Local-action current

Any metal surface, similar to the situation for zinc, is a composite of electrodes electrically short-circuited through the body of the metal itself (Fig. 2.2). So long as the metal remains dry, local-action current and corrosion are not observed. But on exposure of the metal to water or aqueous solutions, local-action cells are able to function and are accompanied by chemical conversion of the metal to corrosion products. Local-action current, in other words, may... 

Whereas auxiliary anodes need not be consumed in order to fulfill their purpose, sacrificial anodes are consumed not less than is required by Faraday s law in order to supply an equivalent electric current. In general, the observed rate of consumption is greater than the theoretical. For zinc the difference is not large, but for magnesium it is appreciable, with the cause being ascribed to local-action currents on the metal surface, to formation of colloidal metal particles [13, 14] or, perhaps more important, to initial formation of univalent magnesium ions [15], The latter ions are unstable and react in part with water in accord with... 

Current between a magnesium anode and a 50-gal steel tank filled with air-saturated hot water is 100 mA. Disregarding local-action currents, what interval of time is required between filling and emptying the tank to ensure minimum corrosion of outlet steel water piping (Solubility of oxygen in inlet water, 25 °C, = 6mL/liter.)... 

Excitability. Excitabihty refers to electrical responsiveness of the heart to various stimuli by the generation of local excitatory currents, action potentials, or fibrillation. 

FIGURE 4-1 Impulse conduction in unmyelinated (top) and myelinated (bottom) fibers. The arrows show the flow of action currents in local circuits into the active region of the membrane. In unmyelinated fibers the circuits flowthrough the adjacent piece of membrane but in myelinated fibers the circuit flow jumps to the next node. 

A major aspect of environmental safeguarding is the abolishment of bulk neutralization of pollutants in a mixed fluid flow. It must be replaced by local action taken, wherever possible, on a component-by-component basis as close to the origin as may be, and preferably inside the source. This is just the opposite of currently prevailing systems where the entire spectrum of pollutants are collected and neutralized and/or recycled. Moreover, the local treatment performed close to the source is less expensive, if one takes into account all relevant costs. Since vehicles mostly rely on combustion for their energy, the safeguarding methods will include ... 

The AChR consists of five subunits surrounding an ion-conducting channel (Fig. 28.2). Activation of the binding sites on the two a-subunits results in a conformational change. This allows the simultaneous inflow of Na+ and Ca++ and outflow of K+, with a net inflow of positive charge. The response to a spontaneously secreted quantum of ACh (that is, activation of several thousand AChRs) is seen as a miniature EPC. With nerve stimulation, many quanta are released synchronously to produce a fuU-sized EPC, which is the sum-mated response of the 200 or so individual miniature EPCs. The EPC is a local graded current that in normal conditions triggers an action potential in the adjacent muscle membrane (Fig. 28.1). 

Most studies of the local actions of opioids on the intestinal mucosa have utilized muscle-stripped sheets of ileal mucosa with attached submucosa mounted in Ussing flux chambers. Peptidase-resistant enkephalin derivatives such as DPDPE decrease short-circuit current, an electrical measure of active transepithelial ion transport, across isolated mucosal sheets from the guinea pig ileum [46,127,128], rabbit ileum [129-131], mouse jejunum [132], and pig distal jejunum/ileum [133]. This effect, which occurs after the application of opioid agonists to the serosal aspect of epithelial sheets, is due to an increase in electroneutral salt absorption and a decrease in electrogenic chloride secretion [46,132,133], In contrast to enkephalin derivatives, opiate alkaloids have limited effects on active transepithelial transport of ions [69]. Pretreat-... 

The equilibrium situation in an electrochemical cell is obtained, if the electrical current is interrupted, if all local actions (e.g. transport in the electrode) have come to an end and no internal short circuits occur. Then, as mentioned (Figure 3.5.10), the cell voltage is determined by the difference in the lithium potential (chemical potential of lithium) between the left-hand side (Ihs) and right-hand side (rhs) of the electrochemical cell (E - open cell voltage, F - Faraday constant) ... 

