancient part of the cerebral cortex, the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...

 (also called archicortex), has at most three cellular layers, and is divided into subfields. Neurons in various layers connect vertically to form small microcircuits, called columns
Cortical column
A cortical column, also called hypercolumn or sometimes cortical module, is a group of neurons in the brain cortex which can be successively penetrated by a probe inserted perpendicularly to the cortical surface, and which have nearly identical receptive fields...

. Different neocortical architectonic fields are distinguished upon variations in the thickness of these layers, their predominant cell type and other factors such as neurochemical
Neurochemical
A neurochemical is an organic molecule, such as serotonin, dopamine, or nerve growth factor, that participates in neural activity. The science of neurochemistry studies the functions of neurochemicals.-Prominent neurochemicals:...

 markers.

Development

The cerebral cortex develops from the most anterior part of the neural plate
Neural plate
In human embryology, formation of neural plate is the first step of neurulation. It is created by a flat thickening opposite to the primitive streak of the ectoderm.-Development:...

, a specialized part of the embryonic ectoderm
Germ layer
A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...

. The neural plate folds and closes to form the neural tube
Neural tube
In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...

. From the cavity inside the neural tube develops the ventricular system
Ventricular system
The ventricular system is a set of structures containing cerebrospinal fluid in the brain. It is continuous with the central canal of the spinal cord.-Components:The system comprises four ventricles:* right and left lateral ventricles* third ventricle...

, and, from the epithelial cells of its walls, the neurons and glia of the nervous system. The most anterior (frontal) part of the neural tube, the telencephalon
Telencephalon
The cerebrum or telencephalon, together with the diencephalon, constitutes the forebrain. The cerebrum is the most anterior region of the vertebrate central nervous system. Telencephalon refers to the embryonic structure, from which the mature cerebrum develops...

, gives rise to the cerebral hemispheres and cortex.

Cortical neurons are generated within the ventricular zone, next to the ventricles. At first, this zone contains "progenitor" cells, which divide to produce glial and neuronal cells. The glial fibers produced in the first divisions of progenitor cells are radially oriented, spanning the thickness of the cortex from the ventricular zone to the outer, pia
Pia mater
Pia mater often referred to as simply the pia, is the delicate innermost layer of the meninges, the membranes surrounding the brain and spinal cord. The word finds its roots in Latin, meaning literally "tender mother." The other two meningeal membranes are the dura mater and the arachnoid mater....

l surface, and provide scaffolding for the migration of neurons outwards from the ventricular zone. The first divisions of the progenitor cells are symmetric, which duplicates the total number of progenitor cells at each mitotic cycle
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

. Then, some progenitor cells begin to divide asymmetrically, producing one postmitotic cell that migrates along the radial glial fibers, leaving the ventricular zone, and one progenitor cell, which continues to divide until the end of development, when it differentiates into a glial cell
Astrocyte
Astrocytes , also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord...

 or an ependymal cell
Ependyma
Ependyma is the thin epithelial membrane lining the ventricular system of the brain and the spinal cord. Ependyma is one of the four types of neuroglia in the central nervous system. It is involved in the production of cerebrospinal fluid ....

. The migrating daughter cells become the pyramidal neurons of the cerebral cortex.

The layered structure of the mature cerebral cortex is formed during development. The first pyramidal neurons generated migrate out of the ventricular zone and subventricular zone, together with Cajal-Retzius cell
Cajal-Retzius cell
The term Cajal–Retzius cell is applied to reelin-producing neurons of the human embryonic marginal zone which display, as a salient feature, radial ascending processes that contact the pial surface, and a horizontal axon plexus located in the deep marginal zone...

s from the preplate. Next, a cohort of neurons migrating into the middle of the preplate divides this transient layer into the superficial marginal zone
Marginal zone
The marginal zone is the region at the interface between the non-lymphoid red pulp and the lymphoid white-pulp of the spleen. A marginal zone also exists in lymph nodes.-Composition and markers:It is composed of cells derived...

, which will become layer one of the mature neocortex, and the subplate
Subplate
The transient fetal subplate zone, together with the marginal zone and the cortical plate, represents the developmental anlage of the mammalian cerebral cortex...

