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cerebral cortex


n.

The extensive outer layer of gray matter of the cerebral hemispheres, largely responsible for higher brain functions, including sensation, voluntary muscle movement, thought, reasoning, and memory.


 
 
World of the Body: cerebral cortex

Seen from the outside, the most obvious component of the human brain is the intricately folded cerebral cortex that covers the pair of cerebral hemispheres, which conceal most of the rest of the brain. The convolutions, or gyri, of the cortex, and the fissures or sulci that separate them, vary enormously from brain to brain, and from one hemisphere to the other in each individual. True cerebral cortex is found in the brains of fishes, reptiles, and birds, but is a major feature of the brain in all mammals. In humans it is relatively enormous in size, even compared with our closest relatives, chimpanzees and gorillas. The dominance of the cortex in the human brain led Thomas Willis, the eminent Oxford physician, to propose, in 1664, that it is the seat of ‘higher’ mental processes, such as perception, memory, and will.

The cerebral cortex is the thin outer cloak of grey matter that covers the external surface of the cerebral hemispheres, like the ‘bark’ of a tree, which is indeed what its Latin root refers to. The total area of cortex in man is estimated at nearly a square meter; it is about 4 mm thick, and it contains 10 000 million or more nerve cells (neurons). The number of synapses (connections between nerve fibres and other neurons) is even more staggering — there are, on average, around 10 000 synapses on every cortical neuron.

When thin sections are viewed under a microscope, with appropriate staining of cell bodies, the cortex is seen to consist of several distinct layers. The almost universally adopted layering scheme is that proposed by the late nineteenth-century German anatomist Korbinian Brodmann. Most of the cortex, the so-called neocortex, has a 6-layered structure, but some areas of the hemisphere are covered with simpler cortex with fewer layers, which is believed to be representative of a relatively primitive stage of brain development.

Cortical neurons and their connections

Cortical neurons are basically classified as pyramidal and non-pyramidal cells. A pyramidal cell can be recognized by a single, fairly thick process, the apical dendrite, which sprouts out of the top of the cell body and extends up toward the cortical surface. The other dendrites (branches of the cell body that receive incoming information from the fibres of other neurons), called basal dendrites, form a skirt around the lower part of the cell body. All dendrites bear large numbers of spines, small excrescences on which incoming nerve fibres terminate to form synapses. Pyramidal cells receive incoming nerve fibres from the thalamus and from other areas of cortex, as well as from nearby neurons. The axon of a pyramidal cell (the process that conveys impulses away from the neuron to other nerve cells) can extend a long way, for example more than a meter down to the spinal cord, as well as sending branches to other cortical neurons.

The axon of a typical pyramidal cell can make thousands of synapses on other neurons. When the cell fires an impulse it sweeps along all the branches of the axon to reach the synapses at the terminals, where it triggers the release of the excitatory neurotransmitter glutamate. This affects receptor molecules in the membrane of the target neuron in such a way that it becomes more permeable to sodium ions, which rush into the cell, making the interior more positively charged. This depolarization increases the probability that the target cell will itself fire off an impulse.

If the target cell of the axon of a pyramidal neuron lies below the cerebral hemispheres, in the brain stem or spinal cord, it is termed a projection axon. If it goes to other cortical areas in the same hemisphere, it is an association axon, while if it innervates cells in the cortex of the opposite hemisphere it is called commissural.

Other neurons in the cortex are all non-pyramidal cells — a very heterogeneous group. They are called interneurons because they have relatively short axons that make local connections and do not leave the cortex itself. Non-pyramidal cells can be either excitatory or inhibitory. If the latter, they commonly use the important inhibitory substance GABA (gamma-aminobutyric acid) as the transmitter at their synapses. Non-pyramidal cells, like pyramidal cells, receive axons from the thalamus, from other cortical areas, and from other local interneurons.

The outermost layer of the neocortex, layer I, consists of a mesh of axons and dendrites with very few cell bodies. The other layers consist of pyramidal and non-pyramidal neurons in varying proportions. Layer IV contains a high proportion of non-pyramidal cells, and receives most of the incoming fibres from the thalamus. Layers V and VI have particularly large pyramidal cells that project to subcortical centres, such as the spinal cord and thalamus.

