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Here are your exam-oriented notes on the Cerebral Hemisphere:
CEREBRAL HEMISPHERE - EXAM-ORIENTED NOTES
1. INTRODUCTION
- Cerebrum = largest part of the brain; fills most of the cranial cavity above floors of anterior & middle cranial fossae and above the tentorium cerebelli.
- Heavily convoluted, bilobed structure.
- Divided into 2 cerebral hemispheres by the longitudinal cerebral fissure (deep median cleft, but incomplete separation).
Each hemisphere has 4 components (HIGH-YIELD):
| Component | Description |
|---|---|
| Cerebral cortex | Outer layer of grey matter |
| White matter | Inner mass |
| Basal ganglia / nuclei | Large grey matter masses embedded in white matter (basal part) |
| Lateral ventricle | Cavity within the hemisphere |
2. EXTERNAL FEATURES
A. POLES (3)
| Pole | Location | Shape |
|---|---|---|
| Frontal pole | Anterior end | More rounded |
| Occipital pole | Posterior end | More pointed |
| Temporal pole | Anterior end of temporal lobe | Fits into anterior part of middle cranial fossa |
B. SURFACES (3)
| Surface | Description |
|---|---|
| Superolateral | Most convex & extensive; faces upward & laterally; conforms to cranial vault |
| Medial | Flat & vertical; shows thick C-shaped cut of corpus callosum |
| Inferior | Irregular; divided by stem of lateral sulcus into - (a) orbital surface (anterior, small) and (b) tentorial surface (posterior, large) |
C. BORDERS (6)
- Superomedial - separates superolateral from medial surface
- Superciliary - junction of superolateral and orbital surfaces
- Inferolateral - between superolateral and tentorial surfaces
- Medial occipital - between medial and tentorial surfaces
- Medial orbital - between medial and orbital surfaces
- Occipital - between two portions of inferior surface
3. MAIN CEREBRAL SULCI
ON SUPEROLATERAL SURFACE
-
Central sulcus (Fissure of Rolando)
- Runs obliquely from the superomedial border downwards towards the lateral sulcus
- Separates frontal from parietal lobe
- Separates precentral gyrus (motor) from postcentral gyrus (sensory)
-
Lateral sulcus (Sylvian fissure)
- Most prominent sulcus on superolateral surface
- Has a stem + 3 rami: anterior horizontal, anterior ascending, posterior ramus
- Stem separates orbital surface from temporal lobe
- Posterior ramus separates temporal from parietal lobe
-
Precentral sulcus - anterior and parallel to central sulcus
-
Postcentral sulcus - posterior and parallel to central sulcus
-
Superior & Inferior frontal sulci - divide frontal lobe into superior, middle, and inferior frontal gyri
-
Intraparietal sulcus - divides parietal lobe into superior parietal lobule (above) and inferior parietal lobule (below)
- Inferior parietal lobule contains: Supramarginal gyrus (caps the upturned end of lateral sulcus) and Angular gyrus (caps the superior temporal sulcus)
ON MEDIAL SURFACE
- Cingulate sulcus - follows the curve of corpus callosum; separates the cingulate gyrus from medial frontal gyrus/paracentral lobule
- Callosal sulcus - separates cingulate gyrus from corpus callosum
- Calcarine sulcus - on posterior part of medial surface; contains the primary visual cortex (Area 17)
- Parieto-occipital sulcus - visible on both medial and superolateral surfaces; separates parietal from occipital lobe
- Suprasplenial sulcus - above splenium; separates precuneus from cingulate gyrus
Key medial surface landmarks:
- Paracentral lobule = area around upper part of central sulcus (between cingulate & superomedial border)
- Precuneus = quadrangular area between parieto-occipital sulcus and paracentral lobule
- Cuneus = triangular area between posterior calcarine sulcus and parieto-occipital sulcus
ON INFERIOR SURFACE
Orbital part:
- Olfactory sulcus - straight, anteroposterior; lodges olfactory bulb & tract; medial area = gyrus rectus
- Orbital sulcus - H-shaped; divides orbital surface into anterior, posterior, medial, lateral orbital gyri