, forming a middle layer called the cortical plate. These cells will form the deep layers of the mature cortex, layers five and six. Later born neurons migrate radially into the cortical plate past the deep layer neurons, and become the upper layers (two to four). Thus, the layers of the cortex are created in an inside-out order. The only exception to this inside-out sequence of neurogenesis occurs in the layer I of primates, in which, contrary to rodents, neurogenesis continues throughout the entire period of corticogenesis.

Evolution

The cerebral cortex is derived from the pallium
Pallium (neuroanatomy)
In a neuroanatomy context, the word pallium refers to the layers of gray and white matter that cover the upper surface of the telencephalon in vertebrates. The non-pallial part of the telencephalon builds the subpallium. In basal vertebrates the pallium is a relatively simple three-layered...

, a layered structure found in the forebrains of all vertebrates. The basic form of the pallium is a cylindrical layer enclosing fluid-filled ventricles. Around the circumference of the cylinder are four zones, the dorsal pallium, medial pallium, ventral pallium, and lateral pallium, which are thought respectively to give rise to the neocortex, hippocampus, amygdala, and olfactory cortex.

Until recently, no counterpart to the cerebral cortex had been recognized in invertebrates. However, a study published in the journal Cell in 2010, based on gene expression profiles, reported strong affinities between the cerebral cortex and the mushroom bodies
Mushroom bodies
The mushroom bodies or corpora pedunculata are a pair of structures in the brain of insects and other arthropods.-Structure:Mushroom bodies are usually described as neuropils, i.e. as dense networks of neuronal processes and glia...

 of ragworms. Mushroom bodies are structures in the brains of many types of worms and arthropods that are known to play important roles in learning and memory; the genetic evidence indicates a common evolutionary origin, and therefore indicates that the origins of the earliest precursors of the cerebral cortex date back to the early Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

 era.

Layered Structure

The different cortical layers each contain a characteristic distribution of neuronal cell types and connections with other cortical and subcortical regions. One of the clearest examples of cortical layering is the Stria of Gennari in the primary visual cortex. This is a band of whiter tissue that can be observed with the naked eye in the fundus of the calcarine sulcus of the occipital lobe. The Stria of Gennari is composed of axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

s bringing visual information from the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...

 into layer four of visual cortex.

Staining cross-sections of the cortex to reveal the position of neuronal cell bodies and the intracortical axon tracts allowed neuroanatomists in the early 20th century to produce a detailed description of the laminar structure of the cortex in different species. After the work of Korbinian Brodmann
Korbinian Brodmann
Korbinian Brodmann was a German neurologist who became famous for his definition of the cerebral cortex into 52 distinct regions from their cytoarchitectonic characteristics.-Life:...

 (1909), the neurons of the cerebral cortex are grouped into six main layers, from outside (pial surface) to inside (white matter):
  1. Layer I, the Molecular layer, contains few scattered neurons and consists mainly of extensions of apical dendritic
    Dendrite
    Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...

     tufts of pyramidal neurons and horizontally-oriented axons, as well as glial cells. Some Cajal-Retzius
    Cajal-Retzius cell
    The term Cajal–Retzius cell is applied to reelin-producing neurons of the human embryonic marginal zone which display, as a salient feature, radial ascending processes that contact the pial surface, and a horizontal axon plexus located in the deep marginal zone...

     and spiny stellate cells can be found here. Inputs to the apical tufts are thought to be crucial for the ‘‘feedback’’ interactions in the cerebral cortex involved in associative learning and attention. While it was once thought that the input to layer I came from the cortex itself, it is now realized that layer I across the cerebral cortex mantle receives substantial input from ‘‘matrix’’ or M-type thalamus cells (in contrast to ‘‘core’’ or C-type that go to layer IV).
  2. Layer II, the External granular layer
    Granular layer (cerebral cortex)
    The external granular layer is one of the layers of the cerebellar cortex consisting of granule cells, which are also known as granular neurons. The EGL is a cell population present during development, which migrates past the purkinje layer to form the internal granular layer . EGL cells migrate to...