Specialized cortical areas

Korbinian Brodmann also recognized that the cortex is divided into a large number of fields or areas, distinguished by slight differences in the appearance of the layers. He suggested that each anatomically distinctive area is specialized for a particular sensory, motor, or associative function.

Each hemisphere is mainly concerned with the control of muscles and with sensory input from the opposite side of the body, and also with visual and auditory information from the opposite side of the outside world. Hence damage of one hemisphere tends to affect sensation and movement on the opposite side. The cerebral cortex can be seen as the terminus of all the sensory pathways of the nervous system, in the sense that the cortex seems to be needed for conscious perception. Only when information originating in the eyes, the ears, the skin, or any other sensory organ finally reaches the cortex can it then be felt as a subjective experience. Equally, the cortex is the origin of our intended actions. From the motor cortex, axons, especially those from the very large ‘Betz cells’ in layer V, project all the way to the spinal cord to contact motor neurons, which relay the signals out to the muscles. But the cortex is also an integrative organ. Large areas of it are associative in function, meaning that they bring together activity from different sensory and motor systems to make higher-level functions possible, such as speech, memory, and thought. We know from studies of patients who have suffered damage to various parts of the cortex that some of this association cortex is intimately related to our character or personality.

The visual cortex

An area of the neocortex that has been particularly well studied is the primary visual cortex (area 17 of Brodmann), found at the back of the occipital lobe, mainly on the banks of a deep sulcus. Both eyes send signals, via the thalamus, to the visual cortex of both hemispheres, in such a way that each hemisphere receives information about the opposite half of the visual field. Thalamic fibres carrying information from the right eye and the left eye are segregated from each other as they enter the cortex and they form alternating patches of right-eye and left-eye input, about 0.3 mm across, to the cells of layer IV. Since connections between cortical cells mainly run up and down, this has the effect of imposing a pattern of functional ‘columns’ on the cortex, the neurons below any particular point on the cortical surface tending to be dominated functionally by either the right or the left eye. Such ‘columnar’ organization is a characteristic feature of the cerebral cortex, reflecting the segregation of different classes of incoming nerve fibres and the arrangement of connections between cortical neurons.

Area 17 is responsible for basic visual feature detection, but there exist dozens of other, interconnected visual areas in the occipital lobe and even in temporal and parietal lobes. Some are concerned with colour discrimination, or complex pattern recognition, certain cells even responding when the eyes view a stimulus as complex as a face. These areas belong to the association cortex, mentioned above, which is a striking feature of the human brain, permitting the integration and further analysis of simple sensory information to form the basis of meaningful, conscious experience, and the accurate control of action.

The primitive cortex

An example of the more ‘primitive’ cortex described earlier is the hippocampus (Latin for ‘sea-horse’, on account of its appearance in cross-section), which is tucked underneath, on the inner aspect of the temporal lobe. It is unusual in that it has white matter on the outside, and its structure is simple compared with the neocortex, with only three layers. The hippocampus, which receives processed information from much of the association cortex, seems to be involved in short-term conscious memory. It is functionally connected with the hypothalamus and the limbic system, parts of the brain that control basic life functions such as hormonal systems and basic body rhythms and appetites.

So the cerebral cortex is important for an amazing range of functions, from basic drives for self-preservation to the highest levels of consciousness.

— Laurence Garey

See nervous system. See also brain; central nervous system; vision.

 
Dental Dictionary: cerebral cortex

n

A thin layer of gray matter on the surface of the cerebral hemisphere, folded into gyri with about two-thirds of its area buried in fissures. The cerebral cortex integrates higher mental functions, general movement, visceral functions, perception, and behavioral reactions.

 
Britannica Concise Encyclopedia: cerebral cortex

Layer of gray matter that constitutes the outer layer of the cerebrum and is responsible for integrating sensory impulses and for higher intellectual functions. It is divided into four lobes, roughly defined by major surface folds; sometimes the limbic system, or limbic lobe, is considered to be a fifth lobe. The frontal lobe controls motor activity and speech, the parietal controls touch and position, and the temporal lobe handles auditory reception and memory. The occipital lobe at the back of the brain holds the brain's major visual-reception area. The limbic lobe controls smell, taste, and emotional responses.

For more information on cerebral cortex, visit Britannica.com.

