Cortical Layers of The Cerebral Cortex

Cerebral Cortex: Development

The cerebral cortex is the cerebrums or brains outer layer of neural tissue. Two cortices divide it. First, the longitudinal fissure dividing the cerebrum into the left cerebral and right cerebral hemisphere. The two region are joined beneath the cortex by the corpus callosum. The cerebral cortex represents a key role in memory, attention, awareness, thought, language, perception, and consciousness.
In People, the cerebral cortex is folded, providing a much greater surface area in the defined capacity of the skull. A wrinkle or ridge in the cortex is called a gyrus (plural gyri), and depression or fissure is named a sulcus (plural sulci). Within the human brain, the cerebral cortex is more than two-thirds, which is buried in the sulci The human cerebral cortex is 2 to 4 millimeters (0.079 to 0.157 in ) thick.

The cerebral cortex is formed of gray matter, consisting essentially of cell bodies (with astrocytes being the most plentiful cell type in the cortex as well as the human brain in general ) and capillaries. It differs with the underlying white matter, consisting chiefly of the white myelinated sheaths of neuronal axons.

The most modern part of the cerebral cortex to form in the evolutionary history of people is the neocortex, which distinguished into six layers. Neurons in numerous layers connect vertically to create small microcircuits, called cortical columns. Various neocortical regions known as Brodmann areas are identified by variation in their cytoarchitectonics (histological structure) and functional roles in sensation cognitive and behavior.

Cortical Structure Layers
The different cortical layers each contain a characteristic distribution of neuronal cell types and connections with other cortical and subcortical regions. For example, there are direct connections between different cortical areas and indirect connection via the thalamus. This is a band of whiter tissue that can be identified with the natural eye in the fundus of the calcarine sulcus of the occipital lobe. The Stria of Gennari consists of axons bringing visual information from the thalamus into layer four of the visual cortex.

Straining cross -section of the cortex to reveal the position of neuronal cell bodies and the intracortical axon tract allowed neuroanatomists in the early 20th century to produce a textile record of the laminar structure of the cortex in different species. The neurons of the cerebral cortex are grouped into six main layers, from outside (pial surface) to the inside (white matter).
1. Layer I, the molecular layer, contains a few scattered neurons and consists mainly of branches of apical dendritic tufts of pyramidal neurons and horizontally oriented axons, as well as glial cells. During development, Cajal-Retzius and subpial granular layer cells are present in this layer. And some spiny stellate cells can be found here. INpust to the apical tufts is thought to be crucial for the “feedback” interactions in the cerebral cortex linked to 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 over the cerebral cortex mantle receives substantial input from “matrix or M0type thalamus cells ( contracts to the core: or C-type that go to layer IV).

2. Layer II, the external granular layer, contains small pyramidal neurons and numerous stellate neurons.
3,Layer III, the external pyramidal layer. Includes 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 primary source of corticocortical efferents.
4, Layer IV, the internal granular layer, consist of different types of pyramidal neurons and stellate, and is the main target of thalamocortical afferents from the thalamus type C neurons as well as intrahemispheric corticocortical afferents.
5. Layer V, the internal pyramidal layer, comprises large pyramidal neurons which cause increases to axons leaving the cortex and running down to subcortical structures ( including the basal ganglia). In the primary motor cortex the frontal lobe, layer V contains Beta cells whose axons travel through the internal capsule, the brains stem and the spinal cord forming the corticospinal tract, this is the main pathway for voluntary motor control.
6.Layer VI, the polymorphic or multiform layer, includes few large pyramidal neurons and many tiny spindle-like pyramidals and multiform neurons; layer VI sends different fiber to the thalamus establishing very precise reciprocal interconnection between the cortex and the thalamus. The layer VI neurons from one cortical column connect with thalamus neurons that provide input into the same cortical column. These connection are both excitatory and inhibitory. Neurons transmit excitatory fibers to neurons in the thalamus and also transmit collaterals to the thalamic reticular nucleus that hinder these same thalamus neurons or ones adjunct to them. One theory si that because the inhibitory output is reduced by cholinergic input to the cerebral cortex, this provides the brainstem the adjustable gain for the relay fo lemniscal inputs.

The cortical layers are not just stacked one on top of the other; there is a characteristic connection between the neuronal types and different layers, which are distinct and span all the thickness of the cortex. These cortical microcircuits are classified into cortical columns and minicolumn. It has been thought that the minicolumns are the basic functional units of the cortex. Functional properties of the cortex can change abruptly between laterally adjacent points, but they continue in the direction perpendicular to the surface.There is also proof of the presence of functionally distinct cortical columns in the visual cortex, auditory cortex, and associative cortex.

Cortical areas that require a layer IV are referred to as granular. Cortical areas that have only a rudimentary layer IV are referred to as deys granular. Information processing within each layer is defined by different temporal dynamics with that in the layers II/II having a slow 2 Hz oscillation while that in layer V having a fast 10-15 HZ one.


Cerebral Cortex Function: Connections, Sensory, Motor,Association

Cerebral Cortex Function: Connections, Sensory, Motor,Association

Cerebral Cortex: Development

Health Life Media Team