Cerebral Cortex; histologyy and its parts

Cerebral cortex is also called pallidum and it consistsof two hemispheres. Surface area of cerebral cortex in human beings is 2.2 sq m. Both the cerebral hemispheres are separated by a deep vertical fissure (deep furrow or groove). The separation is complete anteriorly and posteriorly. But in middle portion, the fissure extends only up to corpus callosum. Corpus callosum is the broad band of commissural fibers, connecting the two hemispheres. Surface of the cerebral cortex is characterized by complicated pattern of sulci (singular = sulcus) andgyri (singular = gyrus). Sulcus is a slight depression or groove and gyrus is a raised ridge.

HISTOLOGY OF CEREBRAL CORTEX

LAYERS OF CEREBRAL CORTEX

Cerebral cortex consists of gray matter that surrounds the deeper white matter. It is formed by different types of nerve cells along with their processes and neuroglia. It is not uniform throughout. It is thickest, i.e. 4.5 cm at the precentral gyrus and thinnest at frontal and occipital poles. According to Economo, the cerebral cortex is formed by six layers of structures. Following are the layers from outside to inside:

Molecular or Plexiform Layer

Molecular layer has few small fusiform cells. It also contains dendrites or axons from cells of deeper

layers.

External Granular Layer

External granular layer consists of large number of close ly packed small cells, which are round, polygonal or triangular in shape. Dendrites of these cells pass into molecular layer. Axons end in the deeper layers. Some axons enter white substance of the hemisphere.

Outer Pyramidal Layer

Outer pyramidal layer is formed by pyramidal cells, which are of two sizes. Medium sized pyramidal cells

are in the outer portion and larger pyramidal cells are in deeper portion.

Internal Granular Layer

Like external granular layer, this layer also has closely packed smaller cells, which are stellate type. But, the nerve fibers are more in this layer than in external granular layer. This layer contains many horizontal

fibers, which appear as a white strip known as outer

strip.

Ganglionic Layer or Internal Pyramidal Layer

Ganglionic layer or internal pyramidal layer consists of pyramidal cells of graded sizes. It is well developed in the precentral (motor) cortex. Pyramidal cells in this region are otherwise known as Betz cells or giant cells. This layer also contains cells of Martinotti. Martinotti cells are peculiar in that their axons pass outward towards the surface of the cortex.

Fusiform Cell Layer

Fusiform cell layer is in contact with white matter of cerebral hemisphere. It is composed of closely packed small spindle-shaped cells.

PARTS OF CEREBRAL CORTEX

Cerebral cortex is divided into two parts based on phylogeny (evolutionary development of a species):

1. Neocortex

2. Allocortex.

1. Neocortex

Neocortex is the phylogenetically new structure of cerebral cortex. It is also called isocortex or neopallium. This part forms the major portion of cerebral cortex. Part of the cerebral cortex that has all six layers of structures is called neocortex.

2. Allocortex

Allocortex is the phylogenetically oldest structure of cerebral cortex. It has less than six layers of structures. It is divided into two divisions namely, archicortex and paleocortex, which form the parts of limbic system.

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