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A drug administered in the same dosage, under similar conditions, to adult individuals from a population homogeneous in race, gender and age, triggers different pharmacological effects. This phenomenon represents the pharmacological variability in a relatively homoge‐ neous population, as a natural expression of the biological variability of the response to any stimulus. The cause of the pharmacological variability to a drug is often considerably differ‐ ent between the individuals of the same population. The pharmacology variability (pharma‐ cokinetics, pharmaco-dynamics and pharmaco-toxicological) is therefore of two types: interindividual (on population level) and, respectively, intra-individual (on individual level).

The neurons represent unique anatomical fractions, with the ability to forward the information in a network system, which justifies the experience-dependent mechanisms such as memorization, learning or consciousness. In order to fulfill these functions, the neurons are, structurally and functionally polarized. This aspect is obvious in their tripartite composition: cell body, axon and dendrites. While the body comprises the biosynthesis structure (nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus and mitochondria for energy storage), the axon is equipped with molecular and under-cellular components for the propagation of the action potential from the cell body to distant targets, the dendrites represent a set of branched endings which prolong from the cell body and have as result an increase of it, in order to receive the signal. The structural connections for the signal transmission are the basis of the neural architecture. Inside the mature nervous system, normally, the neural circuit (neuroarchitecture) supports adaptive changes. Inside an old synapses neural cells can involute (until their disappearance), with simultaneous formation of new branches (new neuritis) which will establish new synapses. These changes in the neurotic and synaptic architecture prove the great plasticity (capacity) of the nervous system to adapt to the environmental conditions. Today we consider that there are two major intracellular classes of factors (signals) which adjust the neuronal development and adaptation: growth factors (the growth factor fibroblasts, the ciliary neutrophil factor, etc.) and the neurotransmitters factors (dopamine, serotonin, acetylcholine, glutamate, etc.). The loss of balance between the two systems of factors leads to the development of the pathological states.