Functional Areas of the Brain

1. Motor Areas (Frontal Lobe)

Primary Motor Cortex - Brodmann Area 4

• Location: Precentral gyrus, just anterior to the central (Rolandic) sulcus
• Function: Voluntary movement control; sends long fibers down the corticospinal tract to spinal cord motor neurons
• Key feature: Somatotopically organized - the motor homunculus maps body parts to cortical regions, with the face and hands having disproportionately large representations

Premotor Cortex - Brodmann Area 6

• Location: Lateral surface, just anterior to primary motor cortex
• Function: Higher-order planning and preparation for limb movements; guides movements based on sensory cues

Supplementary Motor Area (SMA) - Brodmann Area 6 (medial)

• Location: Medial surface of the frontal lobe
• Function: Planning and sequencing of complex voluntary movements; bilateral coordination

Frontal Eye Fields - Brodmann Area 8

• Location: Middle and superior frontal gyri
• Function: Voluntary saccadic (rapid) eye movements

Broca's Area - Brodmann Areas 44 & 45

• Location: Posterior part of the inferior frontal gyrus (triangular and opercular portions), left hemisphere in most right-handers
• Function: Speech production (expressive language); damage causes Broca's aphasia - non-fluent speech with intact comprehension

2. Somatosensory Areas (Parietal Lobe)

Primary Somatosensory Cortex - Brodmann Areas 3, 1, 2

• Location: Postcentral gyrus, immediately posterior to the central sulcus
• Function: Receives and processes touch, pain, temperature, proprioception from the contralateral body
• Key feature: The sensory homunculus maps body surface onto the cortex; hands, lips, and tongue have the largest representations

Secondary Somatosensory Cortex

• Location: Parietal operculum (lower end of postcentral gyrus, abutting the Sylvian fissure)
• Function: Further processing of touch and pain; integrates input from both sides of the body

Posterior Parietal Cortex - Brodmann Areas 5 & 7

• Location: Superior parietal lobule
• Function: Visuomotor integration, spatial perception, stereognosis (recognizing objects by touch); damage causes apraxia and spatial neglect

Inferior Parietal Lobule - Brodmann Areas 39 & 40

• Supramarginal gyrus (Area 40): Reading, phonological processing
• Angular gyrus (Area 39): Language comprehension, reading, writing, arithmetic
• Wernicke's Area (posterior Area 22 + Area 39/40): Language comprehension; damage causes Wernicke's aphasia - fluent but meaningless speech

3. Visual Areas (Occipital Lobe)

• Dorsal stream ("where/how"): V1 → parietal cortex - spatial location and visually-guided movement
• Ventral stream ("what"): V1 → inferotemporal cortex - object and face recognition

4. Auditory Areas (Temporal Lobe)

5. Other Specific Sensory Areas

6. Association Areas

Prefrontal Cortex - Brodmann Areas 9, 10, 11, 12, 46

• Function: Working memory, executive function, decision-making, personality, abstract thinking, impulse control; damage causes frontal lobe syndrome (disinhibition, poor planning)
• Dorsolateral prefrontal cortex (DLPFC): Working memory and planning
• Orbitofrontal cortex: Reward processing and social behavior

Posterior Parietal - Temporal - Occipital (PTO) Association Cortex - Area 37, 39, 40

• Multimodal integration of vision, touch, and spatial information; critical for reading and calculation

Inferotemporal Cortex - Areas 20, 21, 37

• Function: Complex object recognition, face recognition (fusiform face area), form vision

Limbic Association Cortex - Areas 23-32, cingulate gyrus

• Function: Emotion processing, memory consolidation, motivation; forms part of the Papez circuit

7. The Sensory & Motor Homunculi

Summary Table: Brodmann Areas Quick Reference

Wallenberg Syndrome (Lateral Medullary Syndrome)

Anatomy of the Lateral Medulla

Etiology

• Large vessel atherosclerosis (vertebral or PICA stenosis/occlusion): ~50%
• Vertebral artery dissection: ~15% - especially in younger patients, often preceded by neck pain or recent neck manipulation
• Small vessel disease: ~13%
• Cardioembolism: ~5%

