cerebral cortex cerebral cortex the cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. the cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of allocortex. it is separated into two cortices, by the longitudinal fissure that divides the cerebrum into the left and right cerebral hemispheres. the two hemispheres are joined beneath the cortex by the corpus callosum. the cerebral cortex is the largest site of neural integration in the central nervous system. it plays a key role in attention, perception, awareness, thought, memory, language, and consciousness. the cerebral cortex is part of the brain responsible for cognition. cerebral cortextissue slice from the brain of an adult macaque monkey (macaca mulatta). the cerebral cortex is the outer layer depicted in dark violet. source: brainmaps.orggolgi-stained neurons in the cortexdetailspart ofcerebrumidentifierslatincortex cerebrimeshd002540neuronames39neurolex idbirnlex_1494ta98a14.1.09.003 a14.1.09.301ta25527, 5528fma61830anatomical terms of neuroanatomy in most mammals, apart from small mammals that have small brains, the cerebral cortex is folded, providing a greater surface area in the confined volume of the cranium. apart from minimising brain and cranial volume, cortical folding is crucial for the brain circuitry and its functional organisation. in mammals with small brains there is no folding and the cortex is smooth. a fold or ridge in the cortex is termed a gyrus (plural gyri) and a groove is termed a sulcus (plural sulci). these surface convolutions appear during fetal development and continue to mature after birth through the process of gyrification. in the human brain the majority of the cerebral cortex is not visible from the outside, but buried in the sulci. the major sulci and gyri mark the divisions of the cerebrum into the lobes of the brain. the four major lobes are the frontal, parietal, occipital and temporal lobes. other lobes are the limbic lobe, and the insular cortex often referred to as the insular lobe. there are between 14 and 16 billion neurons in the human cerebral cortex. these are organised into horizontal cortical layers, and radially into cortical columns and minicolumns. cortical areas have specific functions such as movement in the motor cortex, and sight in the visual cortex. visual cortex is located in the occipital lobe. structure lateral view of cerebrum showing several cortices the cerebral cortex is the outer covering of the surfaces of the cerebral hemispheres and is folded into peaks called gyri, and grooves called sulci. in the human brain it is between two and three or four millimetres thick, and makes up 40 per cent of the brain's mass. 90 per cent of the cerebral cortex is the six-layered neocortex with the other 10 per cent made up of allocortex. there are between 14 and 16 billion neurons in the cortex, and these are organized radially in cortical columns, and minicolumns, in the horizontally organized layers of the cortex. the neocortex is separable into different regions of cortex known in the plural as cortices, and include the motor cortex and visual cortex. about two thirds of the cortical surface is buried in the sulci and the insular cortex is completely hidden. the cortex is thickest over the top of a gyrus and thinnest at the bottom of a sulcus. folds further information: gyrification the cerebral cortex is folded in a way that allows a large surface area of neural tissue to fit within the confines of the neurocranium. when unfolded in the human, each hemispheric cortex has a total surface area of about 0.12 square metres (1.3 sq ft). the folding is inward away from the surface of the brain, and is also present on the medial surface of each hemisphere within the longitudinal fissure. most mammals have a cerebral cortex that is convoluted with the peaks known as gyri and the troughs or grooves known as sulci. some small mammals including some small rodents have smooth cerebral surfaces without gyrification. lobes the larger sulci and gyri mark the divisions of the cortex of the cerebrum into the lobes of the brain. there are four main lobes: the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. the insular cortex is often included as the insular lobe. the limbic lobe is a rim of cortex on the medial side of each hemisphere and is also often included. there are also three lobules of the brain described: the paracentral lobule, the superior parietal lobule, and the inferior parietal lobule. thickness for species of mammals, larger brains (in absolute terms, not just in relation to body size) tend to have thicker cortices. the smallest mammals, such as shrews, have a neocortical thickness of about 0.5 mm; the ones with the largest brains, such as humans and fin whales, have thicknesses of 2–4 mm. there is an approximately logarithmic relationship between brain weight and cortical thickness. magnetic resonance imaging of the brain (mri) makes it 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 but in general, sensory cortex is thinner than motor cortex. one study has found some positive association between the cortical thickness and intelligence. another study has found that the somatosensory cortex is thicker in migraine patients, though it is not known if this is the result of migraine attacks, the cause of them or if both are the result of a shared cause. a later study using a larger patient population reports no change in the cortical thickness in patients with migraine. a genetic