anatomist-physiological particularities of the cerebrum
functions cortex and pathology of the cerebrum
neuropsychological methods studies of the cortex
Different areas of the cerebral cortex (neocortex) may be distinguished from one another by their histological features and neuroanatomical connections. Brodmann’s numbering scheme for cortical areas has been used for many years and will be introduced in this section.
Projection areas. By following the course of axons entering and leaving a given cortical area, one may determine the other structures to which it is connected by afferent and efferent pathways. The primary projection areas are those that receive most of their sensory impulses directly from the thalamic relay nuclei (primary somatosensory cortex; Brodman areas 1, 2, 3), the visual (area 17), or the auditory (areas 41, 42) pathways. The primary motor cortex (area 4) sends motor impulses directly down the pyramidal pathway to somatic motor neurons within brainstem and the spinal cord. The
primary projection areas are somatotopically organized and serve the contralateral half of the body. Proceeding outward along the cortical surface from the primary projection areas, one encounters the secondary projection areas (motor, areas 6, 8, 44; sensory, areas 5, 7a, 40; visual, area 18; auditory, area 42), which subserve higher functions of coordination and information processing, and the tertiary projection areas (motor, areas 9, 10, 11; sensory, areas 7b, 39; visual, areas 19, 20, 21; auditory, area 22), which are responsible for complex functions such as voluntary movement, spatial organization of sensory input, cognition, memory, language, and emotion. The two hemispheres are connected by commissural fibers, which enable bihemispheric coordination of function. The most important commissural tract is the corpus callosum; because many tasks are performed primarily by one of the two hemispheres (cerebral dominance), interruption of the corpus callosum can produce various disconnection syndromes. Total callosal transection causes splitbrain syndrome, in which the patient cannot name an object felt by the left hand when the eyes are closed, or one seen in the left visual hemifield (tactile and optic anomia), and cannot read words projected into the left visual hemifield (left hemialexia), write with the left hand (left hemiagraphia), or make pantomimicmovements with the left hand (left hemiapraxia). Anterior callosal lesions cause alien hand syndrome (diagonistic apraxia), in which the patient cannot coordinate the movements of the two hands. Disconnection syndromes are usually not seen in persons with congenital absence (agenesis) of the corpus callosum.
Cytoarchitecture. Most of the cerebral cortex consists of isocortex, which has six distinct cytoarchitectural layers. The Brodmann classification of cortical areas is based on distinguishing histological features of adjacent areas of isocortex.
Functional areas. The functional organization of the cerebral cortex can be studied with various techniques: direct electrical stimulation of the cortex during neurosurgical procedures, measurement of cortical electrical cortical activity (electroencephalography and evoked potentials), and measurement of regional cerebral blood flow and metabolic activity. Highly specialized areas for particular functions are found in many different parts of the brain. A lesion in one such area may produce a severe functional deficit, though partial or total recovery often occurs because adjacent uninjured areas may take over some of the function of the lost brain tissue. (The extent to which actual brain regeneration may aid functional recovery is currently unclear.) The specific anatomic patterns of functional localization in the brain are the key to understanding much of clinical neurology.
The subcortical structures include the basal ganglia, thalamus, subthalamic nucleus, hypothalamus, red nucleus, substantia nigra, cerebellum, and brain stem, and their nerve pathways. These structures perform many different kinds of complex information processing and are anatomically and functionally interconnected with the cerebral cortex. Subcortical lesions may produce symptoms and signs resembling those of cortical lesions; special diagnostic studies may be needed for their precise localization.