Organization of the Cerebral Cortex

Organization

• Cortex consists of
  • neurons
    • Types
      • Pyramidal (main)
      • Stellate
      • Fusiform
      • Horizontal
  • neuroglia
  • nerve fibres
  • blood vessels
• Cortex forms a gray matter
  • covers there areas
    • cerebral hemispheres
    • obscures diencephalone
    • upper brain stem
• Surface area of the brain has been increased multiple times to accommodate 10 billion neurons
  • Gyri
    • convolutions
  • Sulci
    • fissues
• Functional aspects
  • Receives sensory information from & send motor commands to opposite half of the body
    • Sensory right cerebral cortex –> left motor
      • left handed, right hemisphere dominant
      • non language functions
        • visuospatial processing
        • dressing sense
        • music appreciation
    • Sensory left cerebral cortex –> right motor
      • right handed, left hemisphere dominant
      • speech & understanding of spoken-written language
        • Wernicke’s area: understanding spoken-written language
        • Broca’s area: speech production
• Broadmann mapped 52 cortical areas, and found functional regions
  • PET scan
  • Functional MRIs
  • No functional area can act alone (interlapping)
    • consciousness
    • memory
    • language
  • Conscious behaviour involves entire cortex!
  • 3 types of functional areas
    • motor
    • sensory
      • act like vertical column?
    • association
      • piece together sensory inputs from previous experiences to produce meaningful perceptions
      • multiple association areas are linked together
        • enabling higher mental functions like learning, producing speech, solving math problems

Lobes of cerebral hemisphere

• Frontal lobes
  • control motor responses
  • pre-frontal part
    • higher mental functions
    • thinking, planning, problem-solving, decision making, stored memories
    • emergency decisions
• Parietal lobes
  • process sensory information
    • Touch
    • Pressure
    • Temperature
    • Body movements/ proprioception: senstation from muscle, joint, tendons
• Temporal lobes
  • process auditory information
  • only part where both sides are linked, unlike the others where sensory and motor are of separate and opposite.
    • if there are lesions on the left lobe or right lobe, both side hearing will be affected
• Occipital lobes
  • process visual sensory information
• Limbic lobe
  • hippocampus & amygdala
    • related to (short term) memory & emotional processing

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Fissures, Sulci & Gyri

2 fissures of the brain:

• Median Longitudinal fissure
  • Separates the left & right hemisphere of the cerebral cortex
  • falx cerebri (dural brain covering) lies within this fissure
• Transverse Cerebral fissure
  • Separates the cerebral hemispheres superiorly from the cerebellum, mid-brain and diencephalon inferiorly
  • Tentorium cerebri (dural brain covering) lies in this fissure

Sulcus:

Superolateral surface

• Central sulcus
  • Also known as Sulcus of Rolando
    • located about 1cm behind the midpoint between the frontal and occipital poles in the superior and medial margin
    • runs down and forward
  • separates frontal & parietal lobe
  • Separated from lateral sulcus by narrow bridge of tissue
• Lateral sulcus (DEEP)
  • separates (parietal & frontal lobe) from temporal lobe
  • Insula buried within this sulcus
• Parieto-occipital sulcus
  • separates parietal lobe & occipital lobe
• Intraparietal sulcus
  • divides parietal lobe into
    • superior parietal lobule
      • Somatosensory association area
        • Broadmann Area 5 & 7
      • Stereognosis
        • integrates different sensory modalities & enables to integrate size & shape of objects being felt along with past sensory experience
        • eyes closed, touch coin and can identify which coin it is
    • inferior parietal lobule

Medial surface:

• Parieto-occipital sulcus
  • prominent in medial surface
  • infront of it: precuneus
  • behind it: cuneus
  • corresponding parts of area 5 , 7
• Calcarine sulcus
  • runs from occipital lobe to reach parieto-occipital sulcus
  • in depth of it lie the primary visual area
    • area 17

Gyrus:

• Precentral gyrus
  • Primary Motor cortex
  • thicker than post central gyrus?
  • run parallel to central sulcus
  • most neurons control:
    • face, tongue, hands of opposite half of body
• Postcentral gyrus
  • Primary somatosensory cortex
  • amount of cortex devoted to particular region depends on sensitivity of region
    • most sensitive: face & fingertips
• Occipital gyrus
• Middle temporal gyrus
• Superior temporal gyrus
• Superior frontal gyrus

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Functional Areas of the Brain