disorder of the cerebral cortex, whereby decreased folding in certain areas results in a microgyrus, where there are four layers instead of six, is in some instances seen to be related to dyslexia. layers of neocortex diagram of layers pattern. cells grouped on left, axonal layers on right. three drawings of cortical lamination by santiago ramon y cajal, each showing a vertical cross-section, with the surface of the cortex at the top. left: nissl-stained visual cortex of a human adult. middle: nissl-stained motor cortex of a human adult. right: golgi-stained cortex of a 1+1⁄2 month-old infant. the nissl stain shows the cell bodies of neurons; the golgi stain shows the dendrites and axons of a random subset of neurons. micrograph showing the visual cortex (predominantly pink). subcortical white matter (predominantly blue) is seen at the bottom of the image. he-lfb stain. the neocortex is formed of six layers, numbered i to vi, from the outermost layer i – near to the pia mater, to the innermost layer vi – near to the underlying white matter. each cortical layer has a characteristic distribution of different neurons and their connections with other cortical and subcortical regions. there are direct connections between different cortical areas and indirect connections via the thalamus. one of the clearest examples of cortical layering is the line of gennari in the primary visual cortex. this is a band of whiter tissue that can be observed with the naked eye in the calcarine sulcus of the occipital lobe. the line of gennari is composed of axons bringing visual information from the thalamus into layer iv of the 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. the work of korbinian brodmann (1909) established that the mammalian neocortex is consistently divided into six layers. layer i layer i is the molecular layer, and contains few scattered neurons, including gabaergic rosehip neurons. layer i consists largely of extensions of apical dendritic tufts of pyramidal neurons and horizontally oriented axons, as well as glial cells. during development, cajal–retzius cells and subpial granular layer cells are present in this layer. also, some 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). layer ii layer ii, the external granular layer, contains small pyramidal neurons and numerous stellate neurons. layer iii layer iii, the external pyramidal layer, contains predominantly small and medium-size 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. layer iv layer iv, the internal granular layer, contains different types of stellate and pyramidal cells, and is the main target of thalamocortical afferents from thalamus type c neurons (core-type) as well as intra-hemispheric corticocortical afferents. the layers above layer iv are also referred to as supragranular layers (layers i-iii), whereas the layers below are referred to as infragranular layers (layers v and vi). layer v layer v, the internal pyramidal layer, contains large pyramidal neurons. axons from these leave the cortex and connect with subcortical structures including the basal ganglia. in the primary motor cortex of the frontal lobe, layer v contains giant pyramidal cells called betz cells, whose axons travel through the internal capsule, the brain stem, and the spinal cord forming the corticospinal tract, which is the main pathway for voluntary motor control. layer vi layer vi, the polymorphic or multiform layer, contains few large pyramidal neurons 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. that is, layer vi neurons from one cortical column connect with thalamus neurons that provide input to the same cortical column. these connections are both excitatory and inhibitory. neurons send excitatory fibers to neurons in the thalamus and also send collaterals to the thalamic reticular nucleus that inhibit these same thalamus neurons or ones adjacent to them. one theory is that because the inhibitory output is reduced by cholinergic input to the cerebral cortex, this provides the brainstem with adjustable "gain control for the relay of lemniscal inputs". columns 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 columns and minicolumns. it has been proposed that the minicolumns are the basic functional units of the 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, 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 layers ii/iii having a slow 2 hz oscillation while that in layer v has a fast 10–15 hz oscillation. types of cortex based on the differences in laminar organization the cerebral cortex can be classified into two types, the large area of neocortex which has six cell layers, and the much smaller area of allocortex that has three or four layers: the neocortex is also known as the isocortex or neopallium and is the part of the mature cerebral cortex with six distinct layers. examples of neocortical areas include the granular primary motor cortex, and the striate primary visual cortex. the neocortex has two subtypes, the true isocortex and the proisocortex which is a transitional region between the isocortex and the regions of the periallocortex. the allocortex is the part of the cerebral cortex with three or four layers, and has three subtypes, the paleocortex with three cortical laminae, the archicortex which has four or five, and a transitional area adjacent to the allocortex, the periallocortex. examples of allocortex are the olfactory cortex and the hippocampus. there is a transitional area between the neocortex