Week 2 - Cortex and Thalamus

Cortex: Gross Anatomy of Cerebral Hemispheres

  • Two cerebral hemispheres separated by the longitudinal fissure
    • Left and right are anatomically similar but functionally asymmetric (hemispheric lateralisation)
  • External surface landmarks
    • Major lobes: Frontal, Parietal, Temporal, Occipital, Limbic (functional), Insula (hidden in lateral sulcus)
    • Key sulci/fissures:
    • Longitudinal fissure (midline)
    • Central sulcus (separates frontal & parietal lobes)
    • Lateral (Sylvian) sulcus (separates temporal from frontal/parietal)
    • Parieto-occipital & Calcarine sulci (occipital boundaries)
  • Vascular reference: branches of the middle cerebral artery course in the lateral sulcus

Grey vs White Matter

  • Grey matter = cortical & deep nuclei cell bodies
  • White matter = myelinated axonal fibre tracts
    • Age-related degeneration of white matter visible on MRI (25 y vs 75 y scans)

Major White-Matter Systems

  • Commissural fibres (inter-hemispheric)
    • Corpus callosum (largest, >200\,\text{million} axons): rostrum, genu, body, splenium
    • Anterior commissure (temporal connections)
    • Fornix (limbic projection but often described separately)
  • Association fibres (intra-hemispheric)
    • Short arcuate fibres between neighbouring gyri
    • Longitudinal & arcuate fasciculi, cingulum, uncinate fasciculus
  • Projection fibres (cortex ⇄ sub-cortex/cord)
    • Corona radiata converges into internal capsule (anterior limb, genu, posterior limb, retrolenticular & sub-lenticular parts)
    • Corticospinal, corticobulbar, thalamo-cortical, etc.
    • Bounded laterally by putamen/globus pallidus & medially by caudate/thalamus

Functional Cortical Areas by Lobe

Frontal Lobe

  • Precentral gyrus = Primary Motor Cortex (M1)
    • Upper motor neurons → corticospinal tract → lower motor neurons → voluntary movement
    • Somatotopic motor homunculus (lower limb medial → face lateral)
  • Premotor & Supplementary Motor Areas (planning)
  • Broca’s area (pars opercularis + pars triangularis of inferior frontal gyrus; dominant hemisphere)
    • Motor speech formulation; lesion → Broca (expressive) aphasia
  • Orbitofrontal cortex (personality, reward, decision; damage → disinhibition, “frontal lobe syndrome”/witzelsucht)
  • Prefrontal cortex (executive functions)
    • Dorsolateral PFC: working memory, planning, problem solving
    • Ventromedial/Orbitomedial PFC: emotional regulation; connections with amygdala & hippocampus
    • Classic case: Phineas Gage

Parietal Lobe

  • Post-central gyrus = Primary Somatosensory Cortex (S1; areas 3,1,2)
    • Receives touch, pressure, pain, temperature, proprioception via thalamo-cortical pathways (e.g. spinothalamic, dorsal column-medial lemniscal)
    • Sensory homunculus mirrors motor
  • Secondary somatosensory cortex (S2) in parietal operculum
  • Inferior parietal lobule (supramarginal + angular gyri)
    • Abstract thinking, reading, maths; lesions → acalculia, agraphia, left-right disorientation (Gerstmann)

Temporal Lobe

  • Superior temporal gyrus
    • Heschl’s transverse gyri = Primary Auditory Cortex (A1) — tonotopic (apex 500 Hz → base 8 kHz)
    • Wernicke’s area (posterior portion; dominant hemisphere) — language comprehension; lesion → receptive aphasia
  • Olfactory (piriform) cortex, amygdala, hippocampal formation (learning & memory)
  • Medial temporal structures linked to limbic system; lesion → explicit memory deficits

Occipital Lobe

  • Primary Visual Cortex (V1) around calcarine sulcus (cuneus above, lingual gyrus below)
    • Input: retina → optic nerve → optic chiasm → optic tract → LGN of thalamus → optic radiations → V1
    • Lesions: homonymous hemianopia, quadrantanopia

Insula

  • Hidden beneath opercula; Short & Long insular gyri
  • Primary Gustatory Cortex (taste): afferents via CN VII, IX, X → nucleus solitarius → thalamus → insula/parietal operculum
  • Viscerosensory integration

Limbic Lobe

  • Cingulate gyrus, parahippocampal gyrus, hippocampus, amygdala
  • Roles: emotion, motivation, learning, memory

