Neurology A Deeper Insight Into The Mind 4

During early development of the vertebrate embryo

, a longitudinal groove on the neural plate gradually deepens as ridges on either side of the groove (the neural folds) become elevated, and ultimately meet, transforming the groove into a closed tube, the ectodermal wall of which forms the rudiment of the nervous system.

This tube initially differentiates into three vesicles (pockets): the pros encephalon at the front, the mesencephalon, and, between the mesencephalon and the spinal cord, the rhomb encephalon. (By six weeks in the human embryo) the pros encephalon then divides further into the telencephalon and diencephalon; and the rhomb encephalon divides into the met encephalon and myelencephalon.

As the vertebrate grows, these vesicles differentiate further still. The telencephalon differentiates into, among other things, the striatum, the hippocampus and the neocortex, and its cavity becomes the first and second ventricles. Diencephalon elaborations include the sub thalamus, hypothalamus, thalamus and epitheliums, and its cavity forms the third ventricle.

The tectum, pretectum, cerebral peduncle and other structures develop out of the mesencephalon, and its cavity grows into the mesencephalon duct (cerebral aqueduct). The met encephalon becomes, among other things, the Pons and the cerebellum, the myelencephalon forms the medulla oblongata, and their cavities develop into the fourth ventricle.

Planarians, members of the phylum Platyhelminthes (flatworms), have the simplest, clearly defined delineation of a nervous system into a central nervous system (CNS) and a peripheral nervous system (PNS). Their primitive brain, consisting of two fused anterior ganglia, and longitudinal nerve cords form the CNS; the laterally projecting nerves form the PNS. A molecular study found that more than 95% of the 116 genes involved in the nervous system of planarians, which includes genes related to the CNS, also exist in humans. Like planarians, vertebrates have a distinct CNS and PNS, though more complex than those of planarians.

The CNS of chordates differs from that of other animals in being placed dorsally in the body, above the gut and notochord/spine. The basic pattern of the CNS is highly conserved throughout the different species of vertebrates and during evolution.

The major trend that can be observed is towards a progressive telencephalisation, the telencephalon of reptiles is only an appendix to the large olfactory bulb, while in mammals it makes up most of the volume of the CNS. In the human brain, the telencephalon covers most of the diencephalon and the mesencephalon. Indeed, the allometric study of brain size among different species shows a striking continuity from rats to whales, and allows us to complete the knowledge about the evolution of the CNS obtained through cranial endocasts.

by: Sabina Kucz

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