Structure of the nervous system

    Despite the extremely sophisticated routing methods axons, a significant fraction of initially established connections will disappear shortly after their formation following a selection process anddisposal.    The nervous system will first remove a fraction of neurons. In many cases, the neuron survival will be suspended in the presence of said neurotrophic molecules. In the absence of Such molecules, neurons trigger a programmed cell suicide, apoptosis (see below). These provide survival signals to the nervous system, on the one hand, to eliminate cells that have established aberrant connections, on the other hand, to control the size of the different populations of neurons. Thus, when neurotrophic molecules are present only in the region of a target population of neurons, only neurons whose axons actually happened to will survive. If, forThe abundance of neurotrophic molecules is limited, supernumerary neurons disappear fault arriving to collect sufficient survival signals. Thus, at the end of embryonic life in rodents, experimental inactivation of the first discovery neurotrophin, NGF ( nerve growth factor ) wiped out virtually all paravertebral sympathetic ganglion neurons. Conversely,injecting NGF doubles the number of these neurons.    Up to this point, the development and connectivity of neurons are determined entirely by genetic programs. However, at this stage of their formation, neural networks do not perform imperfectly function. For example, the visual acuity of a newborn mammal is poor compared to that of an adult subject. The nervous system is going to optimize network operations.This is the electrical activity (see below) of neurons that will afford because it spreads the information through the nervous system. For example, a light stimulus on the retina will cause propagating electrical activity to the parts of the brain in the specialized treatment visual information. The system will use this information to modify transport connections between neurons.The visual system of higher vertebrates thus provides an example of the effect of the electrical activity neural networks. In mammals, the information from the retinal neurons are transmitted in a brain region called the visual cortex. Mammals have binocular vision allows them to judge distances (compared to the vision of the right eye and that of the left eye). For this binocular vision is powerful, it is necessary that the information from each eye arrive in distinct regions of the visual cortex. At birth, the axon terminals with information from both eyes overlap extensively. Gradually, the system will reshape connections so that the activated termini simultaneously gather. As endings from the same eye are often synchronized among themselves and with those from the other eye, they eventually tend to cluster in different regions of the visual cortex.    This example illustrates one of the fundamental principles that guide the optimization training synapse between neurons: Synchronization of activity fosters connections convergent. This principle is implemented during the period of maturation of the nervous system, but it is likely that also governs the evolution of connections throughout the life of the neurons.