Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “bistability”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,369 records · Page 76Linked to original sources
PubMed2006

Biologically based computational models of high-level cognition.

Computer models based on the detailed biology of the brain can help us understand the myriad complexities of human cognition and intelligence. Here, we review models of the higher level aspects of human intelligence, which depend critically on the prefrontal cortex and associated subcortical areas. The picture emerging from a convergence of detailed mechanistic models and more abstract functional models represents a synthesis between analog and digital forms of computation. Specifically, the need for robust active maintenance and rapid updating of information in the prefrontal cortex appears to be satisfied by bistable activation states and dynamic gating mechanisms. These mechanisms are fundamental to digital computers and may be critical for the distinctive aspects of human intelligence.

Brain↗
PubMed1970

Pathways through networks of branched DNA.

Differentiation may be controlled by forks in DNA each having a replicatable protein "switch," stable in either a "left" or a "right" configuration, which determines the path through the DNA network taken by RNA polymerases during transcription. The possibility for dedifferentiation exists, but differentiation could be made irreversible by the exertion of a similar control over parts of the paths through the network taken by DNA polymerases. The concept of bistable switches at DNA branch points can be used to account for antibody variability.

Antibodies↗
PubMed1999

Emergent properties of networks of biological signaling pathways.

Many distinct signaling pathways allow the cell to receive, process, and respond to information. Often, components of different pathways interact, resulting in signaling networks. Biochemical signaling networks were constructed with experimentally obtained constants and analyzed by computational methods to understand their role in complex biological processes. These networks exhibit emergent properties such as integration of signals across multiple time scales, generation of distinct outputs depending on input strength and duration, and self-sustaining feedback loops. Feedback can result in bistable behavior with discrete steady-state activities, well-defined input thresholds for transition between states and prolonged signal output, and signal modulation in response to transient stimuli. These properties of signaling networks raise the possibility that information for "learned behavior" of biological systems may be stored within intracellular biochemical reactions that comprise signaling pathways.

Animals↗
PubMed1999

Remote triggering of waves in an electrochemical system

In the potentiostatic electrochemical oxidation of formic acid on a platinum ring electrode under bistable conditions, an appropriate perturbation at one location of the ring can cause the emergence of a wave on the opposite side (remote triggering). These findings can be rationalized in terms of the nonlocal coupling function of the system and are theoretically reproduced by solution of the corresponding reaction-migration equation.

Journal Article↗
PubMed2005

Positive autoregulation of cI is a dispensable feature of the phage lambda gene regulatory circuitry.

Complex gene regulatory circuits contain many features that are likely to contribute to their operation. It is unclear, however, whether all these features are necessary for proper circuit behavior or whether certain ones are refinements that make the circuit work better but are dispensable for qualitatively normal behavior. We have addressed this question using the phage lambda regulatory circuit, which can persist in two stable states, the lytic state and the lysogenic state. In the lysogenic state, the CI repressor positively regulates its own expression by stimulating transcription from the P(RM) promoter. We tested whether this feature is an essential part of the regulatory circuitry. Several phages with a cI mutation preventing positive autoregulation and an up mutation in the P(RM) promoter showed near-normal behavior. We conclude that positive autoregulation is not necessary for proper operation of the lambda circuitry and speculate that it serves a partially redundant function of stabilizing a bistable circuit, a form of redundancy we term "circuit-level redundancy." We discuss our findings in the context of a two-stage model for evolution and elaboration of regulatory circuits from simpler to more complex forms.

Bacteriophage lambda↗
PubMed1978

Ultrasonic scanning in pancreatic disease.