Classic Clinical Features

1. Sensory

• Ipsilateral facial hypalgesia and thermoanesthesia - loss of pain and temperature on the same side face (spinal nucleus/tract of CN V)
• Contralateral body hypalgesia and thermoanesthesia - loss of pain and temperature in the trunk and limbs on the opposite side (spinothalamic tract)
• Vibration, proprioception, and touch are preserved (medial lemniscus is in medial medulla)

2. Motor/Bulbar

• Dysphagia - difficulty swallowing
• Dysarthria/dysphonia/hoarseness - weakness of palate, pharynx, vocal cord
• Ipsilateral palatal, pharyngeal, and vocal cord paralysis (nucleus ambiguus involvement - CN IX, X)
• No limb weakness (pyramids are spared)

3. Sympathetic

• Ipsilateral Horner's syndrome - ptosis, miosis, anhidrosis (descending hypothalamospinal sympathetic fibers)

4. Vestibular/Cerebellar

• Vertigo, nausea, vomiting (vestibular nuclei)
• Nystagmus (horizontal, torsional, or mixed)
• Ipsilateral cerebellar ataxia - limb ataxia, gait ataxia, falling to the side of the lesion (inferior cerebellar peduncle)
• Lateropulsion - strong tendency to fall/veer toward the side of the lesion

5. Ocular

• Skew deviation and ocular tilt reaction
• Multiple types of nystagmus (horizontal, torsional, mixed)
• Gaze-holding abnormalities
• Smooth pursuit defects

The Triad

Rare/Additional Features

1. Wild arm ataxia (lateral cuneate nucleus involvement)
2. Central neuropathic pain with allodynia
3. Loss of taste (nucleus tractus solitarius - rostral/lateral zone)
4. Inability to sneeze (sneezing center in spinal trigeminal nucleus)
5. Autonomic dysfunction - tachycardia, blood pressure lability
6. Ondine's curse (central hypoventilation syndrome) - failure of automatic breathing; life-threatening if not recognized; due to lesions of nucleus ambiguus and adjacent reticular formation
7. Contralateral hyperhidrosis with ipsilateral anhidrosis (sympathetic disruption)
8. Opalski syndrome - Wallenberg + ipsilateral hemiparesis (lesion extends to pyramidal decussation at the caudal medulla)

Sensory Pattern Variants

• Far lateral lesion: Ipsilateral face + contralateral foot/leg only (selective sacral/lumbar spinothalamic fibers hit, which are located most laterally)
• Mediolateral lesion: Ipsilateral face + contralateral entire hemibody (unilateral pattern) - ventral trigeminothalamic tract also affected
• Rostral medullary lesion: More severe dysphagia, hoarseness, facial paresis
• Caudal medullary lesion: More marked vertigo, nystagmus, gait ataxia

Diagnosis

Imaging

• MRI with diffusion-weighted imaging (DWI) is the investigation of choice - confirms the infarct in the lateral medulla
  • DWI positive from day 1 (earliest indicator)
  • T2-weighted MRI may not show the infarct initially; becomes visible after several days
• MRA (MR Angiography) - shows loss of flow in the vertebral artery or PICA; identifies arterial dissection
• T1-weighted axial MRI through the vertebral arteries - bright intramural signal (crescent sign) indicates vertebral dissection

Treatment

Prognosis

• Wallenberg syndrome is the most common posterior circulation stroke syndrome, accounting for ~2-3% of all ischemic strokes
• It is the most prevalent syndrome related to intracranial vertebral artery occlusion

Memory Aid

Ipsilateral side (same as lesion): Horner's + facial pain/temp loss + cerebellar ataxia + bulbar palsy (dysphagia, hoarseness) Contralateral side (opposite): body pain/temp loss

Circle of Willis - Complete Anatomy Guide

Overview

A complete, full-caliber Circle of Willis is present in only ~34% of individuals - anatomical variants are the rule, not the exception.

