Sedative concentrations of propofol have an important effect on the control of breathing, showing depression of the ventilatory response to hypercapnia. The depression is attributed to an exclusive effect within the central chemoreflex loop at the central chemoreceptors. In contrast to low-dose inhalational anesthetics, the peripheral chemoreflex loop, when stimulated with carbon dioxide, remains unaffected by propofol.
The classical power spectrum, computed in the frequency domain, outranks traditionally used periodograms derived in the time domain (such as the chi2 periodogram) regarding the search for biological rhythms. Unfortunately, classical power spectral analysis is not possible with unequally spaced data (e.g., time series with missing data). The Lomb-Scargle periodogram fixes this shortcoming. However, peak detection in the Lomb-Scargle periodogram of unequally spaced data requires some careful consideration. To guide researchers in the proper evaluation of detected peaks, therefore, a novel procedure and a computer program have recently become available. It is recommended that the Lomb-Scargle periodogram be the default method of periodogram analysis in future biomedical applications of rhythm investigation.
Aggregates of heart cells from chicken embryos beat spontaneously. We used intracellular microelectrodes to record the periodic behavior of the membrane potential that triggers the contractions . Every 5-12 beats, a short current pulse was applied at various points in the cycle to study the phasedependent resetting of the rhythm . Pulses stronger than 2.5 nA caused the final rhythm to be reset to almost the same point in the cycle regardless of the phase at which the pulse was applied (type zero resetting) . Pulses of 51 nA only caused a slight change of the phase. Increasing current intensities to between 1 and 2.5 nA gave rise to an increasing steepness in a small part of the phaseresponse curve. The observation of type zero resetting implies the existence of a critical stimulation that might annihilate the rhythm. Although we did find a phase at which more or less random responses occurred, the longest pause in the rhythm was 758 ms, 2.4 times the spontaneous interval . This suggests that the resting membrane potential was unstable, at least against the internal noise of the system . The conclusions are discussed in terms of the concepts of classical cardiac electrophysiology .
Rabbits that have been exposed for several weeks to a L12:D12 regimen exhibit afterwards, under constant circumstances and in steady darkness for 53 h, the presence of "programmed" circadian rhythms in the RMS value and spectral composition of their (computer-analysed) EEG's, in the RMS value of their electromyograms, and in skin temperature. For the occipital and frontal EEG's the RMS value attains its lowest level during a dip occurring about 3 h before the previous light on-moment and its maximum, which is 1.5 times the minimal value, during a peak at some 3 h before the previously occurring light off-transient. Similar changes occur in the power spectral densities of all frequencies between 0.5 cps and 40 cps, with the result that circadian changes in the spectral composition of the EEG, although present and having the same time course, are rather slight. The results are discussed in terms of the two-oscillator model for the central circadian mechanism.
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