We have analysed retrospectively the pancreatic ultrasound scans (using a bistable machine) in 138 consecutive patients, and have related the results to the clinical status and the final diagnosis in each case. The scans were read without knowledge of the patient's clinical state. When technically unsatisfactory scans were excluded from consideration, the overall diagnostic accuracy of ultrasonography proved to be 82%, with a false positive rate of 8%. The scan was abnormal in all 10 patients with cancer of the pancreas: a positive diagnosis of cancer was made in six. All patients with chronic pancreatitis in relapse had abnormal scans, but in 53% the scans were normal in patients in whom the disease was in clinical remission. In seven patients with chronic pancreatitis who suffered relentless pain, the head of the pancreas was swollen and contained cystic areas or emitted abnormal echoes. In acute pancreatitis ultrasonic scanning proved useful in following the progression of the disease to final resolution, or to development of complicating pseudocyst, abscess, or ascites. Random echoes in the early stages of acute pancreatitis are features of haemorrhagic necrosis. In alcoholic relapsing pancreatitis the persistence of abnormal echoes, disposed linearly along the axis of major ducts, suggests the presence of chronic pancreatitis.

Acute Disease↗
PubMed2003

Static and dynamic electro-optic properties of a SmC* phase in surface stabilized geometry and dispersed in the polymer matrix.

Comparative electro-optical measurements have been made on a ferroelectric liquid crystal (FLC) in surface stabilized geometry and confined to an ellipsoidal cavity within a polymer matrix. The static and dynamic electro-optical characteristics were measured for both systems and show qualitatively similar behaviours. A fast switching and important bistability were observed and characterized as a function of the applied electric field strength. The switching time between the two stable states of the surface stabilized cell was found to be longer than that found for the composite films. We argue that the faster switching dynamic of the FLC in cavities is due to the enhance of the rotational mobility of the molecules, probably (and partly) because of the "soft" anchoring character of the molecules at the cavity walls. Using a collective switching model in the high field regime, which assume a linear coupling between the spontaneous polarization and the local cavity electric field, we give an estimate of the rotational viscosity of the FLC molecules in the droplets.

Journal Article↗
PubMed2005

Anchoring and structural transitions as a function of molecular length in confined liquid crystals.

Using deuteron nuclear magnetic resonance to study liquid crystals confined to cylindrical pores, an anchoring transition has been found. The transition exhibits an unexpected sharp dependence of the anchoring strength on cyanobiphenyl liquid crystal molecular length. A structural transition from a parallel axial to a planar radial configuration occurs due to an anchoring transition from planar to weakly homeotropic orientation at the walls. The anchoring strength is at a minimum near the decylcyanobiphenyl (10CB) liquid crystal length. Long chain liquid crystal configurations depend on thermal cycling and on the equilibrium atmosphere leading to a bistable SmA structure. Orientational order wetting in the isotropic phase also depends on molecular length.

Journal Article↗
PubMed2005

Switches in bacteriophage lambda development.

The lysis-lysogeny decision of bacteriophage lambda (lambda) is a paradigm for developmental genetic networks. There are three key features, which characterize the network. First, after infection of the host bacterium, a decision between lytic or lysogenic development is made that is dependent upon environmental signals and the number of infecting phages per cell. Second, the lysogenic prophage state is very stable. Third, the prophage enters lytic development in response to DNA-damaging agents. The CI and Cro regulators define the lysogenic and lytic states, respectively, as a bistable genetic switch. Whereas CI maintains a stable lysogenic state, recent studies indicate that Cro sets the lytic course not by directly blocking CI expression but indirectly by lowering levels of CII which activates cI transcription. We discuss how a relatively simple phage like lambda employs a complex genetic network in decision-making processes, providing a challenge for theoretical modeling.

Bacteriolysis↗
PubMed1978

Determination of bladder volumes by gray scale ultrasonography.

Gray scale ultrasonography was used to measure bladder volumes. A high degree of accuracy (r = 0.097) in correlating volumes with instilled volumes is attributed to the inherently greater accuracy of gray scale as compared to bistable B-mode scanning.

Aged↗
PubMed2000

Obstetric US imaging: the past 40 years.

