Variable Pulse Amplitude Research Articles

Luteinizing hormone (LH)-releasing hormone in third ventricular cerebrospinal fluid of the ewe: correlation with LH pulses and the LH surge

Morphological evidence suggests that LHRH may be secreted into cerebrospinal fluid (CSF), but only in the rhesus monkey has CSF LHRH been found to reflect changes occurring in the LHRH neuroendocrine system. This study investigated whether LHRH is detectable in ovine CSF and, if so, whether its release profile correlates with peripheral LH profiles during pulse and surge conditions. A polyethylene catheter was threaded through a stainless steel guide cannula previously implanted into the third ventricle of an ovariectomized ewe, which enabled continuous CSF withdrawal on repeated occasions. The first experiment (n = 3) showed that peripheral LH concentrations were unaffected by CSF removal at rates of 12, 30, 50, and 100 microliters/min, and the second (n = 4) established that CSF LHRH secretion was pulsatile, with considerable variation in pulse amplitude (6.3 +/- 1.8 pg/ml; range, 1.3-18.7 pg/ml). In the third experiment (n = 6), an endogenous LH surge was induced after progesterone withdrawal and 17 beta-estradiol administration. Although CSF LHRH (15.3 +/- 1.3 h) and peripheral LH (14.8 +/- 1.0 h) surges occurred simultaneously, CSF LHRH concentrations were greater than half-maximal levels for longer (11.0 +/- 0.6 h; P < 0.005) than LH concentrations (6.0 +/- 0.4 h). This is the first study in sheep to reveal the presence of LHRH in CSF and show that it expresses dynamic and longer term changes coincident with peripheral LH fluctuations. CSF LHRH analysis also permits repeated sampling from individuals and, therefore, long term within-individual comparisons.

Luteinizing hormone (LH)-releasing hormone in third ventricular cerebrospinal fluid of the ewe: correlation with LH pulses and the LH surge.

Morphological evidence suggests that LHRH may be secreted into cerebrospinal fluid (CSF), but only in the rhesus monkey has CSF LHRH been found to reflect changes occurring in the LHRH neuroendocrine system. This study investigated whether LHRH is detectable in ovine CSF and, if so, whether its release profile correlates with peripheral LH profiles during pulse and surge conditions. A polyethylene catheter was threaded through a stainless steel guide cannula previously implanted into the third ventricle of an ovariectomized ewe, which enabled continuous CSF withdrawal on repeated occasions. The first experiment (n = 3) showed that peripheral LH concentrations were unaffected by CSF removal at rates of 12, 30, 50, and 100 microliters/min, and the second (n = 4) established that CSF LHRH secretion was pulsatile, with considerable variation in pulse amplitude (6.3 +/- 1.8 pg/ml; range, 1.3-18.7 pg/ml). In the third experiment (n = 6), an endogenous LH surge was induced after progesterone withdrawal and 17 beta-estradiol administration. Although CSF LHRH (15.3 +/- 1.3 h) and peripheral LH (14.8 +/- 1.0 h) surges occurred simultaneously, CSF LHRH concentrations were greater than half-maximal levels for longer (11.0 +/- 0.6 h; P < 0.005) than LH concentrations (6.0 +/- 0.4 h). This is the first study in sheep to reveal the presence of LHRH in CSF and show that it expresses dynamic and longer term changes coincident with peripheral LH fluctuations. CSF LHRH analysis also permits repeated sampling from individuals and, therefore, long term within-individual comparisons.

