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Physics News Update
Number 673, February 18, 2004 by Phil Schewe, James Riordon, and Ben Stein

A New Approach For Calming Parkinson's Tremors

Many neuroscientists believe that pathological brain rhythms, for example in Parkinson's disease and in epilepsy, arise from an abnormal synchronization of many thousands of nerve cells (neurons). This physical mechanism appears in many physical and biological systems. For example, it enables fireflies to light up in unison.

Sometimes, synchrony is desirable, for instance, when the cells of the heart's main pacemaker (the sino-atrial node) fire all together to stimulate heart contraction.

But in many cases synchrony is harmful. London's Millennium Bridge, which swayed undesirably shortly after it opened in 2000, provides a useful example. Hundreds of pedestrians subconsciously synchronized their pace to the bridge's sideways, left-to-right swaying motions. The bridge oscillations, driven by pedestrians, became dangerously large, and the walkway had to be closed for reconstruction.

In the case of a Parkinson's tremor one also needs to suppress the synchronous oscillations of nerve cells, but one can hardly apply the methods used by engineers for the Millennium Bridge. Thus, researchers need a technique to control the collective synchrony of neurons.

Now, a paper suggests a new approach: one would measure the collective rhythm of nerve cells and, after some delay, electrically
"feed back" this rhythm into the population of nerve cells. Adjusting the delay time and the amplification in the feedback loop, the researchers in principle could either suppress or enhance the collective rhythm.

The researchers (Michael Rosenblum, mros@agnld.uni-potsdam.de, and Arkady Pikovsky, pikovsky@stat.physik.uni-potsdam.de, University of Potsdam, Germany) have tested this idea in simulations that employ mathematical models of neuron populations.

The researchers believe the scheme might be used, in particular, for suppressing Parkinson's tremors by means of the emerging medical technique, called Deep Brain Stimulation, that enables intervention with the use of implanted microelectrodes.

In principle, medical doctors could use an implanted electrode to measure electrical activity of the brain area and stimulate the nerve-cell population via a second electrode with the delayed signal. The advantage of this approach is that individual neurons are not much affected and continue to function, while the pathological collective Parkinsonian rhythm is suppressed noninvasively. (Rosenblum and Pikovsky, Physical Review Letters, upcoming.)

Attogram Mass Detection

Attogram mass detection has been achieved by Harold Craighead and his colleagues at Cornell, with prospects of exquisite detection of very tiny chemical and biological species, possibly with arrays of detectors. With their lithographically fabricated nanoelectromechanical (NEMS) device, the Cornell researchers can measure the mass of a particle with a sensitivity of 10-18 grams, far exceeding the precision of a comparable device with femtogram (10-15 g) sensitivity reported last year (Update 634-2). To get any better measurement of mass you would have to vaporize the particle and shoot its constituent molecules through a mass spectrometer.

At Cornell, mass measurement works this way: when the minuscule particle is absorbed onto a tiny sliver of silicon it alters the sliver's resonant oscillation (see figure). The oscillation in turn is monitored by reflecting laser light off the cantilever. It's as if a particle with a mass of a billionth of a billionth of a gram stepped onto a diving board whose springiness was observed by reflected light.

So far Craighead's group has weighed small gold dots and tiny coatings of molecules on the dots, but the goal is to detect and identify viruses. (Previously the same group detected the immunospecific binding of a single bacterium using the oscillating-cantilever method. They did this by coating their with a specific antibody and therefore could bind and detect the added mass only of the corresponding antigen.)

The mass sensitivity with the present cantilever (4 microns long, 500 nm wide, and at room temperature) is expected to be 0.39 attogram and will only get better as the size of the cantilever is reduced further, extending the sensitivity well into the zeptogram (10-21 g) range. (Ilic et al., Journal of Applied Physics, upcoming article; also, see lab website. For an analysis of the ultimate limits of cantilever sensing of mass, see Ekinci et al., Journal of Applied Physics, 1 March 2004.)