Cerebral cortex

see also Cortical surface anatomy.

The cerebral cortex is the outer surface of the cerebrum and is composed of gray matter.

It is the outermost layered structure of neural tissue of the brain, in humans and other mammals. It covers the brain, and is divided into two cortices, along the sagittal plane, covering the left and right cerebral hemispheres.

The human cerebral cortex contains numerous myelinated fibres, many of which are concentrated in tangentially organized layers and radially oriented bundles.

The cortex is on the outer layer of the brain and is composed of up to 6 layers. The process is observed starting with embryonic days 10 to 11.5 (E10 to E11.5), continuing throughout the gestational period.

The 6 layers are formed in the subventricular zone and then migrate to their final locations in the cortical plate with the help of various signalers, to form various cortical zones and plates.

When Korbinian Brodmann 1) defined a map of the cerebral cortex in 1909, he summarized 43 distinct cytoarchitectonic areas of the brain. His work in defining these histologically derived parcels set the framework for understanding the cerebral cortex. Since then, strategies to revise and improve the mapping of the cerebral cortex have been elusive. Even though Brodmann created a map based on postmortem histological characteristics of the cerebrum, he lacked the element of in vivo characterization of function.

Understanding the amazingly complex human cerebral cortex requires a map (or parcellation) of its major subdivisions, known as cortical areas. Making an accurate areal map has been a century-old objective in neuroscience. Using multi-modal magnetic resonance imaging from the Human Connectome Project (HCP) and an objective semi-automated neuroanatomical approach, Glasser et al delineated 180 areas per hemisphere bounded by sharp changes in cortical architecture, function, connectivity, and/or topography in a precisely aligned group average of 210 healthy young adults. They characterized 97 new areas and 83 areas previously reported using post-mortem microscopy or other specialized study-specific approaches. To enable automated delineation and identification of these areas in new HCP subjects and in future studies, they trained a machine-learning classifier to recognize the multi-modal 'fingerprint' of each cortical area. This classifier detected the presence of 96.6% of the cortical areas in new subjects, replicated the group parcellation, and could correctly locate areas in individuals with atypical parcellations. The freely available parcellation and classifier will enable substantially improved neuroanatomical precision for studies of the structural and functional organization of human cerebral cortex and its variation across individuals and in development, aging, and disease 2).

The implications of a new map of the human cerebrum are profound. This new map can aid neuroscientists and neurosurgeons in their understanding of cognitive and organic disease. Neurosurgeons could integrate this new cortical map to target and treat disease that is specific to the area of clinical concern. Alternatively, neurosurgeons can also use this map to avoid areas of the cerebral cortex that could result in unnecessary neurological deficits from surgical approaches. Cognitive scientists can research patterns of individual, aberrant cortical mapping in cognitive disease such as Alzheimer disease or autistic spectrum disorders. With this new map, diagnosis and management of neurological disease in the near future may be defined with higher degrees of precision and accuracy 3).

The cerebral cortex plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness.

The neocortex.

The human cerebral cortex is 2 to 4 millimetres (0.079 to 0.157 in) thick.

The cerebral cortex (the largest part) is estimated to contain 15–33 billion neurons, each connected by synapses to several thousand other neurons.

These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells.

In large mammals the surface of the cerebral cortex is folded, giving a much greater surface area in a confined space as in the skull. A fold or ridge in the cortex is termed a gyrus (plural gyri) and a groove or fissure is termed a sulcus (plural sulci). In the human brain more than two-thirds is buried in the sulk.

see Primary motor cortex

The internal granular layer of the cortex, also commonly referred to as the granular layer of the cortex, is the layer IV in the subdivision of the mammalian cortex into 6 layers. The adjective internal is used in opposition to the external granular layer of the cortex.

This layer receives the afferent connections from the thalamus and from other cortical regions and sends connections to the other layers.

Brodmann K. Vergleichende Lokalisationslehre der Grosshirnrinde in ihren Prinzipien dargestellt auf Grund des Zellenbaues: Leipzig, Germany: Barth; 1909.
Glasser MF, Coalson TS, Robinson EC, Hacker CD, Harwell J, Yacoub E, Ugurbil K, Andersson J, Beckmann CF, Jenkinson M, Smith SM, Van Essen DC. A multi-modal parcellation of human cerebral cortex. Nature. 2016 Aug 11;536(7615):171-8. PubMed PMID: 27437579; PubMed Central PMCID: PMC4990127.
Hoshide R, Jandial R. Human Cerebral Cortex Map 2.0. Neurosurgery. 2016 Dec;79(6):N16-N17. PubMed PMID: 27861409.
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