Neocortical Layers

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.