 
Sports Science and Medicine: cerebral cortex

The outer, surface layer of grey matter in the cerebral hemispheres of the forebrain. The cerebral cortex contains motor areas, which control complex motor skills and some involuntary movements; sensory areas, which receive information from the sense organs; and association areas, which are responsible for thought, learning language, and personality. See also prefrontal cortex, premotor cortex, primary motor cortex.

 
Health Dictionary: cerebral cortex

The surface layer of gray tissue of the cerebrum, frequently called the gray matter. The large size of the cerebral cortex in humans distinguishes them from other animals. Specific parts of the cortex control specific functions, including sensation, voluntary muscle movement, thought, reasoning, and memory.

 
Wikipedia: cerebral cortex
Location of the cerebral cortex
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Location of the cerebral cortex
Slice of the cerebral cortex, ca. 10.5mm wide
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Slice of the cerebral cortex, ca. 10.5mm wide
Golgi-stained neurons in the somatosensory cortex of the macaque monkey.
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Golgi-stained neurons in the somatosensory cortex of the macaque monkey.

The cerebral cortex is a structure within the vertebrate brain with distinct structural and functional properties. In non-living, preserved brains, the outermost layers of the cerebrum has a grey color, hence the name "grey matter". Grey matter is formed by neurons and their unmyelinated fibers while the white matter below the grey matter of the cortex is formed predominantly by myelinated axons interconnecting different regions of the central nervous system. The human cerebral cortex is 2-4 mm (0.08-0.16 inches) thick and plays a central role in many complex brain functions including memory, attention, perceptual awareness, "thinking", language and consciousness.

The surface of the cerebral cortex is folded in large mammals where more than two thirds of the cortical surface is buried in the grooves, called "sulci". The phylogenetically more ancient part of the cerebral cortex, the hippocampus, is differentiated in five layers of neurons, while the more recent neo-cortex is differentiated in six basic layers. Relative variations in thickness or cell type (among other parameters) allows us to distinguish among different neocortical architectonic fields. The geometry of these fields seems to be related to the anatomy of the cortical folds and, for example, layers in the upper part of the cortical grooves (called gyri) are more clearly differentiated than in its deeper parts (called sulcal "fundi").

Development

The cerebral cortex develops from the neural plate, a specialised part of the embryonic ectoderm. The neural plate folds and closes to form the neural tube. From the cavity inside the neural tube develops the ventricular system, and, from the epithelial cells of its walls, the neurons and glial cells. The most-frontal part of the neural tube, the telencephalon, gives rise to the cerebral hemispheres and the neocortex.

Most cortical neurons are generated within the ventricular zone close to the ventricles. Initially, progenitor cells in the ventricular zone divide symmetrically, producing two progenitor cells by mitotic cycle. Then, some progenitor cells begin to divide asymmetrically, producing one postmitotic cell that migrates radially and leaves the ventricular zone, and a daughter cell that continues to divide or that eventually dies. The migrating cells will become neurons.[1] Recent work has identified radial glial cells (also here) as one population of progenitor cells.[2]

During fetal and neonatal life, the immature cerebral cortex (the cortical plate) is sandwiched between two synaptic zones: the marginal zone above and an area just below the cortical plate, the subplate. The subplate is transient and disappears by approximately 2 months postnatal (Friauf, 1991).

Laminar pattern

The standard areas of cortex (isocortex) is characterized as having six distinct layers. From outside inward:

  1. Molecular layer
  2. External granular layer
  3. External pyramidal layer
  4. Internal granular layer
  5. Internal pyramidal layer
  6. Multiform layer.

After migration, neurons form efferents and receive afferent connections characteristic of their layer. It is interesting to note that during development, the inner layers are formed before the outer layers are.

  1. The molecular layer I contains few scattered neurons and consists mainly of extensions of apical dendrites and horizontally oriented axons, and some Cajal-Retzius and spiny stellate neurons can be found.
  2. The external granular layer II contains small pyramidal neurons and numerous stellate neurons.
  3. The external pyramidal layer III contains predominantly small and medium sized pyramidal neurons, 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. The internal granular layer IV contains different types of stellate and pyramidal neurons, and is the main target of thalamocortical afferents as well as intra-hemispheric corticocortical afferents.
  5. The internal pyramidal layer V contains large pyramidal neurons (as the Betz cells in the primary motor cortex), as well as interneurons, and it is the principal source of efferent for all the motor-related subcortical structures.
  6. The multiform layer VI contains few large pyramidal neurons and many small spindle-like pyramidal and multiform neurons. The layer VI sends efferent fibers to the thalamus establishing a very precise reciprocal interconnection between the cortex and the thalamus (Creutzfeldt, 1995).

