Cortical Divisions & Cerebral Lobes – Comprehensive Study Notes

Central Nervous System – Cortical Divisions

Four Gross Divisions of the Brain

• Cerebrum (largest, “grey wrinkled cap”)

• Diencephalon

• Brain-stem

• Cerebellum

The cerebrum itself contains three evolutionary/structural components:

1. Cerebral hemispheres (cortex + sub-cortical white matter)

2. Limbic system / rhinencephalon

3. Basal (sub-cortical) nuclei / basal ganglia

Functions attributed to the cerebrum: memory, emotion, consciousness, higher cognition, initiation of voluntary action, interpretation of sensation.

Surface Anatomy Terminology

• Cerebral cortex = thin (~2–4 mm) sheet of grey matter that completely covers the hemispheres.

• Fold = gyrus (pl. gyri).

• Shallow groove = sulcus (pl. sulci).

• Deep groove = fissure.

• Most conspicuous fissure is the longitudinal fissure, dividing right and left hemispheres in the mid-line.

Major Named Sulci

1. Central sulcus (Rolandic fissure)

2. Lateral sulcus (Sylvian fissure)

3. Parieto-occipital sulcus

Hemispheric Lateralisation

• Functional asymmetry exists, although the hemispheres work as an integrated unit.

• LEFT: logical, analytical, verbal → dominant for language.

• RIGHT: intuitive, perceptual, spatial.

• \text{~97\%} of people show left-hemisphere language dominance (including most left-handers).

• Sex differences: lateralisation is less pronounced in females; females therefore have lower incidence of aphasia after unilateral (left) lesions.

Deep Cerebral Structures

• Corpus callosum – largest white-matter commissure; principal inter-hemispheric bridge.

• Inner white matter arranged in tracts (association, commissural, projection).

• Basal (sub-cortical) nuclei embedded within white matter.

Classical Cortical Lobes & Boundaries

• Frontal – anterior to central sulcus, superior to lateral sulcus.

• Parietal – posterior to central sulcus, separated from occipital by parieto-occipital sulcus.

• Temporal – inferior to lateral sulcus.

• Occipital – posterior pole, small.

• Insula (5th lobe) – buried deep within lateral sulcus; function still incompletely understood, but left-insular damage correlates with non-fluent speech.

Brodmann (1909) identified 52 cyto-architectonic areas; these numeric labels (e.g., 4, 17, 44) remain a standard shorthand.

Functional Organisation: Primary vs Association Cortex

• Primary sensory & motor areas: first cortical recipients of thalamic sensory input or final cortical origin of descending motor commands.

• Association areas: adjacent expanses that integrate, interpret or plan.

Posterior half = sensory dominant (perception).
Anterior half = motor dominant (action).
Large swathes intermingle ➔ complex behaviours (language, planning, social cognition).

FRONTAL LOBE

1. Primary Motor Cortex – Area 4 (Pre-central gyrus)

• Executes fine voluntary movement of contralateral skeletal muscles.

• Distorted somatotopic map → motor homunculus: disproportionately large face, tongue, hand represent high dexterity demands.

2. Premotor Cortex – Area 6

• Association area; programs learned, sequential motor skills (e.g., signature).

• Interacts with parietal sensory association cortex, basal nuclei, thalamus.

• Lesion (with primary area) ⇒ spastic paralysis, loss of patterned skills.

3. Prefrontal Cortex (Anterior frontal convexity)

• Long reciprocal connections to all other lobes.

• Generates “executive functions”: planning, judgement, prediction, self-monitoring, inhibition.

4. Frontal Eye Field – Area 8

• Initiates rapid conjugate saccades & visual attention shifts.

• Unilateral damage ➔ both eyes deviate toward lesion.

5. Broca’s Speech Area – Areas 44 & 45 (inferior frontal gyrus, dominant hemisphere)

• Motor programming of articulate speech.

• Anterior part (semantic), posterior part (phonologic).

• Sends commands to premotor & primary motor cortex ➔ laryngeal, pharyngeal, oral, respiratory muscles.

• Lesion → non-fluent (Broca) aphasia: intact comprehension, impaired output.

PARIETAL LOBE

1. Primary Somatosensory Cortex – Areas 1-2-3 (Post-central gyrus)

• Receives contralateral touch, pressure, vibration, pain, temperature, proprioception.

• Somatotopic sensory homunculus; density of receptors = cortical area size (large lips, tongue, pharynx).

2. Somatosensory Association Cortex – Areas 5 & 7

• Integrates primary input; allows object identification by feel, spatial relationships, body schema; stores tactile memories.

3. Inferior Parietal Lobule

• Supramarginal gyrus – Area 40

• Angular gyrus – Area 39

Dominant (left) lesions ⇒ anomia, alexia with agraphia, finger agnosia, acalculia, L-R disorientation.
Non-dominant lesions ⇒ visuospatial neglect, impaired selective attention.

