Spatially heterogeneous systems biological structures

The ntility of combining electrochemical and spectroscopic detection has also been demonstrated. Xin and Wightman demonstrated simnltaneons detection of catecholamine exocytosis and calcinm-ion release from single bovine chromaffin cells with a dual amperometric/calcinm-selective dye-bound fluorescence sensor and demonstrated that catecholamine and calcium release were temporally and spatially correlated (25). In microelectrode measnrements, individual exocytotic events associated with neurotransmitter release appear as spikes with the area of the amperometric spikes providing a 

Microelectrode suuctures have been created that mimic aspects of brain function. Amatore and coworkers have described assemblies of paired microband electrodes that behave like a neuronal synapse (26). The generator electrode in these devices mimics a synaptic terminal, and the collector electrode functions as a postsynaptic membrane. These artificial synapses can be designed in several configurations to perform Boolean logical operations, such as AND or OR operations. 

A number of successful bioelectronic devices have been created that exploit miaoelec-trode technology. These include the cochlear implant and peripherally implantable stimulators (27). Other devices including retinal or cortical implants are less developed and not yet applicable. A challenge in this field is to engineer microelectrode surfaces so that they are both biocompatible and can be used to direct cell adhesion and growth (e.g., to stimulate nerve growth across an interrupted neuronal pathway). 

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Disturbances that increase water scarcity promote the physical uniformity of river systems and the decrease of biological diversity in streams and rivers. The structure and functioning of heavily impacted river systems become mutually and strikingly similar, irrespective of the river s origin and the climate. The more intense and persistent the disturbance, the greater is the resemblance. On the other hand, river organisms use resources most efficiently in spatially heterogeneous chaimels, and under moderate disturbance frequencies, rather than in steady conditions, to which they are not adapted. 

In real systems, especially in heterogeneous catalytic and biological sys terns, the reactants are often arranged irregularly in space. Therefore, an arising instability may cause simultaneous diffusion of substances from one point to another inside the system to make the reactant concentration oscillations arranged in a certain manner in space during the occurrence of nonlinear chemical transformations. As a result, a new dissipative structure arises with a spatially nonuniform distribution of certain reac tants. This is a consequence of the interaction between the process of diffusion, which tends to create uniformity of the system composition, and local processes of the concentration variations in the course of nonlinear... 

As we have seen, the need for chemical simulations at a variety of levels is quite clearly demonstrated in the broad class of systems in which molecular and supermolecular interactions lead to macroscopic cooperativity, self-organization, and evolution. Although this class extends far beyond the domain of "classical" chemistry as well, it nevertheless covers the chemical spectrum, from combustion and explosions in gaseous mixtures to biochemical regulation of metabolism and biosynthesis, and from adsorption/ chemisorption and heterogeneous catalysis to chemical oscillations, spatial structures, and travelling chemical waves in condensed phase chemistry and biology. 

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