During early development, there is an additional layer of neurons present in the future white matter. these are called subplate neurons and these neurons disappear during postnatal development.

The cortical layers are not simply stacked one over the other; they develop characteristic connections between different layers, which define the basic structure of the cortical columns in the mature cortex (Mountcastle, 1997).

There are no actual borders between the layers, and neurons cross layer boundaries with their dendrites and axons trees all over. The pyramidal cells (the majority of the neurons) span at least three layers, and in many cases all the layers. Thus, it is not obvious that the layers have any functional significance. However, the flow of current in the cortical layers is consistent and shows inputs principally in layer IV, and the spread of activity, and thus the flow of information, roughly follows the models put forth by Martin, Whitteridge, and Somogyi in 1985.

Connections of the cerebral cortex

The cerebral cortex sends connections (efferents) and receives connections (afferents) from many subcortical structures like the thalamus and basal ganglia. Most of the sensory stimulation arrives at the cerebral cortex indirectly through different thalamic nuclei. This is the case of touch, vision and sound but not of olfactory stimulation, which passes to the olfactory bulb and then to the olfactory (pyriform) cortex. The largest part of the connections arriving at the cerebral cortex do not come from subcortical structures however. The main source of cortical stimulation is the cerebral cortex itself: maybe 99% of the total connections (Braitenberg and Schüz, 1991).

Other areas receive impulses from the primary sensory areas and integrate the information coming in from different types of receptors (i.e., modalities). These are often called association areas and make up a great deal of the cortex in all primates, humans included. Thus, the cortex is commonly described as comprised of the primary sensory areas, the motor areas and the association area.

Motor areas

The motor areas are located in both hemispheres of the cortex. They are shaped like a pair of headphones 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, which executes voluntary movements
  • Supplementary motor areas and premotor cortex, which select voluntary movements.

In addition, motor functions have been described for:

  • Posterior Parietal Cortex, which guides voluntary movements in space
  • Dorsolateral Prefrontal Cortex, which decides which voluntary movements to make according to higher-order instructions, rules, and self-generated thoughts.

Sensory areas

Areas that receive that particular information are called sensory areas. Parts of the cortex that receive sensory inputs from the thalamus are called primary sensory areas. The senses of vision, audition and touch are served by the primary visual cortex, primary auditory cortex and primary somatosensory cortex. In general, the two hemispheres receive the information from the opposite sides of the body. 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 organisation of sensory maps in the cortex reflects that of the corresponding sensing organ, in which is known as a topographic map. Neighbouring points in the primary visual cortex, for example, correspond to neighbouring points in the retina. This topographic map is called a retinotopic map. In the same way, there exists a tonotopic map 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 deformed human representation, the somatosensory homunculus, where the size of different limbs reflects the importance of their innervation.

Association areas

Association areas comprise three major groups:

  1. Parietal, temporal, and occipital lobes - all located in the posterior part of the brain - are involved in producing our perceptions resulting from what our eyes see, ears hear, and other sensory organs inform us about the position of different parts of our body and relate them to the position of other objects in the environment
  2. Frontal lobe - called prefrontal association complex and involved in planning actions and movement, as well as abstract thought

In humans, the association areas of the left hemisphere, especially the parietal-temporal-occipital complex, are responsible for our understanding and use of language.

Classification

Based on the differences in lamination the cerebral cortex can be classified into two major groups:

Auxiliary classes are:

Based on supposed developmental differences the following classification also appears:

In addition, cortex may be classified on the basis of gross topographical conventions into the following:

  • Temporal Cortex
  • Parietal Cortex
  • Frontal Cortex
  • Occipital Cortex

Cortical thickness

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. One study has found some positive association between the cortical thickness and intelligence.[3]

See also

Further reading

  • Kandel, E.R., Schwartz, J. H., and Jessell, T.M. Principles of Neural Science (Fourth Edition). 2000. New York, McGraw Hill. ISBN 0-8385-7701-6.
  • Zigmond, M. J., Bloom, F. E., Landis, S.C., Roberts, J.L, and Squire, L.R. Fundamental Neuroscience. 1999. San Diego, Academic Press. ISBN 0-12-780870-1.

