Increase In Amplitude Of Oscillations Research Articles

Evidence for bursting pacemaker neurones in cultured spinal cord cells

Intracellular recordings were made from dissociated mouse spinal cord cells in primary culture. One type of spinal cord neurone, with a large cell body (40–50 μm), 3–5 short neurites, and a mean resting potential of −65 mV, was found to fire rhythmic bursts of action potentials with a phase duration of approximately 1s when the membrane potential was depolarized to −55 mV. These bursts did not arise from spontaneous synaptic input, but appeared to result from endogenous ionic conductance properties of the membrane resembling those observed in molluscan bursting pacemaker neurones. Ionic conductances underlying this bursting activity were studied pharmacologically by local application of ionic conductance blockers. Pacemaker potentials depended on Na + conductance, since tetrodotoxin and Na-free medium were the most potent agents for blocking spontaneous rhythmic activity. However, a Ca 2+ conductance was involved in the depolarizing phase of membrane potential oscillations, since Ba 2+ application increased oscillation amplitude. Action potentials observed during the bursts were Na +- and Ca 2+-dependent. They did not differ significantly from those observed in other spinal cord neurones in culture. Application of tetraethylammonium, CoCl 2, BaCl 2 and 4-aminopyridine revealed at least three different potassium conductances which controlled this bursting pacemaker activity. A delayed potassium conductance controlled spike duration, a Ca-dependent potassium conductance controlled the duration of the burst and underlay the hyperpolarizing phase terminating the burst, and finally, a transient potassium conductance appeared to be involved in the control of phase duration. The demonstration that spinal cord neurones growing in monolayer culture display typical bursting pacemaker activity raises the possibility that bursting pacemaker neurones in the mammalian spinal cord may be involved in a phasic pattern generator that could control such activities as walking and the respiratory rhythm.

Strength of hydrostation trash rack bars

Trash racks are one of the important parts of the mechanical equipment of hydroelectric stations since their breakage causes shutdown of the turbines until they are repaired. Therefore, the provision of trouble-free operation of racks is of importance. The author states that the most vulnerable part of the rack structure is the bars, which can be damaged both from the static load as a result of clogging and from the stresses occurring during vibration of the bars under the effect of the water passing through them. Used for explaining the processes in the case reported here is the Milovich theory, which does not however touch on all the causes and conditions of increase of oscillation amplitude as it is observed in the laboratory.

Stability of an experimental atrioventricular reentrant tachycardia in dogs

The factors leading to spontaneous block and instability of a reentrant tachycardia were studied with a computer model and in eight anesthetized dogs with an electronically simulated atrioventricular (AV) bypass tract. The bypass tract delay could be changed in a programmed sequence. AV nodal refractoriness in the dogs was increased with verapamil. Stability of the reentrant tachycardia was assessed by noting how rapidly cycle-length changes from a transient alteration in the bypass-tract delay time would dampen out and the tendency of the reentrant tachycardia to spontaneously block. There was close agreement between computer model predictions and experimental results. After a transient change in bypass tract delay time, dampened oscillations of the cycle length were noted. The degree of dampening was related to the slope of the AV nodal refractory curve. When the reentrant tachycardia operated on a steep portion of the refractory curve (slope steeper than -1), spontaneous oscillations and block developed. In the dog, the reentrant tachycardias terminated spontaneously either by an increase in amplitude of oscillations or by an increase in AV nodal refractoriness. We conclude that, for stable reentry, a reentrant circuit must have no conduction block and must be able to dampen out transient perturbations.

Two major dust storms, one Mars year apart: Comparison from Viking data

The Viking Mars Landers have been on the Mars surface for over two Mars years. During the first year two major, probably global, dust storms occurred. The first was unusually early compared to most previous Earth‐based observations. A major storm occurred during the second year, almost precisely one year after the first storm of the first year. Meteorological data show roughly similar atmospheric behavior for the two early storms. Of particular note is the increase in amplitude of pressure oscillations (probably of baroclinic origin) and concurrent increases in wind speed during the build‐up phase of all three storms. The generation of these waves appears to be a natural consequence of seasonal effects not associated with the dust storms. It is suggested that ‘baroclinic’ waves, should they exist in the southern hemisphere during the time of dust storm generation, could be an important factor in growth and development of the dust storms.

Respiratory oscillations in discharge frequency of chemoreceptor afferents in sinus nerve and anaesthetized cats at normal and low arterial oxygen tensions.