    , contains small pyramidal neurons
    Pyramidal cell
    Pyramidal neurons are a type of neuron found in areas of the brain including cerebral cortex, the hippocampus, and in the amygdala. Pyramidal neurons are the primary excitation units of the mammalian prefrontal cortex and the corticospinal tract. Pyramidal neurons were first discovered and...

     and numerous stellate neurons.
  3. Layer III, the External Pyramidal layer, contains predominantly small and medium-size pyramidal neurons
    Pyramidal cell
    Pyramidal neurons are a type of neuron found in areas of the brain including cerebral cortex, the hippocampus, and in the amygdala. Pyramidal neurons are the primary excitation units of the mammalian prefrontal cortex and the corticospinal tract. Pyramidal neurons were first discovered and...

    , as well as non-pyramidal neurons with vertically-oriented intracortical axons; layers I through III are the main target of interhemispheric corticocortical afferents, and layer III is the principal source of corticocortical efferents.
  4. Layer IV, the Internal Granular layer, contains different types of stellate
    Stellate cell
    In neuroscience, stellate cells are neurons with several dendrites radiating from the cell body giving them a star shaped appearance. The three most common stellate cells are the inhibitory interneurons found within the molecular layer of the cerebellum, excitatory spiny stellate interneurons and...

     and pyramidal neurons
    Pyramidal cell
    Pyramidal neurons are a type of neuron found in areas of the brain including cerebral cortex, the hippocampus, and in the amygdala. Pyramidal neurons are the primary excitation units of the mammalian prefrontal cortex and the corticospinal tract. Pyramidal neurons were first discovered and...

    , and is the main target of thalamocortical afferents from thalamus type C neurons. as well as intra-hemispheric corticocortical afferents.
  5. Layer V, the Internal Pyramidal layer, contains large pyramidal neurons
    Pyramidal cell
    Pyramidal neurons are a type of neuron found in areas of the brain including cerebral cortex, the hippocampus, and in the amygdala. Pyramidal neurons are the primary excitation units of the mammalian prefrontal cortex and the corticospinal tract. Pyramidal neurons were first discovered and...

     (such as the Betz cells in the primary motor cortex); it is the principal source of subcortical efferents, as such, there are large pyramidal cells which give rise to axons leaving the cortex and running down through the basal ganglia
    Basal ganglia
    The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...

    , the brain stem and the spinal cord.
  6. Layer VI, the Polymorphic or Multiform layer, contains few large pyramidal neurons
    Pyramidal cell
    Pyramidal neurons are a type of neuron found in areas of the brain including cerebral cortex, the hippocampus, and in the amygdala. Pyramidal neurons are the primary excitation units of the mammalian prefrontal cortex and the corticospinal tract. Pyramidal neurons were first discovered and...

     and many small spindle-like pyramidal and multiform neurons; layer VI sends efferent fibers to the thalamus, establishing a very precise reciprocal interconnection between the cortex and the thalamus. These connections are both excitatory and inhibitory. Neurons send excitatory
    Excitatory postsynaptic potential
    In neuroscience, an excitatory postsynaptic potential is a temporary depolarization of postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell as a result of opening of ligand-sensitive channels...

     fibers to neurons in the thalamus and also from collateral to them ones via the thalamic reticular nucleus that inhibit
    Inhibitory postsynaptic potential
    An inhibitory postsynaptic potential is a synaptic potential that decreases the chance that a future action potential will occur in a postsynaptic neuron or α-motoneuron...

     these thalamus neurons or ones adjacent to them. Since the inhibitory output is reduced by cholinergic
    Cholinergic
    The word choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation. Found in most animal tissues, choline is a primary component of the neurotransmitter acetylcholine and functions with inositol as a basic constituent of lecithin...

     input to the cerebral cortex, this provides the brainstem with adjustable "gain control for the relay of lemnsical
    Posterior column-medial lemniscus pathway
    The posterior column-medial lemniscus pathway is the sensory pathway responsible for transmitting fine touch, vibration and conscious proprioceptive information from the body to the cerebral cortex; as well as tactile pressure, barognosis, graphesthesia, stereognosis, recognition...

     inputs".


