Mechanisms, ionic coupling

Studied in both an experimental and theoretical manner. The link between glycolytic oscillations and the pulsatile secretion of insulin in pancreatic p cells [53] is another topic of current concern. Models for the latter phenomenon rely on the coupling between intracellular metabolic oscillations and an ionic mechanism generating action potentials. Such coupling results in bursting oscillations of the membrane potential, which are known to accompany insulin secretion in these cells [54, 55]. 

All polarizable models share the ability to polarize, by varying their charge distribution in response to their environment. In addition, shell models and EE models with charge-dependent radii have the ability to modify their polarizability—the magnitude of this polarization response—in response to their local environment. Consequently, it is reasonable to expect that shell models and mechanically coupled EE models may be slightly more transferable to different environments than more standard PPD and EE models. To date, it is not clear whether this expectation has been fully achieved. Although some shell-based models for both ionic and molecular compounds have been demonstrated to be transferable across several phases and wide ranges of phase points, " it is not clear that the transferability displayed by these models is better than that demonstrated in PPD- or EE-based models. And even with an environment-dependent polarizability, it has been demonstrated that the basic shell model cannot fully capture all of the variations in ionic polarizabilities in different crystal environments. ... 

The reactions illustrated in equations (62-64) are each catalyzed by rhodium acetate. Diazo compounds, especially diazoalkanes, can also react by a simple ionic mechanism. The esterification of a carboxylic acid with diazomethane is a familiar example. The ionic pathway is especially likely when the intermediate carbocation would be stabilized. Thus, diazine (158) couples smoothly with phenols, presumably by thermal rearrangement to the corresponding diazo sugar, followed by acid-catalyzed N2 loss... 

Acetylcholine Approximately 5% of brain neurons have receptors for ACh. Most CNS responses to ACh are mediated by a large family of G protein-coupled muscarinic M receptors that lead to slow excitation when activated. The ionic mechanism of slow excitation involves a decrease in membrane permeability to potassium. Of the nicotinic receptors present in the CNS (they are less common than muscarinic receptors), those on the Renshaw cells activated by motor axon collaterals in the spinal cord are the best-characterized. Drugs affecting the activity of cholinergic systems in the brain include the acetylcholinesterase inhibitors used in Alzheimer s disease (eg, tacrine) and the muscarinic blocking agents used in parkinsonism (eg, benztropine). 

The corrected ion and charged particle profiles in Figure 6, coupled with the information in Figure 1, could be looked at as a smooth progression from small ions to large molecular ions to large charged soot particles which produce neutral particles on recombination, consistent with an ionic mechanism of soot formation. 

Shaw RM, Rudy Y (1997) Ionic mechanisms of propagation in cardiac tissue. Roles of the sodium and L-type calcium currents during reduced excitability and decreased gap junctimi coupling. Circ Res 81(5) 727-741... 

As mentioned in the previous section the end-groups may be either hydroxyl or thiol (mercaptan) and as a result somewhat different vulcanization processes are used. The older elastomers, such as Thiokol FA, are considered to have hydroxyl end-groups and these are coupled by the use of a zinc compound. The actual mechanism is not clear but since zinc hydroxide, zinc borate and zinc stearate as well as the normally used zinc oxide are all effective it has been suggested (Fettes et al., 1954) that an ionic mechanism may be involved. 

Many C(sp )—C(sp ) coupling reactions not transition metal-mediated involve the two electron oxidation of ethers or amines to oxocarbenium ions or iminium ions. These are then attacked in an ionic mechanism by in i dM-formed enolates or other carbanion nucleophiles to generate product. As such, they will not be covered in this review. 

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