Tentorial part:
- Collateral sulcus (medial) and Occipitotemporal sulcus (lateral) - both run anteroposteriorly
- Lingual gyrus = area between collateral and calcarine sulci (posteriorly)
- Parahippocampal gyrus = continuation of lingual gyrus anteriorly; its hooked anterior end = UNCUS
- Parahippocampal gyrus is continuous with cingulate gyrus via the isthmus posteriorly
4. LOBES OF THE CEREBRAL HEMISPHERE
The superolateral surface is divided by 3 main sulci + 2 imaginary lines into 4 lobes:
Sulci used: Central, Lateral, Parieto-occipital
Imaginary lines:
- 1st: Vertical line joining parieto-occipital sulcus to pre-occipital notch
- 2nd: Horizontal line from lateral sulcus to the 1st imaginary line
| Lobe | Boundaries |
|---|---|
| Frontal | Anterior to central sulcus, above lateral sulcus |
| Parietal | Behind central sulcus, above lateral sulcus, in front of 1st imaginary line |
| Temporal | Below lateral sulcus, in front of 1st imaginary line |
| Occipital | Behind both imaginary lines |
5. CEREBRAL CORTEX
- Made of grey matter - contains nerve cell bodies, dendrites, synapses
- Thickness: 1.5 - 4.5 mm
- Total area: ~2200 cm²
- Contains ~10 billion neurons
Types of cortex:
- Isocortex (Neocortex) - 6-layered; makes up ~90% of human cortex
- Allocortex - less than 6 layers (includes paleocortex and archicortex)
6 layers of neocortex (superficial to deep):
| Layer | Name | Key contents |
|---|---|---|
| I | Molecular (plexiform) | Few neurons, mostly dendrites & axons |
| II | External granular | Small pyramidal & stellate cells |
| III | External pyramidal | Medium-sized pyramidal cells |
| IV | Internal granular | Stellate (granule) cells; receives thalamocortical input |
| V | Internal pyramidal (ganglionic) | Large pyramidal cells; Betz cells in motor cortex |
| VI | Multiform (fusiform/polymorphic) | Fusiform cells; projects to thalamus |
Types of cortex based on layers:
- Agranular cortex - prominent layers III & V (pyramidal layers), absent IV; found in motor areas
- Granular cortex (Koniocortex) - prominent layer IV; found in sensory areas
6. FUNCTIONAL AREAS (Brodmann's Areas)
MOTOR AREAS
| Area | Brodmann No. | Location | Function |
|---|---|---|---|
| Primary motor cortex | Area 4 | Precentral gyrus | Voluntary movement; contains Betz cells |
| Premotor cortex | Area 6 | Anterior to precentral gyrus | Planning & coordinating movements |
| Supplementary motor area | Part of Area 6 | Medial surface, paracentral lobule | Bilateral coordinated movements |
| Frontal eye field | Area 8 | Posterior part of middle frontal gyrus | Voluntary eye movements (contralateral) |
| Broca's area | Areas 44 & 45 | Inferior frontal gyrus (dominant hemisphere) | Motor speech (expressive) |
Motor homunculus: Inverted representation - foot/leg at top (paracentral lobule), head at bottom; hand & face occupy largest areas.
SENSORY AREAS
| Area | Brodmann No. | Location | Function |
|---|---|---|---|
| Primary somatosensory cortex | Areas 3, 1, 2 | Postcentral gyrus | Conscious somatic sensation from contralateral body |
| Primary visual cortex | Area 17 | Banks of calcarine sulcus | Vision; shows stria of Gennari |
| Visual association cortex | Areas 18, 19 | Surrounding Area 17 | Color discrimination, object recognition |
| Primary auditory cortex | Areas 41, 42 | Transverse temporal gyri (Heschl's gyri) in lateral sulcus | Hearing |
| Auditory association cortex | Area 22 | Superior temporal gyrus | Sound interpretation |
| Wernicke's area | Area 22 (posterior part) | Posterior superior temporal gyrus (dominant hemisphere) | Comprehension of speech |
| Primary olfactory cortex | Area 28 | Piriform cortex / uncus / amygdala | Olfaction (no thalamic relay) |
| Gustatory cortex | Area 43 | Inferior postcentral gyrus | Taste |
Sensory homunculus: Similar inverted representation as motor; largest areas = lips, tongue, face, hand.
