Chapter 027. Aphasia, Memory Loss, and Other Focal Cerebral Disorders (Part 1)

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Harrison's Internal Medicine Chapter 27. Aphasia, Memory Loss, and Other Focal Cerebral Disorders Aphasia, Memory Loss, and Other Focal Cerebral Disorders: Introduction The cerebral cortex of the human brain contains ~20 billion neurons spread over an area of 2.5 m2. The primary sensory areas provide an obligatory portal for the entry of sensory information into cortical circuitry, whereas the primary motor areas provide final common pathways for coordinating complex motor acts. The primary sensory and motor areas constitute 10% of the cerebral cortex. The rest is subsumed by unimodal, heteromodal, paralimbic, and limbic areas, collectively known as the association cortex (Fig....

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Chapter 027. Aphasia, Memory Loss, and Other Focal Cerebral Disorders (Part 1) Harrison's Internal Medicine > Chapter 27. Aphasia, Memory Loss, and Other Focal Cerebral Disorders Aphasia, Memory Loss, and Other Focal Cerebral Disorders: Introduction The cerebral cortex of the human brain contains ~20 billion neurons spread over an area of 2.5 m2. The primary sensory areas provide an obligatory portal for the entry of sensory information into cortical circuitry, whereas the primary motor areas provide final common pathways for coordinating complex motor acts.
The primary sensory and motor areas constitute 10% of the cerebral cortex. The rest is subsumed by unimodal, heteromodal, paralimbic, and limbic areas, collectively known as the association cortex (Fig. 27-1). The association cortex mediates the integrative processes that subserve cognition, emotion, and behavior. A systematic testing of these mental functions is necessary for the effective clinical assessment of the association cortex and its diseases. Figure 27-1
Lateral (top) and medial (bottom) views of the cerebral hemispheres.
The numbers refer to the Brodmann cytoarchitectonic designations. Area 17 corresponds to the primary visual cortex, 41–42 to the primary auditory cortex, 1– 3 to the primary somatosensory cortex, and 4 to the primary motor cortex. The rest of the cerebral cortex contains association areas. AG, angular gyrus; B, Broca's area; CC, corpus callosum; CG, cingulate gyrus; DLPFC, dorsolateral prefrontal cortex; FEF, frontal eye fields (premotor cortex); FG, fusiform gyrus; IPL, inferior parietal lobule; ITG, inferior temporal gyrus; LG, lingual gyrus; MPFC, medial prefrontal cortex; MTG, middle temporal gyrus; OFC, orbitofrontal cortex; PHG, parahippocampal gyrus; PPC, posterior parietal cortex; PSC, peristriate cortex; SC, striate cortex; SMG, supramarginal gyrus; SPL, superior parietal lobule; STG, superior temporal gyrus; STS, superior temporal sulcus; TP, temporopolar cortex; W, Wernicke's area.According to current thinking, there are no centers for "hearing words," "perceiving space," or "storing memories." Cognitive and behavioral functions (domains) are coordinated by intersecting large-scale neural networks that contain interconnected cortical and subcortical components. The network approach to higher cerebral function has at least four implications of clinical relevance: (1) a single domain such as language or memory can be disrupted by damage to any one of several areas, as long as these areas belong to the same network; (2) damage confined to a single area can give rise to multiple deficits, involving the functions of all networks that intersect in
that region; (3) damage to a network component may give rise to minimal or transient deficits if other parts of the network undergo compensatory reorganization; and (4) individual anatomic sites within a network display a relative (but not absolute) specialization for different behavioral aspects of the relevant function. Five anatomically defined large-scale networks are most relevant to clinical practice: a perisylvian network for language; a parietofrontal network for spatial cognition; an occipitotemporal network for face and object recognition; a limbic network for retentive memory; and a prefrontal network for attention and behavior.