During the evolution of the living being we saw that the first neurons appeared on the outer surface of the organism, given that the primary function of the nervous system is to relate the animal to the environment. Of the three embryonic leaflets the ectoderm is the one that is in contact with the external environment of the organism and it is from this leaflet that the nervous system originates.
The first pointThe formation of the nervous system consists of a thickening of the ectoderm, located above the notochord, forming the so-called neural plaque. It is known that the formation of this plaque and the subsequent formation of the neural tube plays an important role in the inducing action of the notochord and mesoderm. Notochords implanted in the abdominal wall of amphibian embryos induce neural tube formation there. Notochord or mesoderm extirpations in young embryos resulted in major spinal anomalies.
The neural plate grows progressively, thickens and acquires a longitudinal groove called neural groove that deepens to form the neural gutter. The lips of the neural gutter fuse to form the neural tube.
The undifferentiated ectoderm then closes over the neural tube, thus isolating it from the external environment. At the point where this ectoderm meets the lips of the neural gutter, cells develop that form on each side a longitudinal lamina called the neural crest. The neural tube gives rise to elements of the central nervous system, while the crest gives rise to elements of the peripheral nervous system as well as non-nervous elements.
From the beginning of its formation, the caliber of the neural tube is not uniform. The cranial part, which gives rise to the adult brain, becomes dilated and constitutes the primitive brain, or archencephalon; the caudal part, which gives rise to the adult marrow, remains of uniform caliber and constitutes the early marrow of the embryo. In the archencephalon, three dilations are initially distinguished, which are the primordial encephalic vesicles called the forebrain, midbrain, and rhombencephalon. With the subsequent development of the embryo, the forebrain gives rise to two vesicles, telencephalon and diencephalon. The midbrain does not change, and the rhomboencephalon gives rise to the metencephalon and the myelencephalon.
The telencephalon comprises a median part, from which two lateral portions, the lateral telencephalic vesicles, are enveloped. The median part is anteriorly closed by a lamina that constitutes the cranial portion of the nervous system and is called terminal lamina. The lateral telencephalic vesicles grow very large to form the cerebral hemispheres and almost completely hide the middle part and the diencephalon.
The diencephalon has four small diverticula: two lateral, the optical vesicles, which form the retina; one dorsal, which forms the pineal gland; and a ventral, the infundibulum, which forms the neurohypophysis.
Neural Tube Cavity: The light from the neural tube remains in the adult nervous system, undergoing in some parts various modifications. Primitive marrow light in the adult forms the central canal of the marrow. The dilated cavity of the rhombencephalon forms the IV ventricle. The cavity of the diencephalon and the middle part of the telencephalon forms the III ventricle.
The light of the midbrain remains narrow and forms the cerebral aqueduct that joins the III to the IV ventricle. The light from the lateral telencephalic vesicles forms, on either side, the lateral ventricles, joined to the III ventricle by the two interventricular foramina. All cavities are lined by a cuboidal epithelium called the epithelium and, with the exception of the central canal of the medulla, contain a cerebrospinal fluid, or cerebrospinal fluid.
Flexures: During the development of the various parts of the archencephalon, flexures or curvatures appear on its ceiling or floor, mainly due to different growth rates. The first flexion to appear is the cephalic flexure, which appears in the region between the midbrain and the forebrain. Then, between the primitive medulla and the archencephalon, a second flexure, called cervical flexure, arises. It is determined by a ventral flexion of the entire embryo head in the region of the future neck. Finally, a third flexion appears, directed against the first two, at the point of union between the goal and the myelencephalon: the pontine flexure. With development, the two caudal flexions come apart and practically disappear. However, the cephalic flexure remains determined in the adult male brain an angle between the brain, derived from the forebrain, and the rest of the neuro axis.
Nervous system division based on anatomical and functional criteria
The central nervous system is that located within the axial skeleton (cranial cavity and vertebral canal); The peripheral nervous system is one that is located outside this skeleton. The brain is the part of the central nervous system within the neural skull; and the medulla is located within the spinal canal. The brain and spinal cord constitute the neuro axis. In the brain we have brain, cerebellum and brainstem.
One can divide the nervous system into nervous system of the relationship life, or somatic and nervous system of vegetative life, or visceral. The nervous system of relationship life is that which relates to organism with the environment. It has an afferent and an efferent component.
The afferent component leads to nerve centers impulses originating in peripheral receptors, informing them of what is going on in the environment. The efferent component leads to skeletal striated muscles the command of nerve centers resulting in voluntary movements.
The visceral nervous system is one that relates to innervation and control of the viscera. The afferent component conducts nerve impulses originating from viscera receptors to specific areas of the nervous system. The efferent component takes impulses from nerve centers to the viscera. This efferent component is also called autonomic nervous system and can be divided into sympathetic and parasympathetic nervous system.
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