New concepts for the design of large area gas proportional scintillation counters

New concepts are presented that allow the design of gas proportional scintillation counters with the large areas and medium-size photomultipliers. These designs are based on the compensation of the radial decrease of the scintillation light collected in the photomultiplier due to solid angle and reflection effects, with a radial increase of the light production due to a nonuniform electric field produced by a properly shaped curved grid. An implementation of these concepts based on a 2-in-diameter photomultiplier is presented. It shows a pulse amplitude variation along the radial position (0 to 10 mm) of less than 0.5%, and an energy resolution of 8.0% for a broad 5.9-keV X-ray beam entering the detector through the full 25-mm-diameter window.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Microstimulation in visual area MT: effects of varying pulse amplitude and frequency

We have previously shown that perceptual judgements of motion direction are based in part on the activity of direction selective neurons in extrastriate visual area MT (Salzman et al., 1990, 1992). In those experiments, we applied low-amplitude microstimulation pulses (10 microA, 200 Hz) to clusters of MT neurons whose preferred directions were similar. The effect of microstimulation was to bias the monkeys' choices on a direction discrimination task toward the preferred direction of neurons at the stimulation site. The results suggest that microstimulation generated a directionally specific cortical signal by activating selectively neurons near the electrode tip. To test this notion more directly, we have now examined the behavioral effects of varying current amplitude, current frequency, and electrode position. In the majority of experiments, the directional bias in the monkeys' choices was reduced or eliminated as current amplitude increased to 80 microA. In addition, 80 microA stimulating pulses frequently impaired overall performance as measured by the percentage of correct responses. This decrement in performance indicated that 80 microA pulses introduced "noise" into the neural circuitry encoding motion direction, presumably by increasing current spread to activate a larger population of neurons representing all directions of motion. In contrast, increasing current frequency to 500 Hz (10 microA pulses) preserved the directional specificity of microstimulation effects. The precise position of the stimulating electrode also influenced the magnitude of microstimulation effects; in some cases, differences in position on the order of 100 microns determined whether an experiment yielded a very large effect or no effect at all. Thus, directionally specific activation of cortical circuitry within MT can be disrupted by increases in current spread or by small changes in electrode position. These observations suggest that the effects of low-amplitude microstimulation depend upon direct activation of a well-localized population of neurons.

Pulsatile Prolactin Secretion in Hyperprolactinemia due to Presumed Pituitary Stalk Interruption

PRL, like other anterior pituitary hormones, is normally secreted in a pulsatile fashion. However, it is not known whether such pulses depend on dopamine and/or other hypothalamic factors. This question can be addressed by investigating patients with large pituitary mass lesions, since such patients often have hyperprolactinemia due to disruption of normal hypothalamic input to the pituitary gland. Six such patients (5 with non-PRL-secreting tumors and 1 with a craniopharyngioma) and 11 healthy control subjects had PRL levels measured every 15 min over 24 h. PRL pulses were located by cluster analysis. All patients had PRL pulses of normal frequency, but increased amplitude. Circadian variation in PRL pulse amplitude, present in healthy women, was abolished in tumor patients. These results imply that normal pituitary levels of dopamine do not control the generation of PRL pulses. Instead, PRL pulses may arise from the pituitary gland, with pulse amplitude and circadian rhythm modulation by dopamine and other hypothalamic factors. Alternatively, the mild hyperprolactinemia associated with large hypothalamic-pituitary tumors may represent partial impairment of dopamine secretion, with sufficient pituitary dopamine levels to maintain normal PRL pulse frequency.

Experimental determination of three-dimensional RF magnetic field distribution of nmr coils

RF magnetic field homogeneity is an important consideraxion in all NMR experiments. In this work, an extension to an existing method is suggested for experimental evaluation of RF field homogeneity of NMR coils in three dimensions. The technique is founded on Fourier analysis of signal intensity modulation produced by variation of RF pulse length or amplitude. The frequency of signal modulation produced by RF pulse length variation is proportional to the strength of the local RF field. The three-dimensional spatial information is encoded by application of frequency- and phase-encoding magnetic field gradients. RF field profiles of commonly used coil configuration obtained using this technique are presented. The method was found to be more convenient and sensitive than other techniques currently used for generating RF field profiles.

