Department of Bioscience & Bioengineering

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IIT Jodhpur

We present a molecular-dynamics-based study on certain structural features of disordered two-dimensional lattices of (Formula presented) particles with screened Coulomb interactions which are placed on rigid substrates with (Formula presented) randomly distributed Gaussian pinning centers. We find that over a wide temperature range, for (Formula presented) and for various pinning intensities (Formula presented) the lattice structures are typically described by domains of triangular-lattice-like regions punctuated by domains of clustered topological defects. Our calculations reveal that as (Formula presented) increases, sizable triangular-lattice-like regions coexist with increased topological defect densities. This result suggests that the particles remain correlated even at strong pinning regimes. Its implications are discussed in relation to recent experiments on heavy-ion-irradiated high-temperature superconductors and the assumptions used in various flux pinning theories of type-II superconductors. © 1998 The American Physical Society.

World University root node

Physical Review B - Condensed Matter and Materials Physics

Domain structure in two-dimensional lattices with randomly distributed pinning centers: Application to type-II superconductors

An isomorph of the glabrous hand is visible in primary somatosensory cortex (area 3b) of owl monkeys in brain sections cut parallel to the surface and stained for myelin. A mediolateral row of five ovals, separated by myelin-light septa, represents digits and corresponds precisely with cortical sites activated by light touch on individual digits in microelectrode recordings. A number of caudal ovals relate to pads of the palm. A more distinct septum separates the hand from the more lateral face representation. Within the face representation, two large myelin-dense ovals can be identified that are activated by the upper or lower face in a caudo-rostral sequence. Accidental finger loss or dorsal column section, deafferentations that result in reorganization of the physiological map in area 3b, do not alter the morphological map. The proportions for each digit and palm in the morphological map do not vary across normal and deafferented animals. Similar isomorphs were also seen in area 3b of squirrel and macaque monkeys. We conclude that the anatomical isomorph for the body surface representation in area 3b is a reliable reflection of normal cortical organization and may be a common feature of the primate area 3b. The isomorph can provide a reference in studies of somatotopic reorganization.

Cerebral Cortex

A histologically visible representation of the fingers and palm in primate area 3b and its immutability following long-term deafferentations

We describe an experimental and theoretical project centered on the use of nonlinear acoustic shock impulses and microelectromechanical systems (MEMS) sensors to detect and image buried, regular-shaped inclusions at various depths in soil. The use of shock waves is partly motivated by the perceived limited capability of conventional acoustic delivery modes, and by recent research which shows that shock impulses travel efficiently in granular media.

IEE Conference Publication

Detection of non-metallic landmines using shock impulses and MEMS sensors

We report extensive numerical studies of the dynamics of a classical particle in an anharmonic one-dimensional potential while it is harmonically coupled to three different many-particle systems. The studies address the comparatively simpler dynamical problem when the energy of the anharmonic oscillator is sufficiently low. The first model is one in which the many-particle system is a chain of harmonic oscillators that have nearest-neighbor harmonic interactions. The anharmonic oscillator is connected to every harmonic oscillator via harmonic springs. The second model is identical to the first except that each oscillator is subjected to an additional one-body harmonic potential. The third model is identical to the second except that the nearest-neighbor couplings between the individual harmonic oscillators are absent, i.e., the oscillators are decoupled with respect to one another. In the first model we find that the lowest frequency of the anharmonic oscillator, while possessing a lower bound, increases as [Formula Presented] where [Formula Presented] is the number of oscillators in the bath and [Formula Presented] is the force constant describing the strength of interaction between the anharmonic oscillator and the interacting harmonic many-body system. In the second model we find aspects of dynamical correlations as seen in the canonical ensemble and a dominant frequency that increases as [Formula Presented] We recover the dynamical correlations of the anharmonic oscillator as obtained within the framework of a canonical ensemble [S. Sen, R. S. Sinkovits, and S. Chakravarti, Phys. Rev. Lett. 77, 4855 (1996)] in the third model for sufficiently many harmonic oscillators. We comment briefly on an alternate way to correctly model heat baths for numerical studies on the dynamics of physical systems using canonical ensembles. © 1998 The American Physical Society.

Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics

Dynamics of an anharmonic oscillator that is harmonically coupled to a many-body system and the notion of an appropriate heat bath

We have recently shown that the low-temperature velocity power spectrum of an anharmonic oscillator (AHO) in a canonical ensemble is recovered when one considers the AHO as coupled via harmonic springs to a system of noninteracting harmonic oscillators (HO’s), each with the same characteristic frequency as that of the AHO [D.P. Visco, Jr. and S. Sen, Phys. Rev. E 57, 224 (1998)]. In the present work, we generalize our earlier study by establishing the following points. (i) We show that when the AHO is coupled via anharmonic springs to a system of noninteracting HO’s, each with characteristic frequency as that of the AHO, the dynamics of the AHO is strongly affected by the altered coupling and hence we contend that the bath particles must be connected via harmonic springs to preserve the dynamics of the AHO. (ii) We consider an AHO with a characteristic frequency that differs from that of the bath particles and show that the correct dynamics of the new AHO is recovered when (a) the harmonic oscillators that make up the bath particles have constrained movement and (b) the bath particles are harmonically coupled at significantly weakened strength compared to the study in case (i) above. © 1998 The American Physical Society.

