Cerebral cortex là gì

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Keywords: Cerebral cortexCerebellumDevelopmentEvolutionHumanNonhuman primateNeuroanatomyParcellation


Abstract

Cerebral cortex và cerebellar cortex both vary enormously across species in their form size và complexity of convolutions. We discuss the development và evolution of cortical structures in terms of anatomy and functional organization. We propose that the distinctive sầu shapes of cerebral & cerebellar cortex can be explained by relatively few developmental processes, notably including mechanical tension along axons & dendrites. Regarding functional organization, we show how maps of myelin content in cerebral cortex are evolutionarily conserved across primates but differ in the proportion of cortex devoted to lớn sensory, cognitive sầu, & other functions. We summarize recent progress và challenges in (i) parcellating cerebral cortex into a mosaic of distinct areas, (ii) distinguishing cortical areas that correspond across species from those that are present in one species but not another, & (iii) using this information along with surface-based interspecies registration lớn gain deeper insights inlớn cortical evolution. We also bình luận on the methodological challenges imposed by the differences in anatomical và functional organization of cerebellar cortex relative sầu khổng lồ cerebral cortex.

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Introduction

The cerebral cortex is the dominant structure of the mammalian brain và plays key roles in a remarkably diverse range of behaviors, including perception, volitional movement, cognition, memory, and emotion. In humans, it is vital for capabilities such as language & tool use that make us chất lượng both as a species và as individuals. Its pint-sized partner, the cerebellar cortex, is less well understood functionally but is widely presumed lớn play a coordinating role in most or all of the above sầu functions . Figure 1 illustrates the dramatic differences in brain size, neuronal numbers, và complexity of cerebral convolutions for four primate species (human, chimpanzee, macaque, và marmoset) plus the intensively studied mouse. In each species, cerebral và cerebellar cortex, & the subcortical nuclei they surround, are highly interconnected và function as a well-integrated system. This Review discusses cerebral và cerebellar cortices from both developmental and evolutionary perspectives, with an emphasis on primates.


Adapted, with permission, from Herculano-Houzel <2009>. Approximate ratios are based on weight differences; ratquả táo for neuronal number differ modestly.


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Basic Brain Numbers

The human brain contains ∼86 billion neurons, which are distributed nonuniformly across the cerebellum, cerebral cortex, & subcortical nuclei . The cerebellum contains ∼80% of the brain’s neurons, but it constitutes only ∼10% of brain mass because most of its neurons are tiny granule cells packed inlớn a dense cellular layer within the cerebellar cortical ribbon. In contrast, the cerebral cortex contains only ∼20% of all neurons (∼16 billion), but it constitutes ∼80% of brain mass as a result of its larger neurons, more abundant neuropil (dendrites, axons, synapses, & glial cells), and the extensive sầu axonal projections that course through the underlying cerebral white matter. The remainder of the brain comprises many diverse subcortical nuclei that, in aggregate, constitute ∼8% of brain mass (including deep subcortical trắng matter tracts) but contain only 0.8% of neurons – because these neurons tend khổng lồ be large and surrounded by neuropil.

Cerebral and cerebellar cortices are both sheet-lượt thích structures but otherwise exhibit stark differences. Human cerebral cortex has a total surface area of 1,843 ± 196 cmét vuông , about the kích thước of a medium pizza for each hemisphere; it is 2.6 mm thiông xã on average, but its thickness varies more than twofold across different cortical areas . Cerebellar cortex is a single sheet, fused along the midline, and it is less than 1 milimet thick (i.e., about one third that of cerebral cortex) (https://msu.edu/∼brains/brains/human/coronal/ 2800_cell.html). Its major folds (lamellae) are more reg­ular, akin khổng lồ those of an accordion, & its total surface area roughly matches that of a single cerebral hemisphere . In contrast, the subcortical domain includes a collection of nuclei and subnuclei within the thalamus, hypothalamus, basal ganglia, midbrain, và brain stem, with an estimated ∼400 anatomically distinct entities . Though some nuclei are irregularly shaped (e.g., the long-tailed caudate nucleus), most are blob-lượt thích rather than sheet-like.