The Broadmann functional areas

• 3 , 1 , 2
  • Primary somatosensory cortex
    • receives pain, touch, temperature senses, proprioceptive senses from muscles joints and opposite half of body
  • Location: Post-central gyrus
• 4
  • Primary motor cortex
    • controls the skillful voluntary movements of opposite half of body
  • Location: Pre-central gyrus
  • Large pyramidal cells give rise to Pyramidal tracts
• 1 , 7
• 5 , 7
  • Somatosensory association area
    • integrates different sensory modalities & enables to integrate size & shape of objects being felt along with past sensory experience (Stereognosis)
  • Location: Superior parietal lobule
• 6 , 8
  • Premotor area
    • receives input from thalamus, sensory areas & basal ganglia
    • programs motor activity
      • to be executed by area 4
• 17
  • Primary visual area
    • left cortex receives right half of visual field
    • vice versa
  • Location: wall of calcarine sulus & occipital pole
• 18 , 19
  • Secondary visual area
    • recognise & appreciate what is seen
• 22
  • Wernicke’s sensory speech area
    • enables understanding of spoken-written language
    • read a sentence, understand and speak it loudly
    • Lesion in this area:
      • Receptive aphasia
        • major impairment of language comprehension
  • Location: left dominant hemisphere on superior temporal gyrus
• 41 , 42
  • Primary auditory area
    • unilateral lesion produces partial deafness in BOTH ears
  • Secondary auditory area
    • interpretation of sound
  • Location: inferior wall of lateral sulcus (superior temporal sulcus)
• 44 , 45
  • Broca’s motor speech area
    • forms words of speech
      • by connection with parts of area 4
        • stimulating muscles of larynx, palate & tongue
    • Lesion in this area:
      • Motor aphasia
        • cant use right hand (left dominant area)
      • Expressive aphasia
        • In stroke
        • loss of ability to produce spoken-written language
  • Location: inferior frontal gyrus

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Corpus Callosum & Paracentral lobule

• Corpus callosum
  • largest commissure connecting corresponding areas of
    • right hemisphere
    • left hemisphere
  • connection between the hemispheres
  • Parts
    • Genu
    • Rostrum
    • Body/Midbrain
    • Splenium
• Paracentral lobule
  • continuation of precentral & postcentral gyrus
    • on medial surface
    • around tip of central sulcus
  • controls:
    • legs
    • defaecation
    • micturition reflex

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Histological structure of cerebral cortex

6 layers of the cerebral cortex:

• Molecular (plexiform) layer
  • apical dendrites of pyramidal cells
  • large no. of synapses happen here
• OUTER granular layer
  • stellate cells
• OUTER pyramidal cell layer
  • pyramidal cells smaller
• INNER granular layer
  • closely packed stellate cells
  • horizontal fibres (of Baillarger)
• INNER pyramidal cell later (ganglionic layer)
  • large pyramidal cells
    • particularly in motor area
  • inner fibres of Baillarger
• Multiform cell layer
  • fusiform cells
  • many nerve fibres entering white matter

Granular & Agranular cortex

• Granular cortex
  • Sensory area
    • area 3,1,2 & area
      5,7
  • Prominent in
    • outer granular layer
    • inner granular layer
• Agranular cortex
  • Motor area
  • Prominent in
    • Outer pyramidal cell layer
    • Inner pyramidal cell layer

Pyramidal Cells

• 2/3 of cortical neurons
• Giant BETZ cells
  • longer pyramidal cells
  • mostly in area 4
• Multiple dendrites from the apex and base
• Dendritic spines
  • synapse with axons/dendrites of other neurons
• Single axon starts from base
  • transverses deeper cortical structures & enters white matter as projection, association or commissural fibres

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White fibres of cerebral cortex

3 Types of white fibres

• Commissural fibres
  • Connect corresponding/identical regions of LEFT & RIGHT hemispheres
  • Found in:
    • Corpus callosum
      • Function: for learned discrimination, sensory experience and memory
      • Extensions:
        • forceps minor
        • forceps major
        • radiation
        • tapetum
      • Corpus callosum removed
        • can easily name objects presented to their right visual field
        • but deny awareness of objects presented to their left visual filed
          • but if given the word DOG, the patient can select a model of dog from other animals
    • Anterior commissure
    • Posterior commissure
    • Fornix
    • Habenular commissure
• Association fibres
  • Connect different regions of SAME hemisphere
  • Types:
    • Short association fibres
    • Long association fibres
      • Uncinate fasciculus
      • cingulum
        • lying within the white matter of the cingulate gyrus
      • superior longitudinal fasciculus
        • largest bundle of nerve fibres
        • connects the frontal lobe to (occipital & temporal lobe)
      • interior longitudinal fasciculus
        • runs anteriorly from the occipital lobe, passing lateral to the optic radiation, and it distributed to the temporal lobe
      • fronto-occipital fasciculus
        • connect the frontal lobe to the (occipital & temporal lobe)
• Projection fibres
  • The internal capsule
    • Interconnecting axons
      • Importance: high concentration of important nerve fibres within the capsule
        • even a small amount of haemorrhage can cause widespread effects on the contralateral side of the body
    • Ascending projection from the thalamus to the cortex & descending projections from the cortex to subcortical structures
      • thalamus
      • brain stem
      • spinal cord
    • V-shaped when cut coronally & horizontally
    • 5 parts of internal capsule
      • Genu
        • the bend in the V
        • contains corticospinal fibres
        • run between cortex & brainstem
      • Anterior limb
        • contains:
          • frontopontine
          • thalamocortical fibres
      • Posterior limb
        • contains corticospinal fibres & sensory fibres from the body
      • Retrolenticular part
        • contains fibres from the optic system
          • visual fibres
          • from the lateral geniculate nucleus of the thalamus
          • most posterioly, this becomes the optic radiation
      • Sublenticular part
        • contains fibres
          • from the medial geniculate nucleus of the talamus

• Discuss relevant clinical neuroanatomy