Although diagnostic ultrasonography (US) was developing in the late 1940s and early 1950s, it was not until the 1960s, with the availability of commercial equipment, that its usefulness in obstetrics began to be realized fully by radiologists and obstetricians around the world. Advances from A-mode to bistable and then to gray-scale static imaging were followed by the introduction of automated compound imaging and real-time US. Also, the development and initial use of Doppler US for the detection of fetal heart motion and the eventual use of pulsed and color Doppler US for the evaluation of such fetal structures as the major vessels and heart chambers contributed to increasing the usefulness of US in obstetrics. The development of specialized transducers--in particular, endovaginal probes--resulted in images of the early fetus. At the present time, the development of multiplanar, three-dimensional imaging shows great promise for more complete imaging of the fetus. The importance of US in the examination of the pregnant patient and, in particular, of the fetus has led to its worldwide dominance as the imaging modality of choice. The contributions of obstetric US to improving maternal well-being and fetal health have been recognized as a key component in all countries around the world.

Female↗
PubMed2003

Impedance, gas mixing, and bimodal ventilation in constricted lungs.

To evaluate the effect of increasing smooth muscle activation on the distribution of ventilation, lung impedance and expired gas concentrations were measured during a 16-breath He-washin maneuver in five nonasthmatic subjects at baseline and after each of three doses of aerosolized methacholine. Values of dynamic lung elastance (El,dyn), the curvature of washin plots, and the normalized slope of phase III (S(N)) were obtained. At the highest dose, El,dyn was 2.6 times the control value and S(N) for the 16th breath was 0.65 liter(-1). A previously described model of a constricted terminal airway was extended to include variable muscle activation, and the extended model was tested against these data. The model predicts that the constricted airway has two stable states. The impedances of the two stable states are independent of smooth muscle activation, but driving pressure and the number of airways in the high-resistance state increase with increasing muscle activation. Model predictions and experimental data agree well. We conclude that, as a result of the bistability of the terminal airways, the ventilation distribution in the constricted lung is bimodal.

Administration, Inhalation↗
PubMed2002

Short-term plasticity of descending synaptic input to phrenic motoneurons in rats.

Respiratory afferent stimulation can elicit increases in respiratory motor output that outlast the period of stimulation by seconds to minutes [short-term potentiation (STP)]. This study examined the potential contribution of spinal mechanisms to STP in anesthetized, vagotomized, paralyzed rats. After C(1) spinal cord transection, stimulus trains (100 Hz, 5-60 s) of the C(1)-C(2) lateral funiculus elicited STP of phrenic nerve activity that peaked several seconds poststimulation. Intracellular recording revealed that individual phrenic motoneurons exhibited one of three different responses to stimulation: 1) depolarization that peaked several seconds poststimulation, 2) depolarization during stimulation and then exponential repolarization after stimulation, and 3) bistable behavior in which motoneurons depolarized to a new, relatively stable level that was maintained after stimulus termination. During the STP, excitatory postsynaptic potentials elicited by single-stimulus pulses were larger and longer. In conclusion, repetitive activation of the descending inputs to phrenic motoneurons causes a short-lasting depolarization of phrenic motoneurons, and augmentation of excitatory postsynaptic potentials, consistent with a contribution to STP.

Animals↗
PubMed2005

Transient depression of excitatory synapses on interneurons contributes to epileptiform bursts during gamma oscillations in the mouse hippocampal slice.

Persistent gamma frequency (30-70 Hz) network oscillations occur in hippocampal slices under conditions of metabotropic glutamate receptor (mGluR) activation. Excessive mGluR activation generated a bistable pattern of network activity during which epochs of gamma oscillations of increasing amplitude were terminated by synchronized bursts and very fast oscillations (>70 Hz). We provide experimental evidence that, during this behavior, pyramidal cell-to-interneuron synaptic depression takes place, occurring spontaneously during the gamma rhythm and associated with the onset of epileptiform bursts. We further provide evidence that excitatory postsynaptic potentials (EPSPs) in pyramidal cells are potentiated during the interburst gamma oscillation. When these two types of synaptic plasticity are incorporated, phenomenologically, into a network model previously shown to account for many features of persistent gamma oscillations, we find that epochs of gamma do indeed alternate with epochs of very fast oscillations and epileptiform bursts. Thus the same neuronal network can generate either gamma oscillations or epileptiform bursts, in a manner depending on the degree of network drive and network-induced fluctuations in synaptic efficacies.

