The
cerebral cortex contains three main types of neurons arranged in a layered
structure:
1. Pyramidal
cells (shaped like a tepee, with an apical dendrite reaching from the upper
end toward the cortical surface, and basilar dendrites extending horizontally
from the cell body);
2. Stellate
neurons (star shaped, with dendrites extending in all directions); and
3. Fusiform neurons (found in deeper layers, with a large dendrite that ascends toward the surface of the cortex).
In addition, following two types of cells are also present:
- Horizontal cells of Cajal, small, fusiform, horizontally place cells are found in the most superficial layer of the cerebral cortex.
- Cells of Martinotti, small multipolar cells are found in all the layers of cerebral cortex.
The
axons of pyramidal and fusiform neurons form the projection and association
fibers.
- large layer V pyramidal neurons projecting their axons to the spinal cord and brain stem,
- smaller layer II and layer III pyramidal cells sending association axons to other cortical areas, and
- fusiform neurons giving rise to corticothalamic projections.
- Stellate neurons are interneurons whose axons remain within the cortex.
Types of Cortices
1. Allocortex (archicortex) is found predominantly
in the limbic system cortex and contains fewer layers than the isocortex (three
in most regions).
2. Isocortex
(neocortex) is more commonly found in most of the cerebral hemisphere and
contains six layers.
3. Juxtallocortex
(mesocortex) forms the transition between the allocortex and isocortex. It
contains three to six layers and is found in such regions as the cingulate
gyrus and the insula.
The
isocortex consists of up to six well-defined layers of cells. The organization
of these layers is referred to as cytoarchitecture.
The outermost molecular or plexiform layer (I) contains nonspecific afferent fibers that come from within the cortex or from the thalamus.
The internal granular layer (IV) is usually a thin layer with closely packed stellate cells. This layer also presents high concentration of horizontally arranged nerve fibers that form external band of Baillarger.
The internal pyramidal or ganglionic layer (V) contains, in most areas, pyramidal cells that are fewer in number but larger in size than those in the external pyramidal layer. These cells project to distal structures (eg, brain stem and spinal cord). In the motor cortex of the precentral gyrus, the pyramidal cells of this layer are very large and are known as Betz cells which account for about 3% of the projection fibers of the corticospinal or pyramidal tract. This layer also presents large bundles of horizontal fibers that form inner band of Baillarger.
The fusiform (multiform) layer (VI) consists of irregular fusiform cells whose axons enter the adjacent white matter.
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| Structure of the cerebral cortex. Left and Middle: Nissl cellular stain of visual or sensory cortex and motor cortex respectively; Right: Golgi neuronal stain. Source: Click on the image to see the name of different layers. By User: Looie496 created file, Santiago Ramon y Cajal created artwork [Public domain], via Wikimedia Commons |
Variation
in cortical structure:
Not all the areas in cerebral cortex has the uniformity in terms of possessing all these six layers. The cerebral cortex in a particular area may be homotypical (possessing all six layers) in contrast to heterotypical areas where the basic six layers cannot be recognized.
Not all the areas in cerebral cortex has the uniformity in terms of possessing all these six layers. The cerebral cortex in a particular area may be homotypical (possessing all six layers) in contrast to heterotypical areas where the basic six layers cannot be recognized.
Granular
type
- contains packed stellate cells
- generally three layers are recognized, layers 2 through 5 merge into a single layer of granualr cells
- found in postcentral gyrus, superior temporal gyrus and partly in the hippocampal gyrus
Agranular
type
- as in precentral and other areas in the frontal lobe
- layers 2 and 4 are almost absent as the granular cells are poorly developed
- pyramidal cells in layers 3 and 5 are large and densely packed
- the fibers given off from these areas are associated with motor function
- found in precentral gyrus and other areas of frontal lobe
The
constituent group of neurons with similar functions in cerebral cortex are organized into vertical units or columns of functional
activity.
Read More:
Gross Anatomy and Blood Supply of Cerebrum.
White Matters of Cerebrum.
Functional areas in cerebral hemisphere.
REFERENCES:
Gray’s Anatomy, 39th Edition
Snell’s Clinical Neuroanatomy 7th Edition
Lange Clinical Neuroanatomy 25th Edition
Read More:
Gross Anatomy and Blood Supply of Cerebrum.
White Matters of Cerebrum.
Functional areas in cerebral hemisphere.
REFERENCES:
Gray’s Anatomy, 39th Edition
Snell’s Clinical Neuroanatomy 7th Edition
Lange Clinical Neuroanatomy 25th Edition