Morphogenesis – How the Brain Gets in Shape

Each brain structure attains it kích thước and shape through a tightly choreographed mix of morphogenetic events, including neuronal và glial proliferation by cell division, migration from where cells were born lớn their target structure, & the elaboration of axons, dendrites và glial processes. The overall volume of each brain structure can be estimated by its aggregate number of neurons và glial cells (dictated primarily by proliferation but modulated by cell death) multiplied by the average cell volume (including dendrites, axons, synapses, and glial processes). Individual neurons and glial cells have sầu diverse shapes, as their processes extkết thúc in characteristic patterns for each tissue and cell type. Dendritic arbors are often biased in orientation (anisotropic), & axonal projections typically extend asymmetrically from the cell body.

In a classic study, Thompson <1917> described how tension và pressure interact with structural anisotropies và asymmetries khổng lồ determine the shape of many biological structures. If neuronal and glial processes generate mechanical tension, it could be a powerful morphogenetic influence throughout the nervous system . In particular, we hypothekích thước that mechanical tension combined with differential neuronal proliferation plays pivotal roles in answering the following four questions: (i) Why are cerebral và cerebellar cortices sheet-lượt thích structures, whereas subcortical nuclei are mostly blob shaped? (ii) Why is cerebral cortex smooth in most small-brained mammals but convoluted in most large-brained mammals? (iii) What causes the species-specific pattern of convolutions of cerebral cortex? (iv) Why does folding of cerebellar cortex differ so dramatically from that of cerebral cortex?

Are Neurons Tense?

As mechanical tension is central to lớn our proposed framework, it is important to note that neurons indeed can generate tension. The strongest evidence comes from classic in vitro studies: neurons grown in tissue culture extover neurites whose growth cones attach to lớn the underlying substrate và generate robust mechanical tension . In response khổng lồ external perturbations (e.g., tugging or relaxation), neurites adjust their length by a negative sầu feedback process that tends lớn maintain a steady tension , much as a fishing line is reeled in or out to regulate tension . There is evidence that many axons in subcortical Trắng matter are indeed under tension in brain slices from early postnatal và adult ferrets , though it is unclear from this study whether axonal tension actually drives cortical folding. Without more fully reviewing a diverse và controversial toàn thân of evidence, it suffices for our present purposes khổng lồ posit that axonal và dendritic tension in vivo is an attractive và highly plausible mechanism, though its widespread occurrence in vivo has yet to be rigorously established.

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A Role for Tension in Forming Sheet-Like & Blob-Like Structures

Cerebral cortical neurons proliferate in the ventricular and subventricular zones & migrate outward along radial glial processes khổng lồ size the cortical plate & later the laminated cortical sheet . Shortly after arrival, cortical pyramidal neurons extover a prominent apical dendrite followed by many subsidiary dendrites ; the early elaboration of apical dendrites may arise from a diffusible chemoattractant (Sem3A) preferentially released from superficial cortical layers . If these dendrites generate mechanical tension, and if the tufts at apical dendritic tips are anchored by adhesion at synaptic junctions, tension would tkết thúc to keep superficial and deep layers cthua together . As cortex expands (by adding neurons, dendrites, synapses, axons, and glial cells), radial tension would keep the cortex thin, và growth would be manifested mainly by tangential expansion along the more compliant dimensions parallel to the surface. In addition, the steady CSF production by the choroid plexus elevates intraventricular hydrostatic pressure and would tover lớn inflate the embryonic cerebral ventricles like a water balloon, thereby enhancing tangential cortical expansion .

The unique architecture of cerebellar cortex arises from developmental events that differ radically from those in cerebral cortex . Early on, Purkinje cell precursors migrate from the ventricular surface, khung a monolayer below the cortical surface, & extkết thúc dendrites anisotropically towards the pial surface. Precursors of the far more numerous granule cells emerge from the rhombic lip, migrate, continue proliferating in the transient external granule cell layer, and leave a trailing axon while migrating past the Purkinje cells khổng lồ form the granule cell layer. Radial anisotropies associated with the ascending axons of granule cells, the trellis-like dendritic arbors of Purkinje neurons, & the Bergmann radial glial cells may help keep the cerebellar cortex thin if there is mechanical tension along any or all of these anisotropic components, just as proposed above for cerebral cortex. Additionally, cerebellar cortex has a second, transverse anisotropy involving granule cell axons that branch after ascending, forming parallel fibers that run long distances highly parallel to lớn one another within the superficial cerebellar cortical layer, most commonly along the mediolateral axis. If parallel fibers generate tension, they would resist expansion along their long axis, so that tangential expansion would preferentially occur along the more compliant axis orthogonal khổng lồ the parallel fibers (i.e., along the rostrocaudal axis). Consistent with this hypothesis and reflecting the fact that lamellar folds near the midline tover khổng lồ run orthogonal to it, the unfolded cerebellar cortex is a highly elongated ribbon after flattening schematically or computationally .