Animals↗
PubMed2002

Sustained firing of alpha and gamma hind limb motoneurons induced by stimulation of the pudendal nerve.

Axons from receptors in the cat vaginal wall run in the sensory pudendal nerve (SPN), and brief (<10 s) vaginal probing (VP) in the decerebrate cat produces a long-lasting (>1 min) contraction of the triceps surae (TS) muscles. The aim of the present project was to find out whether brief SPN stimulation also produces sustained TS response and, eventually, to study the mechanisms involved in it. Decerebrate female cats were used. In some cats, TS electromyography (EMG) and tension response were recorded; stimulation of left SPN with single or repetitive trains of shocks produced a bilateral TS response that outlasted the stimulus >1 min as VP did. In paralyzed cats (pancuronium; Panc), intracellular recordings were made from hind limb motoneurons (MNs). SPN stimulation produced a depolarization 1 min) electroneurographic (ENG) postdischarge in a small filament of the medial gastrocnemius (MG) nerve; the MG EMG postdischarge was also recorded. Large spikes (LS) and small spikes (SS) were distinguished in the ENG. During the postdischarge, LS frequency and the integrated EMG activity correlated well (r > 0.9); no correlation was found between SS and EMG. After Panc injection, LS postdischarge was absent but the SS postdischarge remained. LS followed by EMG potential were also evoked by brief TS stretch (reflex LS); single shocks to SPN only elicited SS that were not followed by EMG potential. It is concluded that alpha axons and gamma axons produced LS and SS, respectively, and that SPN activates gamma axons. It is proposed that, in the nonparalyzed cats, the stimulation of SPN with trains of shocks might cause an increase in the afferent inflow from muscle spindles to alpha MNs through the sustained firing of gamma MNs. The increased excitatory inflow would depolarize alpha MNs and allow bistable MN firing; Panc would decrease this inflow by blocking transmission to the spindle fibers.

Action Potentials↗
PubMed2002

Fine temporal resolution of analytic phase reveals episodic synchronization by state transitions in gamma EEGs.

The analytic signal given by the Hilbert transform applied to an electroencephalographic (EEG) trace is a vector of instantaneous amplitude and phase at the temporal resolution of the digitizing interval (here 2 ms). The transform was applied after band-pass filtering for extracting the gamma band (20-80 Hz in rabbits) to time series from up to 64 EEG channels recorded simultaneously from high-density arrays giving spatial "windows" of 4 x 4 to 6 x 6 mm onto the visual, auditory, or somatosensory cortical surface. The time series of the analytic phase revealed phase locking for brief time segments in spatial patterns of nonzero phase values from multiple EEG that was punctuated by episodic phase decoherence. The derivative of the analytic phase revealed spikes occurring not quite simultaneously (within +/- 4 ms) across arrays aperiodically at mean rates in and below the theta range (3-7 Hz). Two measures of global synchronization over a group of channels were derived from analytic phase differences between pairs of channels on the same area of cortex. One was a synchronization index expressing phase locking. The other was a decoherence index estimating the variance in phase among multiple channels. Spectral analyses of the indices indicated that decoherence events recurred aperiodically at rates in and below the theta range of the EEGs. The results provide support for the hypothesis that neurons in mesoscopic neighborhoods in sensory cortices self-organize their activity by synaptic interactions into wave packets that have spatial patterns of amplitude (AM) and phase (PM) modulation of their spatially coherent carrier waves in the gamma range and that form and dissolve aperiodically at rates in and below the theta range. Each AM pattern is formed by a nonlinear state transition in the cortical dynamics, as shown by spikes in the derivative. Phase locking within each PM pattern is not at zero phase lag but over a fixed distribution of phase values that is consistent with the radially symmetric phase gradients already reported called "phase cones" detected by Fourier-based methods. The insight is suggested that sensory cortices are bistable comparably to cardiac dynamics, with a diastolic state that accepts sensory input and an abrupt transition to a systolic state that transmits perceptual output. Further support for this inference will require improvements in methods for temporal resolution of the times of onset of spatial patterns of phase modulation.