The foregoing represent currently accepted therapeutic uses of aerosols. Other uses have been or are being investigated and these exploit the lungs as a site for delivery of a drug to bypass absorption and metabolic processes that limit their use by other routes or to exert a local action in the airways. 

Conduction velocity of an action potential can be increased dramatically by the presence of myelin, a fatty sheath that surrounds the axon and that is interrupted into gaps every millimeter or so at the nodes of Ranvier. Myelin is elaborated by Schwann cells in the peripheral nervous system and by oligodendrocytes in the central nervous system (the biochemistry of myelin will be discussed later in the article). The presence of myelin will dramatically alter the mode and velocity of conduction of the action potential in the axon. As in unmyelinated nerves, the action potential is still transmitted from one section of the axon to another by the presence of local circuit currents. However, the fatty sheath of myelin has poor conduction properties and therefore acts as an insulator. Hence, the local circuit currents jump from one gap to another at the nodes of Ranvier and the rate of conduction is enhanced as local circuit currents travel faster than the action potential itself This process of discontinuous conduction is known as saltatory conduction. Numerous diseases involving myelin deficiency have been described clinically. As one might predict, demyelinating diseases have profound effects on neuronal conduction and on the well-being of the patient. A few of these conditions will be described briefly in the upcoming section on myelin biochemistry. 

Mesalamine (5-aminosalicylic acid asacol, others) is a salicylate that is used for its local effects in the treatment of inflammatory bowel disease (see Chapter 38). It currently is available as a suppository and rectal suspension enema (rowasa) for treatment of mild-to-moderate proctosigmoiditis Cl rectal suppository (canasa, others) for the treatment of distal ulcerative colitis, proctosigmoiditis, or proctitis. Oral formulations and controlled-release capsule that deliver drug to the lower intestine are efficacious in treatment of inflammatory bowel disease, in particular ulcerative colitis. Sulfasalazine (salicylazosulfapyridine azulfidine) contains mesalamine linked covalently to sulfapyridine (see Chapter 38) it is absorbed poorly after oral administration, but it is cleaved to its active components by bacteria in the colon. The drug is of benefit in the treatment of inflammatory bowel disease, principally because of the local actions of mesalamine. 

CRYPTORCHIDISM Cryptorchidism, the failure of one or both testes to descend into the scrotum, affects up to 3% of fuU-term male infants, becoming less prevalent with advancing postnatal age. Cryptorchid testes have defective spermatogenesis and are at increased risk for developing germ cell tumors. Hence, the current approach is to reposition the testes as early as possible, typically at 1 year of age but definitely before 2 years of age. The local actions of androgens stimulate descent of the testes thus, hCG can be used to induce testicular descent if the cryptorchidism is not secondary to anatomical blockage. Therapy usually consists of injections of hCG (3000 lU/m body surface area) intramuscularly every other day for 6 doses. 

Cathodic protection (CP) is achieved by applying electrochemical principles to metallic components buried in soil or immersed in water. It is accomplished by flowing a cathodic current through a metal-electrolyte interface, favoring the reduction reaction over the anodic metal dissolution. This enables the entire structure to work as a cathode. Cathodic protection is accomplished by supplying an external current to the corroding metal on the snrface of which local action cells operate [10]. 

Both resistance of the electrolyte and polarization of the electrodes limit the magnitude of current produced by a galvanic cell. For local-action cells on the surface of a metal, electrodes are in close proximity to each other consequently, resistance of the electrolyte is usually a secondary factor compared to the more important factor of polarization. When polarization occurs mostly at the anodes, the corrosion reaction is said to be anodically controlled (see Fig. 5.7). Under anodic control, the corrosion potential is close to the thermodynamic potential of the cathode. A practical example is impure lead immersed in sulfuric add, where a lead sulfate film covers the anodic areas and exposes cathodic impurities, such as copper. Other examples are magnesium exposed to natural waters and iron immersed in a chromate solution. 

At low current densities, local-action corrosion supplements stray-current corrosion. At high current densities in some environments, oxygen may be evolved, reducing the amount of metal corroding per faraday of electricity. 

If a noncorroding metal (minimum local action) that polarizes only slightly (high value of is polarized anodically at moderate current densities, then ioa can be substituted for in Fig. 29.1, and... 

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