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Current understanding of neuronal growth is mostly qualitative, as the staggering number of physical and chemical guidance cues involved prohibit a fully quantitative description of axonal dynamics. We report on a general approach that describes axonal growth in vitro, on poly-D-lysine coated glass substrates, as diffusion in an effective external potential, representing the collective contribution of all causal influences on the growth cone. We use this approach to obtain effective growth rules that reveal an emergent regulatory mechanism for axonal pathfinding on these substrates., Comment: Accepted for publication in Physical Review E

Detailed knowledge of mechanical parameters such as cell elasticity, stiffness of the growth substrate, or traction stresses generated during axonal extensions is essential for understanding the mechanisms that control neuronal growth. Here we combine Atomic Force Microscopy based force spectroscopy with Fluorescence Microscopy to produce systematic, high-resolution elasticity maps for three different types of live neuronal cells: cortical (embryonic rat), embryonic chick dorsal root ganglion, and P-19 (mouse embryonic carcinoma stem cells) neurons. We measure how the stiffness of neurons changes both during neurite outgrowth and upon disruption of microtubules of the cell. We find reversible local stiffening of the cell during growth, and show that the increase in local elastic modulus is primarily due to the formation of microtubules. We also report that cortical and P-19 neurons have similar elasticity maps, with elastic moduli in the range 0.1-2 kPa, with typical average values of 0.4 kPa (P-19) and 0.2 kPa (cortical). In contrast, DRG neurons are stiffer than P-19 and cortical cells, yielding elastic moduli in the range 0.1-8 kPa, with typical average values of 0.9 kPa. Finally, we report no measurable influence of substrate protein coating on cell body elasticity for the three types of neurons.

We describe an undergraduate laboratory that combines an accurate measurement of the speed of light, a fundamental investigation of a basic laser system, and a nontrivial use of statistical analysis. Students grapple with the existence of longitudinal modes in a laser cavity as they change the cavity length of an adjustable-cavity HeNe laser and tune the cavity to produce lasing in the TEM$_{00}$ mode. For appropriate laser cavity lengths, the laser gain curve of a HeNe laser allows simultaneous operation of multiple longitudinal modes. The difference frequency between the modes is measured using a self-heterodyne detection with a diode photodetector and a radio frequency spectrum analyzer. Asymmetric effects due to frequency pushing and frequency pulling, as well as transverse modes, are minimized by simultaneously monitoring and adjusting the mode structure as viewed with a Fabry-Perot interferometer. The frequency spacing of longitudinal modes is proportional to the inverse of the cavity length with a proportionality constant equal to half the speed of light. By changing the length of the cavity, without changing the path length within the HeNe gas, the speed of light in air can be measured to be ($2.9972 \pm0.0002) \times 10^{8}$ m/s, which is to high enough precision to distinguish between the speed of light in air and that in a vacuum., Comment: 6 pages; 6 figures; Published in the American Journal of Physics

An Oklahoma study sought to identify specific benefits that accrue to students with special needs in secondary Supervised Agricultural Experience (SAE) programs and to determine teachers' perceptions of SAE programs for students with special needs. Data were gathered in the fall of 1993 through a mailed survey of all 362 agricultural education departments in the state; 249 responses were received (69 percent). The study found that only 68 percent (956) of 1,401 students with special needs were involved in SAE. The most common types of SAE programs conducted by students with special needs included livestock exhibition, horticulture, agricultural mechanics, and job placement. The teachers were very experienced, with an average age of 38 and with 14 years of teaching experience. Overall, teachers' perceptions of students with special needs involved in SAE programs were positive. They felt that SAE for students with special needs was similar to SAE for mainstream students in most respects, although special needs students needed more help and had fewer options. The study concluded that SAE offered multiple opportunities and benefits for special needs students and related directly to the objectives of special education. Several difficulties were cited: poor student recordkeeping, lack of parental support, family's socioeconomic status, supervision time, and students' abilities and behaviors. Recommendations were made to involve more special needs students in SAE based on their needs. (Contains 23 references.) (KC)