Cerebral Cortex Study Notes

I. BRAIN

A. PARTS OF THE BRAIN

  • Cerebrum: Largest part of the brain, includes the cerebral cortex.

  • Cerebellum: Coordinates voluntary movements such as posture, balance, coordination, and speech.

  • Brain Stem: Connects the cerebrum with the spinal cord, controls involuntary functions like breathing, heart rate, and blood pressure.

B. LOBES OF THE BRAIN

  • MAJOR lobes (4):   - Frontal Lobe: Responsible for reasoning, planning, movement, emotions, and problem solving.   - Parietal Lobe: Processes sensory information such as touch, temperature, and pain from the body.   - Occipital Lobe: Responsible for visual perception and processing.   - Temporal Lobe: Involved in processing auditory information, memory, and speech.

  • MINOR/Synthetic Lobes (2):   - Limbic Lobe:     - Cingulate Gyrus: Regulates emotions and pain, involved in emotional response.     - Parahippocampal Gyrus: Associated with memory encoding and retrieval.     - Dentate/Dentate Gyrus: Plays a role in the formation of new memories.   - Insula (Central Lobe): Involved in the perception of taste and visceral sensations.

C. CEREBRAL CORTEX

  • Gray matter that surrounds the white matter of the cerebrum, notable for its darker color.

  • Compromises approximately 90% of the cerebrum.

  • Gray Matter: Consists of cell bodies of neurons.

  • White Matter: Composed of myelinated nerve fibers (axons).

  • Gyri: Folds/convolutions of the cerebral cortex.

  • Sulci: Fissures/depressions between the gyri.

II. FIBERS (WHITE MATTER)

  • The cerebrum contains neurons and axons primarily composed of white matter, which facilitates communication between different brain regions.

TYPES OF NERVE FIBERS

  • Commissural Fibers: Connect homologous areas of the two hemispheres.   - Direction: Side to side (right to left and left to right).   - Examples:     - Corpus Callosum: Largest commissure of the brain, connects both hemispheres.       - Divisions of Corpus Callosum:         - Rostrum: Connects the anterior part of the frontal lobe.         - Genu: Connects anterior parts of the frontal lobe together with Rostrum.         - Body/Trunk: Connects posterior parts of the frontal and parietal lobes, and superior part of the temporal lobe.         - Splenium: Connects the occipital lobe.     - Anterior Commissure: Connects temporal gyri of both hemispheres, located in front of the fornix and above hypothalamus.     - Posterior Commissure: Important for bilateral pupillary light reflex; lies above the cerebral aqueduct.     - Habenular Commissure: Crosses stria medullaris fibers to contralateral habenular nuclei.     - Crura of the Fornix: Connects similar parts of the fornix across hemispheres.

  • Association Fibers: Connect areas within the same hemisphere.   - Short association fibers (Arcuate loops) connect adjacent gyri.   - Long association fibers include:     - Superior Longitudinal Fasciculus: Connects frontal, parietal, and occipital lobes (above insula).     - Inferior Occipitofrontal Fasciculus: Interconnects frontal, temporal, and occipital lobes (below insula).     - Arcuate Fasciculus: Connects frontal and temporal lobes.     - Uncinate Fasciculus: Connects frontal lobe to temporal lobe.     - Cingulum: Part of the limbic system beneath the cingulate and parahippocampal gyri.

  • Projection Fibers: Connect cortex to subcortical nuclei.   - Includes:     - Internal Capsule: Tracts run from cortex to subcortical nuclei, divided into Anterior limb, Genu, Posterior limb.     - External Capsule: Composed of cholinergic fibers between lentiform nucleus and claustrum.     - Corona Radiata: Extensions of internal capsule fibers that rise towards the cortex.     - Optic Radiations: Located within corona radiata, project to the visual cortex.

III. CORTEX (GRAY MATTER)

  • Grey matter is crucial as it is where neurons reside.

A. SUBDIVISIONS

  • The cortex is categorized based on developmental order:   - Neocortex: Newest part, comprises 90% of cortex.   - Paleocortex: Related to olfactory system, located at the base of the hemisphere.   - Archicortex: Oldest part, includes hippocampal formation; part of the limbic system.

B. REGIONS

  • Divided based on layer counts:   - Isocortex: 6 layers.   - Mesocortex: 3-6 layers; serves as a transition between isocortex and allocortex.   - Allocortex: 3 layers; found in hippocampal formation and olfactory cortex.

