Editing Cerebral cortex (section)

=== {{anchor|Layers}}{{anchor|Laminar pattern}}Layers of neocortex ===
[[File:Minute structure of the cerebral cortex.jpg|thumb|Diagram of layers pattern. Cells grouped on left, axonal layers on right.]]
[[File:Cajal cortex drawings.png|thumb|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 stain|Nissl]]-stained visual cortex of a human adult. Middle: Nissl-stained motor cortex of a human adult. Right: [[Golgi stain|Golgi]]-stained cortex of a {{frac|1|1|2}} month-old infant. The Nissl stain shows the cell bodies of neurons; the Golgi stain shows the [[dendrite]]s and axons of a random subset of neurons.]]
[[File:Visual cortex - low mag.jpg|thumb|[[Micrograph]] showing the [[visual cortex]] (predominantly pink). Subcortical [[white matter]] (predominantly blue) is seen at the bottom of the image. [[LFB stain|HE-LFB stain]].]]
[[File:NeuronGolgi.png|thumb|[[Golgi's method|Golgi-stained]] neurons in the cortex ([[macaque]])]]

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 [[laminar organization|cortical layering]] is the [[line of Gennari]] in the [[Visual cortex#Primary visual cortex (V1)|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 [[axon]]s 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====
{{See also|Core-matrix theory of thalamus}}
Layer I is the '''molecular layer''', and contains few scattered neurons, including [[GABAergic]] [[rosehip neuron]]s.<ref name="Allen">{{cite web |title=Scientists identify a new kind of human brain cell |url=https://www.alleninstitute.org/what-we-do/brain-science/news-press/articles/scientists-identify-new-kind-human-brain-cell |website=Allen Institute |date=27 August 2018}}</ref> Layer I consists largely of extensions of apical [[dendrite|dendritic]] tufts of [[pyramidal cell|pyramidal neurons]] and horizontally oriented axons, as well as [[glial cells]].<ref name="Shipp_2007">{{cite journal | vauthors = Shipp S | title = Structure and function of the cerebral cortex | journal = Current Biology | volume = 17 | issue = 12 | pages = R443–R449 | date = June 2007 | pmid = 17580069 | pmc = 1870400 | doi = 10.1016/j.cub.2007.03.044 | bibcode = 2007CBio...17.R443S }}</ref> During development, [[Cajal–Retzius cell]]s<ref>{{cite journal | vauthors = Meyer G, Goffinet AM, Fairén A | title = What is a Cajal-Retzius cell? A reassessment of a classical cell type based on recent observations in the developing neocortex | journal = Cerebral Cortex | volume = 9 | issue = 8 | pages = 765–775 | date = December 1999 | pmid = 10600995 | doi = 10.1093/cercor/9.8.765 | doi-access =  }}</ref> and subpial granular layer cells<ref>{{cite journal | vauthors = Judaš M, Pletikos M |title=The discovery of the subpial granular layer in the human cerebral cortex |journal=Translational Neuroscience |date=2010 |volume=1 |issue=3 |pages=255–260 |doi=10.2478/v10134-010-0037-4 |s2cid=143409890|doi-access=free }}</ref> 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.<ref>{{cite journal | vauthors = Gilbert CD, Sigman M | title = Brain states: top-down influences in sensory processing | journal = Neuron | volume = 54 | issue = 5 | pages = 677–696 | date = June 2007 | pmid = 17553419 | doi = 10.1016/j.neuron.2007.05.019 | hdl-access = free | doi-access = free | hdl = 11336/67502 }}</ref>