References

  1. ^ P. Rakic (1988). "Specification of cerebral cortical areas". Science 241 (4862): 170–176. DOI:10.1126/science.3291116. 
  2. ^ Stephen C. Noctor, Alexander C. Flint, Tamily A. Weissman, Ryan S. Dammerman & Arnold R. Kriegstein (2001). "Neurons derived from radial glial cells establish radial units in neocortex". Nature 409 (6821): 714–720. DOI:10.1038/35055553. PMID 11217860. 
  3. ^ Katherine L. Narr, Roger P. Woods, Paul M. Thompson, Philip Szeszko, Delbert Robinson, Teodora Dimtcheva, Mala Gurbani, Arthur W. Toga and Robert M. Bilder (2007). "Relationships between IQ and Regional Cortical Gray Matter Thickness in Healthy Adults". Cerebral Cortex 17 (9): 2163–2171. 
  • Angevine, J. and Sidman, R. 1961. Autoradiographic study of cell migration during histogenesis of cerebral cortex in the mouse. Nature, 192:766-768
  • Creuzfeldt, O. 1995. Cortex Cerebri. Springer-Verlag.
  • Marin-Padilla, M. 2001. Evolución de la estructura de la neocorteza del mamífero: Nueva teoría citoarquitectónica. Rev. Neurol, 33(9):843-853
  • Mountcastle, V. 1997. The columnar organization of the neocortex. Brain, 120:701-722
  • Noctor SC, Flint AC, Weissman TA, Dammerman RS, Kriegstein AR. (2001) Neurons derived from radial glial cells establish radial units in neocortex. "Nature" 409(6821):714-720. PMID 11217860
  • Ogawa, M. et al. 1995. The reeler gene-associated antigen on Cajal-Retzius neurons is a crucial molecule for laminar organization of cortical neurones. Neuron, 14:899-912
  • Rakic, P. 1988. Specification of cerebral cortical areas. Science, 241:170-176
  • Friauf, J. 1991. Changing patterns of synaptic input to subplate and cortical plate during development of visual cortex.
  • Braitenberg, V and Schüz, A 1991. "Anatomy of the Cortex: Statistics and Geometry" NY: Springer-Verlag

External links

Brain: telencephalon (cerebrum, cerebral cortex, cerebral hemispheres)
Primary sulci/fissures Medial longitudinal, Lateral, Central, Parietoöccipital, Calcarine, Cingulate, Callosal Collateral fissure
Frontal lobe Precentral gyrus (Primary motor cortex, 4), Precentral sulcus, Superior frontal gyrus/Frontal eye fields (6, 8, 9), Middle frontal gyrus (46), Inferior frontal gyrus (44-Pars opercularis, 45-Pars triangularis), Orbitofrontal cortex (10, 11, 12, 47)
Parietal lobe Somatosensory cortex (Primary (1, 2, 3, 43), Secondary (5)), Precuneus (7m), Parietal lobules (Superior (7l), Inferior (40)), Angular gyrus (39), Intraparietal sulcus, Marginal sulcus
Occipital lobe Primary visual cortex (17), Cuneus, Lingual gyrus, 18, 19 - Lateral occipital sulcus
Temporal lobe Primary auditory cortex (41, 42), Superior temporal gyrus (38, 22), Middle temporal gyrus (21), Inferior temporal gyrus (20), Fusiform gyrus (37) Medial temporal lobe (Amygdala, Hippocampus, Parahippocampal gyrus (27, 28, 34, 35, 36)
Cingulate cortex/gyrus Subgenual area (25), anterior cingulate (24, 32, 33), Posterior cingulate (23, 31), Retrosplenial cortex (26, 29, 30), Supracallosal gyrus
white matter tracts Corpus callosum (Splenium, Genu, Rostrum, Tapetum), Septum pellucidum, Ependyma, Internal capsule, Corona radiata, External capsule, Olfactory tract, Fornix (Commissure of fornix), Anterior commissure, Posterior commissure Terminal stria
Basal ganglia Striatum (Putamen,Caudate nucleus, Nucleus accumbens), Globus pallidus, Claustrum, Subthalamic nucleus, Substantia nigra
Other Insular cortex Olfactory bulb, Anterior olfactory nucleus Septal nuclei Basal optic nucleus of Meynert
Some categorizations are approximations, and some Brodmann areas span gyri.

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