Current-controlled stimulation

During current-controlled stimulation, the current is constant throughout the period of the pulse thus the Ve at any point in space is constant during the pulse. During Vwe-ce control, current is not constant throughout the period of the pulse (Fig. 12) and the Ve at any point decreases during the pulse. 

The pulses may be of unidireetional eurrent (interrupted DC, monophasic), which implies that the current has a DC component. High-voltage pulsed galvanic stimulation is also used, with pulse currents up to some amperes, but pulse duration only a few microseconds. If DC effects are to be avoided (e.g., to reduce electrolytic effects or electrode metal corrosion), the current is biphasic. Faradic currents are biphasic currents of the type generated by an induction coil. If the pulses are slowly increased in amplitude, then reduced, and after a pause again increased, we have a ramp or surged current. Because many effects are current controlled, it is often better to use a constant amplitude current mode than a constant amplitude voltage mode of the stimulator output. 

Charge injection from an electrode into an electrolyte (e.g., extracellular fluid) is commonly controlled by one of three methods. In the current-controlled (also called galvanostatic) method, a current source is attached between the working and counter electrodes and a user-defined current is passed. In the voltage-controlled (also called potentiostatic) method, current is driven between the working electrode and counter electrode as required to control the working electrode potential with respect to a third (reference) electrode. This may be used for electrochemical measurements of certain neurotransmitters [19]. This method is most often not used for stimulation... 

Although blood pressure control follows Ohm s law and seems to be simple, it underlies a complex circuit of interrelated systems. Hence, numerous physiologic systems that have pleiotropic effects and interact in complex fashion have been found to modulate blood pressure. Because of their number and complexity it is beyond the scope of the current account to cover all mechanisms and feedback circuits involved in blood pressure control. Rather, an overview of the clinically most relevant ones is presented. These systems include the heart, the blood vessels, the extracellular volume, the kidneys, the nervous system, a variety of humoral factors, and molecular events at the cellular level. They are intertwined to maintain adequate tissue perfusion and nutrition. Normal blood pressure control can be related to cardiac output and the total peripheral resistance. The stroke volume and the heart rate determine cardiac output. Each cycle of cardiac contraction propels a bolus of about 70 ml blood into the systemic arterial system. As one example of the interaction of these multiple systems, the stroke volume is dependent in part on intravascular volume regulated by the kidneys as well as on myocardial contractility. The latter is, in turn, a complex function involving sympathetic and parasympathetic control of heart rate intrinsic activity of the cardiac conduction system complex membrane transport and cellular events requiring influx of calcium, which lead to myocardial fibre shortening and relaxation and affects the humoral substances (e.g., catecholamines) in stimulation heart rate and myocardial fibre tension. 

Cholecystokinin (CCK) is produced in the intestine and the brain. It appears to be an important mediator of anxiety. It also stimulates vasopressin secretion and slows gastric emptying. In addition, it is an important humoral satiety signal (appetite control). Various antagonists have been developed and are currently being investigated with regard to their therapeutic potential. 

These equations describe the full oxidation of a conducting polymer Submitted to a potential step under electrochemically stimulated confer-mational relaxation control as a function of electrochemical and structural variables. The initial term of /(f) includes the evolution of the current consumed to relax the structure. The second term indicates an interdependence between counter-ion diffusion and conformational changes, which are responsible for the overall oxidation and swelling of the polymer under diffusion control. 

Figure 4.5 Influence of oxidant stress on action potentials recorded In an isolated rabbit ventricular myocyte, (a) Control action potential, (b) Action potential recorded 3 min after exposure to oxidant stress induced by the photoactivation of rose bengal (50 nu). (c) Spontaneous and repetitive action potential discharges induced 6.5 min after exposure to rose bengal. Action potentials were recorded via a 2.5 MQ suction electrode and a current-clamp amplifier. The cell was stimulated at 0.1 Hz with a 2 ms suprathreshold current pulse and, when the cell showed automaticity (after 6 min), stimulation was stopped. Redrawn from Matsuura and Shattock (1991b).

The disadvantages described above in terms of the irreversibility of the polyion response stimulated further research efforts in the area of polyion-selective sensors. Recently, a new detection technique was proposed utilizing electrochemically controlled, reversible ion extraction into polymeric membranes in an alternating galvanostatic/potentiostatic mode [51]. The solvent polymeric membrane of this novel class of sensors contained a highly lipophilic electrolyte and, therefore, did not possess ion exchange properties in contrast to potentiometric polyion electrodes. Indeed, the process of ion extraction was here induced electrochemically by applying a constant current pulse. 

Thermally stimulated creep (TSCr) method, 21 742-743 Thermally stimulated current spectrometry (TSC), 21 743 Thermal mass meters, 20 681 Thermal mechanical analysis (TMA), of polyester fibers, 20 21 Thermal motion, in silicon-based semiconductors, 22 237-238 Thermal noise, silicon-based semiconductors and, 22 237 Thermal oxidation, 10 77-78, 79 in VOC control, 20 683-685 Thermal oxidation rates, silicon, 22 490 Thermal oxidizers... 

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