Regulation of Anterior Pituitary Gonadotrophin Subunit mRNA Levels by Gonadotrophin‐Releasing Hormone in the Ewe

Abstract Levels of mRNA for the common a subunit and for the beta subunits of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were measured in the pituitary glands of ovariectomized hypothalamo-pituitary disconnected ewes. A control group (n = 7) received 250 ng pulses of gonadotrophin-releasing hormone (GnRH) each hour for one week. To examine the effects of changing GnRH pulse amplitude four sheep were given 250 ng pulses of GnRH for one week and then 25 ng pulses for one week. Plasma LH and FSH concentrations were lowered by reducing the GnRH pulse amplitude but pituitary levels of mRNA for a subunit were increased. Levels of mRNA for FSHbeta and LHbeta were similar with 25 ng and 250 ng pulses of GnRH. To examine the importance of pulsatile versus continuous GnRH inputs, a group of sheep was given a constant infusion of 250 ng/h GnRH for one week. Compared to sheep given 250 ng pulses of GnRH the mRNA levels for LHbeta and FSHbeta were lower in sheep given a constant infusion of GnRH; levels of a subunit mRNA were similar in the two groups. To examine the short-term effects of removing GnRH inputs, ovariectomized, hypothalamo-pituitary disconnected ewes that had been receiving 250 ng pulses of GnRH each hour were deprived of GnRH for 6 h (n = 4) or 30 h prior to slaughter; levels of mRNA for the three subunits were similar to control values in both of these groups. These studies show that wide variation in GnRH pulse amplitude has little effect on mRNA levels for the gonadotrophin subunits but message levels are affected by the mode of GnRH input (constant versus pulsatile). The maintenance of gonadotrophin subunit mRNA levels for at least 30 h after GnRH deprivation suggests that these mRNA species have a long half-life or that transcription continues after GnRH withdrawal.

Quantal currents evoked by graded intracellular depolarization of crayfish motor axon terminals.

1. Quantal transmitter release was examined at nerve terminals of the excitatory motor axon of the crayfish opener muscle. The magnitude of synaptic currents, recorded with macro-patch electrodes at a nerve terminal, served as a measure of quantal size. Transmitter release was initiated by pulses of depolarizing current applied intracellularly to the axonal terminals after application of tetrodotoxin. Quantal release was altered by a variety of methods and the resulting quantal output and quantal size were measured. 2. Amplitude distributions of quantal events were obtained during experimental manipulations which altered the rate of quantal release by up to 25-fold. These manipulations consisted of: varying pulse amplitude or pulse duration; facilitating the release by prolonged depolarization; and application of a potassium channel blocker, 4-aminopyridine. 3. The amplitude of quantal events is impervious to marked changes in presynaptic depolarization and is not affected by experimental procedures which promote accumulation of calcium ions in the terminals. The vesicular mechanism of release, in which transmitter substance is prepackaged in vesicles which individually undergo exocytosis at a release zone, could account for the observed results.

Noise autocorrelation in high density recording on metal film disks

This paper presents measurement and analysis of noise associated with closely spaced transitions. The measured noise autocorrelation function is found to be similar to noise calculated with both pulse amplitude variation and position jitter. The amplitude variation is larger for closely spaced transitions.

Noninvasive determination of central blood pressure by impedance plethysmography.

A new algorithm based on impedance plethysmography provides reliable determinations, on an experimental basis, of arterial blood pressure. Signals over the brachial artery are picked up by four Velcro-type electrodes attached to the skinward side of a regular blood pressure cuff. Mathematical formulas are used to define those impedance pulses that correspond to systolic, diastolic, and mean arterial blood pressure values. The envelope of the impedance pulses recorded during cuff deflation can be defined by linear regressions, function of just one normalized independent variable, which is the ratio between the amplitude of the pulse with maximum amplitude and of the amplitude of the pulses with constant amplitude. These linear regressions are assumed to be universally applicable, the influence of individual anatomic variations having been eliminated by the use of a normalized variable. The results of this study prove that the variable pulse amplitude obtained during a cuff deflation is a quantifiable reaction of the circulatory system to arterial constriction.

