567 research outputs found
A gradient method for the quantitative analysis of cell movement and tissue flow and its application to the analysis of multicellular Dictyostelium development
We describe the application of a novel image processing method, which allows quantitative analysis of cell and tissue movement in a series of digitized video images. The result is a vector velocity field showing average direction and velocity of movement for every pixel in the frame. We apply this method to the analysis of cell movement during different stages of the Dictyostelium developmental cycle. We analysed time-lapse video recordings of cell movement in single cells, mounds and slugs. The program can correctly assess the speed and direction of movement of either unlabelled or labelled cells in a time series of video images depending on the illumination conditions. Our analysis of cell movement during multicellular development shows that the entire morphogenesis of Dictyostelium is characterized by rotational cell movement. The analysis of cell and tissue movement by the velocity field method should be applicable to the analysis of morphogenetic processes in other systems such as gastrulation and neurulation in vertebrate embryos
4D topology optimization: Integrated optimization of the structure and self-actuation of soft bodies for dynamic motions
Topology optimization is a powerful tool utilized in various fields for
structural design. However, its application has primarily been restricted to
static or passively moving objects, mainly focusing on hard materials with
limited deformations and contact capabilities. Designing soft and actively
moving objects, such as soft robots equipped with actuators, poses challenges
due to simulating dynamics problems involving large deformations and intricate
contact interactions. Moreover, the optimal structure depends on the object's
motion, necessitating a simultaneous design approach. To address these
challenges, we propose "4D topology optimization," an extension of
density-based topology optimization that incorporates the time dimension. This
enables the simultaneous optimization of both the structure and self-actuation
of soft bodies for specific dynamic tasks. Our method utilizes multi-indexed
and hierarchized density variables distributed over the spatiotemporal design
domain, representing the material layout, actuator layout, and time-varying
actuation. These variables are efficiently optimized using gradient-based
methods. Forward and backward simulations of soft bodies are done using the
material point method, a Lagrangian-Eulerian hybrid approach, implemented on a
recent automatic differentiation framework. We present several numerical
examples of self-actuating soft body designs aimed at achieving locomotion,
posture control, and rotation tasks. The results demonstrate the effectiveness
of our method in successfully designing soft bodies with complex structures and
biomimetic movements, benefiting from its high degree of design freedom.Comment: 36 pages, 27 figures; for supplementary video, see
https://youtu.be/sPY2jcAsNY
Characteristic Scales of Baryon Acoustic Oscillations from Perturbation Theory: Non-linearity and Redshift-Space Distortion Effects
An acoustic oscillation of the primeval photon-baryon fluid around the
decoupling time imprints a characteristic scale in the galaxy distribution
today, known as the baryon acoustic oscillation (BAO) scale. Several on-going
and/or future galaxy surveys aim at detecting and precisely determining the BAO
scale so as to trace the expansion history of the universe. We consider
nonlinear and redshift-space distortion effects on the shifts of the BAO scale
in -space using perturbation theory. The resulting shifts are indeed
sensitive to different choices of the definition of the BAO scale, which needs
to be kept in mind in the data analysis. We present a toy model to explain the
physical behavior of the shifts. We find that the BAO scale defined as in
Percival et al. (2007) indeed shows very small shifts ( 1%) relative
to the prediction in {\it linear theory} in real space. The shifts can be
predicted accurately for scales where the perturbation theory is reliable.Comment: 21 pages, 9 figures, references and supplementary sections added,
accepted for publication in PAS
Inhibition of tibialis anterior spinal reflex circuits using frequency-specific neuromuscular electrical stimulation
Arai S., Sasaki A., Tsugaya S., et al. Inhibition of tibialis anterior spinal reflex circuits using frequency-specific neuromuscular electrical stimulation. Artificial Organs , (2024); https://doi.org/10.1111/aor.14737.Background: Neuromuscular electrical stimulation (NMES) can generate muscle contractions and elicit excitability of neural circuits. However, the optimal stimulation frequency for effective neuromodulation remains unclear. Methods: Eleven able-bodied individuals participated in our study to examine the effects of: (1) low-frequency NMES at 25 Hz, (2) high-frequency NMES at 100 Hz; and (3) mixed-frequency NMES at 25 and 100 Hz switched every second. NMES was delivered to the right tibialis anterior (TA) muscle for 1 min in each condition. The order of interventions was pseudorandomized between participants with a washout of at least 15 min between conditions. Spinal reflexes were elicited using single-pulse transcutaneous spinal cord stimulation applied over the lumbar enlargement to evoke responses in multiple lower-limb muscles bilaterally and maximum motor responses (Mmax) were elicited in the TA muscle by stimulating the common peroneal nerve to assess fatigue at the baseline and immediately, 5, 10, and 15 min after each intervention. Results: Our results showed that spinal reflexes were significantly inhibited immediately after the mixed-frequency NMES, and for at least 15 min in follow-up. Low-frequency NMES inhibited spinal reflexes 5 min after the intervention, and also persisted for at least 10 min. These effects were present only in the stimulated TA muscle, while other contralateral and ipsilateral muscles were unaffected. Mmax responses were not affected by any intervention. Conclusions: Our results indicate that even a short-duration (1 min) NMES intervention using low- and mixed-frequency NMES could inhibit spinal reflex excitability of the TA muscle without inducing fatigue
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