C. LAYERS

  • Histological visualization shows six layers:   - Layer I: Molecular Layer   - Layer II: External granular layer   - Layer III: External pyramidal layer   - Layer IV: Internal granular layer   - Layer V: Internal pyramidal layer/Ganglionic layer   - Layer VI: Multiform layer

D. CELLS

  • Pyramidal Cells: Principal cortical neurons, largest cells; apical dendrites reach surface, basal dendrites extend horizontally.   - Betz Cell: Largest pyramidal cell with a size of 150 μm.

  • Stellate Cells: Smaller interneurons, numerous short dendrites, prevalent in sensory areas.

  • Fusiform Cells: Vertical orientation, located primarily in deep layers of cortex.

  • Horizontal Cells (of Cajal): Small, fusiform neurons aligned parallel to the surface.

  • Cell of Martinotti: Present in all layers, axon projects toward the surface of the cortex.

E. ANATOMICAL AREAS

  • Cortical/Vertical Column: Functional unit containing vertically oriented groups of neurons; vital for processing information.

F. VARIATIONS ACCORDING TO CORTICAL STRUCTURE

  • Homotypical: Equal layer development, mainly in association areas.

  • Heterotypical/Idiotypical:   - Granular: Well-developed layers II & IV; sensory areas.   - Agranular: Well-developed layers III & V; motor areas.

G. FUNCTIONAL AREAS

  • Primary Areas: Respond to stimuli.   - Example areas include Primary visual (BA 17), primary auditory (BA 41), primary motor (BA 4).

  • Association Areas: Process information, categorized into unimodal and heteromodal areas.

H. BRODMANN AREAS

  • Cytoarchitectural subdivisions numbered during discovery; examples include:   - BA 3, 1, 2: Primary Somatosensory/ Somesthetic.   - BA 4: Primary Motor.   - BA 22: Wernicke’s Area (auditory association).

IV. LANGUAGE PRODUCTION

  • Language: Vocabulary and rules for effective communication.

  • Speech: Involves production; requires several brain structures such as Broca’s Area and Wernicke’s Area.

  • Cerebral Dominance: Refers to the left hemisphere being primarily responsible for language processing.

A. AUDITORY PATHWAY

  • Sound reception begins in the Organ of Corti, projecting to the Primary Auditory Cortex (BA 41).   - Signals then sent to Wernicke’s Area (BA 22) for comprehension, and subsequently to Broca’s Area (BA 44, 45) for motor planning of speech, which is executed via the Primary Motor Cortex (BA 4).

B. VISUAL PATHWAY

  • Visual processing follows a similar pathway from Rods & Cones to Primary Visual Cortex (BA 17) and then to secondary areas for interpretation before reaching Wernicke’s Area.

V. DISORDERS OF CORTICAL NETWORKS

A. AGNOSIAS

  • Agnosias are classified by impairment in recognizing stimuli despite intact sensory functions.   - Visual Agnosia: Failure to recognize objects visually, with lesions located at the temporal aspect of the visual unimodal association area.   - Tactile Agnosia: Inability to recognize by touch, lesions at the supramarginal gyrus (BA 40).   - Auditory Agnosia: Failure to recognize sounds, lesions at posterior part of the superior temporal gyrus (BA 22).  

B. APRAXIA

  • Characterized as a loss of ability to perform tasks due to lesions disrupting motor planning.   - Ideomotor Apraxia: Inability to perform actions on command; lesion at the supramarginal gyrus of the dominant hemisphere.   - Ideational Apraxia: Failure to execute a series of actions; lesion at the dominant parietal lobe/corpus callosum.   - Motor/Kinetic Apraxia: Inability to execute learned movements; lesion at the contralateral frontal lobe.   - Gait Apraxia: Difficulty with initiating movement; lesions in bilateral frontal lobes.

C. APHASIAS

  • Language production impairments.   - Motor Aphasia (Broca's Aphasia): Lesion at Broca's Area (BA 44, 45); characterized by difficulty producing speech while comprehension remains intact.   - Speech Sensory Aphasia (Wernicke’s Aphasia): Lesions at Wernicke’s Area (BA 22); speech is fluent but lacks meaning.   - Global Aphasia: Involves multiple areas including Broca and Wernicke areas, resulting in comprehensive communication deficits.

VI. CEREBRAL DOMINANCE

  • Dominant Hemisphere: Left hemisphere drives language and analytical functions; established by age 10.

  • Non-Dominant Hemisphere: Right hemisphere correlates with artistic and spatial functions.

VII. REVIEW QUESTIONS

  • Example questions on the identification of Brodmann areas, understanding of functions related to lesions, and connections among cortical areas.

VIII. REFERENCES

  • Rivera, I. (2024). Asynchronous Lecture on Cerebral Cortex and Higher Cortical Functions.