Cerebral Cortex – Structure, Brodmann Areas, Functional Maps & White Matter

General Features of the Cerebral Cortex
  • Covers the paired, prominent cerebral hemispheres; forms an outer “bark”‐like layer
  • Mostly composed of the highly evolved, complex neocortex
  • Grey-matter ribbon showing sulci & gyri
  • 50–100 billion50\text{–}100\,\text{billion} neuronal cell bodies in a laminated sheet (3–6 layers) that, if unfolded, spans ≈2.5 ft2\approx 2.5\,\text{ft}^2
  • Variable thickness: 1.5 mm1.5\,\text{mm} (thin, e.g. calcarine sulcus) → 4.5 mm4.5\,\text{mm} (thick, motor/association areas)
  • Functions: memory, attention, perceptual awareness, thought, language, consciousness
  • Highest CNS level; always operates in concert with sub-cortical “lower” centres
  • Behaviour shaped by both inherited programs and learned programs stored in cortex
Classification of Cortex
  • Archicortex (allocortex)
    • 33 layers
    • Example: hippocampus + dentate gyrus ("hippocampal formation")
  • Neocortex (isocortex)
    • 66 cellular layers (detailed below)
    • Represents the majority of human cerebral cortex
Six Layers of the Neocortex (outside → inside)
  1. Molecular (plexiform) layer
    • Mostly neuropil (axons, dendrites, synapses)
  2. External granular layer
    • Packed stellate (granule) cells
  3. External pyramidal layer
    • Small/medium pyramidal neurons
  4. Internal granular layer
    • Densely packed stellate cells; prominent external band of Baillarger (tangential fibres)
  5. Internal (ganglionic) pyramidal layer
    • Large pyramidal cells (Betz cells in motor cortex); contains inner band of Baillarger
  6. Multiform (polymorphic) layer
    • Heterogeneous fusiform & Martinotti cells; blends into underlying white matter
  • Afferent vs. efferent streams traverse specific layers (e.g. thalamocortical afferents terminate mainly in layer 4; corticofugal efferents originate mainly layers 5/6)
Cortical Neuronal Cell Types
  • Pyramidal cells (principal excitatory projection neurons)
  • Stellate (granule) cells (interneurons, esp. layers 2 & 4)
  • Fusiform cells (layer 6)
  • Horizontal cells of Cajal (rare; layer 1)
  • Cells of Martinotti (inhibitory interneurons, deep layers)
Surface Landmarks & Sulci/Gyri
  • Central (Rolandic) sulcus → separates frontal & parietal lobes
  • Precentral gyrus (motor), Postcentral gyrus (somatosensory)
  • Lateral (Sylvian) fissure → separates temporal lobe from frontal/parietal
  • Parieto-occipital sulcus & pre-occipital notch mark occipital lobe
Brodmann’s Numerical Map & Functional Triad
  • 4747 cytoarchitectonic areas demarcated by Brodmann
  • Functional grouping:
    1. Motor areas
    2. Sensory areas
    3. Association areas (>75%75\% of human cortex; integrative & cognitive roles)
MOTOR AREAS (Frontal Lobe)
  • Primary Motor Cortex (M1) – Area 4
    • Location: precentral gyrus & anterior part of paracentral lobule
    • Somatotopic homunculus (inverted body representation; leg → superomedial, face → inferolateral)
    • Initiates precise voluntary movements
  • Premotor Cortex – Area 6
    • Anterior to M1 on superior, middle & inferior frontal gyri + medial surface
    • Plans & programs complex movements; stimulation evokes postural/axial patterns
  • Supplementary Motor Area (part of Area 6, medial surface)
  • Frontal Eye Field – Area 8
    • Posterior middle frontal gyrus, just in front of precentral facial area
    • Controls conjugate horizontal gaze; lesions cause transient ipsilateral gaze deviation
  • Broca’s Motor Speech Area – Areas 44 & 45 (pars opercularis & pars triangularis, IFG, dominant hemisphere)
    • Generates motor programs for articulation; lesion → expressive (non-fluent) aphasia
PRIMARY SOMATOSENSORY CORTEX (Parietal Lobe)
  • Postcentral gyrus, posterior paracentral lobule – Areas 3, 1 & 2
  • Receives contralateral exteroceptive (pain, temperature, touch) & proprioceptive input
SOMATOSENSORY ASSOCIATION CORTEX
  • Superior parietal lobule – Areas 5 & 7
    • Interprets size, shape, texture; enables stereognosis
LANGUAGE AREAS
  • Wernicke’s Sensory Speech Area – Area 22 plus 39 & 40 (posterior STG, angular gyrus, supramarginal gyrus; dominant hemisphere)
    • Comprehension of spoken & written language
    • Lesion → receptive aphasia (fluent but meaningless speech, impaired comprehension)
  • Arcuate Fasciculus
    • Association bundle linking Wernicke’s ↔ Broca’s areas
    • Damage → conduction aphasia (intact comprehension & fluent speech, poor repetition)
