Chapter 32: Cerebral Hemispheres

The cerebral cortex contains primary motor, premotor, and sensory areas, which process initial conscious sensory input or control motor movements, as well as higher-level association cortices (unimodal or multimodal) that facilitate complex tasks like language, social communication, and planning, particularly in the frontal lobe. The limbic system is a specialized multimodal region involved in memory, emotion, and behavior, incorporating structures like the hippocampal formation and cingulate cortex, and integrating wide-ranging physiological reactions via connections with the hypothalamus. The brain's external surface is marked by gyri (convolutions) separated by sulci, the deepest of which are termed fissures, with the main sulci displaying depths of 1–3 cm and exhibiting significant variability among individuals. The fundamental organizational structure differentiates between the older, layered allocortex and the evolutionary younger, six-layered isocortex (neocortex), which forms the majority of the human cortex. The microscopic structure features various neuronal types, including pyramidal cells, which primarily use glutamate as an excitatory transmitter, and specialized pyramidal cells such as the large Betz cells located in lamina V of the primary motor cortex and Meynert cells in the primary visual cortex. Inhibitory neurotransmission relies heavily on GABA, secreted by cortical interneurones like basket cells, chandelier cells, and Martinotti cells, often classified based on their co-expression of calcium-binding proteins (e.g., PV, SOM, CR) and neuropeptides (e.g., VIP, NPY). Cortical areas are segregated using architectonic maps, such as the classical Brodmann map of approximately 50 areas, which is now complemented by modern quantitative methods analyzing unique receptor fingerprints—the characteristic expression patterns of multiple neurotransmitter receptors—to delineate functional networks. Structurally distinct areas include the primary visual cortex (BA17), where the myelin-rich horizontal stripe of Gennari divides layer IV, and the amygdala, particularly the basolateral group, which exhibits quasi-cortical properties and dense receptor distributions. Connectivity within the brain is achieved via projection fibres (including the corticospinal tract, which originates in lamina V and shows a prominent ventral shift in spinal termination in primates), commissural fibres (like the corpus callosum), and numerous association tracts in the white matter, such as short U-fibres connecting adjacent gyri, and longer tracts like the cingulum and the hook-shaped uncinate fasciculus, the latter being vital for lexical retrieval and social cognition.