Cerebrum and Functional Cortical Areas Study Guide

Physical Structure and Anatomy of the Cerebrum\n\n* General Characteristics: The cerebrum represents the largest and most anterior portion of the brain. It is responsible for a vast array of high-order functions, including the initiation and coordination of all voluntary bodily activities, emotional control, hearing, vision, personality, thinking, speech, and intellectual processes.\n* Gross Structure: It comprises two cerebral hemispheres separated in the midline by the longitudinal fissure. At the base of this fissure, the hemispheres are connected by the corpus callosum, a thick band of nerve fibers.\n* Surface Anatomy: The surface of the cerebrum is characterized by folds and grooves to maximize surface area within the cranial vault:\n * Gyrus (plural gyri): A fold or ridge on the cortical surface.\n * Sulcus (plural sulci): A groove or fissure between gyri.\n * Fissure: A deep groove, such as the longitudinal or transverse fissures.\n* Cortical Landmarks: \n * Main Sulci: The Central sulcus separates the frontal and parietal lobes. The Lateral sulcus (Sylvian fissure) separates the frontal and parietal lobes from the temporal lobe. The Parieto-occipital sulcus separates the parietal lobe from the occipital lobe.\n * Major Gyri: The Precentral gyrus (located in the frontal lobe) serves as the primary motor area. The Postcentral gyrus (located in the parietal lobe) serves as the primary somatosensory area.\n* Borders and Poles: The brain is described as having a Frontal pole, Temporal pole, and Occipital pole. Territorial boundaries include the Superomedial border and the Inferolateral border. A distinct indentation on the inferior-lateral aspect is the Preoccipital notch.\n\n# The Cerebral Cortex: Histology and Layers\n\n* Composition: The cerebral cortex is the outer layer of the cerebrum, primarily composed of nerve cell bodies (grey matter), neuroglia, and capillaries. It measures approximately 2−4 mm2-4\,mm in thickness.\n* Functional Significance: It is essential for consciousness, memory, attention, perception, awareness, thought, and language.\n* Horizontal Laminar Organization: The cortex is divided into six distinct histological layers from superficial to deep:\n 1. Molecular (Plexiform) Layer: Consists mainly of fibers (axons and dendrites) with only a few sparse cells.\n 2. External Granular Layer: Contains small pyramidal neurons and numerous stellate neurons.\n 3. External Pyramidal Layer: Predominantly contains small and medium-sized pyramidal neurons, along with non-pyramidal neurons characterized by vertically oriented intracortical axons.\n 4. Internal Granular Layer: Contains diverse types of stellate and pyramidal neurons.\n 5. Internal Pyramidal Layer: Contains large pyramidal neurons.\n 6. Multiform Layer: Contains a few large pyramidal neurons and many small spindle-like pyramidal and multiform neurons. It also houses specialized cells known as the cells of Martinotti and horizontal cells of Cajal.\n\n# Cerebral White Matter and Connectivity\n\n* Composition: Located beneath the grey matter, white matter consists of myelinated axons and glia (mostly oligodendrocytes). These fibers act as a relay and coordination network for communication between different brain regions and lower brain centers.\n* Physical Properties: In a freshly cut brain, the tissue appears pinkish-white due to myelin lipids and capillary networks. Preservation with formaldehyde gives it a distinct white color. Myelin serves as an insulator, increasing the speed of signal transmission.\n* Connectivity Principles: According to Schuz and Braitenberg, the number of fibers of a specific range of length is inversely proportional to their length. Long-range fibers constitute only 2%2\% of the total cortico-cortical fibers, a number roughly equivalent to those in the corpus callosum.\n* Fiber Classification: Nerve fibers are grouped into three categories based on their connections:\n 1. Association Fibers: Connect different cortical regions within the same hemisphere.