AUDITORY AREAS (Temporal Lobe)
  • Primary Auditory Cortex – Areas 41 & 42
    • Location: Heschl’s (anterior transverse temporal) gyrus on superior surface of STG
    • Processes basic sound characteristics (tone, pitch)
  • Auditory Association Cortex – Area 22 (surrounds primary area on lateral STG)
    • Higher-order sound interpretation; integrates with language (part of Wernicke’s)
VISUAL AREAS (Occipital Lobe)
  • Primary Visual Cortex (Striate Cortex) – Area 17
    • Medial occipital surface within walls & floor of posterior calcarine sulcus; extends to occipital pole
    • Characteristic myelinated “line of Gennari” (stria)
  • Secondary / Association Visual Cortex – Areas 18 & 19
    • Surrounds Area 17 on medial & lateral surfaces; interprets form, colour, motion; interfaces with dorsal (“where”) & ventral (“what”) streams
OTHER SIGNIFICANT ASSOCIATION AREAS
  • Prefrontal Cortex – Areas 9, 10, 11, 12, 46, 47
    • Executive functions: planning, reasoning, decision-making, personality, social behaviour, working memory
  • Orbitofrontal Cortex – emotional & reward processing
  • Cingulate Gyrus – motivation, error detection, limbic integration
  • Posterior Parietal Association Cortex – visuospatial attention, hand-eye coordination
CORTICAL HOMUNCULUS (Motor & Sensory Maps)
  • Disproportionate representation: face & hands occupy large cortical areas, reflecting fine motor control & rich sensory innervation
  • In both motor (Area 4) & somatosensory (Areas 3-2) cortices; body mapped mediolaterally: leg → trunk → arm → face → intraoral/pharyngeal structures
WHITE MATTER OF THE CEREBRUM
  • Association Fibres (intracortical)
    • Short (U-fibres) connect adjacent gyri
    • Long bundles: superior longitudinal fasciculus, inferior longitudinal fasciculus, uncinate fasciculus, cingulum
  • Commissural Fibres (inter-hemispheric)
    • Corpus callosum (rostrum, genu, body, splenium)
    • Anterior commissure (connects inferior & middle temporal gyri + olfactory areas)
    • Posterior commissure (interconnects pre-tectal/visual nuclei)
    • Habenular commissure (olfactory–limbic link)
  • Projection Fibres (corticofugal & corticopetal)
    • Form corona radiata → internal capsule → brain-stem tracts
    • Internal capsule subdivisions & contents:
      • Anterior limb – frontopontine, frontothalamic fibres
      • Genu – corticobulbar fibres (face motor)
      • Posterior limb – corticospinal (arm/leg), somatosensory thalamocortical, visual & auditory radiations (retrolenticular/sub-lenticular parts)
    • Continue as cerebral peduncles, pyramids, etc.; decussate (lateral corticospinal tract)
Corticofugal Fibre System (schematic)
  • Prefrontal, frontal eye field, secondary motor, posterior parietal, primary sensory, visual & auditory cortices all send descending outputs that modulate brain-stem + spinal circuits
Clinical Correlates & Lesion Syndromes
  • Area 4 lesion → contralateral paresis/plegia (UMN signs)
  • Area 6 lesion → apraxia (deficit in motor planning)
  • Frontal eye field lesion → eyes deviate toward lesion side
  • Broca’s area lesion → expressive (non-fluent) aphasia; comprehension preserved
  • Wernicke’s area lesion → receptive (fluent) aphasia; impaired comprehension
  • Angular gyrus (39) lesion → alexia & agraphia
  • Somatosensory cortex lesion → contralateral anaesthesia/hypoesthesia
  • Superior parietal lobule lesion → astereognosis, contralateral neglect (non-dominant)
  • Visual cortex destruction → contralateral homonymous hemianopia
Numerical / Statistical Facts & Miscellaneous
  • Total cortical neurons: ≈1011\approx 10^{11} (range 50–100 billion50\text{–}100\,\text{billion})
  • Cortical surface area (unfolded): ≈2.5 ft2\approx 2.5\,\text{ft}^2 (≈ 0.23 m20.23\,\text{m}^2)
  • Layer thickness range: 1.5 mm→4.5 mm1.5\,\text{mm} \rightarrow 4.5\,\text{mm}
  • Cortex occupies >75%75\% of brain volume devoted to association processing
Key Take-Home Connections
  • Neocortex = six‐layered sheet hosting specialised functional modules (motor, sensory, association)
  • Brodmann numbers provide a universal cytoarchitectural shorthand to locate function
  • White-matter architecture (association, commissural, projection fibres) integrates hemispheres, lobes & sub-cortical centres, enabling coordinated perception, cognition & action
  • Clinical localisation of cortical lesions relies on understanding somatotopic/functional maps described above