A Threshold Equation for Action Potential Initiation

In central neurons, the threshold for spike initiation can depend on the stimulus and varies between cells and between recording sites in a given cell, but it is unclear what mechanisms underlie this variability. Properties of ionic channels are likely to play a role in threshold modulation. We examined in models the influence of Na channel activation, inactivation, slow voltage-gated channels and synaptic conductances on spike threshold. We propose a threshold equation which quantifies the contribution of all these mechanisms. It provides an instantaneous time-varying value of the threshold, which applies to neurons with fluctuating inputs. We deduce a differential equation for the threshold, similar to the equations of gating variables in the Hodgkin-Huxley formalism, which describes how the spike threshold varies with the membrane potential, depending on channel properties. We find that spike threshold depends logarithmically on Na channel density, and that Na channel inactivation and K channels can dynamically modulate it in an adaptive way: the threshold increases with membrane potential and after every action potential. Our equation was validated with simulations of a previously published multicompartemental model of spike initiation. Finally, we observed that threshold variability in models depends crucially on the shape of the Na activation function near spike initiation (about −55 mV), while its parameters are adjusted near half-activation voltage (about −30 mV), which might explain why many models exhibit little threshold variability, contrary to experimental observations. We conclude that ionic channels can account for large variations in spike threshold

Assessment of applied microwave power of intermittent microwave-dried carrot powders from Colour and NIRS

ArticleApplied microwave (MW) power level is an essential factor on the quality of the dried agricultural products. Even if higher MW powers result in shorter drying times, they lead to quality degradations. It is almost impossible to know the applied MW power of a dried and powdered product by human vision. Thus, the aim of this study was to predict the applied MW power of carrot powders by using two different instruments, a chromameter and FT-NIRS. The experiments were carried out at nine different power levels (100–500 W) with three replications (N = 27). The colour and NIR reflectance was measured using a chromameter and NIRS system. The data was analysed using PLS regression. The drying time of intermittent MW drying at the highest applied power of 500 W was 1.12–5.47 times shorter than those of other lower applied powers. Applied MW power was a crucial factor on all colour parameters of the powdered carrots. Brightness (L*) decreased significantly with the increase of applied MW power resulting in darker product colours. Data analysis results showed that the NIRS system (R2 = 0.99; SEP = 16.1 W) can predict the microwave power of powdered carrots with significantly better performance than a chromameter (R2 = 0.95; SEP = 29.9 W). But, the chromamater is far more inexpensive when compared with the NIRS system and hence, it can also be used to predict the applied MW power from the colour data relatively well. Also, a mathematical model was developed to predict applied MW power from the colour parameters

Characterization of aroma and phenolic composition of carrot (Daucus carota ‘Nantes’) powders obtained from intermittent microwave drying using GC–MS and LC–MS/MS

Carrot is an important food in human nutrition. The effects of different microwave (MW) powers on the aroma, phenolic and antioxidant properties of MW-dried and powdered carrot samples were investigated. Fresh carrot samples (Daucus carota ‘Nantes’) were dried using seven different MW power levels of 150, 200, 250, 300, 350, 400 and 450 W (0.50, 0.67, 0.83, 1.00, 1.17, 1.33 and 1.50 W/g, respectively). The aroma compounds of the carrot samples were extracted by purge and trap method and determined by gas chromatography-mass spectrometry (GC[sbnd]MS). In fresh samples, terpenes were identified as the dominant aroma group and a significant reduction was found in the amount of these compounds depending on the applied MW power level. Terpinolene, (E)-?-bisabolene, elemicin and myristicin were identified as dominant terpenes. In the dried samples, aldehydes, furans, alcohols, acids and pyrazines were determined as major components depending on the applied MW power levels. Hexanal, 3-methyl-butanal, acetic acid and hexanoic acid were dominant in the dried samples. For the determination of the phenolic compounds, the method of liquid chromatography coupled with tandem mass spectrometry (LC[sbnd]MS/MS) was employed. As a result of the analysis, a total of seven phenolic compounds, two of which were more dominant (3-caffeoylquinic acid and di-caffeic acid derivative) were identified. The total phenolic content of the carrot samples dried with lower MW power level of 150 and 200 W (0.50 and 0.67 W/g) was found to be relatively better preserved. When both aroma and phenolic compounds were evaluated, it was observed that they were better preserved in the carrot samples dried with low MW power levels (150 and 200 W). © 2019 Institution of Chemical Engineer