Animals↗
PubMed2006

Dynamical mechanisms of odor processing in olfactory bulb mitral cells.

In the olfactory system, the contribution of dynamical properties such as neuronal oscillations and spike synchronization to the representation of odor stimuli is a matter of substantial debate. While relatively simple computational models have sufficed to guide current research in large-scale network dynamics, less attention has been paid to modeling the membrane dynamics in bulbar neurons that may be equally essential to sensory processing. We here present a reduced, conductance-based compartmental model of olfactory bulb mitral cells that exhibits the complex dynamical properties observed in these neurons. Specifically, model neurons exhibit intrinsic subthreshold oscillations with voltage-dependent frequencies that shape the timing of stimulus-evoked action potentials. These oscillations rely on a persistent sodium conductance, an inactivating potassium conductance, and a calcium-dependent potassium conductance and are reset via inhibitory input such as that delivered by periglomerular cell shunt inhibition. Mitral cells fire bursts, or clusters, of spikes when continuously stimulated. Burst properties depend critically on multiple currents, but a progressive deinactivation of I(A) over the course of a burst is an important regulator of burst termination. Each of these complex properties exhibits appropriate dynamics and pharmacology as determined by electrophysiological studies. Additionally, we propose that a second, inconsistently observed form of infrathreshold bistability in mitral cells may derive from the activation of ATP-activated potassium currents responding to hypoxic conditions. We discuss the integration of these cellular properties in the larger context of olfactory bulb network operations.

Algorithms↗
PubMed2005

Persistent sodium currents and repetitive firing in motoneurons of the sacrocaudal spinal cord of adult rats.

Months after sacral spinal transection in rats (chronic spinal rats), motoneurons below the injury exhibit large, low-threshold persistent inward currents (PICs), composed of persistent sodium currents (Na PICs) and persistent calcium currents (Ca PICs). Here, we studied whether motoneurons of normal adult rats also exhibited Na and Ca PICs when the spinal cord was acutely transected at the sacral level (acute spinal rats) and examined the role of the Na PIC in firing behavior. Intracellular recordings were obtained from motoneurons of acute and chronic spinal rats while the whole sacrocaudal spinal cord was maintained in vitro. Compared with chronic spinal rats, motoneurons of acute spinal rats were more difficult to activate because the input resistance was 22% lower and resting membrane potential was hyperpolarized 4.1 mV further below firing threshold (-50.9 +/- 6.2 mV). In acute spinal rats, during a slow voltage ramp, a PIC was activated subthreshold to the spike (at -57.2 +/- 5.0 mV) and reached a peak current of 1.11 +/- 1.21 nA. This PIC was less than one-half the size of that in chronic spinal rats (2.79 +/- 0.94 nA) and usually was not large enough to produce bistable behavior (plateau potentials and self-sustained firing not present), unlike in chronic spinal rats. The PIC was composed of two components: a TTX-sensitive Na PIC (0.44 +/- 0.36 nA) and a nimodipine-sensitive Ca PIC (0.78 +/- 0.82 nA). Both were smaller than in chronic spinal rats (but with similar Na/Ca ratio). The presence of the Na PIC was critical for normal repetitive firing, because no detectable Na PIC was found in the few motoneurons that could not fire repetitively during a slow ramp current injection and motoneurons that had large Na PICs more readily produced repetitive firing and had lower minimum firing rates compared with neurons with small Na PICs. Furthermore, when the Na PIC was selectively blocked with riluzole, steady repetitive firing was eliminated, even though transient firing could be evoked on a rapid current step and the spike itself was unaffected. In summary, only small Ca and Na PICs occur in acute spinal motoneurons, but the Na PIC is essential for steady repetitive firing. We discuss how availability of monoamines may explain the variability in Na PICs and firing in the normal and spinal animals.

Animals↗