Figure 4. Architectonics of the human cerebral cortex.

Figure 4

Architectonics of the human cerebral cortex. (a) Allocortex and neocortex make up the human cerebral cortex. The allocortex is small and includes the olfactory bulb, hippocampal formation, pre and parasubiculum, and entorhinal region (red). Periallocortical areas (rose) accompany the allocortex proper, whereas proneocortical areas (deep yellow) are allied with homotypical association areas of the mature neocortex (light yellow). Periallocortex and proneocortex comprise the mesocortex. The expansive neocortex of the parietal, occipital, and temporal lobes respectively consists of a primary field (deep blue), a belt of first order association areas (light blue), and related high-order association areas (light yellow). The frontal lobe is similarly structured into a primary motor field (deep green), premotor areas (light green), and prefrontal areas (light yellow). The same color code applies to (Fig. 5). (b) Two allocortical core regions. The olfactory bulb, the first core, is connected with the adjoining periallocortex: anterior olfactory nucleus, olfactory tubercle, piriform and periamygdalar regions (rose). Essential components of the autonomic loop include the agranular (rose) and dysgranular (deep yellow) areas of the insular and subgenual mesocortex. The hippocampal formation (deep red) is the second and much more important allocortical core region. The periallocortex is represented anteriorly by the transentorhinal region (rose). (c) The basic hierarchical organization of the cerebral cortex composed of four fundamental elements: the allocortex (red), then periallocortical transitional zones (rose) and proneocortex (deep yellow), and mature neocortex (light yellow). Expansive association areas that adjoin the proneocortex are, architectonically relatively simple, and consist of a mostly nonspecialized homotypic neocortex (light yellow in 1a). There follow, by degrees, more sophisticated heterotypic areas in the form of first order association areas (light blue) and primary sensory fields (deep blue), or premotor areas (light green) and primary motor field (deep green). White arrows indicate this gradual increase in cortical differentiation. They also indicate in the frontal, insular, and temporal cortex the gradual increase in average myelin content of cortical projection cells when traced from the sparsely myelinated periallocortex and proneocortex through the high-order and first order association areas to the primary fields of the neocortex. (d) Supplementary arrows show major interconnectivities and how exteroceptive input is relayed from the most highly differentiated neocortical primary sensory fields to the primary motor field and allocortex. Visual, auditory, and somatosensory information (blue arrow) passes through the respective primary fields (deep blue) and first order sensory association areas (light blue) to related high-order association areas (light yellow). Then, data are transferred to the prefrontal cortex. Tracts generated from this highest organizational level of the brain transfer the information — chiefly via the striatal and cerebellar loops (small black semicircular arrows) — back to the primary motor field, which relays motor programs into brain stem and spinal premotor and motor neurons (deep green arrow). Some of the data-flow from the sensory association areas to the prefrontal cortex leaves the mainstream and converges on the entorhinal region and amygdala via proneocortical and periallocortical transitional areas (the afferent arm of the limbic circuit, indicated here by the descending part of the black U-shaped arrow). The entorhinal allocortex, hippocampal formation, and amygdala are closely linked. Their efferent projections head mainly toward the prefrontal cortex (forming the efferent arm of the limbic loop, indicated by the ascending part of the black U-shaped arrow). In addition to exteroceptive information, limbic loop centers receive interoceptive stimuli from the internal organs and subcortical nuclei involved in viscerosensory data processing. On the output side, they influence the endocrine system via the hypothalamo-hypophyseal axis and direct both brain stem and spinal cord nuclei, which regulate visceromotor functions (Fig. 5b). In sum, the limbic loop integrates exteroceptive (tactile, olfactory, auditory, visual) and interoceptive impressions with the regulation of the organism's internal homeostasis and modifies voluntary, including affect-related, somatomotor behavior in response to any given situation.125,173 Abbreviation: MD — mediodorsal nuclei of the thalamus.

From: Idiopathic Parkinson's Disease: Staging an α-Synucleinopathy with a Predictable Pathoanatomy

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