Brain Lobes, Blood Supply, and Stroke Basics

From Sound Recognition to Human Speech

• Evolutionary sequence
  • Early animals (e.g., sheep, raccoon) first had to interpret sounds before they ever produced meaningful ones.
  • Example: Sheep must distinguish the sound of a predator (e.g.
    coyote) from that of prey (e.g.
    mouse).
  • Once brains realized sound could carry meaning, production of meaningful sound (speech) evolved.
• Take-home method for the course
  • Treat every topic as “How would a brain / kidney / etc. think?” rather than memorizing trivia.
  • If you understand function + logic, you can predict answers even with new diagrams on the exam.

External Landmarks: Four Cerebral Lobes & Cerebellum

• Frontal Lobe
  • Voluntary movement (primary motor cortex).
  • Language expression (Broca’s area).
  • Higher executive tasks: planning, organizing, complex math.
• Parietal Lobe
  • Somatosensation ➔ specifically touch (much richer than only “pressure”).
• Temporal Lobe — the “junk drawer”
  • Audition (hearing).
  • Language comprehension (Wernicke’s area).
  • Taste & smell.
  • Face recognition (prosopagnosia results if damaged).
  • Place/scene recognition: instantly knowing you’re in the anatomy lab vs. your bedroom.
• Occipital Lobe
  • Vision.
• Cerebellum (not cortical)
  • Balance, equilibrium, coordination.

Internal Anatomy Revealed on Mid-Sagittal Cut

• Diencephalon (core / “dead center”)
  • Thalamus — hub of conscious sensory relay (“makes sense of senses”).
  • Hypothalamus — immediately under thalamus; autonomics, hormones, temp, hunger, etc.
  • Subthalamus — basal ganglia components (motor modulation).
  • Epithalamus — surrounds thalamus; contains pineal gland → melatonin → sleep.
• Brain-Stem Components
  • Midbrain, Pons, Medulla Oblongata (ascending ↔ descending tracts + basic life functions).

Vascular Supply & the Circle of Willis

• Arterial path: Heart → \text{aorta} → common carotid arteries (palpable pulse) → base of skull.
• Traffic-circle analogy (NJ style)
  • Left & right carotids empty into a circular anastomosis so one side can feed both hemispheres if an input fails.
  • Eponym: Circle of Willis (Dr Willis).
• Main cerebral exit ramps
  1. Middle Cerebral Artery (MCA) — largest lumen; supplies broad lateral cortex.
  2. Anterior Cerebral Artery (ACA) — medial frontal-parietal surface.
  3. Posterior Cerebral Artery (PCA) — occipital + inferior temporal.
  4. Anterior Choroidal Artery (AChA) — smallest; tiny deep territory.
• Hemodynamic rule
  • Embolus entering the circle follows the path of least resistance → MCA most commonly occluded (largest opening).

Stroke Pathophysiology

• Two fundamental mechanisms
  1. Ischemic stroke — vessel blocked by clot.
  2. Hemorrhagic stroke — vessel ruptures; blood leaks & compresses tissue.
• Common denominator: local brain region deprived of adequate blood.

Functional Territories vs. Stroke Signs

• Motor Homunculus (pre-central gyrus)
  • Mouth/face = inferior part.
  • Hand/arm = mid-lateral.
  • Leg = medial/intra-hemispheric.
• Typical MCA (lateral) ischemia → contralateral
  • Slurred or aphasic speech (Broca + Wernicke zones in MCA area).
  • Facial droop: lower facial motor fibers in lateral motor strip.
  • Arm/hand weakness > leg weakness (leg sits in ACA zone).
• ACA stroke ➔ leg ≥ arm weakness ± personality change (frontal medial).
• PCA stroke ➔ visual field deficits, face/place recognition issues.
• Cerebellar stroke
  • Ataxia, intention tremor, dysmetria (misses target), “drunk” gait, vertigo, nausea.
• Hemorrhagic presentation
  • Diffuse ↑ICP signs, bilateral deficits possible, sudden severe headache.

Classic Clinical Presentation Checklist (FAST + more)

• Face droop.
• Arm weakness/“arm drift” on pronator test.
• Speech: slurred, aphasia.
• Time: urgent; tissue dies ~ 1\,\text{min} ≈ 1.9\,\text{million} neurons.
• Extras mentioned:
  • Balance issues / dizziness.
  • Sudden severe headache.
  • Nausea/vomiting (esp. cerebellar/hemorrhagic).

Laterality Rules

• Each cerebral hemisphere controls the contralateral body side.
• Therefore right-sided deficits → left hemisphere lesion and vice-versa.

Sample Exam-Style Reasoning (mentioned in class)

• Image with numbered dots: placing a dot between motor hand & leg zones narrows answer choices.
• MCA lesion = unilateral facial droop + arm drift but spared leg.
• Bilateral leg deficit suggests medial (ACA) or diffuse bleed.
• Pronator drift test: patient closes eyes, arms extended supinated; weak arm pronates + falls.

Practical / Philosophical Implications

• Evolution built redundancy (Circle of Willis) to mitigate single-artery loss.
• Understanding functional logic removes need for rote memorization.
• Ethical urgency: recognizing signs quickly saves cortex before irreversible damage.

Key Terms & Quick Definitions

• Somatosensation – collective term for diverse touch modalities.
• Prosopagnosia – inability to recognize faces (temporal damage).
• Contralateral – opposite side control.
• Ischemia vs. Hemorrhage – block vs. bleed.
• Homunculus – distorted body map on cortex.
• Bilateral – affecting both sides of body.
• Ataxia/Dysmetria – cerebellar coordination failures.

Mnemonics & Memory Hooks

• “Junk Drawer Temporal Lobe” – holds sensory odds & ends.
• “Traffic Circle of Willis” – any closed lane still routes cars to all exits.
• FAST – Face, Arm, Speech, Time.

Quick Formulas / Numbers

• Neuronal loss estimate: \approx 1.9\,\text{million neurons min}^{-1} during untreated stroke.
• Average conscious awareness relay time via thalamus: fractions of a second (qualitative, not given numerically).