Membrane time constant

Barker A.T., Garnham C.W., and Freeston I.L. Magnetic nerve stimulation — the effect of waveform on efficiency, determination of neural membrane time constants and the measurement of stimulator output, in magnetic motor stimulation basic principles and clinical experience. Electroenceph. Clin. Neurophysiol. 1991 43 227-237. 

Cardioversion or defibrillation is the electrical termination of arrhythmias using field stimulation. Unlike pacing, in which cardiac excitation is initiated in and propagates from a small region of tissue near the electrode, cardioversion must arrest electrical activity by simultaneous stimulation of most of the heart. In practice, this means establishing a critical field across a critical mass of cardiac tissue. This requires a compromise between the electrical response of the tissue and the electrical capabilities of the device. The electrical response of cardiac cells is complex, but stimulation mostly depends on the first-order properties of the membrane [6]. Theoretical and experimental studies have shown that the optimum voltage waveform for stimulation of cardiac tissue is a waveform with a characteristic rise time comparable to the cell membrane time constant [7,8]. 

The smallest current amplitude required to cause excitation is known as the rheobase current (/rh). T is the membrane time constant of the axon if the axon is stimulated intracellularly. For extracellular stimulation, T is a time constant which takes into account the extracellular space resistance. The relationship between current amplitude and pulse width can also be derived theoretically using the cable equation by assuming that total charge on the cable for excitation is constant [Jack et al., 1983]. 

In the case of the voltage clamp, the magnitude of the membrane potential is imposed and one monitors the resulting current which flows through the membrane. The current clamp method is of limited use when the time constants of ionic conductances are larger than the membrane time constant or when ionic conductances change at a threshold potential because in this case unstable potentials are arising which are not attainable by a constant current. 

Ishida et al. were some of the first investigators to propose a method for investigating the adhesive properties of tablets [151]. Their method involved placing a tablet onto a membrane under constant pressure for one minute and then measuring the force required to remove it. Most methods published since that time involve essentially the same principle, with variations in the type of membrane used and the... 

While in vivo studies assess absorption rates as process-lumped time constants from blood level versus time data, these rate parameters encompass the kinetics of dosage-form release, GI transit, metabolism, and membrane permeation. The use of isolated tissue and cellular preparations to screen for drug absorption potential and to evaluate absorption rate limits at the tissue and cellular levels has been expanded by the pharmaceutical industry over the past several years. For more detail in this regard, the reader is referred to an article by Stewart et al. [68] for references on these preparations and for additional details on the various experimental techniques outlined below. 

In the previous paragraph, the basic considerations of FEM modelling have been laid out. The outcome of a static thermal simulation based on this model is a 3-d temperature field T x,y,z). In this section it is discussed, how the characteristic figures, such as thermal resistance and thermal time constant of the membrane, can be deduced. 

In order to determine the thermal time constant of the microhotplate in dynamic measurements, a square-shape voltage pulse was applied to the heater. The pulse frequency was 5 Hz for uncoated and 2.5 Hz for coated membranes. The amplitude of the pulse was adjusted to produce a temperature rise of 50 °C. The temperature sensor was fed from a constant-current source, and the voltage drop across the temperature sensor was amplified with an operational amplifier. The dynamic response of the temperature sensor was recorded by an oscilloscope. The thermal time constant was calculated from these data with a curve fit using Eq. (3.29). As already mentioned in the context of Eq. (3.37), self-heating occurs with a resistive heater, so that the thermal time constant has to be determined during the cooHng cycle. 

The deviation between the time constants for membrane heating and cooling was measured as well (Eq. (3.37)). The heater of a single microhotplate was driven with a rectangular-shape current pulse. The pulse amplitude was adjusted to produce a temperature rise of 50 °C. In this case the measured time constant for cooling was... 

This gives a time constant between 0.1 and 1 s. The slowest process, however, is membrane hydration, the time constant of which can be estimated by... 

Dynamic characteristics of a fuel cell engine are of paramount importance for automotive application. Three primary processes govern the time response of a PEFC. They are (1) electrochemical double-layer discharging, (2) gas transport through channel and GDL, and (3) membrane hydration or dehydration (i.e., between a dry and a hydrated state). The time constant of double-layer discharging is between micro- and milliseconds, sufficiently short to be safely ignored for automotive fuel cells. The time constant for a reactant gas to transport through GDL can be estimated simply by its diffusion time, i.e.,... 

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