Cluster analysis: a simple, versatile, and robust algorithm for endocrine pulse detection.

Endocrine signaling provides one critical means of physiological communication within an organism. Many endocrine signals exhibit an episodic or pulsatile configuration. In an effort to provide a versatile and statistically based algorithm for investigating the regulation of endocrine pulse signals, we have formulated a computerized algorithm in which a pulse is defined as a statistically significant increase in a "cluster" of hormone values followed by a statistically significant decrease in a second cluster of values. The increase or decrease is judged in relation to the actual experimental error expressed by the replicates in the presumptive nadir and peak data clusters. The program permits the operator to specify the cluster sizes of test peaks and pre- and postpeak nadirs. This method is largely insensitive to unstable base-line hormone concentrations and is not adversely affected by varying pulse amplitudes, widths, or configurations within the endocrine series. In addition, the simple statistical basis for this algorithm renders it minimally dependent on explicit or a priori assumptions about rates of hormone secretion or disappearance. The program has been validated for false-positive errors against a wide range of intraseries coefficients of variation (4-52%). We have illustrated its performance for profiles of luteinizing hormone, follicle-stimulating hormone, growth hormone, prolactin, adrenocorticotropic hormone, and cortisol and compared these episodic patterns with those of stable serum constituents (total serum protein and calcium), which do not exhibit pulsatile fluctuation.

Brass keyer system for electronic organ

A keying arrangement for electronic organs in which a tone signal consisting of a square wave having, for example, a fifty percent duty cycle is connected to the input of the keyer circuit, and the output thereof, upon the depression of a playing key, consists of a pulse train with the amplitude rising at a controlled rate and the pulse width or duty cycle decreasing at a controlled rate but lagging the increase in amplitude so as to closely duplicate the playing of a muted brass instrument in the attack fashion. A muted brass tremulant affect is achieved by applying a cyclically time variant signal to modulate the pulse amplitude and pulse duty cycle, again with the change in duty cycle lagging the change in amplitude. The circuitry for accomplishing this comprises a diode keyer wherein a pair of resistor-capacitor circuits having different time constants control the keyer to effect variations in pulse amplitude and duty cycle over respective intervals of time.

Optical range finder for 15–10-m distances

A time of flight method is suggested for measuring distances in large manipulator range-finding applications. Light pulses of a SH laser diode are used. Special attention was paid to reducing the pulse amplitude variations in the transit time measurement. Preliminary results with a prototype have proved the method to be highly usable in measuring position, orientation, and shape of the objects within a 1.5–10-m range. The resolution of the distance measurement is ±1 mm with a measuring time of 10 msec.

Scalable visible Nd:YAG pumped Raman laser source

Experimental and analytical studies of a 60 Hz, 0.45 W, 630 nm Raman laser source with a 12 ns pulse duration have demonstrated an overall electrical to optical efficiency of 0.12 percent. This is the first demonstration of a short pulse, high repetition rate red laser at such a high average output power without the need for a visible pump laser. Additional significance arises from the fact that simple extensions of the present work will produce many wavelengths in the visible and near infrared (IR) spectral region. A 1064 nm Nd:YAG pump laser operating at 60 Hz was used to pump a methane gas Raman laser operating at 1544 nm. This wavelength was mixed with the remaining 1064 nm laser output in noncritically phase matched lithium niobate to produce 630 nm radiation. The optical energy conversion efficiencies for the three steps were 1.4, 30, and 20 percent, respectively, for output energies of 86 mJ at 1064 nm, 15 mJ at 1544 nm, and 7.5 mJ at 630 nm. rms pulse amplitude variation measured 6 percent or less. A 10.7 million pulse life test was conducted, and the average output energy did not vary more than ±10 percent from its initial value.

