931 research outputs found
Afferent Convergence from Divergent Sources Appears to Enhance the Spastic Patient\u27s Ability to Inhibit Antagonist Motoneurons During an Agonist Contraction
Spasticity and excessive muscular cocontraction during movement are common impairments following upper motor neuron (UMN) lesions. For example, during voluntary ankle dorsiflexion, individuals with spastic type cerebral palsy (CP) do not inhibit the antagonist triceps surae motoneuron pool
Antagonist Muscle Inhibition During Voluntary And Automatic Movements: Implications For Clinical Intervention
Various rehabilitation techniques encourage eliciting automatic reactions in the hope that muscle activations elicited in this way will influence voluntary movement. Unlike voluntary movements that consist of willed initiation and involvement of descending corticofugal and and basal ganglia pathways, automatic postural reactions resulting from external perturbations primarily involve peripheral receptors arising from the vestibular apparatus, muscles, joints, and eyes. We were interested to determine whether voluntary and automatic movements, which are initiated very differently, involved similar temporal activation of muscles and the same pattern of reciprocal inhibition between agonist and antagonist motor neuron pools
Quenched Narrow-Line Laser Cooling of 40Ca to Near the Photon Recoil Limit
We present a cooling method that should be generally applicable to atoms with
narrow optical transitions. This technique uses velocity-selective pulses to
drive atoms towards a zero-velocity dark state and then quenches the excited
state to increase the cooling rate. We demonstrate this technique of quenched
narrow-line cooling by reducing the 1-D temperature of a sample of neutral 40Ca
atoms. We velocity select and cool with the 1S0(4s2) to 3P1(4s4p) 657 nm
intercombination line and quench with the 3P1(4s4p) to 1S0(4s5s)
intercombination line at 553 nm, which increases the cooling rate eight-fold.
Limited only by available quenching laser power, we have transferred 18 % of
the atoms from our initial 2 mK velocity distribution and achieved temperatures
as low as 4 microK, corresponding to a vrms of 2.8 cm/s or 2 recoils at 657 nm.
This cooling technique, which is closely related to Raman cooling, can be
extended to three dimensions.Comment: 5 pages, 4 figures; Submitted to PRA Rapid Communication
Doppler cooling and trapping on forbidden transitions
Ultracold atoms at temperatures close to the recoil limit have been achieved
by extending Doppler cooling to forbidden transitions. A cloud of ^40Ca atoms
has been cooled and trapped to a temperature as low as 6 \mu K by operating a
magneto-optical trap on the spin-forbidden intercombination transition.
Quenching the long-lived excited state with an additional laser enhanced the
scattering rate by a factor of 15, while a high selectivity in velocity was
preserved. With this method more than 10% of pre-cooled atoms from a standard
magneto-optical trap have been transferred to the ultracold trap. Monte-Carlo
simulations of the cooling process are in good agreement with the experiments
Two-Photon Doppler cooling of alkaline-earth-metal and ytterbium atoms
A new possibility of laser cooling of alkaline-earth-metal and Ytterbium
atoms using a two-photon transition is analyzed. We consider a -
transition, with excitation in near resonance with the
level. This greatly increases the two-photon transition rate, allowing an
effective transfer of momentum. The experimental implementation of this
technique is discussed and we show that for Calcium, for example, two-photon
cooling can be used to achieve a Doppler limit of 123 microKelvin. The
efficiency of this cooling scheme and the main loss mechanisms are analyzed.Comment: 7 pages, 5 figure
Nonclassical Interference Effects In The Radiation From Coherently Driven Uncorrelated Atoms
We demonstrate the existence of new nonclassical correlations in the
radiation of two atoms, which are coherently driven by a continuous laser
source. The photon-photon-correlations of the fluorescence light show a spatial
interferene pattern not present in a classical treatment. A feature of the new
phenomenon is, that bunched and antibunched light is emitted in different
spatial directions. The calculations are performed analytically. It is pointed
out, that the correlations are induced by state reduction due to the
measurement process when the detection of the photons does not distinguish
between the atoms. It is interesting to note, that the phenomena show up even
without any interatomic interaction.Comment: 4 pages, 6 Figure
Deterministic delivery of externally cold and precisely positioned single molecular ions
We present the preparation and deterministic delivery of a selectable number
of externally cold molecular ions. A laser cooled ensemble of Mg^+ ions
subsequently confined in several linear Paul traps inter-connected via a
quadrupole guide serves as a cold bath for a single or up to a few hundred
molecular ions. Sympathetic cooling embeds the molecular ions in the
crystalline structure. MgH^+ ions, that serve as a model system for a large
variety of other possible molecular ions, are cooled down close to the Doppler
limit and are positioned with an accuracy of one micrometer. After the
production process, severely compromising the vacuum conditions, the molecular
ion is efficiently transfered into nearly background-free environment. The
transfer of a molecular ion between different traps as well as the control of
the molecular ions in the traps is demonstrated. Schemes, optimized for the
transfer of a specific number of ions, are realized and their efficiencies are
evaluated. This versatile source applicable for broad charge-to-mass ratios of
externally cold and precisely positioned molecular ions can serve as a
container-free target preparation device well suited for diffraction or
spectroscopic measurements on individual molecular ions at high repetition
rates (kHz).Comment: 11 pages, 8 figure
Single photon generation by pulsed excitation of a single dipole
The fluorescence of a single dipole excited by an intense light pulse can
lead to the generation of another light pulse containing a single photon. The
influence of the duration and energy of the excitation pulse on the number of
photons in the fluorescence pulse is studied. The case of a two-level dipole
with strongly damped coherences is considered. The presence of a metastable
state leading to shelving is also investigated.Comment: 17 pages, 4 figures, submitted to PR
Postoperative Immune Suppression in Visceral Surgery: Characterisation of an Intestinal Mouse Model
Background: Postoperatively acquired immune dysfunction is associated with a higher mortality rate in case of septic complications. As details of this severe clinical problem are still unknown, animal models are essential to characterise the mechanisms involved. Methods: Mice were laparotomised and the small intestine was pressed smoothly in antegrade direction. For extension of trauma, the intestine was manipulated three times consecutively. Following this, the ex vivo cytokine release of splenocytes was determined. The degree of surgical trauma was analysed by detection of HMGB1 and IL-6 in serum and by neutrophil staining in the muscularis mucosae. Results: We adapted the previously described animal model of intestinal manipulation to provide a model of surgically induced immune dysfunction. Following intestinal manipulation, the mice showed elevated serum levels of HMGB1 and IL-6 and increased infiltration of granulocytes into the muscularis mucosae. Ex vivo cytokine release by splenocytes was suppressed in the postoperative period. The degree of suppression correlated with the extent of surgical trauma. Conclusions: In this study, we describe a surgically induced immune dysfunction animal model, in which a significant surgical trauma is followed by an immune dysfunction. This model may be ideal for the characterisation of the postoperative immune dysfunction syndrome
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