In contrast khổng lồ cortical structures, subcortical neurons typically elaborate dendrites that either radiate quasi-isotropically from the soma or are elongated (e.g., bipolar) but are not aligned consistently relative khổng lồ their neighbors (see http://neuromorpho.org). If these dendrites also generate mechanical tension, this would tend to lớn keep each subcortical nucleus relatively compact rather than flattening into a sheet. Altogether, mechanical tension along cellular processes in gray matter can account for macroscopic shape differences based on the pattern of anisotropic versus isotropic arborizations.

To Fold or Not?

Cerebral cortex tends to lớn be convoluted (gyrencephalic) in large brains & smooth (lissencephalic) in small brains (Fig. 1). We hypothesize that cerebral cortex is convoluted to the degree that proliferation of cortical neurons outpaces that of the underlying subcortical nuclei and that this reflects an important correlation between the kích cỡ of different brain regions & the order of neurogenesis. In an important study, Finlay and Darlington <1995> reported that:

“the sizes of brain components, from medulla to lớn forebrain, are highly predictable from absolute brain size by a nonlinear function. The order of neurogenesis was found khổng lồ be highly conserved across a wide range of mammals & lớn correlate with the relative sầu enlargement of structures as brain size increases, with disproportionately large growth occurring in late-generated structures.”

Aspects of this “late equals large” hypothesis have sầu been challenged . However, for present purposes, it is sufficient to lớn presume that neurogenesis occurs later & more extensively for cerebral neocortex than for various subcortical nuclei &, accordingly, that the ratio of neocortical lớn subcortical neurons increases disproportionately with brain kích cỡ. Thus, for small brains, the modest number of cortical neurons forms a sheet whose surface area is just sufficient khổng lồ surround the subcortical core without major folding. For large brains, the larger number of neocortical neurons and larger cortical surface area exceeds what is needed khổng lồ envelop the subcortical core, thus predicting the existence of convolutions , though the specific pattern of folds depends on mechanisms discussed below. In rare situations, large brains may have sầu reduced cortical folding, as in the lissencephalic manatee & also human clinical lissencephaly. However, in these cases, the cortex is unusually thiông xã, resulting in a smaller surface area for a given volume and, hence, fewer convolutions needed to surround the subcortical core, even if cortical volume and neuronal number is in the normal range. Indeed, in humans, thinner regions such as the visual cortex (∼2 mm thickness) tend khổng lồ be folded more tightly than thicker regions lượt thích the temporal pole (∼4 milimet thickness).

Cerebellar cortex differs from cerebral cortex in being convoluted in all mammals (and in many nonmammalian vertebrates) . By the ngắn gọn xúc tích articulated above sầu, this may reflect the fact that the cerebellar cortex envelops deep cerebellar nuclei whose aggregate volume is small. Hence, even species with small brains have a convoluted cerebellar cortex because its surface area far exceeds that needed lớn surround the deep nuclei.

How & Where the Cortex Folds

Much has been written about how the cerebral cortex gets its distinctive sầu folds. Here, we summarize key issues và observations without attempting lớn be comprehensive. In brief, four main mechanisms have sầu been proposed.

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(i) Mechanical tension along the length of axons coursing through the white matter would tover to lớn bring strongly connected regions closer together, thus forming a gyral fold that reduces the wiring length of these connections (e.g., a gyrus between strongly connected areas V1 và V2), whereas sulci would be more likely to lớn khung between weakly connected regions (e.g., the central sulcus between weakly connected areas 3b & 4) as schematized in Figure 2 . This hypothesis has great explanatory power, including the ability to lớn explain consistency of folding in regions dominated by large areas with svào and consistent connections và variability of folding in “balkanized” mosaics of smaller areas. Also, tension-based folding would naturally lead to compact wiring (“wiring length minimization”), which is as important for brains as it is for computer chips. However, there is skepticism in some quarters about tension-based folding ; space limitations here preclude detailed discussion of these criticisms and our specific counterarguments.