\n * Short Association (U-fibers): Lie immediately beneath the cortex and connect adjacent gyri. These include intracortical and subcortical fibers.\n * Long Association: Bundled into named structures: Cingulum, Uncinate Fasciculus, Arcuate Fasciculus, Superior Longitudinal Fasciculus, Inferior Longitudinal Fasciculus, and Fronto-occipital Fasciculus.\n 2. Commissural Fibers: Connect corresponding regions between the two hemispheres.\n * Corpus Callosum: The largest brain commissure. It consists of the Rostrum (thin lamina connecting the genu to the lamina terminalis), Genu (anterior end connecting frontal lobes), Body/Trunk (connects posterior frontal, parietal, and superior temporal lobes), and Splenium (connects occipital lobes). Fibers of the rostrum/genu form the Forceps minor, while splenium fibers form the Forceps major.\n * Anterior Commissure: Connects inferior/middle temporal gyri and olfactory structures.\n * Posterior Commissure: Carries fibers from pretectal nuclei.\n * Fornix (Hippocampal Commissure): Interconnects the posterior columns of the fornix.\n * Habenular Commissure: Small bundle crossing near the pineal gland, associated with the habenular nucleus.\n 3. Projection Fibers: Afferent and efferent fibers linking the cortex with the diencephalon, brain stem, cerebellum, and spinal cord.\n * Ascending (Afferent/Corticopetal): Includes Thalamic, Optic, and Auditory radiations.\n * Descending (Efferent/Corticofugal): Includes Pyramidal tracts (corticospinal and corticobulbar), corticopontine, corticoreticular, and corticorubral tracts.\n\n# The Internal Capsule\n\n* Structure: A thick, V-shaped band of projection fibers with its concavity directed laterally toward the lentiform nucleus.\n* Anterior Limb: Located between the head of the caudate nucleus and the lentiform nucleus. It contains the anterior thalamic radiation and frontopontine fibers.\n* Genu: The angle of the V pointing medially. It contains corticonuclear (corticobulbar) fibers and the anterior part of the superior thalamic radiation.\n* Posterior Limb: Divided into two parts:\n * Lenticothalamic part: Located between the thalamus and lentiform nucleus. Contains corticospinal, frontopontine, and frontorubral fibers, plus the majority of the superior thalamic radiation to the somatosensory cortex.\n * Retrolenticular part: Contains fibers from the medial and lateral geniculate nuclei. The optic radiation travels to the visual cortex around the calcarine sulcus, and the auditory radiation travels to the superior temporal gyrus.\n\n# Functional Cortical Areas (Brodmann’s Classification)\n\n* Functional Overview: Areas are categorized into Sensory (receiving input), Motor (controlling muscle movement), and Association (complex processing like learning or writing).\n* Sensory Areas and Numbers:\n * Somatosensory: Areas 11, 22, and 33 (Primary somatosensory cortex in the postcentral gyrus).\n * Visual: Area 1717 (Primary) and Area 1919 (Visual association).\n * Auditory: Area 4141 (situated in the temporal lobe, also called Heschl’s gyrus).\n * Gustatory (Taste): Area 4343 (located in the insula on the roof of the lateral sulcus).\n * Olfactory (Smell): Area 2020 (located in the uncus on the medial temporal lobe). Also known as the Rhinencephalon.\n * Wernicke’s Area: Area 2222 (involved in language comprehension and naming).\n* Motor Areas and Numbers:\n * Primary Motor Cortex: Area 44 (precentral gyrus). Contains pyramidal neurons projecting to the medulla.\n * Premotor Cortex: Areas 66 and 88. Involved in planning complex movements.\n * Frontal Eye Field: Area 99. Controls voluntary eye movements.\n * Prefrontal Area: Areas 1010 and 1111. Involved in thinking, judgment, and social behavior.\n * Broca’s Area: Areas 4444 and 4545 (inferior frontal gyrus). Controls motor speech production.\n\n# Specialized Association Areas\n\n* Primary Sensory Cortex: Responsible for basic awareness of sensations (e.g., smell, sight, touch).