Proof of Concept for an Ultrasensitive Technique to Detect and Localize Sources of Elastic Nonlinearity Using Phononic Crystals.

The appearance of nonlinear effects in elastic wave propagation is one of the most reliable and sensitive indicators of the onset of material damage. However, these effects are usually very small and can be detected only using cumbersome digital signal processing techniques. Here, we propose and experimentally validate an alternative approach, using the filtering and focusing properties of phononic crystals to naturally select and reflect the higher harmonics generated by nonlinear effects, enabling the realization of time-reversal procedures for nonlinear elastic source detection. The proposed device demonstrates its potential as an efficient, compact, portable, passive apparatus for nonlinear elastic wave sensing and damage detection.

Tacticity in chiral phononic crystals.

The study of vibrational properties in engineered periodic structures relies on the early intuitions of Haüy and Boscovich, who regarded crystals as ensembles of periodically arranged point masses interacting via attractive and repulsive forces. Contrary to electromagnetism, where mechanical properties do not couple to the wave propagation mechanism, in elasticity this paradigm inevitably leads to low stiffness and high-density materials. Recent works transcend the Haüy-Boscovich perception, proposing shaped atoms with finite size, which relaxes the link between their mass and inertia, to achieve unusual dynamic behavior at lower frequencies, leaving the stiffness unaltered. Here, we introduce the concept of tacticity in spin-spin-coupled chiral phononic crystals. This additional layer of architecture has a remarkable effect on their dispersive behavior and allows to successfully realize material variants with equal mass density and stiffness but radically different dynamic properties.

Proof of concept of a frequency-preserving and time-invariant metamaterial-based nonlinear acoustic diode.

Acoustic filters and metamaterials have become essential components for elastic wave control in applications ranging from ultrasonics to noise abatement. Other devices have been designed in this field, emulating their electromagnetic counterparts. One such case is an acoustic diode or rectifier, which enables one-way wave transmission by breaking the wave equation-related reciprocity. Its achievement, however, has proved to be rather problematic, and current realizations display a number of shortcomings in terms of simplicity and versatility. Here, we present the design, fabrication and characterization of a device able to work as an acoustic diode, a switch and a transistor-like apparatus, exploiting symmetry-breaking nonlinear effects like harmonic generation and wave mixing, and the filtering capabilities of metamaterials. This device presents several advantages compared with previous acoustic diode realizations, including versatility, time invariance, frequency preserving characteristics and switchability. We numerically evaluate its efficiency and demonstrate its feasibility in a preliminary experimental realization. This work may provide new opportunities for the practical realization of structural components with one-way wave propagation properties.

Place navigation in rats guided by a vestibular and kinesthetic orienting gradient.

The role of the slope of terrain in orientation was examined in rats trained to find, among 4 equidistant feeders, the 1 located in the upper left quadrant of a 10% tilted arena (1-m radius). Rats started from the center in light and with randomly changing slope direction reached the correct goal in 90% of 1st choices after 29 sessions. The same rats maintained 83% correct choices when the experiment was conducted in darkness. On a horizontal arena, their performance became random. After training, successful navigation was also observed (71% correct 1st choices) when the rats were started from different points at about 30 cm from the wall. This finding suggests that the slope of terrain may be used to establish a cognitive map based primarily on kinesthetic and vestibular signals. The flexibility of such a map seems to be rather limited, however, because changing the goal position with respect to inclination requires prolonged retraining.

Presynaptic modulation by group III metabotropic glutamate receptors (mGluRs) of the excitatory postsynaptic potential mediated by mGluR1 in rat cerebellar Purkinje cells.

Purkinje neurons were recorded from rat cerebellar slices. Parallel fibres stimulation elicited a fast excitatory postsynaptic potential (EPSP) mediated by ionotropic glutamate (iGluR) -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors followed by the inhibitory gamma-aminobutyric acidA (GABAA)-dependent postsynaptic potential. In the presence of antagonists for iGluRs and for GABAA receptors, brief tetanic activation evoked a slow metabotropic glutamate receptor (mGluR)-dependent EPSP (mGluR-EPSP). This mGluR-EPSP was blocked by the selective mGluR1 antagonists LY367385 and CPCCOEt, but not by the mGluR5 antagonist MPEP. Group II agonists affected neither iGluR-EPSP nor mGluR-EPSP. Conversely, L-AP4 and L-SOP, group III mGluR agonists, inhibited both iGluR- and mGluR-EPSPs. The depolarisations evoked by both AMPA and group I agonists were unaffected, indicating a presynaptic action of group III mGluRs. These data suggest that glutamate released by parallel fibres activates group III mGluR autoreceptors, depressing both iGluR- and mGluR1-mediated EPSPs.

Reduction of cutaneous blood flow in cold fingers.

Blood flow to fingers is reduced during cold exposure. This is generally attributed to vasoconstriction. We tested the hypothesis that increased blood viscosity, not vasoconstriction, accounts for reductions of cutaneous flow after fingers cool. Blood viscosity was higher at 10 degree C than at 27 degree C and independent of hematocrit at low shear rates. The increase of finger vascular resistance may be due to increased vascular hindrance early in cold exposure (< 15 min) and is more likely due to increased viscosity after 20-30 min, a factor that may dominate the peripheral microcirculaton during prolonged cold exposure.

Postsynaptic current mediated by metabotropic glutamate receptors in cerebellar Purkinje cells.

In rat cerebellar slices, repetitive parallel fiber stimulation evokes an inward, postsynaptic current in Purkinje cells with a fast component mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors and a slower component mediated by metabotropic glutamate receptors (mGluR). The mGluR-mediated excitatory postsynaptic current (mGluR-EPSC) is evoked selectively by parallel fiber stimulation; climbing fiber stimulation is ineffective. The mGluR-EPSC is elicited most effectively with increasing frequencies of parallel fiber stimulation, from a threshold of 10 Hz to a maximum response at approximately 100 Hz. The amplitude of the mGluR-EPSC is a linear function of the number of stimulus pulses without any apparent saturation, even with >10 pulses. Thus mGluRs at the parallel fiber-Purkinje cell synapse can function as linear detectors of the number of spikes in a burst of activity in parallel fibers. The mGluR-EPSC is present from postnatal day 15 and persists into adulthood. It is inhibited by the generic mGluR antagonist (RS)-a-methyl-4-carboxyphenylglycine and by the group I mGluR antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid at a concentration selective for mGluR1. Although the intracellular transduction pathway involves a G protein, the putative mediators of mGluR1 (phospholipase C and protein kinase C) are not directly involved, indicating that the mGluR-EPSC studied here is mediated by a different and still unidentified second-messenger pathway. Heparin, a nonselective antagonist of inositol-trisphosphate (IP3) receptors, has no significant effect on the mGluR-EPSC, suggesting that also IP3 might be not required for the response. Buffering intracellular Ca2+ with a high concentration of bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid partially inhibits the mGluR-EPSC, indicating that Ca2+ is not directly responsible for the response but that resting Ca2+ levels exert a tonic potentiating effect on the mGluR-EPSC.