While it was once thought that the input to layer I came from the cortex itself,<ref>{{cite journal | vauthors = Cauller L | title = Layer I of primary sensory neocortex: where top-down converges upon bottom-up | journal = Behavioural Brain Research | volume = 71 | issue = 1–2 | pages = 163–170 | date = November 1995 | pmid = 8747184 | doi = 10.1016/0166-4328(95)00032-1 | s2cid = 4015532 }}</ref> it is now known that layer I across the cerebral cortex receives substantial input from ''matrix'' or M-type thalamus cells,<ref>{{cite journal | vauthors = Rubio-Garrido P, Pérez-de-Manzo F, Porrero C, Galazo MJ, Clascá F | title = Thalamic input to distal apical dendrites in neocortical layer 1 is massive and highly convergent | journal = Cerebral Cortex | volume = 19 | issue = 10 | pages = 2380–2395 | date = October 2009 | pmid = 19188274 | doi = 10.1093/cercor/bhn259 | doi-access = free }}</ref> as opposed to ''core'' or C-type that go to layer IV.<ref name="Jones">{{cite journal | vauthors = Jones EG | title = Viewpoint: the core and matrix of thalamic organization | journal = Neuroscience | volume = 85 | issue = 2 | pages = 331–345 | date = July 1998 | pmid = 9622234 | doi = 10.1016/S0306-4522(97)00581-2 | s2cid = 17846130 }}</ref>

It is thought that layer I serves as a central hub for collecting and processing widespread information. It integrates ascending sensory inputs with top-down expectations, regulating how sensory perceptions align with anticipated outcomes. Further, layer I sorts, directs, and combines excitatory inputs, integrating them with neuromodulatory signals. Inhibitory interneurons, both within layer I and from other cortical layers, gate these signals. Together, these interactions dynamically calibrate information flow throughout the neocortex, shaping perceptions and experiences.<ref name="s309">{{cite journal | vauthors = Huang S, Wu SJ, Sansone G, Ibrahim LA, Fishell G | title = Layer 1 neocortex: Gating and integrating multidimensional signals | journal = Neuron | volume = 112 | issue = 2 | pages = 184–200 | date = January 2024 | pmid = 37913772 | doi = 10.1016/j.neuron.2023.09.041 | pmc = 11180419 }}</ref>

====Layer II====
Layer II, the '''[[External granular layer (cerebral cortex)|external granular layer]]''', contains small [[Pyramidal cell|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 [[commissural fiber|commissural]] corticocortical [[afferent nerve fiber|afferents]], and layer III is the principal source of corticocortical [[efferent nerve fiber|efferents]].

====Layer IV====
Layer IV, the '''[[internal granular layer]]''', contains different types of [[stellate cell|stellate]] and pyramidal cells, and is the main target of [[thalamocortical radiations|thalamocortical afferents]] from thalamus type C neurons (core-type)<ref name="Jones"/> 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). [[African elephant|African elephants]], [[Cetacea|cetaceans]], and [[hippopotamus]] do not have a layer IV with axons which would terminate there going instead to the inner part of layer III.<ref name="u035">{{cite journal | vauthors = Bhagwandin A, Molnár Z, Bertelsen MF, Karlsson KÆ, Alagaili AN, Bennett NC, Hof PR, Kaswera-Kyamakya C, Gilissen E, Jayakumar J, Manger PR | title = Where Do Core Thalamocortical Axons Terminate in Mammalian Neocortex When There Is No Cytoarchitecturally Distinct Layer 4? | journal = The Journal of Comparative Neurology | volume = 532 | issue = 7 | pages = e25652 | date = July 2024 | pmid = 38962882 | doi = 10.1002/cne.25652 }}</ref> 

====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 cell]]s, 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 layer''' or '''multiform layer''', contains few large pyramidal neurons and many small spindle-like pyramidal and multiform neurons; layer VI sends [[Efferent nerve fiber|efferent fibers]] to the thalamus, establishing a very precise reciprocal interconnection between the cortex and the thalamus.<ref>Creutzfeldt, O. 1995. ''Cortex Cerebri''. Springer-Verlag.</ref> 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 postsynaptic potential|excitatory]] fibers to neurons in the thalamus and also send collaterals to the [[thalamic reticular nucleus]] that [[Inhibitory postsynaptic potential|inhibit]] these same thalamus neurons or ones adjacent to them.<ref name="Lam">{{cite journal | vauthors = Lam YW, Sherman SM | title = Functional organization of the somatosensory cortical layer 6 feedback to the thalamus | journal = Cerebral Cortex | volume = 20 | issue = 1 | pages = 13–24 | date = January 2010 | pmid = 19447861 | pmc = 2792186 | doi = 10.1093/cercor/bhp077 }}</ref> 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 [[Posterior column-medial lemniscus pathway|lemniscal]] inputs".<ref name="Lam"/>