Formation - From Aorta to Circle

Anterior Circulation (Internal Carotid System)

• Aorta → brachiocephalic artery (right) / direct (left) → common carotid arteries
• Common carotid bifurcates → internal carotid artery (ICA) + external carotid artery
• ICA ascends through carotid canal → cavernous sinus (carotid siphon S-bend) → pierces dura → supraclinoid segment
• ICA terminal branches (mnemonic OPAAM):
  • Ophthalmic artery
  • Posterior communicating artery (PComm)
  • Anterior choroidal artery
  • Anterior cerebral artery (ACA)
  • Middle cerebral artery (MCA)

Posterior Circulation (Vertebrobasilar System)

• Subclavian arteries → vertebral arteries (ascend through foramina transversaria of C6-C1)
• Vertebral arteries enter foramen magnum → unite at pontomedullary junction to form basilar artery
• Basilar artery ascends along ventral pons → terminates at midbrain by bifurcating into posterior cerebral arteries (PCA)

Components of the Circle

The MCA does NOT form part of the circle - it is a terminal branch of the ICA that exits laterally.

Architecture: Anterior vs. Posterior

           AComm
     ACA ——|—— ACA
      |              |
     ICA            ICA
      |              |
    PComm          PComm
      |              |
     PCA ——basilar—— PCA
           (top of basilar)

Anterior part of the circle:

• Right ACA (A1) + AComm + Left ACA (A1)

Posterior part of the circle:

• Right PCA (P1) + Right PComm → Right ICA ← Left ICA ← Left PComm + Left PCA (P1)

The Three Main Cerebral Arteries and Their Territories

Anterior Cerebral Artery (ACA)

• Passes forward and medially, then curves up and over the corpus callosum in the interhemispheric fissure
• Major branches: pericallosal and callosomarginal arteries
• Territory: Medial surface of frontal and parietal lobes - supplies the leg/foot area of the motor and sensory homunculi
• Occlusion: Contralateral leg weakness/sensory loss, urinary incontinence, abulia (if bilateral)

Middle Cerebral Artery (MCA)

• Turns laterally into the Sylvian fissure; divides into superior and inferior divisions
• Superior division: Cortex above Sylvian fissure - lateral frontal lobe, peri-Rolandic (face, arm motor/sensory)
• Inferior division: Cortex below Sylvian fissure - lateral temporal, parietal
• Territory: Most of the dorsolateral convexity - the largest territory of any cerebral artery
• Occlusion: Contralateral hemiplegia/hemisensory loss (face + arm > leg), homonymous hemianopia, aphasia (dominant hemisphere), neglect (non-dominant)

Posterior Cerebral Artery (PCA)

• Curves back from top of basilar, sweeps around the midbrain
• Territory: Inferior and medial temporal lobe, medial occipital lobe (visual cortex)
• Occlusion: Contralateral homonymous hemianopia (with macular sparing), visual agnosia, alexia without agraphia (left PCA), memory deficits (hippocampus)

Penetrating (Deep) Vessels Arising Near the Circle

High-yield: The lenticulostriate arteries are "end arteries" with no collateral supply - they are especially prone to hypertensive damage (lacunar infarcts + hypertensive hemorrhage).

Branches of the Basilar Artery (Posterior Circulation)

Clinical Significance - High-Yield for Exams

1. Berry (Saccular) Aneurysms

2. Collateral Flow

3. CN III Compression

4. Anterior Choroidal Artery Occlusion

Memory Mnemonics

Circle of Willis — Concise

What It Is

9 Vessels That Form the Circle

MCA does NOT form part of the circle - it exits laterally.

Simple Diagram

        AComm
  ACA ——+—— ACA
   |              |
  ICA            ICA
   |              |
 PComm         PComm
   |              |
  PCA —basilar— PCA

ICA Branches Mnemonic — OPAAM

3 Main Cerebral Artery Territories

Deep Penetrating Arteries (High-Yield)

Basilar Artery Branches

Berry Aneurysm Sites (Most → Least Common)

1. AComm (~35%) - most common
2. PComm (~30%) - compresses CN III (blown pupil)
3. MCA bifurcation (~20%)
4. Basilar tip (~5%)