The cerebral cortex is built in layers. Beneath the gross anatomy of gyri and sulci, the cortex has a microscopic architecture that is highly conserved and exquisitely revealing of function. The six-layered pattern of neocortex is one of the most rigorously documented and most clinically useful organizing principles in neuroanatomy: a specific layer dominates in primary motor cortex, a different layer dominates in primary sensory cortex, association cortex shows yet a different pattern. Each pattern reflects the connections and functions of the region. The cytoarchitectonic map first developed by Brodmann in 1909 has been refined but remains the standard framework for connecting microscopic anatomy to functional regions.

This page covers the cellular architecture of the cerebral cortex, the Brodmann numbering system as a clinical shorthand, and the functional mapping of major cortical regions. The point is to give the trainee a working vocabulary for the cellular architecture of cortex — enough to read a paper that mentions “agranular cortex” or “Brodmann area 4” and know what is meant.

The Six-Layered Neocortex

Most of the cerebral cortex (about 90%) is six-layered neocortex (isocortex). From the surface inward:

• Layer I (molecular layer): nearly devoid of cell bodies; contains tangential axons (particularly from the apical dendrites of pyramidal neurons in deeper layers) and the terminations of certain afferent inputs. Despite its sparseness, this layer plays a major role in cortical computation.
• Layer II (external granular layer): small neurons including granule cells and small pyramidal neurons. Receives some thalamic input.
• Layer III (external pyramidal layer): medium and large pyramidal neurons. The principal source of corticocortical projections — fibers connecting one cortical region to another, including across the corpus callosum.
• Layer IV (internal granular layer): dense population of small granule cells (stellate cells). The principal recipient layer for specific thalamic inputs. Most prominent in primary sensory cortices (visual, auditory, somatosensory).
• Layer V (internal pyramidal layer): large pyramidal neurons including the giant Betz cells in motor cortex. The principal output to subcortical structures including the striatum, brainstem, and spinal cord.
• Layer VI (multiform layer): heterogeneous cell types. The principal source of corticothalamic feedback projections.

The relative thickness of these layers varies dramatically by region, producing the cytoarchitectonic variations that Brodmann mapped.

Types of Cortex by Cytoarchitecture

Three broad types of cortex are recognized:

• Granular cortex (koniocortex): prominent layer IV, sparse layer V. Characteristic of primary sensory cortices (visual, auditory, somatosensory). Layer IV is so prominent in primary visual cortex that it includes a visible white stripe — the “stria of Gennari” — visible to the unaided eye.
• Agranular cortex: layer IV is reduced or absent; layer V is prominent. Characteristic of primary motor cortex (Brodmann area 4) and the supplementary motor area. Layer V is the output layer that produces the corticospinal tract.
• Homotypical (six-layered, balanced) cortex: layers are present in standard proportions. Most association cortex.

Recognition of these patterns underlies the Brodmann map: the boundaries between Brodmann areas are based largely on transitions in cytoarchitecture from one pattern to another.

The Brodmann Areas

Korbinian Brodmann’s 1909 cytoarchitectonic map divided the human cerebral cortex into 47 numbered areas based on differences in cellular architecture. Some areas are clinically and pedagogically essential; others remain references in research papers. The most clinically important:

The Functional Hierarchy

Cortical processing flows in a hierarchy from primary cortices through association cortices to higher-order areas:

• Primary cortices: receive direct thalamic input or send direct subcortical output. Primary motor cortex (M1), primary sensory cortices (S1, V1, A1, gustatory).
• Unimodal association cortices: process information from a single sensory modality. Visual association cortex, auditory association cortex, somatosensory association cortex.
• Heteromodal association cortices: integrate information across modalities. Posterior parietal cortex, prefrontal cortex, lateral temporal cortex.
• Paralimbic and limbic cortices: subserve emotion, memory, and motivation. Cingulate, parahippocampal, orbitofrontal, insular cortices.

Hierarchical processing means that damage at different levels produces different clinical pictures: primary cortex lesions produce simple deficits (a primary visual cortex lesion produces hemianopia); association cortex lesions produce more complex syndromes (a fusiform face area lesion produces prosopagnosia); heteromodal cortex lesions produce the most complex syndromes (a dorsolateral prefrontal lesion produces executive dysfunction).

Columnar Organization

Within the layered structure, the cortex is also organized into vertical columns. Each column extends from layer I to layer VI and is several hundred micrometers in diameter. Neurons within a column share connectivity and functional properties:

• In primary visual cortex: ocular dominance columns alternate left-eye and right-eye dominant zones; orientation columns process specific stimulus orientations.
• In primary somatosensory cortex: columns process input from a single submodality (touch, vibration, joint position) from a single skin region.
• In primary motor cortex: columns control specific muscle synergies.

The columnar organization is one of the most consistently observed features of mammalian cortex, though its precise functional significance and developmental origin remain active research areas.

The Allocortex

About 10% of the cerebral cortex is not six-layered. The allocortex has fewer layers and a different cellular organization. Two main types:

• Paleocortex: three- to five-layered. Includes the olfactory cortex and adjacent regions.
• Archicortex: three-layered. Includes the hippocampus.

Allocortex is found in the most phylogenetically ancient parts of the cerebral cortex. The transition zones between allocortex and neocortex (the periallocortex and proisocortex) include the parahippocampal gyrus, entorhinal cortex, and parts of the cingulate.

