Modeling Action Potential Reversals in Tunicate Hearts

Tunicates are small invertebrates which possess a unique ability to reverse flow in their hearts. Scientists have debated various theories regarding how and why flow reversals occur. Here we explore the electrophysiological basis for reversals by simulating action potential propagation in an idealized model of the tubelike tunicate heart. Using asymptotic formulas for action potential duration and conduction velocity, we propose tunicate-specific parameters for a two-current ionic model of the action potential. Then, using a kinematic model, we derive analytical criteria for reversals to occur. These criteria inform subsequent numerical simulations of action potential propagation in a fiber paced at both ends. In particular, we explore the role that variability of pacemaker firing rates plays in generating reversals, and we identify various favorable conditions for triggering retrograde propagation. Our analytical framework extends to other species; for instance, it can be used to model competition between the sinoatrial node and abnormal ectopic foci in human heart tissue

High Curie Temperature Achieved in the Ferromagnetic MnxGe1−x/Si Quantum Dots Grown by Ion Beam Co-Sputtering

Ferromagnetic semiconductors (FMSs) exhibit great potential in spintronic applications. It is believed that a revolution of microelectronic techniques can take off, once the challenges of FMSs in both the room-temperature stability of the ferromagnetic phase and the compatibility with Si-based technology are overcome. In this article, the MnxGe1−x/Si quantum dots (QDs) with the Curie temperature (TC) higher than the room temperature were grown by ion beam co-sputtering (IBCS). With the Mn doping level increasing, the ripening growth of MnGe QDs occurs due to self-assembly via the Stranski–Krastanov (SK) growth mode. The surface-enhanced Raman scattering effect of Mn sites observed in MnGe QDs are used to reveal the distribution behavior of Mn atoms in QDs and the Si buffer layer. The Curie temperature of MnxGe1−x QDs increases, then slightly decreases with increasing the Mn doping level, and reaches its maximum value of 321 K at the doping level of 0.068. After a low-temperature and short-time annealing, the TC value of Mn0.068Ge0.932 QDs increases from 321 K to 383 K. The higher Ge composition and residual strain in the IBCS grown MnxGe1−x QDs are proposed to be responsible for maintaining the ferromagnetic phase above room temperature