Dynamics of a nonlinear oscillator which is coupled to various model heat baths

Over recent years, we have come to the surprising realization that sensory cortex is highly plastic in functional organization, even in adult brains. Much of the evidence for this conclusion comes from studies of the effects of peripheral deafferentation or sensory experience on the somatotopy of primary somatosensory cortex (area 3b) of monkeys. Local modifications in cortical representations occur rapidly after sensory loss or more gradually during altered sensory experience. These changes depend on reductions in lateral inhibition and other dynamic adjustments in sensory networks, as well as Hebbian-like modifications of synaptic strengths. Activity-dependent alterations in the expression of neurotransmitters and modulators may also play a role. After major deactivations, such as those produced by amputation or section of dorsal column afferents, cortex regains responsiveness over a much longer time period as a result, at least in part, from the growth of new connections.

Seminars in the Neurosciences

Plasticity of somatosensory cortex in primates

Sensory stimuli to the body are conveyed by the spinal cord to the primary somatosensory cortex. It has long been thought that dorsal column afferents of the spinal cord represent the main pathway for these signals, but the physiological and behavioural consequences of cutting the dorsal column have been reported to range from mild and transitory to marked. We have re-examined this issue by sectioning the dorsal columns in the cervical region and recording the responses to hand stimulation in the contralateral primary somatosensory cortex (area 3b). Following a complete section of the dorsal columns, neurons in area 3b become immediately and perhaps permanently unresponsive to hand stimulation. Following a partial section, the remaining dorsal column afferents continue to activate neurons within their normal cortical target territories, but after five or more weeks the area of activation is greatly expanded. After prolonged recovery periods of six months or more, the deprived hand territory becomes responsive to inputs from the face (which are unaffected by spinal cord section). Thus, area 3b of somatosensory cortex is highly dependent on dorsal spinal column inputs, and other spinal pathways do not substitute for the dorsal columns even after injury.

Nature

Deactivation and reactivation of somatosensory cortex after dorsal spinal cord injury

The orderly representations of sensory surfaces in the brains of adult mammals have the capacity to reorganize after injuries that deprive these representations of some of their normal sources of activation. Such reorganizations can be produced by injury that occurs peripherally, such as nerve damage or amputation, or after injury to the CNS, such as spinal cord damage or cortical lesion. These changes likely are mediated by a number of different mechanisms, and can be extensive and involve the growth of new connections. Finally, some types of reorganizations may help mediate the recovery of lost functions, whereas others may lead to sensory abnormalities and perceptual errors.

Neuroscientist

Reorganization of sensory systems of primates after injury

We report particle dynamical simulations of a two-dimensional grain pile in a box with the base being slowly tilted from horizontal to [Formula Presented] angle at which the pile undergoes a large layer sliding event. When dissipation between the grains is negligible, the distribution [Formula Presented] of displacements [Formula Presented] of the surface grains shows decays that are consistent with [Formula Presented] with [Formula Presented] for [Formula Presented] At time [Formula Presented] at [Formula Presented] we find a crossover in τ to [Formula Presented] Dissipation appears to play a key role in the system dynamics only when [Formula Presented] We find that the time for the onset of avalanches, [Formula Presented] where [Formula Presented] is the time when the surface roughness of the sliding pile is minimized. © 1997 The American Physical Society.

Avalanche dynamics in model two-dimensional grain piles

The bgl operon of Escherichia coli is rendered cryptic and uninducible in wild-type cells by the presence of DNA structural elements that negatively regulate transcription. We have carried out a detailed analysis of the sequences implicated in negative regulation. Fine-structure deletion analysis of the upstream sequences showed the presence of at least two elements involved in silencing the promoter. Chemical probing of genomic DNA in vivo showed that a region of dyad symmetry, present upstream of the promoter, is hypersensitive to KMnO4. The hypersensitive region detected corresponds to the potential cruciform structure implicated earlier in negative regulation. Enhancement of transcription from the wild-type promoter, observed in the presence of the gyrase inhibitor novobiocin, was absent in a mutant that carried point mutations in the inverted repeat. This observation suggests that the activation seen in a gyrase mutant is mediated by destabilization of the cruciform because of reduced supercoiling. Deletion of sequences downstream of the potential cruciform also resulted in an increase in transcription, indicating the presence of a second regulatory element. Measurement of transcription from the bgl promoter carrying the deletion, in a strain that has a mutation in the hns gene, indicated that this region is likely to be involved in binding to H-NS or a protein regulated by H-NS, which acts as a non-specific repressor. We also provide evidence which suggests that transcriptional activation by mutations at the cAMP receptor protein (CRP)-binding site is mediated partly by antagonization of the negative effect of H-NS by CRP-cAMP as result of its increased affinity for the mutant site.

Fulltext

Molecular Microbiology

Characterization of the negative elements involved in silencing the bgl operon of Escherichia coli: Possible roles for DNA gyrase, H-NS, and CRP- cAMP in regulation