Cerebral Cortex Structure, Function & Memory

Anatomical Overview

• Cerebral cortex = outermost gray matter of cerebrum; only ≈ 3\,\text{mm} thick, yet houses millions of neurons.
• True seat of the “conscious mind”. Responsible for:
  • Personality & self-awareness
  • Sensory perception
  • Voluntary motor initiation
  • Communication (spoken & written)
  • Memory storage & retrieval
• Cortex is highly folded:
  • Hills = gyri; Valleys = sulci.
  • Folding triples surface area → more room for neurons → greater processing power.

Gross Lobes & Landmarks

• Each hemisphere contains five lobes (named by overlying skull bones, except insula):
  • Frontal
  • Parietal
  • Temporal
  • Occipital
  • Insula (deep to frontal/parietal/temporal junction)
• Major surface grooves
  • Longitudinal fissure – separates left vs right hemispheres.
  • Central sulcus – divides frontal from parietal lobes (Rolandic fissure).
  • Additional sulci mark boundaries of temporal & occipital lobes.

Microscopic vs Macroscopic Architecture

• Gray matter (neuron bodies, dendrites) sits superficially; white matter (myelinated axons) lies deep.
• Different regions specialize for different modal functions but operate holistically—complete cognition requires cooperation of all areas.

Functional Categories of Cortical Areas

1. Sensory Areas – receive raw input (“IN” doors).
2. Association Areas – interpret & give meaning by linking to memory.
3. Motor Areas – plan & issue commands (“OUT” doors).

1. Primary Sensory Cortices (first stop)

• Provide conscious awareness of simple qualities only.
  • \text{Primary somatosensory cortex} – post-central gyrus; “I feel heat”.
  • \text{Primary visual cortex} – occipital lobe; “I see something orange and moving”.
  • Similar primary areas exist for auditory, gustatory, vestibular, and visceral senses.
• Damage → loss of ability to detect that modality in the corresponding body region or field.

Somatotopic (“Homunculus”) Map

• Post-central gyrus contains an ordered body map.
  • Medial → genitals, leg; lateral → face, tongue, intra-abdominal, etc.
• Size of cortical territory ∝ sensitivity (e.g., fingers > trunk).

2. Unimodal Sensory Association Cortices

• Immediately adjacent to each primary area; supply recognition.
  • Somatosensory association cortex: “Yikes, FIRE!” (heat + pain recognized as burning object)
  • Visual association cortex: combines edges, color, movement → “flame”.

3. Multimodal Association Areas

• Integrate multiple senses + memories → produce understanding, prediction, emotion.
• Spread over large regions (especially parietal & temporal). Example functions:
  • Assign meaning: “Fire is dangerous!”
  • Wernicke’s area (temporal/parietal) – interprets spoken & written language (“How many plants did she say?”)

4. Pre-frontal Cortex

• Seat of personality, motivation, abstract reasoning, social judgment, planning.
• Internal dialog in fire scenario: “I’d better call 9-1-1.”

5. Pre-motor Cortex (Secondary Motor)

• Formulates action plans & sequences; rehearses skilled patterns.
• Decides: “Drop to floor and crawl toward exit.”

6. Primary Motor Cortex

• Pre-central gyrus; issues final commands via corticospinal tract → skeletal muscles.
• Somatotopic motor homunculus mirrors sensory map.
• Also contains Broca’s area (dominant hemisphere, frontal lobe): converts thought to grammatically correct motor speech patterns – e.g., afterwards: “Dang, that was scary!”

Flow of Information – Fire Example (Narrative Demonstration)

1. Heat reaches skin → primary somatosensory cortex.
2. Somatosensory association cortex recognizes threat.
3. Multimodal association areas + Wernicke’s integrate with memory → understand danger.
4. Pre-frontal cortex sets goal: survive → call help.
5. Pre-motor cortex designs movement plan.
6. Primary motor cortex activates muscles & Broca’s area vocalizes.
   → Illustrates sequential yet overlapping cortical processing.

Hemispheric Lateralization

• Each hemisphere:
  • Receives sensory input from contralateral body side.
  • Sends motor output to contralateral side.
• Specialized (but not exclusive) higher functions:
  • LEFT (typical right-handed): Analytic thought, logic, language, science, math.
  • RIGHT: Intuition, creativity, spatial/artistic skills, music.
• Not fixed: left-handed individuals often reverse language dominance; brain can access skills wherever stored.

Interhemispheric Communication

• Corpus callosum = large commissural tract linking hemispheres; allows instant sharing (“walking to kitchen for spoon, bathroom for aspirin”).
• Myth-busting: we all use 100 % of both sides in daily life.

Language Centers – Clinical Correlate

• Wernicke’s area (temporal): comprehension of auditory + written words.
  • Lesion → receptive aphasia → fluent but meaningless speech & inability to understand.
• Broca’s area (frontal, premotor): grammar & motor programming of speech.
  • Lesion → expressive (non-fluent) aphasia → understands language but struggles to form words.

Memory Systems

• Memory = storage + retrieval of information.

Temporal Stages

1. Short-Term Memory (STM) – “working memory”
   • Holds limited items (≈7 ± 2) for seconds–minutes.
   • Example: phone number before writing it down.
   • Location: cortical circuits with rapid synaptic activity.
2. Long-Term Memory (LTM) – essentially limitless \left(\text{capacity} \rightarrow \infty\right) after consolidation.
   • Repetition converts STM → LTM (e.g., friend’s number after many calls).

Information Flow Diagram

Outside Stimuli → Sensory Receptors → STM (very temporary) → (Selection for recall + Influences: Excitement, Rehearsal, Association) → LTM (permanent)
                             ↓
                        Data lost / unretrievable

Factors Enhancing Encoding – “AMP UP” Formula

• Arousal – emotional interest boosts norepinephrine release.
• Association – link new info to existing knowledge.
• Motivation – personal relevance/goal (career requirement).
• Practice – spaced repetition, novel methods keep cortex active.
• WARNING: “Going through the motions” ≠ studying; the brain cannot be tricked.

Categories of Long-Term Memory & Disease Links

• Declarative (Explicit) Memory
  • Facts, concepts, events.
  • Neural substrate: hippocampus; neurotransmitter: ACh.
  • Selectively lost in Alzheimer’s disease (degeneration of hippocampus & cholinergic neurons).
• Procedural (Implicit) Memory
  • Skills acquired by doing (instrument playing, ball throwing).
  • Neural substrate: cerebellum & basal nuclei; neurotransmitter: Dopamine.
  • Degrades in Parkinson’s disease (dopaminergic neuron loss in substantia nigra).

Energetics & Plasticity of Learning

• Learning is energy-intensive:
  • Requires increased ATP for:
  • Growth of new dendrites & axonal branches (structural plasticity).
  • Up-regulation of neurotransmitter synthesis & release.
• Fatigue after study sessions is physiological, not merely psychological.

Clinical & Practical Takeaways

• Damage to a localized cortical area → loss of its specific function (e.g., inability to feel a limb, speak fluently, recognize faces).
• However, due to plasticity & inter-connectedness, other regions can sometimes compensate over time.
• Comprehensive understanding of cortical organization aids diagnosis, rehabilitation, and optimizing study strategies.