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Connections of the Cerebral cortex The cerebral cortex sends connections (efferents) and receives connections (afferents) from many subcortical structures like the thalamus and basal ganglia. Most of the sensory stimulation arrives at the cerebral cortex indirectly through different thalamic nuclei. This is the case of touch, vision and sound but not of olfactory stimulation, which passes to the olfactory bulb and then to the olfactory (pyriform) cortex. The largest part of the connections arriving at the cerebral cortex do not come from subcortical structures however. The main source of cortical stimulation is the cerebral cortex itself: up to 75% of the total connections. Areas that receive that particular information are called sensory areas. Parts of the cortex that receive sensory inputs from the thalamus are called primary sensory areas. The senses of vision, audition and touch are served by the primary visual cortex, primary auditory cortex and primary somatosensory cortex. In general, the two hemispheres receive the information from the opposite sides of the body. For example the right primary somatosensory cortex receives information from the left limbs and the right visual cortex receives information from the left visual field. Other areas receive impulses from the primary sensory areas and integrate the information coming in from different types of receptors (i.e., modalities). These are often called association areas and make up a great deal of the cortex in all primates, humans included. Thus, the cortex is commonly described as comprised of the primary sensory areas, the motor areas and the association areas. Association areas Association areas comprise three major groups: In humans, the association areas of the left hemisphere, especially the parietal-temporal-occipital complex, are responsible for our understanding and use of language. Motor areas The motor areas are located in both hemispheres of the cortex. They are shaped like a pair of headphones stretching from ear to ear. The motor areas are very closely related to the control of voluntary movements, especially fine fragmented movements performed by the hand. The right half of the motor area controls the left side of your body and vice versa. Two areas of the cortex are commonly referred to as motor: In addition, motor functions have been described for: Development The cerebral cortex develops from the neural plate, a specialised part of the embryonic ectoderm. The neural plate folds and closes to form the neural tube. From the cavity inside the neural tube develops the ventricular system, and, from the epithelial cells of its walls, the neurons and glial cells. The most-frontal part of the neural tube, the telencephalon, gives rise to the cerebral hemispheres and the neocortex. Most cortical neurons are generated within the ventricular zone close to the ventricles. Initially, progenitor cells in the ventricular zone divide symmetrically, producing two progenitor cells by mitotic cycle. Then, some progenitor cells begin to divide asymmetrically, producing one postmitotic cell that migrates radially and leaves the ventricular zone, and a daughter cell that continues to divide or that eventually dies. The migrating cells will become neurons (Rakic, 1988). Recent work has identified radial glia radial glial cells (also here) as one population of progenitor cells (Noctor et al. 2001). Laminar pattern The standard areas of cortex (isocortex) is characterized as having six distinct layers. From outside inward: After migration, neurons form efferents and receive afferent connections characteristic of their layer. It is interesting to note that, during development, the inner layers are formed before the outer layers are. During early development, there is an additional layer of neurons present in the future white matter. these are called subplate neurons and these neurons disappear during postnatal development. The cortical layers are not simply stacked one over the other; they develop characteristic connections between different layers, which define the basic structure of the cortical columns in the mature cortex (Mountcastle, 1997). There are no actual borders between the layers, and neurons cross layer boundaries with their dendrites and axons trees all over. The pyramidal cells (the majority of the neurons) span at least three layers, and in many cases all the layers. Thus, it is not obvious that the layers have any functional significance. However, the flow of current in the cortical layers is consistent and shows inputs principally in layer IV, and the spread of activity, and thus the flow of information, roughly follows the models put forth by Martin, Whitteridge, and Somogyi in 1985. Classification Based on the differences in lamination the cerebral cortex can be classified into two major groups: Auxiliary classes are: Based on supposed developmental differences the following classification also appears: In addition, cortex may be classified on the basis of gross topographical conventions into the following: See also Further reading | ||||||||||
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