It is important to note that the cortical layers are not simply stacked one over the other; there exist characteristic connections between different layers and neuronal types, which span all the thickness of the cortex. These cortical microcircuits are grouped into cortical column
Cortical column
A cortical column, also called hypercolumn or sometimes cortical module, is a group of neurons in the brain cortex which can be successively penetrated by a probe inserted perpendicularly to the cortical surface, and which have nearly identical receptive fields...

s and minicolumns
Cortical minicolumn
A cortical minicolumn is a vertical column through the cortical layers of the brain, comprising perhaps 80–120 neurons, except in the primate primary visual cortex , where there are typically more than twice the number. There are about 2×108 minicolumns in humans...

, the latter of which have been proposed to be the basic functional units of cortex. In 1957, Vernon Mountcastle showed that the functional properties of the cortex change abruptly between laterally adjacent points; however, they are continuous in the direction perpendicular to the surface. Later works have provided evidence of the presence of functionally distinct cortical columns in the visual cortex (Hubel and Wiesel
Torsten Wiesel
Torsten Nils Wiesel was a Swedish co-recipient with David H. Hubel of the 1981 Nobel Prize in Physiology or Medicine, for their discoveries concerning information processing in the visual system; the prize was shared with Roger W...

, 1959), auditory cortex
and associative cortex.

Cortical areas that lack a layer IV are called agranular. Cortical areas that have only a rudimentary layer IV are called dysgranular. Information processing within each layer is determined by different temporal dynamics with that in the layers II/III having a slow 2 Hz oscillation while that in layer V having a fast 10–15 Hz one.

Connections

The cerebral cortex is connected to various subcortical structures such as the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...

 and the basal ganglia
Basal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...

, sending information to them along efferent connections and receiving information from them via afferent connections. Most sensory information is routed to the cerebral cortex via the thalamus. Olfactory information, however, passes through the olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...

 to the olfactory cortex (piriform cortex
Piriform cortex
In anatomy of animals, the piriform cortex, or pyriform cortex is a region in the brain.-Anatomy and function:The piriform cortex is part of the rhinencephalon situated in the telencephalon....

). The vast majority of connections are from one area of the cortex to another rather than to subcortical areas; Braitenberg and Schüz (1991) put the figure as high as 99%.

The cortex is commonly described as comprising three parts: sensory, motor, and association areas.

Sensory areas

The sensory areas are the areas
Cortical area
A cortical area is a part of the cerebral cortex.-Functionally Defined:Often, a cortical area is functionally defined, i.e. its neurons share certain distinguishing functional properties...

 that receive and process information from the senses. Parts of the cortex that receive sensory inputs from the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...

 are called primary sensory areas. The senses of vision, audition, and touch are served by the primary visual cortex
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....

, primary auditory cortex and primary somatosensory cortex. In general, the two hemispheres receive information from the opposite (contralateral) side of the body
Body
With regard to living things, a body is the physical body of an individual. "Body" often is used in connection with appearance, health issues and death...

. For example the right primary somatosensory cortex receives information from the left limbs, and the right visual cortex receives information from the left visual field. The organization of sensory maps in the cortex reflects that of the corresponding sensing organ, in what is known as a topographic map
Topographic map (Neuroanatomy)
A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system...

. Neighboring points in the primary visual cortex
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....

, for example, correspond to neighboring points in the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

. This topographic map
Topographic map (Neuroanatomy)
A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system...

 is called a retinotopic map
Retinotopy
Retinotopy describes the spatial organization of the neuronal responses to visual stimuli. In many locations within the brain, adjacent neurons have receptive fields that include slightly different, but overlapping portions of the visual field. The position of the center of these receptive fields...

. In the same way, there exists a tonotopic map
Tonotopy
In physiology, tonotopy is the spatial arrangement of where sounds of different frequency are processed in the brain. Tones close to each other in terms of frequency are represented in topologically neighbouring regions in the brain...

 in the primary auditory cortex and a somatotopic map in the primary sensory cortex. This last topographic map of the body onto the posterior central gyrus has been illustrated as a deformed human representation, the somatosensory homunculus
Homunculus
Homunculus is a term used, generally, in various fields of study to refer to any representation of a human being. Historically, it referred specifically to the concept of a miniature though fully formed human body, for example, in the studies of alchemy and preformationism...