TEMPORAL LOBE

1. Primary Auditory Cortex – Areas 41 & 42 (Heschl’s gyrus)

• Conscious perception of sound; bilateral representation – unilateral lesion → partial loss both ears.

2. Auditory Association Cortex – Area 22

• Analyses harmonic & rhythmic patterns; categorises sounds as speech, music, noise.

3. Wernicke’s Area – Posterior part of 22 ± 39/40 (dominant side)

• Comprehension & formulation of language.

• Lesion → fluent but meaningless speech, poor comprehension (Wernicke aphasia).

4. Medial Temporal Structures (Hippocampus, Parahippocampal gyrus)

• Declarative memory, learning, emotional colouring of experience.

5. Perisylvian Zone

• Cortex encircling Sylvian fissure in dominant hemisphere encompassing Broca, Wernicke, angular & supramarginal gyri plus inter-connecting tracts (notably arcuate fasciculus).

• Core network for language formulation, comprehension, repetition.

OCCIPITAL LOBE

1. Primary Visual Cortex – Area 17 (Calcarine region)

• Receives thalamic (lateral geniculate) input.

• Retinotopic; right cortex maps left visual field & vice-versa.

• Lesion → homonymous visual field deficits / cortical blindness.

2. Visual Association Cortex – Areas 18 & 19

• Interprets form, colour, movement, fixation reflexes; matches present input with stored visual memories for recognition.

INSULA

• Deep “5th lobe” within lateral sulcus.

• Roles postulated in gustation, visceral sensation, risk prediction, articulation fluency.

• Dominant-insular lesion may impair well-articulated speech.

Cerebral White-Matter Pathways

1. Association Tracts (within ONE hemisphere)

• Short U-fibres link neighbouring gyri; long fasciculi link distant lobes.
  • Arcuate fasciculus (AF) – posterior temporal ⇄ frontal premotor/Broca; lesion → conduction aphasia (poor repetition).
  • Uncinate fasciculus (UF) – rostral temporal ⇄ orbitofrontal / prefrontal; connects memory & emotion with decision-making; mediates reward/punishment influence.
  • Inferior longitudinal fasciculus (ILF) – occipital ⇄ temporal; object recognition, visual discrimination, memory linking.

2. Commissural Tracts (between hemispheres)

• Corpus callosum – massive bridge; transfers right-ear auditory input to left language cortex, among countless bilateral exchanges.

• Anterior commissure – olfactory bulbs, amygdalae, inferior/medial temporal lobes.

• Posterior commissure – interconnects occipital mid-brain circuits; pupil & eye-movement reflexes.

3. Projection Tracts (cortex ⇄ sub-cortical or spinal levels)

• Corticospinal (motor cortex → spinal anterior horn).

• Corticobulbar (motor cortex → cranial nerve nuclei).

• Corticopontine (motor cortex → pontine nuclei → cerebellum).

• Thalamo-cortical sensory radiations ascend conversely.

Clinical / Functional Correlations & Examples

• Motor & sensory homunculi illustrate disproportionate cortical allocation (example metaphor: “grotesque little human” emphasising hands & face – correlates with speech articulation muscles).

• Damage to premotor + primary motor ⇒ spastic limbs.

• Left-frontal Broca lesion ⇒ clear thought but inability to articulate (non-fluent aphasia).

• Right-parietal lesion ⇒ spatial neglect, attention deficit.

• Female brain’s more bilateral language representation ➔ protective effect (lower aphasia incidence).

• Lesion of dominant insula ⇒ dysfluent, poorly articulated speech (suggesting insular role in speech motor-planning).

Ethical/Philosophical consideration: understanding lateralisation nuances cautions against over-simplistic “left-brain/right-brain” stereotypes; also underscores sex-based neurological variability – important for equitable clinical assessment.

Numerical / Statistical Facts

• Language dominance: \approx97\% left hemisphere.

• Brodmann mapped 52 cyto-architectonic areas.

• Frontal lobe occupies \approx \tfrac{1}{3} of hemispheric surface.

Summary Connections to Speech-Language Pathology

1. Frontal lobe lesions can affect speech initiation (Broca), motor planning (premotor), executive functions (prefrontal).

2. Parietal lobe lesions disrupt multimodal integration needed for reading, writing, calculation (angular & supramarginal gyri).

3. Temporal lobe is critical for auditory comprehension (Wernicke) and memory links.

4. Association tracts, particularly AF, underpin repetition and fluent inter-area communication – their damage creates characteristic aphasic syndromes.

5. Corpus callosum ensures bilateral sensorimotor coordination and linguistic transfer.

Comprehending cortical divisions and white-matter highways allows clinicians to localise lesions, predict deficits, and plan evidence-based interventions for speech, language, and cognitive disorders.