1. The discharge of chemoreceptor afferents in preparations of the sinus nerve in spontaneously breathing anaesthetized cats has been subjected to an averaging procedure in records obtained when the animals breathed (a) air and (b) a hypoxic gas mixture. 2. The mean discharge frequency was higher in hypoxia than at normal oxygen tension. 3. Oscillations in chemoreceptor discharge frequency with the same period as respiration were obtained by the averaging procedure both at normal arterial oxygen tensions and in hypoxia, but there was no significant increase in oscillation amplitude with hypoxia. 4. The carotid body response to arterial PCO2 oscillations does not therefore appear to be amplified by hypoxia. This finding is discussed in relation to the reported dependence upon hypoxia of the ventilatory effects of tube breathing in man.

Phase-space diagrams for a migma fusion device

Results are presented from a systematic study of three-dimensional orbits in a Migma fusion device. As the vertical oscillation amplitude increases, the frequency of the oscillations decreases rapidly, so that a sequence of nonlinear coupling resonances is encountered. The periodic orbits and vortex boundaries associated with these resonances have been calculated, and the results are displayed in a series of phase-space diagrams, together with an explanation of their significance. The vertical confinement limit and its effect on these diagrams are also described. The problems of beam injection via self-ensnaring are examined, and a special injection orbit is proposed which should provide an exceptionally long confinement time together with a high central density. Finally, the effect of multiple scattering on the ion distribution in phase space is discussed, and arguments are presented in favor of a rather broad distribution under steady-state conditions.

Hydrodynamic theory of the stability of a self-focusing relativistic beam

The stability of a self-focusing relativistic electron beam (REB) is investigated in a quasi-hydrodynamic approximation. It is demonstrated that the inclusion of density inhomogeneity of the REB particles leads to a nonlinear relationship between longitudinal and transverse electron motion. The analysis indicates that the amplitude of electron transverse displacement increases sharply within a certain range of REB parameters which might be interpreted as a specific instability. The energy of electron transverse oscillations increases at the expense of the energy of longitudinal oscillations, which, in their turn, acquire their energy from that of the REB translational motion. The conditions for instability and growth rate of unstable oscillations are obtained. It is also shown that nonlinear interactions between high-frequency electron and low-frequency ion oscillations result in the increase of ion oscillation amplitude.

Nitrogen-14 Nuclear Quadrupole Resonance Study of Lithium Hydrazinium Sulfate

Nitrogen-14 nuclear quadrupole resonance (NQR) transition frequencies of the NH2 group of the hydrazinium ion in lithium hydrazinium sulfate have been measured as a function of temperature from 4.2 to 360°K. The usual treatment of temperature dependence, which considers the effect on the transition frequencies of molecular oscillations about the principal field gradient axes, is generalized by considering oscillations about an arbitrarily oriented axis system. This generalization leads to the determination of the orientation of the field gradient axes in the NH2 group, and to a clarification of the existing model of the dynamics of the hydrazinium ion. It is found that between 40 and 320°K the NH2 group undergoes small oscillations about the N–N axis whose amplitude increases linearly with temperature. Above 320°K the amplitude of these oscillations increases more rapidly than a linear function. It is also found that above 150°K, proton exchange in the H–N–H plane of the NH2 group takes place rather than rigid rotation of the protons about the bisector of the H–N–H angle. The linear increase of oscillation amplitude with temperature between 40 and 320°K is found to be inconsistent with the Bayer theory, unless and ad hoc assumption is made about the temperature dependence of the oscillation frequencies.

Nonlinear longitudinal oscillations of relativistic plasma

The investigation has been made of nonlinear forced longitudinal oscillations of a relativistic plasma. It was demonstrated that: a) the nonlinear terms (v▽)p arising in the equations of motion do not restrict the electron oscillation amplitude increase in the region of plasma resonance; b) the nonlinear terms due to the relativistic nature of oscillations, firstly, limit the oscillation amplitude in the resonance region and, secondly, lead to the development of a parametric instability. The harmonic, sub- and ultraharmonic oscillations turned out to be unstable. The conditions for the instability as well as expressions for the growth rates of unstable oscillations were obtained.