\n* Association Areas: Draw upon stored memories to give meaning to sensations (e.g., identifying the sound of an ambulance).\n* Multimodal Association Areas:\n * Posterior Association Area: Where visual, somatosensory, and auditory areas meet. Provides spatial awareness. In the left hemisphere, it includes Wernicke’s Area for reading/naming. In the right hemisphere, it interprets emotional overtones.\n * Anterior Association Area (Prefrontal Cortex): Integrates information from the posterior area with past experiences via the limbic system. Controls judgment and socially acceptable behavior.\n * Limbic Association Area: Located on the medial frontal lobe. Forms memories, translates them to motor responses, and facilitates social interactions and personality expression.\n\n# Visual Pathway and Lesions\n\n* Pathway: Photoreceptors in the retina $\rightarrow$ Optic nerve $\rightarrow$ Optic chiasm (partial decussation) $\rightarrow$ Optic tract $\rightarrow$ Lateral Geniculate Nucleus (LGN) of the thalamus $\rightarrow$ Optic radiations (including Meyer’s loop) $\rightarrow$ Primary visual cortex (V1V1) in the occipital lobe.\n* Lesion Effects:\n * Lesion 1 (Optic Nerve): Complete blindness in one eye.\n * Lesion 2 (Optic Chiasm): Loss of peripheral vision (bitemporal hemianopsia) due to damage to nasal retina fibers; often caused by pituitary tumors.\n * Lesion 3 (Optic Tract): Loss of the entire opposite hemifield (homonymous hemianopsia).\n * Lesion 4 (Meyer’s Loop): Loss of vision in the upper quadrant of the opposite hemifield (quadrantanopia).\n * Lesion 5 (Parietal Optic Radiations): Affects the lower quadrant of the visual world on the opposite side.\n * Lesion 6 (Cortex Lesion): Results in a notched hemifield with macular sparing—vision at the fovea is preserved due to overlapping blood supply or large cortical representation.\n\n# Clinical Correlations and Pathologies\n\n* Cerebral Dominance: The Left hemisphere typically handles language, logic, math, and details. The Right hemisphere handles visual-spatial skills, facial expressions, intuition, and emotions.\n* Speech Disorders:\n * Broca’s Aphasia (Expressive Aphasia): Difficulty producing speech despite intact comprehension. Can involve Apraxia.\n * Wernicke’s Aphasia (Receptive Aphasia): Inability to understand written or spoken language despite producing fluid (but often nonsensical) speech.\n * Global Aphasia: Loss of both expressive and receptive speech functions.\n * Arcuate Fasciculus: The nerve bundle connecting Broca’s and Wernicke’s areas; damage disrupts the link between comprehension and speech production.\n* Cognitive and Sensory Deficits:\n * Anomia: Loss of the power to name objects or people (Webb, 20082008).\n * Alexia: Difficulty reading.\n * Agraphia: Difficulty writing.\n * Acalculia: Difficulty with arithmetic.\n * Astereognosis: Inability to recognize objects by touch with eyes closed (lesion of the somesthetic association area).\n * Agnosia: Inability to identify complex objects, shapes, or smells.\n * Prosopagnosia: Inability to recognize faces (temporal lobe damage).\n* Specific Diseases:\n * Alzheimer’s Disease: A degenerative disease linked to genetic predisposition. Characteristics include atrophy of the cortex, loss of gyri/sulci in the temporal and frontal lobes, and the accumulation of beta amyloid plaques. Prevalence exceeds 4 million4\,million in the USA.\n * Lafora Disease: Characterized by atrophy of the grey matter; patients rarely live past 25 years25\,years of age.\n* Internal Capsule Stroke: Typically involves the posterior limb (supplied by the Middle Cerebral Artery - MCA). Symptoms include contralateral spastic hemiplegia, contralateral hemianesthesia, and contralateral lower facial paralysis. Damage to the retrolenticular part causes contralateral homonymous hemianopsia. The anterior limb is supplied by the Anterior Cerebral Artery - ACA.","title":"Cerebrum and Functional Cortical Areas Study Guide"}