Sensory Pathways & Cortices Summary

  • Vision: retina → LGN (thalamus) → occipital V1
  • Audition: cochlea → MGN (thalamus) → temporal A1
  • Somatosensation: peripheral receptors → spinal/brainstem pathways → VPL/VPM (thalamus) → parietal S1
  • Olfaction: nasal epithelium → olfactory bulb → piriform cortex (bypasses thalamus!)
  • Gustation: CN VII/IX/X → solitary nucleus → VPM (thalamus) → insular/parietal operculum

Hemispheric Lateralisation

  • Most (~90%) individuals: Left hemisphere = dominant (speech, language, calculation, logic, writing)
  • Right hemisphere = spatial perception, music, holistic processing, recognition of faces/objects by multi-sensory cues
  • Language centres are distinct from primary motor or auditory areas despite proximity
  • All sensory & motor pathways (except olfactory) decussate; results in contralateral representation

Thalamus: Macro-anatomy & Position

  • Largest (≈80 %) component of diencephalon; egg-shaped bilateral masses flanking the 3rd ventricle
    • Anterior pole forms posterior wall of interventricular foramen (Monro)
    • Medial surface: forms lateral wall of 3rd ventricle; may fuse via interthalamic adhesion
    • Lateral surface: bounded by posterior limb of internal capsule (massive corticothalamic/thalamo-cortical traffic)
    • Superior surface: underlies fornix & lateral ventricle
    • Inferior surface: contiguous with hypothalamus

Internal Organisation

  • Internal medullary lamina (Y-shaped) divides thalamus into three nuclear groups
    1. Anterior group — limbic relay (emotion, recent memory)
    2. Medial (mediodorsal) group — mood, affect, cognition
    3. Lateral group — specific relay nuclei (sensory/motor)
    • Ventral anterior (VA) & ventral lateral (VL): motor (basal ganglia & cerebellum → motor cortex)
    • Ventral posterolateral (VPL): somatic from body
    • Ventral posteromedial (VPM): somatic from face & gustatory
    • Lateral geniculate nucleus (LGN): vision
    • Medial geniculate nucleus (MGN): audition
  • Intralaminar & reticular nuclei (arousal, pain, thalamo-cortical modulation)

Functional Scheme

sense modality→specific thalamic nucleus  ⟷  modality-specific cortex\text{sense modality} \rightarrow \text{specific thalamic nucleus} \; \longleftrightarrow \; \text{modality-specific cortex}

  • Connections are reciprocal; cortex sends regulatory feedback to thalamus (modulates gating of information)

Key Relationships on Sections

  • Sagittal: thalamus inferior to corpus callosum, posterior to IVF, superior to hypothalamus, anterior to pineal
  • Coronal (through posterior limb of IC): thalamus medial to internal capsule, lateral to 3rd ventricle
  • Horizontal: thalamus medial to IC, posterior to caudate head

Clinical & Age-Related Notes

  • Case study: individual born without temporal lobe demonstrates cortical plasticity but specific deficits (Tuckte et al., 2022)
  • White-matter disease increases with age; loss of inter-area communication contributes to cognitive decline
  • Lesions
    • Internal capsule: dense motor/sensory deficits due to concentrated fibres
    • Thalamic stroke: contralateral sensory loss +/- pain, arousal & memory problems owing to neighbouring limbic structures
    • Prefrontal damage: abulia, impaired executive control
    • Unilateral temporal (dominant) damage: aphasia; non-dominant: prosody & spatial neglect

Ethical & Practical Considerations

  • Understanding lateralisation guides neurosurgical planning & stroke rehabilitation
  • Ageing white-matter degeneration emphasises lifestyle & vascular risk modification
  • Cortical specialisation illustrates evolutionary optimisation yet vulnerability (small focal lesions → profound deficits)

Key Numbers & Equations

  • Corpus callosum axons >! 200\,\text{million}
  • Tonotopic mapping frequency range 500 Hz→8000 Hz500\,\text{Hz} \rightarrow 8000\,\text{Hz} along Heschl’s gyrus
  • Homuncular representation reflects inverse size relation to receptive field size: Cortical area∝1Receptor field size\text{Cortical area} \propto \dfrac{1}{\text{Receptor field size}} (conceptual)

Study Tips & Connections

  • Re-draw homunculi and internal capsule cross-sections to cement somatotopy
  • Map each sensory modality to its thalamic nucleus & cortical location (repeat as flashcards)
  • Compare Broca vs Wernicke aphasias clinically
  • Use MRI/CT atlases to identify corpus callosum, internal capsule, thalamus in all planes
  • Link limbic structures (hippocampus – memory; amygdala – emotion) to prefrontal decision loops for integrative understanding