Icon module 22

Brief summary

From a functional point of view, the nervous system can be subdivided into somatic and autonomous systems.  On the other hand, from an anatomic perspective, we distinguish between peripheral components, the nerves and ganglia, as well as central portions in the brain and spinal cord. The peripheral components contain the afferent (sensory) and the efferent (motor) connections between the receptors and the central nervous system, on the one hand, as well as the central nervous system and their target organs on the other. This module has been concerned mainly with the development of the central nervous system.

Due to the influence of the notochord and the prechordal plate (chorda-mesoderm complex) a thickening develops in the dorsal ectoderm, the neural plate (stage 7).  Between the 19th and the 32nd day of embryonic development, this neural plate is transformed into the neural tube (stage 10). From this, the brain and spinal cord emerge.  Already before the neural tube closes, cells of the neural crest migrate away (stage 9).  In essence, they end up forming the peripheral nervous system.
The transformation of cells of the surface ectoderm into neurectoderm cells is the result of the missing secretion of an inhibiting factor. There are, in addition, various signal molecules that are secreted and determine the polarity, the inner organization and the segmentation of the neural tube. The dorso-ventral differentiation manifests in the spinal cord in a separation of the sensory and motor components

From the 25th day (stage 9), three vesicles can be distinguished at the rostral end of the neural tube: the prosencephalon, the mesencephalon and the rhombencephalon.
With the 5th week, the further subdividing of the prosencephalon into the telencephalon and diencephalon and the rhombencephalon into the metencephalon and myelencephalon commences. Since at the mesencephalon no further subdivision takes place, 5 secondary cerebral vesicles result from it.

The nervous tissue arises in the region of the ependymal proliferation zone that borders on the inner cavity. In the first half of embryonic development, the glial cells and the nerve cells, the neurites, which only then become myelinized, arise from this layer.

Out of the central cavity system the cerebral ventricles and the central spinal cord channel that belongs to it emerge.

The spinal cord differentiates rapidly between the 6th and 10th weeks. Analogous to the formation of the mesoderm somites, a segmented structuring (metamerization) in the spinal cord also takes place. The sensory dorsal roots and the motor ventral roots bind themselves in each segment thereby forming the paired spinal nerves. This segmented organization becomes supplemented by the ascending and descending pathways that connect the brain and the spinal cord.

At around the 6th week, on the side of the telencephalon two lateral vesicles appear out of which, later, the cerebral hemispheres develop (stage 14). In the dorsal part of the vesicle, the pallium, the cerebral cortex arises via radial and tangential migration into it by the various nerve cells. This cellular differentiation occurs during the first two trimesters of the development. In the metencephalon region, the cerebellum emerges from two different germinal zones: an interior one in the region of the ventricular zone of the alar plates and an exterior one in the rostral part of the rhombic lips.

The myelencephalon gives rise to the cerebral nerves IX to XII. In its structure the caudal segment of the myelencephalon resembles the spinal cord whereby, dorsally, the interfaces of the sensory pathways develop. Rostrally, the myelencephalon widens due for the forming of the roof of the IVth ventricle. The choroid plexus that arises therein produces the cerebrospinal flui

At the metencephalon a floor (pons) and a roof (cerebellum) can be distinguished.  Through the dorsal spreading of the anterior rhombic lips the alar plate contributes to the formation of the cerebellum. The pontine nuclei also arise through the tangential cell migration out of the alar plate. Together with the basal plate they are involved in forming the nucleus regions of cerebral nerves V to VIII.

In the roof of the mesencephalon two masses of cells differentiate to become the superior and inferior colliculi, important interfaces for the visual and auditory pathways. In the mesencephalon tegmentum lie the nucleus regions of the cerebral nerves III and IV as well as the suprasegmental nucleus regions of the motor system (substantia nigra and red nucleus). Ventrally, the marginal zone of the mesencephalon becomes thickened through descending fiber systems to the cerebral peduncles.

In the prosencephalon the basal plate disappears in favor of the alar plate.

The diencephalon surrounds the IIIrd ventricle and connects the telencephalon with the mesencephalon. In the diencephalon roof the choroid plexus of the IIIrd ventricle arises as well as the epithalamus (epiphysis). On both sides of the IIIrd ventricle the nucleus regions of the thalamus arise as polymodal interfaces in the service of the cerebral cortex. The subthalamic sulcus marks the boundary between the thalamus and hypothalamus. Out of the ventral region of the hypothalamus a hormonal gland, the hypophysis, as well as the eye and the optic nerve proceed.

In the lateral hemisphere vesicles of the telencephalon the strong growth of the dorsolateral proliferation zone and the migration of the nerve cells into the surface layers lead to the formation of the cerebral cortices. These expand laterally, dorsally, caudally and ventrally so that the cerebral lobes arise and through the formation of the sulcus lateralis, the insula is enclosed. In addition, the expansion of the cortex leads to the emergence of the cerebral convolutions and furrows.
The commissures of the telencephalon in the service of the olfactory and temporal cortex areas (rostral commissure), the hippocampus (commissure of fornix) and the neocortex (corpus callosum) develop out of the lamina terminalis.
The olfactory system forms through the mutual inductions between the ventral surface of the frontal lobe and the nasal mucous membrane.

The blood supply of the brain begins early during the formation of the neural tube in the 4th week. In this, two input systems exist: the internal carotid artery and the vertebral artery.  The two vertebral arteries are connected to the basilar artery, which supplies the brainstem. They also release segmented spinal arteries into the spinal cord. The blood supply of the telencephalon occurs via 3 brain arteries, each of which is responsible for its own area. The venous return flow takes place independent of the arterial system via a sinus network within dura duplicates and flows into the internal jugular vein.