Cortical Connectivity

Each cortical region receives and sends thousands of connections. Three main types:

• Corticocortical (association) fibers: connect different regions of cortex. Short association fibers (U-fibers) connect adjacent gyri; long association fibers (superior longitudinal fasciculus, inferior longitudinal fasciculus, uncinate fasciculus, arcuate fasciculus, cingulum) connect distant cortical regions.
• Commissural fibers: connect the two hemispheres. The largest commissure is the corpus callosum; smaller commissures include the anterior commissure (connecting temporal lobes) and the hippocampal commissure.
• Projection fibers: connect cortex with subcortical structures. The internal capsule carries most projection fibers — both corticospinal/corticobulbar (descending) and thalamocortical (ascending).

White matter tracts are covered in detail on their own page; the relevant point here is that cortical neurons in different layers are differentially connected: layer III gives rise to most corticocortical fibers, layer V gives rise to most corticospinal and corticostriatal fibers, layer VI gives rise to most corticothalamic feedback.

Functional Networks and the Default Mode

Beyond the classical map of discrete functional areas, modern neuroimaging has revealed that the brain functions through coordinated activity across distributed networks. Several networks have been identified:

• Default mode network: active during rest and self-referential thought. Includes the posterior cingulate/precuneus, medial prefrontal cortex, angular gyri, and medial temporal lobes. Implicated in Alzheimer disease (early degeneration).
• Salience network: detects behaviorally relevant stimuli. Includes the anterior insula and dorsal anterior cingulate.
• Central executive (frontoparietal) network: subserves attention and working memory during cognitive tasks. Includes the dorsolateral prefrontal cortex and posterior parietal cortex.
• Sensorimotor network: includes the primary motor and somatosensory cortices and the supplementary motor area.
• Visual network: includes primary and association visual cortices.

Network dysfunction is increasingly recognized as the substrate of various neurological and psychiatric diseases. The clinical examination’s traditional focus on focal deficits is being supplemented (not replaced) by an understanding of distributed network function.

Clinical Cytoarchitecture: Localization on MRI

The cytoarchitectonic patterns above translate into clinical localization on MRI:

• Stria of Gennari in calcarine cortex: a visible white stripe on high-resolution MRI, marking layer IV of primary visual cortex.
• Cortical thickening or thinning: focal cortical dysplasia (with congenital lamination abnormalities), Alzheimer disease (with cortical thinning especially in the posterior cingulate and lateral temporal), corticobasal syndrome (asymmetric cortical atrophy).
• Laminar necrosis on MRI: after hypoxic-ischemic injury, T1 hyperintensity in the cortical ribbon reflects neuronal injury preferentially in layer III and layer V — the layers most metabolically demanding and most vulnerable to ischemia.
• Cortical ribbon sign on diffusion-weighted MRI: in prion disease, the cortical ribbon shows diffusion restriction, often striking and asymmetric.

🔍 Did You Know?

The stria of Gennari in primary visual cortex is the only cytoarchitectonic feature of the human cerebral cortex visible to the naked eye. It is a thin white stripe in layer IV of the calcarine cortex, formed by the heavy myelination of the geniculocalcarine fibers terminating there. Francesco Gennari, a medical student at Parma, described it in 1782 — long before the term “Brodmann area” existed. The presence of this stripe is what defines V1 (“striate cortex”) and what made the boundary between V1 and V2 the very first cytoarchitectonic distinction documented in the cerebral cortex.

Pitfalls and Pearls

• Six-layered neocortex covers about 90% of the cerebral cortex. The remaining 10% is allocortex (paleocortex, archicortex) in phylogenetically ancient regions.
• Granular cortex (prominent layer IV) is primary sensory cortex. Agranular cortex (reduced layer IV, prominent layer V) is primary motor cortex.
• Layer V is the principal output layer to subcortical structures, including the corticospinal tract from primary motor cortex.
• Layer IV is the principal recipient layer for thalamic input. Its prominence in primary sensory cortices reflects this connectivity.
• The stria of Gennari marks V1, the primary visual cortex (Brodmann area 17).
• Brodmann areas 44 and 45 (dominant) constitute Broca area. Damage produces non-fluent aphasia.
• Brodmann area 22 (dominant posterior portion) constitutes Wernicke area. Damage produces fluent aphasia with impaired comprehension.
• Brodmann area 4 is primary motor cortex. The giant Betz cells of layer V give rise to a substantial portion of the corticospinal tract.
• Cortical layers are differentially vulnerable to insult. Hypoxic-ischemic injury affects layers III and V preferentially, producing laminar necrosis visible on MRI.
• Network-based thinking complements localization. Many cortical syndromes reflect network dysfunction rather than focal damage; the default mode network is particularly involved in Alzheimer disease.

References

1. Brodmann K. Vergleichende Lokalisationslehre der Grosshirnrinde in ihren Prinzipien dargestellt auf Grund des Zellenbaues. Leipzig: Barth; 1909.
2. Zilles K, Amunts K. Centenary of Brodmann’s map — conception and fate. Nat Rev Neurosci. 2010;11(2):139-145.
3. Mesulam MM. From sensation to cognition. Brain. 1998;121(6):1013-1052.
4. Mountcastle VB. The columnar organization of the neocortex. Brain. 1997;120(4):701-722.
5. Buckner RL, Andrews-Hanna JR, Schacter DL. The brain’s default network: anatomy, function, and relevance to disease. Ann N Y Acad Sci. 2008;1124:1-38.
6. Raichle ME. The brain’s default mode network. Annu Rev Neurosci. 2015;38:433-447.