, where the size of different body parts reflects the relative density of their innervation. Areas with lots of sensory innervation, such as the fingertips and the lips, require more cortical area to process finer sensation.

Motor areas

The motor areas are located in both hemispheres of the cortex. They are shaped like a pair of headphones
Headphones
Headphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...

 stretching from ear to ear. The motor areas are very closely related to the control of voluntary movements, especially fine fragmented movements performed by the hand. The right half of the motor area controls the left side of the body, and vice versa.

Two areas of the cortex are commonly referred to as motor:
  • Primary motor cortex
    Primary motor cortex
    The primary motor cortex is a brain region that in humans is located in the posterior portion of the frontal lobe. Itworks in association with pre-motor areas to plan and execute movements. M1 contains large neurons known as Betz cells, which send long axons down the spinal cord to synapse onto...

    , which executes voluntary movements
  • Supplementary motor area
    Supplementary motor area
    The supplementary motor area is a part of the sensorimotor cerebral cortex . It was included, on purely cytoarchitectonic arguments, in area 6 of Brodmann and the Vogts...

    s and premotor cortex
    Premotor cortex
    The premotor cortex is an area of motor cortex lying within the frontal lobe of the brain. It extends 3 mm anterior to the primary motor cortex, near the Sylvian fissure, before narrowing to approximately 1 mm near the medial longitudinal fissure, which serves as the posterior border for...

    , which select voluntary movements.


In addition, motor functions have been described for:
  • Posterior parietal cortex
    Posterior parietal cortex
    The posterior parietal cortex plays an important role in producing planned movements. Before an effective movement can be initiated, the nervous system must know the original positions of the body parts that are to be moved, and the positions of any external objects with which the body is going to...

    , which guides voluntary movements in space
  • Dorsolateral prefrontal cortex
    Dorsolateral prefrontal cortex
    The dorsolateral prefrontal cortex , according to a more restricted definition, is roughly equivalent to Brodmann areas 9 and 46. According to a broader definition DL-PFC consists of the lateral portions of Brodmann areas 9 – 12, of areas 45, 46, and the superior part of area 47. These regions...

    , which decides which voluntary movements to make according to higher-order instructions, rules, and self-generated thoughts.


Buried deep in the white matter of the cerebral cortex are interconnected subcortical masses of cerebral gray matter called basal nuclei. The basal nuclei receive input from the substantia nigra of the midbrain and motor areas of the cerebral cortex, and send signals back to both of these locations. They are involved in motor control. They are found lateral to the thalamus. They are often called basal ganglia
Basal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...

, but the word ganglion is best restricted to clusters of neurons outside the Central Nervous System (CNS). Most neuroanatomists disagree on how many brain centers to classify basal nuclei, but agree on at least three: the caudate nucleus
Caudate nucleus
The caudate nucleus is a nucleus located within the basal ganglia of the brains of many animal species. The caudate nucleus is an important part of the brain's learning and memory system.-Anatomy:...

, putamen
Putamen
The putamen is a round structure located at the base of the forebrain . The putamen and caudate nucleus together form the dorsal striatum. It is also one of the structures that comprises the basal ganglia. Through various pathways, the putamen is connected to the substantia nigra and globus pallidus...

, and globus pallidus
Globus pallidus
The globus pallidus also known as paleostriatum, is a sub-cortical structure of the brain. Topographically, it is part of the telencephalon, but retains close functional ties with the subthalamus - both of which are part of the extrapyramidal motor system...

. The putamen and globus pallidus are also collectively known as the lentiform nucleus, because together they form a lens-shaped body. The putamen and caudate nucleus are also collectively called the corpus striatum after their striped appearance.

Association areas

Association areas function to produce a meaningful perceptual experience
Perception
Perception is the process of attaining awareness or understanding of the environment by organizing and interpreting sensory information. All perception involves signals in the nervous system, which in turn result from physical stimulation of the sense organs...

 of the world, enable us to interact effectively, and support abstract thinking and language. The parietal
Parietal lobe
The parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...