OTHER IMPORTANT AREAS
| Area | Location | Function |
|---|---|---|
| Prefrontal cortex | Anterior to premotor area | Personality, judgment, planning, working memory |
| Parietal association cortex | Superior & inferior parietal lobules | Spatial awareness, body image |
| Angular gyrus (Area 39) | Inferior parietal lobule | Reading, writing, calculation |
| Supramarginal gyrus (Area 40) | Inferior parietal lobule | Language, praxis |
7. WHITE MATTER OF THE CEREBRUM
Types of fibres:
A. PROJECTION FIBRES
- Connect cortex to subcortical structures (thalamus, brainstem, spinal cord)
- Form the internal capsule and corona radiata
- Internal capsule has: anterior limb, genu, posterior limb, retrolenticular, sublenticular parts
| Part of Internal Capsule | Fibres carried |
|---|---|
| Anterior limb | Frontopontine fibres, anterior thalamic radiation |
| Genu | Corticobulbar (corticonuclear) fibres |
| Posterior limb | Corticospinal fibres (anterior 2/3), thalamocortical (posterior 1/3) |
| Retrolenticular | Optic radiation (posterior), parieto-occipito-pontine fibres |
| Sublenticular | Auditory radiation, temporopontine fibres |
B. COMMISSURAL FIBRES
- Connect the two hemispheres
| Structure | Connection |
|---|---|
| Corpus callosum | Largest commissure; connects corresponding areas of both hemispheres |
| Anterior commissure | Olfactory regions + temporal lobes |
| Posterior commissure | Pretectal nuclei |
| Hippocampal commissure (fornical) | Hippocampal formations |
| Habenular commissure | Habenular nuclei |
Parts of corpus callosum (anterior to posterior):
Rostrum → Genu → Trunk (body) → Splenium
- Genu = connects prefrontal areas (fibres form forceps minor)
- Splenium = connects occipital lobes (fibres form forceps major)
- Tapetum = fibres from trunk & splenium forming lateral wall of lateral ventricle
C. ASSOCIATION FIBRES
- Connect different cortical areas within the same hemisphere
| Fasciculus | Course | Connects |
|---|---|---|
| Superior longitudinal fasciculus | Lateral to corona radiata | Frontal - Parietal - Occipital - Temporal |
| Uncinate fasciculus | Hooks around lateral sulcus | Frontal pole to temporal pole |
| Inferior longitudinal fasciculus | Along lateral wall of occipital horn | Occipital to temporal lobe |
| Cingulum | Inside cingulate gyrus | Cingulate to parahippocampal gyrus (part of limbic system) |
| Arcuate fasciculus | Part of superior longitudinal | Broca's to Wernicke's area |
8. CLINICAL CORRELATIONS (HIGH-YIELD)
| Lesion | Area Damaged | Clinical Feature |
|---|---|---|
| Broca's aphasia (Expressive) | Area 44/45 (dominant) | Can understand speech, CANNOT speak fluently; non-fluent aphasia |
| Wernicke's aphasia (Receptive) | Area 22 posterior (dominant) | Fluent but meaningless speech; CANNOT understand; word salad |
| Conduction aphasia | Arcuate fasciculus | Can speak & understand, CANNOT repeat |
| Contralateral hemiplegia | Area 4 (motor cortex) or internal capsule | Weakness of opposite side of body |
| Contralateral hemianesthesia | Areas 3,1,2 (somatosensory) | Loss of sensation on opposite body |
| Homonymous hemianopia | Optic radiation / Area 17 | Loss of same half of visual field bilaterally |
| Alexia + agraphia + acalculia | Angular gyrus (Area 39) | Inability to read, write, calculate |
| Prosopagnosia | Visual association cortex (right hemisphere) | Inability to recognize faces |
| Apraxia | Parietal lobe | Inability to perform learned skilled movements despite intact motor/sensory function |
| Neglect syndrome | Non-dominant parietal lobe | Ignoring contralateral half of body/space |
| Upper motor neuron lesion signs | Corticospinal tract damage | Spasticity, hyperreflexia, +ve Babinski, no muscle wasting |
Internal Capsule Lesion:
A small lesion can produce:
- Contralateral hemiplegia (corticospinal fibres in posterior limb)
- Contralateral hemianesthesia (thalamocortical fibres)
- Contralateral homonymous hemianopia (optic radiation)
- Together = "Capsular syndrome"
9. QUICK REVISION TABLE - BRODMANN'S AREAS
| Area | Function |
|---|---|
| 1, 2, 3 | Primary somatosensory |
| 4 | Primary motor |
| 6 | Premotor + SMA |
| 8 | Frontal eye field |
| 17 | Primary visual |
| 18, 19 | Visual association |
| 22 | Auditory association / Wernicke's |
| 28 | Primary olfactory |
| 41, 42 | Primary auditory |
| 43 | Gustatory |
| 44, 45 | Broca's (motor speech) |
10. KEY MNEMONICS
-
Layers of cortex (I-VI): My God, Every Poor Intern Makes it = Molecular, (external) Granular, (external) Pyramidal, (internal) (granular), (internal) Pyramidal, Multiform
-
Parts of corpus callosum (A to P): Roughly Going Through Something = Rostrum, Genu, Trunk, Splenium
-
Internal capsule limbs - fibres at Genu: "Genu = General" (Genu carries corticobulbar/corticonuclear = fibres to cranial nerve nuclei)
-
Broca vs Wernicke: Broca = Broken speech (can't produce); Wernicke = Word salad (can't understand)
-
Dominant vs Non-dominant parietal:
- Dominant = Language functions (reading, writing - angular gyrus)
- Non-dominant = Spatial/visuospatial, neglect
Source: Cerebral Hemisphere lecture notes by Vysakh M Kumar, MPT Orthopaedics, KMCT CAHS
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