Spin-lattice relaxation in anhydrous sodium and lithium soaps between 23° and 180°C

The influence of molecular motions on the spin-lattice relaxation in even- and odd-numbered sodium and lithium soaps between 23 and 180δC has been investigated. The crystalline transition in most sodium soaps can hardly be observed from T 1 measurements as a result of a broadened correlation-time distribution. The T 1 values show that the oscillation amplitude increases and the correlation-time distribution narrows with increasing chain length. The Tl values alternate such that at this temperature the even-numbered soaps presumably have a closer packing than the odd-numbered ones. The crystalline-subwaxy and subwaxy-waxy transitions are observable in only one T 1 minimum. The amplitude of the motions increases as the chain becomes longer. There is a slight alternation in the T 1 minima of the even- and odd-numbered soaps, which again points to a closer packing of the even-numbered sodium soaps. The lithium soaps show a transition to another crystalline phase between 85 and 115°C in which phase the chains are less tilted. The increased mobility is observable from a decrease of T 1.

Current instability in an inhomogeneous plasma

The authors investigate the development of instability when a current is passed through a strongly ionized potassium plasma of non-uniform density. The collisionless plasma, having a density of 109 to 1010 cm−3, is subjected to magnetic fields of 400 to 2000 Oe. In accordance with earlier experiments, “universal” instability in the form of drift waves is observed when no current is present. The coefficient of diffusion across the magnetic field is some two orders of magnitude greater than the classical coefficient. When a current is passed through the plasma two oscillation branches are excited: drift and ion-sound waves. With the first branch the oscillation frequency is inversely proportional to the magnetic field and the waves have an azimuthal component, the azimuthal phase velocities of all the harmonics being the same. With the second branch the oscillation frequency is independent of the magnetic field and varies inversely with the length of the system. For practical purposes, the phase velocity of the longitudinal component coincides with the ion-sound velocity. The critical drift velocity of the electrons and of the ions, at which ion-sound waves are excited, is approximately 2 × 106 cm/ sec, an order of magnitude less than the theoretical critical drift velocity for an homogeneous, almost isothermal, plasma. When the current instability is excited, the diffusion coefficient increases by a factor of 1.5 to 2.5, diffusion beginning to increase simultaneously with the increase in oscillation amplitude.

Measurement of Betatron Oscillation Frequencies on the UCLA 50-MeV Cyclotron

The vertical and radial betatron oscillation frequencies have been measured on the UCLA 50-MeV spiral-ridge cyclotron. The technique used was that of rf knockout, in which the internal beam was passed through a pair of exciter plates which provided an rf electric field. When the frequency was equal to the betatron frequency a detectable increase in betatron oscillation amplitude or ``knockout'' occurred. The knockout as a function of exciter frequency shows a number of resonances rather than a single one. This provides a measurement of radial as well as vertical betatron frequency. Comparisons of the measured frequencies with those calculated from magnetic field measurements are given and show satisfactory agreement.

Steady Rotational Flow of an Incompressible, Viscous Fluid Enclosed between Two Coaxial Cylinders

This paper gives a theoretical study of the two-dimensional flow of an incompressible, viscous fluid enclosed between two coaxial cylinders, one of which is oscillating. The Reynolds number is assumed to be small, and the Navier-Stokes differential equations are solved by means of the method of successive proximations. Both for the first and second order solution the proper boundary conditions are applied. We have shown that there exist two stationary circulation systems in every quadrant, and that the direction of circulation is opposite in these two systems. The circulation velocities are calculated in detail, and the results are compared with experimental results referred to in an earlier paper. Some new experimental results, which are not published yet, are briefly mentioned. The agreement between theoretical and experimental results is excellent. In a discussion at the end of the paper we give the explanation of the remarkable fact that in some experiments the direction of circulation seems to reverse when the oscillation amplitude increases and exceeds a certain critical value.

Oscillators with Automatic Control of the Threshold of Regeneration

The frequency of an oscillator operating just on the threshold of the oscillation generation is determined-as is well known-by the constants of the circuit only; i.e., it does not depend on the operative conditions (supply voltages) of the system. The operation in this critical state for an ordinary dynatron oscillator as well as for a dynatron with back coupling can be continuously secured by means of an arrangement working automatically. Its principle is as follows: The oscillating circuit of the dynatron feeds a rectifier which, in turn, supplies the rectified voltage to the inner grid circuit of the dynatron tube (tetrode) in such a way that its bias becomes more and more negative when the oscillation amplitude increases. Owing to this arrangement the amplitude maintains itself continuously in the critical region for which the frequency stability is very high.