, temporal
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....

, and occipital lobe
Occipital lobe
The occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...

s - all located in the posterior part of the cortex - organize sensory information into a coherent perceptual model of our environment centered on our body image
Body image
Body image refers to a person's perception of the aesthetics and sexual attractiveness of their own body. The phrase body image was first coined by the Austrian neurologist and psychoanalyst Paul Schilder in his masterpiece The Image and Appearance of the Human Body...

. The frontal lobe
Frontal lobe
The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...

 or prefrontal association complex is involved in planning actions and movement, as well as abstract thought. In the past it was theorized that language abilities are localized in the left hemisphere in areas 44/45, the Broca's area
Broca's area
Broca's area is a region of the hominid brain with functions linked to speech production.The production of language has been linked to the Broca’s area since Pierre Paul Broca reported impairments in two patients. They had lost the ability to speak after injury to the posterior inferior frontal...

, for language expression and area 22, the Wernicke's area
Wernicke's area
Wernicke's area is one of the two parts of the cerebral cortex linked since the late nineteenth century to speech . It is involved in the understanding of written and spoken language...

, for language reception. However, language is no longer limited to easily identifiable areas. More recent research suggests that the processes of language expression and reception occur in areas other than just the perisylvian structures, such as the prefrontal lobe, basal ganglia
Basal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...

, cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...

, pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...

, caudate nucleus
Caudate nucleus
The caudate nucleus is a nucleus located within the basal ganglia of the brains of many animal species. The caudate nucleus is an important part of the brain's learning and memory system.-Anatomy:...

, and others.

Classification

Based on the differences in lamination the cerebral cortex can be classified into two major groups:
  • Isocortex (neocortex or homogenetic cortex), the part of the mature cerebral cortex with six distinct layers (homotypic cortex), or that passes through a six-layered stage during development to have more or less than six layers (heterotypic cortex) in the mature brain. Examples of heterotypic isocortex are agranular area 4 of Brodmann and striate area 17 of Brodmann.
  • Allocortex
    Allocortex
    The allocortex is a part of the cerebral cortex characterized by fewer cell layers than the neocortex The allocortex (also known as heterogenetic cortex) is a part of the cerebral cortex characterized by fewer cell layers than the neocortex The allocortex (also known as heterogenetic cortex) is a...

     (or heterogenetic cortex), the part of the cerebral cortex with less than six layers (varying in number). Examples of allocortex are the olfactory cortex and the hippocampus
    Hippocampus
    The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...

    .


Auxiliary classes are:
  • Mesocortex, classification between isocortex and allocortex where layers 2, 3, and 4 are merged
  • Proisocortex
    Proisocortex
    Proisocortex is an area in the cerebral cortex. It is a transitional area between the isocortex and periallocortex.It is found in cingulate cortex , insula and parahippocampal gyrus.- External links :* , BrainInfo....

    , Brodmann area
    Brodmann area
    A Brodmann area is a region of the cerebral cortex defined based on its cytoarchitectonics, or structure and organization of cells.-History:...

    s 24
    Brodmann area 24
    -Human:In the human this area is known as ventral anterior cingulate area 24, and it refers to a subdivision of the cytoarchitecturally defined cingulate cortex region of cerebral cortex . It occupies most of the anterior cingulate gyrus in an arc around the genu of corpus callosum. Its outer...

    , 25
    Brodmann area 25
    Brodmann area 25 is an area in the cerebral cortex of the brain and delineated based on its cytoarchitectonic characteristics.It is also called the subgenual area, area subgenualis or subgenual cingulate....

    , 32
    Brodmann area 32
    The Brodmann area 32, also known in the human brain as the dorsal anterior cingulate area 32, refers to a subdivision of the cytoarchitecturally defined cingulate region of cerebral cortex. In the human it forms an outer arc around the anterior cingulate gyrus...

  • Periallocortex, comprising cortical areas adjacent to allocortex
    Allocortex
    The allocortex is a part of the cerebral cortex characterized by fewer cell layers than the neocortex The allocortex (also known as heterogenetic cortex) is a part of the cerebral cortex characterized by fewer cell layers than the neocortex The allocortex (also known as heterogenetic cortex) is a...

