Development And Evolution

The functional flexibility of the neocortex is related to its capacity to be altered in evolution and development. Neocortex develops from a thin sheet of germinal cells along the margin of the cerebral ventricle. The germinal cells undergo mitosis and divide to form more germinal cells, some of which ultimately differentiate into neurons or glial cells. A marginal zone of glial processes soon forms over the germinal cells, and neuron precursors migrate from the deeper germinal layer toward the surface to form a cortical plate and then cortical layers. Neurons reach the cortex largely by migrating along glial processes that extend radially from the inner ventricular surface to the outer margin of the developing hemisphere. Surprisingly, the neurons that arrive first form the deeper layers because later arriving neurons pass through the earlier arrivals to reach the surface. Most neurons pass from a specific location along the ventricle to a specific location in the cortex by migrating along the process of possibly a single radial glial cell. A row of neurons across the thickness of the cortex may largely originate from a single germinal cell that has divided over and over again to produce a series of migrating offspring. Thus, the thickness of the cortex, constituting surface to white matter rows of roughly 100 or so neurons, may depend on roughly 100 cell divisions. Of course the number varies across cortical areas and the same cortical area across species, but not by that much. One of the reasons for the relative stability of the thickness of the cortex and the number of neurons across this thickness is the developmental process of repeatedly dividing the parent cell to produce offspring in series. Thus, a large change in the final number of neurons in a row would depend on a major change in the number of germinal cell divisions that produce neuronal precursors. Many of the inhibitory neurons of the neocortex do migrate in from a lateral location, but the major point to row migration pattern appears to provide some stability in one feature of cortical organization, the thickness. It may also be important that neurons migrate to specific regions of the cortex, because this might initiate the process of forming cortical areas.

In another way, the generation of cortex is a flexible process that easily allows increases in the size of the neocortical sheet. As neurobiologist Pasko Rakic has stressed, each symmetrical division of germinal cells doubles the number of germinal cells. When each germinal cell finally changes its role to produce a row of cortical neurons, then the number of rows and the size of the cortical sheet will be doubled by each of the previous doublings of the population of germinal cells. Whereas cortical thickness may be hard to change in development, increases in cortical size appear to depend on only a few more cell divisions in the early stages of development. Small genetic changes in the control of the number of cell divisions to form more germinal cells would have major consequences. Thus, we see great variability in the size of the neocortex, but little in the thickness.

Another important factor in the evolution of variability in the cortex is that developing cortical neurons are very sensitive to environmental influences. Developing neurons use information in neural activity patterns to form functional groups. As is sometimes stated, "neurons that fire together wire together.'' Correlated activity in interconnected neurons maintains connections, whereas discorrelations remove those connections. This allows developing cortical neurons to respond in an adaptive way to variable features in the environment. The somatosensory cortex is normally subdivided so that groups of neurons process information from each finger or other subdivisions of the body surface. If a change in development alters the number of fingers or the number of sensory whiskers on the face, the areas of the cortex develop a matching number of modular subdivisions. In other words, brain regions develop to match each other in modular subdivisions that correspond to input from segregated groups of receptors. Each match is based on the transfer of information from one structure to another, rather than on the gradual selection for a long sequence of matches across nuclei and areas by the accumulation of changes in genetics. By using information from the environment, the rapid evolution of major alterations in the functional organization of the neocortex is possible. The monotreme mammal, the duck-billed platypus, has evolved a novel class of receptors on its bill, electroreceptors that are sensitive to weak electrical currents in water produced by the contracting muscles of swimming prey. The brain and neocortex adjusted to this new receptor by subdividing the primary sensory area, S1, into one set of modules related to tactile input and another set of modules related to electroreception. The developmental plasticity of the cortex allows useful adjustments for such changes in input, and the somatosensory system of monotremes was able to acquire new functions. The neocortex is designed not only to readily accommodate new or changed input originating from the receptor surfaces but also to accommodate input from functionally new classes of neurons that are created in the cortex by cortical circuits.

The ability of the neocortex to accommodate alterations in the nature of activating input by forming new circuits and modules may also relate to the evolution of new cortical areas. How the number of cortical areas increased in some lines of evolution is uncertain, but there are several logical possibilities. First, body parts sometimes duplicate in evolution so that suddenly extra parts emerge. A mutation in genes for some control mechanism could lead to the duplication of a cortical area. The two daughter areas could then be gradually differentiated by subsequent changes in genetic control, so that the two daughter areas come to mediate different functions. Another possibility is that new functions emerge as classes of modules are formed within areas. Gradually, over many generations, modules of each class could merge, reducing the lengths of the interconnections between modules. A full merger with complete separation of the two classes of modules would result in two areas emerging from one.

Whatever the mechanisms of cortical evolution, they work by modifying development. In order to better understand the evolution of the neocortex, it will be productive to further investigate the great variability in neocortical organization that exists and the nature of the developmental changes that allow this variability.

Given the great potential for the neocortex to vary in size, accommodate changes in input, and subdivide into areas and modules specialized for functionally adaptive tasks, one might wonder why all mammals do not have expanded sheets of neocortex with many subdivisions. The major reasons for having less neocortex seem to be that the neocortex takes a long time to develop and mature and that the neocortex is metabolically costly. Other constraints undoubtedly also exist. For example, the weight of a large brain would be difficult to manage for bats as flying mammals, for whom all weight is costly. Mammals must be able to compensate for the metabolic and developmental costs of having more neocortex by having the gain in computational ability lead to getting better and more food and longer life spans, so that successful reproduction is improved or at least maintained. Humans, with exceptionally large sheets of neocortex, have used these sheets in ways that have allowed considerable reproductive success, with our species now exceeding 6 billion individuals, but most of this reproductive success has been exceedingly recent, over the last few hundred years. Great apes, as our closest relatives, have a large amount of neocortex, although much less than us. However, they have not done well and border on the verge of extinction. In contrast, small-brained rats and mice thrive over most parts of the earth. The rats and mice are not very bright and they make many mistakes, but they are able to replace themselves in large numbers very rapidly. However, mammals with little neocortex often specialized their small sheets of neocortex for particular functions. Thus, echolocating bats devote much of their small neocortex to auditory input and the sounds used for echolocating objects and prey. The large, costly sheets of neocortex that characterize the brains of higher primates remain unusual because they require long periods of safe development, long reproduction cycles, and consistently available foods of high metabolic value.

Understanding And Treating Autism

Understanding And Treating Autism

Whenever a doctor informs the parents that their child is suffering with Autism, the first & foremost question that is thrown over him is - How did it happen? How did my child get this disease? Well, there is no definite answer to what are the exact causes of Autism.

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