Depolarization current

E. Fukada, Depolarization Current and Molecular Motion in Polyethylene and Polytetrafluoro-ethylene Electrets , Rfept No AST-18401-120-75, FSTC-1901-75, Army For Sci Tech Center, Charlottesville (1976)... 

Dielectric Polarization under electric Hied Temperature step or scan Depolarization current... 

Even when they have a partial crystallinity, conducting polymers swell and shrink, changing their volume in a reverse way during redox processes a relaxation of the polymeric structure has to occur, decreasing the crystallinity to zero percent after a new cycle. In the literature, different relaxation theories (Table 7) have been developed that include structural aspects at the molecular level magnetic or mechanical properties of the constituent materials at the macroscopic level or the depolarization currents of the materials. 

FIGURE 30.3 Changes in membrane potential cp of a cell membrane occurring upon application of depolarizing current pulses of different amplitude / (a,b) below threshold (c) excitation of the membrane during an above-threshold pulse. 

Extracellular ATP has been demonstrated to activate a depolarizing current in different neuronal and non-neuronal cell types. These receptors are also referred to as P2 receptors. The receptors can further be divided into the G-protein-coupled P2Y receptors and the ligand-gated ion channels P2X. Currently, seven P2X receptors (P2XJ-P2X- ) have been cloned (Table 3.3). The receptors exhibit between 26 and 50% overall amino-acid identities, with the highest level of conservation in the extracellular and transmembrane regions. P2X7 (also called P2Z) is the most distant member of the family. 

In vitro Disaggregated cells Repolarizing currents (e.g., IKs, IK1, Ito), depolarizing currents (e.g., INa) currents, ICa (whole cell patch-clamp) Disaggregated cells ventricular myocytes mouse atrial tumor cells (AT-1) immortalized cardiac muscle cells (HL-1) Jost et al.,-65 Liu and Antzelevitch 66 Jurkiewicz and Sanguinetti 67 Li et al. 68 Yang and Roden 69 Banyasz et al. 70 Xia et al.71... 

The majority of the inputs to the granule cells are excitatory, each of which provides a small depolarizing current to the membrane of the cell body. The point of contact between the axonal projection from the neuron and an adjacent cell is termed the s)mapse which under the electron microscope appears as a swelling at the end of the axon. Most synapses are excitatory and are usually located along the dendritic branches of the neuron. The contributions of the individual excitatory s)mapses are additive and, as a result, when an excitatory stimulus occurs a wave of depolarizing current travels down the axon to stimulate the adjacent cell body. However, some synapses are inhibitory, usually fewer in number and strategically located near the cell body. These synapses, when activated, inhibit the effects of any excitatory currents which may travel down the dendritic processes and thereby block their actions on the neuron (Figure 2.1). 

Automaticity can be defined as the ability of a cell to alter its resting membrane potential toward the excitation threshold without the influence of an external stimulus. The characteristic feature of cells with automaticity is a slow decrease in the membrane potential during diastole (phase 4) such that the membrane potential reaches threshold (Figure 16.2). During phase 4 in these pacemaker cells, the background potassium leak current decreases and an inward depolarizing current (h) is... 

A detailed discussion of the statistical thermodynamic aspects of thermally stimulated dielectric relaxation is not provided here. It should suffice to state that kinetics of most of the processes are again complicated and that the phenomenological kinetic theories used to described thermally stimulated currents make use of assumptions that, being necessary to simplify the formalism, may not always be justified. Just as in the general case, TSL and TSC, the spectroscopic information may in principle be available from the measurement of thermally stimulated depolarization current (TSDC). However, it is frequently impossible to extract it unambiguously from such experiments. 

Thermally Stimulated Depolarization Currents in Amorphous Chalcogenides... 

The po and Pi ratio in equation (2.3) determines which of two factors—namely, equilibrium or nonequilibrium (due to emission from traps) carriers—dominate in the relaxation process. That is, the depolarization current contains two maximum one is related to release of carriers from trap the origin of the other lies in the change of conductivity with temperature [14-18]. Although only one of the peaks mentioned contains information about trap parameters, it is possible to discriminate between simultaneously occurring processes, e.g., thermally stimulated depolarization and thermally stimulated dielectric relaxation. 

In pacemaker cells (whether normal or ectopic), spontaneous depolarization (the pacemaker potential) occurs during diastole (phase 4, Figure 14-1). This depolarization results from a gradual increase of depolarizing current through special hyperpolarization-activated ion channels (If, also called If,) in pacemaker cells. The effect of changing extracellular potassium is more complex in a pacemaker cell than it is in a nonpacemaker cell because the effect on permeability to potassium is much more important in a pacemaker (see Effects of Potassium). In a pacemaker—especially an ectopic one—the end result of an increase in extracellular potassium is usually to slow or stop the pacemaker. Conversely, hypokalemia often facilitates ectopic pacemakers. 

When the film is short-circuited and heated to high temperatures at which the molecules attain a sufficiently high mobility, a current is observed in the external circuit. This phenomenon is called pyroelectric effect, thermally stimulated current, or, when the film has been polarized by a static field prior to measurement, depolarization current. The conventional definition of pyroelectricity is the temperature dependence of spontaneous polarization Ps, and the pyroelectric constant is defined as dPJdd (6 = temperature). In this review, however, the term will be used in a broader definition than usual. The pyroelectric current results from the motion of true charge and/or polarization charge in the film. Since the piezoelectricity of a polymer film is in some cases caused by these charges, the relation between piezoelectricity and pyroelectricity is an important clue to the origin of piezoelectricity. 

The pyroelectricity or depolarization current is closely related to the piezoelectricity of Groups (B) and (C), because the pyroelectric current is caused by the thermal activation of charges in the film. The application of polypeptide film to an acoustic transducer was proposed by Fukada, Tamura, and Yamamoto (1968). 

FIGURE 18.11 Thermally stimulated depolarization currents of PVP K30 demonstrating two different global relaxation peaksPi is the (5-relaxation peak (representing molecular motion belfry, and P2 is the a-relaxation peak (representing mobility ). 

Shmeis, R. A., Z. V fong, and S. L. Krill. 2004. A mechanistic investigation of an amorphous pharmaceutical and its solid dispersion, Part I Acornparative analysis by thermally stimulated depolarization current and differential scanning calorimetl harm Re 1 2025-2030. 

Effects of three antiseizure drugs on sustained high-frequency firing of action potentials by cultured neurons. Intracellular recordings were made from neurons while depolarizing current pulses, approximately 0.75 s in duration, were applied (on-off step changes indicated by arrows). 

There are at least two ways to obtain information by t.s.c. The first is that for the global spectra where the polymeric film is polarized by a static electric field at the polarization temperature Tp and then quenching down to the freezing temperature. With the field turned off and the sample short circuited the depolarization current due to dipolar reorientation is measured as the temperature increases from T0 to the final temperature Tf > Tp. 

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