    .


Based on supposed developmental differences the following classification also appears:
  • Neocortex
    Neocortex
    The neocortex , also called the neopallium and isocortex , is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI...

     or Neopallium, which corresponds to the isocortex.
  • Archicortex
    Archicortex
    Archicortex is categorized under allocortex. It is any cortex with fewer than six layers, specifically three layered hippocampal cortexes. It is necessary for hippocampus formation....

    , which phylogenetically
    Phylogenetics
    In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...

     is the oldest cortex
  • Paleocortex
    Paleocortex
    The paleocortex is a layer of the cerebral cortex intermediate phylogenetically between the neocortex and archicortex.; contains 3-5 layers.Paleocortex includes anterior olfactory nucleus, anterior perforated substance, prepyriform area and periamygdalar area....



In addition, cortex may be classified on the basis of gross topographical conventions into four lobes: the Temporal lobe
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....

, Occipital lobe
Occipital lobe
The occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...

, Parietal lobe
Parietal lobe
The parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...

 and Frontal lobe
Frontal lobe
The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...

:

Cortical thickness

For mammals, species with larger brains (in absolute terms, not just in relation to body size) tend to have thicker cortices. The range, however, is not very great — only a factor of 7 between the thickest and thinnest cortices. The smallest mammals, such as shrew
Shrew
A shrew or shrew mouse is a small molelike mammal classified in the order Soricomorpha. True shrews are also not to be confused with West Indies shrews, treeshrews, otter shrews, or elephant shrews, which belong to different families or orders.Although its external appearance is generally that of...

s, have a neocortical thickness of about 0.5 mm; the ones with the largest brains, such as humans and fin whales, have thicknesses of 2.3—2.8 mm. There is an approximately logarithmic relationship between brain weight and cortical thickness; dolphins, however, have considerably thinner cortices than the overall relationship would predict.

With magnetic resonance brain scanners, it is possible to get a measure for the thickness of the human cerebral cortex and relate it to other measures. The thickness of different cortical areas varies. In general, sensory cortex is thinner and motor cortex is thicker. One study has found some positive association between the cortical thickness and intelligence
Intelligence
Intelligence has been defined in different ways, including the abilities for abstract thought, understanding, communication, reasoning, learning, planning, emotional intelligence and problem solving....

.
Another study has found that the somatosensory cortex is thicker in migraine
Migraine
Migraine is a chronic neurological disorder characterized by moderate to severe headaches, and nausea...

 sufferers.

See also

  • Cortical column
    Cortical column
    A cortical column, also called hypercolumn or sometimes cortical module, is a group of neurons in the brain cortex which can be successively penetrated by a probe inserted perpendicularly to the cortical surface, and which have nearly identical receptive fields...

  • Frontal lobe
    Frontal lobe
    The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...

  • Limbic system
    Limbic system
    The limbic system is a set of brain structures including the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex and fornix, which seemingly support a variety of functions including emotion, behavior, long term memory, and olfaction. The term "limbic" comes from the Latin...

  • List of regions in the human brain
  • Microgyrus
    Microgyrus
    A microgyrus is an area of the cerebral cortex that includes only four cortical layers instead of six.Microgyria are believed by some to be part of the genetic lack of prenatal development which is a cause of, or one of the causes of, dyslexia....

  • Occipital lobe
    Occipital lobe
    The occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...

  • Parietal lobe
    Parietal lobe
    The parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...

  • Temporal lobe
    Temporal lobe
    The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....

  • Cerebral hemisphere
    Cerebral hemisphere
    A cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...

  • Neocortex
    Neocortex
    The neocortex , also called the neopallium and isocortex , is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI...

  • Subplate
    Subplate
    The transient fetal subplate zone, together with the marginal zone and the cortical plate, represents the developmental anlage of the mammalian cerebral cortex...

  • Brain-computer interface
    Brain-computer interface
    A brain–computer interface , sometimes called a direct neural interface or a brain–machine interface , is a direct communication pathway between the brain and an external device...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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