Threshold of excitation

In response to stimulation, the neuronal membrane becomes more permeable to Na+, which is drawn into the neuron down the electrical and chemical diffusion gradients. This depolarizes the neuron and makes the membrane further permeable to Na+. If sufficiently depolarized, a threshold of excitation is reached (approximately -60 mV), where voltage-gated Na+ channels open and the membrane becomes highly permeable... 

The quote, however, is as true today as it was nearly twenty years ago. We stand on the threshold of exciting new applications for computers both within the field of education and elsewhere. The subject of this paper is a computer program which represents one totally different approach for the use of computers in chemical education. We hope that it is only one new approach out of many that we will see in the future. 

At least a third of the chapters in this book deal with the use of vanadium in one of its several oxidation states. New analytical techniques have become available that make it possible to characterize these materials more precisely and to determine the exact nature of the active centers. We appear to be on the threshold of exciting breakthroughs in fundamental research that should result in signihcant improvements to industrial oxidation processes. Our hope is that this book will provide insights and stimulate additional research that will achieve these goals. 

Bellermann, M.R.W., Koch, P.M., Mariani, D.R. and Richards, D. (1996). Polarization independence of microwave ionization thresholds of excited hydrogen atoms near the principal resonance, Phys. Rev. Lett. 76, 892-895. 

Mexiletine hydrochloride, like cla.ss I antiarrhythmic agents, blocks the fast Na channel in cardiac cells. It is especially effective on the Purkinje fibers in the heart. The drug increases the threshold of excitability of myocardial cells by reducing the rale of rise and amplitude of the action potential and decreases aulomaliciiy. 

This conjecture is corroborated by the fact that the enzymic model possesses other properties shared by neurons of the thalamus. Reminiscent of the phenomenon described in section 3.3, two distinct thresholds of excitability, linked to two voltage-dependent Ca conductances, have been demonstrated in these neurons by Jahnsen Llinas (1984a). Moreover, the evolution towards a stable steady state takes a different form, depending on the initial state of the system. Thus, in... 

Autoionization by configuration interaction is usually quite fast, and there are many cases in which it predominates over predissociation so that the thresholds of excited states are hidden. However, ethylene " provides an example in which predissociation is sufficiently rapid that the thresholds for the first excited electronic state are apparent in the cross section. 

Excitability is an important property in many biological systems, such as neurons, and it is also closely connected with traveling waves in chemical systems. Excitability has two important characteristics a threshold of excitation and a refractory period. The threshold is the minimum perturbation that will cause... 

To create model A, we must link the two subsystems in an appropriate fashion. We introduce two coupling parameters, gi and gj, and a third parameter, k, that adjusts the relative time scales of the two subsystems. The first coupling parameter causes changes in the variable V of system II to affect the control parameter p in system I. To accomplish this, we first identify P with the constant term k A in eq. (8.3) and then replace p hy p + g V. We also introduce a coupling in the other direction by replacing Vby V + g2B on the right-hand sides of eqs. (8.5) and (8.6). This modification causes oscillations of sufficient magnitude in system I to push system II beyond the threshold of excitability. 

The estimated maximum probability of Hg dissociation is P 0.1 [295]. It can be concluded from the various electron energies measui ed that the excitation function of the level has a maximum at 8.8 eV, close to the threshold of excitation. This corresponds to the most probable transition according to the Franck-Condon principle (Fig. 39). The threshold of molecular nitrogen dissociation has been found [335] to be 9.6 i 0.05 eV, a value virtually coinciding with the Ng dissociation energy (9.76 eV). (Concerning electron impact excitation of atoms and molecules see e.g. [181, 441].)... 

The threshold of excitation, depending on the bromide ion production, increases with rise of both oxygen and malonic acid concentrations. As a result, a nonmonotonic profile of the excitability threshold can appear, with its maximum in the middle of the layer. If the maximum of the excitability threshold is high enough, a poorly excitable sublayer appears between the two excitable sublayers at the top and bottom. 

This behavior was simulated in computer experiments with a simplified model of two excitable layers with linear coupling [52, 59]. Mutual acceleration of waves in crossing points was obtained resulting in a saw-tooth shape of the wave fronts. Development of spiral waves was also observed due to a partial annihilation of colliding waves propagating in both layers when different thresholds of excitability were taken for two layers. 

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