There are 3 modalities for intercellular communications: the chemical substances secreted by some cells are transported at distance where they act as signals on other cells; the surface molecules of a group of cells interact with the neighbouring cells; some special junctions or nexus provide direct relations between cells. In the first modality the chemical signals operate in 3 ways: a) many cells secrete one ore more chemical signals which act as local mediators (paracrine model); these mediators act immediately or are destroyed after they influence the neighbouring cells; b) some specialised cells--endocrine cells--secrete hormones, which are liberated in small amounts into the blood and exert their effects on some target cells, able to recognise and to respond to the hormonal signal; c) the neurones secrete chemical mediators--neurotransmitters, which act at the level of some special junctions--the chemical synapses. Most biologic phenomena are under the overlapping control of both systems--thus they are regarded as neuroendocrine system. The nervous cells transmit the informations much more rapidly than the endocrine cells. The chemical signals are various, as regarding the structure and function: they are large polypeptides, small polypeptides, glycoproteins, amino-acids, steroid molecules derived from cholesterol and fatty acids. The ability of the cells to respond to an extracellular signal molecule depends on the existence of some specific proteins, included in the plasma membrane, called receptors. The chemical signals influence the target cells both by altering the properties or the synthesis rate of their own proteins or by initiating the synthesis of new proteins. The chemical signals induce rapid and transient or slow- and long-lasting responses. All the neurotransmitters and the majority of hormones are water-soluble; the steroid and thyroid hormones are relatively water insoluble; the mechanisms of influencing the target cells are dependent of this feature: the water-soluble molecules do not pass through the target cell membrane, they bind to the surface specific receptor while the insoluble molecules cross the plasma membrane of the target cell and bind to the cytoplasmatic receptors. It results that the water-soluble molecules mediate short-time responses while those insoluble--long-lasting responses. As regarding the local chemical mediators they are secreted by mast cells or they are represented by the large category of prostaglandins. They produce a great diversity of biological effects, they are rapidly destroyed, and this way, they don't penetrate into the blood stream in significant amounts. The majority of the receptors from the surface of activated cells generate some intracellular signals both by altering the activity of some membrane enzyme (adenilate cyclase) with the accumulation of cyclic MPA and by modifying the permeability of some membrane channels(Ca2+ channels). The target cell exposed to a signal for a long period of time loose often the ability to respond to this signal. This process called desensibilization is reversible and is explained by endocytosis of surface receptors together with the ligand and by their lysosomal destruction, by the degradation of the receptor molecular conformation which becomes unable to bind the ligand or by the lack of activation of membrane enzymes or the channels. The gap or nexus junctions are composed by some proteic particles which form a hydrophilic channel to assure the communication between 2 neighbouring cells. These junctions allow some molecules (amino-acids, monosaccharides, cAMP, nucleotides) to pass from a cell to another one, facilitating the chemical and electrical coupling. These structures show a low electrical resistance, but they are dynamic, some junctions have the capacity to change from a low resistance to a high resistance state, isolating the cells from communicating with their neighbours. Oxygen deprivation, the increase of intracellul