Wednesday, October 6, 2010

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

image

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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:

image

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

image

  • 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

image

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

image

  • 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

image

image

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

image

  • Discuss relevant clinical neuroanatomy

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