Cerebral Cortex Layers (Microanatomy) Simplified

The neocortex have 6 layers and allocortex have only 3 layers.

The 6 layers of Neocortex:

Orientation of layers:

• Outer: Towards meaninges
• Inner: Towards white matter

Idea about the layers:

• Molecular or plexiform: Only cell processes
• Granular layer: Densely packed stellate cells
• Pyramidal layer: Medium and Large pyramidal cells
• Multiform layer: Different types of cells
• Pyramidal cells are absent in inner granular layer

Layers	Components	Afferents		Efferents
I – Molecular	Axons and Dendrites (Cell processes)	From other regions of Cortex and Brainstem		To other regions of cortex (Intra-cortical Association functions)
II – External granular	Densely packed Stellate cells + Small pyramidal cells
III – External pyramidal	Loosely packed Stellate cells + Medium pyramidal cells
IV – Internal granular	Densely packed Stellate cells only		+ From Thalamus
V – Internal pyramidal	Large pyramidal cells only (few stellate cells) – Giant Pyramidal cells of Betz		+ From Brain stem	To Brain stem & Spinal cord (Projection fibers)
VI – Multiform	Multiple sized pyramidal cells + Loosely packed stellate cells			To Thalamus

The 3 layers of allocortex:

1. Molecular layer
2. Pyramidal layer
3. Multiform layer

Allocortex is found in: Limbic system

1. Olfactory cortex
2. Hippocampal formation
3. Subiculum

Mesocortex: A transitional type of 3 to 6-layered cortex between neocortex and allocortex –

1. Parahippocampal gyrus
2. Pre- and Para-subiculum
3. Insula

Neurons of the cortex:

1. Pyramidal cells:
   • Large layer V pyramidal cells project axons to brainstem and spinal cord.
   • Smaller layer II and III pyramidal cells project axons to other cortical areas.
2. Stellate cells: Interneurons whose axons remain within the cortex
3. Fusiform cells (in deeper layers): Gives rise to corticothalamic projections
4. Horizontal cells of Cajal
5. Cells of Martinotti

Bands of Baillarger: Formed by high concentration of horizontally arranged nerve fibers.

• External band: In layer IV
• Internal band: In layer V