Neocortical Layers
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The six cytoarchitectural layers of the neocortex
From outside to inside (leptomeninges on the outside and white matter on the inside):
Layer I
- The Molecular (or plexiform) Layer
- A nerve fiber layer, meaning that it is cell sparse and predominantly comprises axons and dendritic processes.
Layer II
- The External Granular Layer
- Contains non-pyramidal cells 5-15 micrometers, and relatively few, small pyramidal cells.
Layer III
- The External Pyramidal Layer
- Predominantly contains pyramidal cells of varying sizes: 10 to 80 micrometers. Also, layer III is sparsely populated with non-pyramidal cells.
Layer IV
- The Internal Granular Layer
- Densely packed with non-pyramidal cells.
- Contains the horizontally oriented external band of Baillarger: a prominent thalamocortical nerve fiber layer. In the primary visual cortex, this nerve fiber band is called the line of Gennari.
Layer V
- Internal Pyramidal (or ganglionic) Layer
- Contains the largest pyramidal cells, most notably the Betz cells of the primary motor cortex – the major cortical motor neurons.
- The horizontally-oriented internal band of Baillarger lies deep within this layer.
Layer VI
- The Multiform (or fusiform) Layer
- Contains a wide variety of pyramidal and non-pyramidal cells.
Although the specific cellular constitution and nerve fiber density of each histologic layer varies across the cerebral cortex, certain areas of similarity exist. In 1909, Korbinian Brodmann published the most widely recognized cytoarchitectural maps of the human brain, which distinguish 52 different cytoarchitectural areas. Each area or group of areas subserves a unique function, which we address elsewhere.