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Number 427, May 10, 1999 by Phillip F. Schewe and Ben Stein
WATCHING PACEMAKERS FORM IN A PETRI DISH. The bandleaders of biological systems, pacemakers are cells or groups of cells capable of generating regularly repeating activity on their own or in response to outside signals. In humans, roughly 5000 cells in the sinoatrial node, located in the right roof of the atrium, generate the signals that regulate the rhythmic contractions of the heart. In efforts that may improve understanding of how natural pacemakers form and provide steady signals, researchers at Technion University in Israel (Yoav Soen, yoav@technion.ac.il) excised muscle cells and connective tissue (fibroblasts) from the ventricles of rats. Spreading these cells on a petri dish under the proper conditions caused the cells to proliferate, move around and eventually form a hardy network of fibers after 1-3 weeks. Using a CCD camera and real-time computer processing , the researchers detected rhythmic contractions in the cells. They also noticed rhythm disorders, such as alternations between irregular and regular rates of contraction. This suggested to them that one or more pacemakers had formed within the network. While in-vitro pacemaker activity has been observed before, the Technion optical technique is the first that monitors the cells noninvasively and continuously long enough to watch a cell network evolve and form a pacemaker system. Although the cell network is very different from a biological heart, it can provide insights into how its structure and density affects the development of a pacemaker. (Soen et al., Physical Review Letters, 26 April; See figure at Physics News Graphics)
CONTROLLING STOCHASTIC RESONANCE. In some systems, such as radio receivers, turning up the volume in order to hear a faint signal amidst much noise usually only results in turning up the noise as well. However, in other systems increasing the amount of ambient noise actually enhances (up to a certain point) the signal-to-noise ratio through a complicated nonlinear cooperation between the system and detector. This effect, known to operate in neurons, lasers, and tunnel diodes, is called stochastic resonance: the noise fluctuations might be stochastic (meaning totally random) but the detection of a desired signal can be maximized by tuning the noise. Now, researchers at Georgia Tech (Bill Ditto, 404-894-5216, wditto@acl.gatech.edu) have not only adjusted the noise knob to advantage but also the detector threshold. This can make the signal-to-noise ratio even better, with a bearing on the study of how sensory systems such as touch or hearing can pick out faint signals. Conversely the extra control mechanism can be used to undo the stochastic resonance effect, which just might be a desirable step in, for example, military applications (jamming) or in the suppression of unwanted interactions between electromagnetic radiation and biological tissue. (Gammaitoni et al., Physical Review Letters, 7 June.)
FERMI QUESTIONS, named for the Enrico Fermi who reveled in the exercise, are problems that call upon the art of approximation. Example: How many liters of water are drunk in the US per year? Answer---10^11 liters (250 x 106 people times 1.5 liters of water per day times 365 days). How long would it take to walk the distance between Earth and Moon? 104 days (2.4 x 105 miles divided by 24 miles/day). The number of postage stamps covering a football field? 107 (stamp area of 4 cm2 over a field of 100 x 50 m2). (The Physics Teacher magazine, May 1999; contact columnist Karen Bouffard, at kbouff@aol.com.)
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