Improved resolving time in n - γ coincidence measurements with high pressure 3He ionization chambers

Resolving times in n - γ coincidence measurements with high-pressure, gridded 3He ionization chambers are shown to be reduced from about 5 us to less than 0.4 us when grid signals are utilized for timing of neutron events. Compared to that found in the normal case when anode signals are utilized, the coincidence efficiency is reduced to about 70%. Lack of compensation for variable pulse amplitude and rise time is a major factor limiting further improvement in the resolving time.

Exploiting the unique time-structure of synchrotron radiation at SSRL

Synchrotron radiation arrives at SSRL in subnanosecond pulses with an interpulse period of 780 ns. The unique combination of short pulses and a relatively long interpulse period makes SSRL a highly attractive source for timing experiments. The discussion covers time-resolved photoluminescence emission and excitation spectroscopy as well as noise-reduction techniques and lifetime measurements. In the measurement of lifetimes less than ≈ 1 ns, problems related to variations in pulse duration, amplitude and period are discussed along with potential solutions.

Photon counting system for subnanosecond fluorescence lifetime measurements

A new photon counting system has been developed for subnanosecond fluorescence lifetime measurements. The system incorporates a nanosecond light pulser, a dual counter unit, and a constant-fraction discriminator. The operating conditions of the light pulser have been adjusted to minimize the spread of the light pulse waveshape. The discriminator has upper and lower level adjustments and a time walk of no more than ±35 psec over a 50-mV to 5-V input pulse amplitude variation. The measuring system has a total system time resolution, expressed as the FWHM of the light pulse, of 800 and 1480 psec using photomultipliers 8850 and 8852, respectively, with full photocathode illumination and optimized operating conditions. The system will measure both single and multiple decay components, and it is designed and optimized for experiments involving measurements of decay time constants as short as 90 psec.

Optical integration of the exciting laser pulse and ratio circuit for fast emission measurements

An economical ratio circuit for the measurement of fast emission signals induced by laser pulses is described. The laser pulse is first integrated by the use of a long-lived phosphor, and a photodetector is employed to provide the reference signal. The apparatus is used in combination with a single-beam sampling oscilloscope and a computer of average transients or with a single-channel boxcar integrator. In either case the ratio circuit can be used to correct for laser pulse amplitude variations.

Measurement techniques for assessing the influence of electromagnetic fields on implanted pacemakers.

A method is described for assessing quantitatively the influence of electromagnetic fields on the function of implanted cardiac pacemakers. This influence may occur either indirectly by the action as an antenna of the catheter, or directly by induction into the pacemaker's electronic circuit. The technique comprises: measurement of the influence of electric voltages of various frequencies applied directly across the generator's output connections; determination of the aerial factor of the catheter configuration; combination of the data acquired to a diagram of overall sensitivity of the combination, in which a 10% variation of maximum output pulse amplitude, pulse width, or repetition rate of the pacemaker, whichever occurs first, has been chosen as a limit; comparison of this sensitivity/frequency curve with the electromagnetic field strength produced by several commonly used apparatus; and measurement of the direct influence of fields on the pacemaker itself. It is concluded, that disturbance of pacemaker action may occur in the vicinity of apparatus designed to produce electromagnetic fields, such as broadcast transmitters, diathermy applicators, and radar sets. Normal household appliances appear to be safe in the respect investigated.

A Search for Deep Trap Levels in Ge(Li) Detectors

We have attempted to measure the level of deep electron trapping centres in Ge(Li) detectors by observing the variation of pulse amplitude with temperature over a wide temperature range. The mean time TD for the re-emission of a trapped carrier from a given level decreases exponentially with increasing temperature; as TD becomes shorter than the amplifier time constants, pulse amplitude should show an increase over a narrow temperature interval. No such transition was observed between 80 and 160 K. We conclude that the main electron trap is deeper than 0.175 eV.