Physics News Update No. 688

The American Institute of Physics Bulletin of Physics News
Number 688 June 11, 2004 by Phillip F. Schewe, Ben Stein

A NEW CHEMOTAXIS ASSAY reveals nerve cells' surprising sensitivity. A new method for studying the guidance (change in direction) of neurons amid a sea of protein molecules shows how sensitive this process is to the surrounding protein gradient.

Chemotaxis is the process by which living cells sniff out their local environment and act accordinglyChemotaxis is the process by which living cells sniff out their local environment and act accordingly, which usually means moving or growing toward higher concentrations of beneficial molecules. In the case of neurons removed from their natural setting and put down on a bed of collagen gel in a dish, growth will follow the increasing gradient of proteins in their vicinity, such as the nerve growth factor (NGF) protein.

Neuronal growth, the way in which the long axon bodies of a nerve cells wire themselves into a network, is of great interest since this aids in knowing how brains form. Now a team of scientists at Georgetown University has developed a new method for measuring the gradient of local proteins (which have been fluorescently tagged) and the axon's response. In this case the neural cells come originally from a rat's brain.

The Georgetown team of neuroscientists and physicists find that axon growth is sensitive to gradients so small (0.1%) that they correspond to about one additional molecule across the spatial extent of the axon's "growth cone," the sensing device at the tip of the growing axon. This is a remarkable feat considering that, at any one instant, there are large statistical fluctuations in the 1000 or so NGF molecules in the vicinity of the growth cone.

The researchers suggest that axons may thus be "nature's most-sensitive gradient detectors." (Rosoff et al., Nature Neuroscicence, June 2004; contact Jeffrey Urbach, urbach@physics.georgetown.edu, 202-687-6594; or Geoffrey Goodhill, geoff@georgetown.edu).

PERFORMING BOOLEAN SURGERY TO UNLOCK BIOSONAR'S SECRETS. Over the last approximately 60 million years of evolutionary history, bats have developed highly optimized biosonar systems in which they broadcast ultrasound at various frequencies and then detect the echoes to sense their surroundings.

At last month's meeting of the Acoustical Society of America in New York, researchers (Rolf Mueller, University of Southern Denmark, +45-6550-3655, rolfm@mip.sdu.dk) presented the first high-resolution, three-dimensional maps to depict spatial regions in which the ears are sensitive to low- mid-, and high-frequency ultrasound.

These biologically based ultrasound-sensitivity maps vary considerably over the studied sample of bat species and are likely to vary even more over the approximately 1000 species which exist in total. They may help inspire much better designs for artificial antennas of any type, from the acoustic ones in ship sonar systems and medical devices to the electromagnetic antennas in cell phones.

In their approach the researchers perform CT scans of bat ears to obtain highly detailed images and 3D shapes which are then rendered on a computer. Next they model the interaction between each ear shape and ultrasound waves from the bat's surroundings. The researchers can understand how the anatomical features of an ear shape bring about the spatial sensitivity patterns by performing painless "Boolean surgery," in which they can modify an ear's shape on a computer (often by removing some features and--as part of their future plans--mixing features from different species) and see how the modifications change the ear's detection of ultrasound. (Paper 4aAB6 at meeting; lay-language paper at http://www.acoustics.org/press/147th/Mueller.html).

MICROWAVE TISSUE WELDING. A conventional microwave oven uses an antenna to squirt microwaves into a reflective box where they preferentially excite and heat anything rich in water molecules.

A new experiment performed in the group of Michael Golosovsky and Dan Davidov at the Racah Institute of Physics, the Hebrew University of Jerusalem reduces the antenna size and dispenses with the resonant box and, by getting really close to the sample of soft matter, can heat a tiny spot, one half by one quarter of a millimeter in size, up to temperatures of 120 C (or 250 F).

One possible application would be "tissue welding," the process of binding together edges of cut tissue using "biological solder" such as albumin. Infrared lasers can do such welding, but Golosovsky (golos@vms.huji.ac.il, 972-2658-6551) says that the microwave approach uses much lower power, can do the job faster, can deposit radiation at deeper levels in the wound, and bandages are transparent to the microwaves. Also collateral tissue damage would be better controlled. (Copty et al., Applied Physics Letters, 14 June 2004)

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