Biophysics

Sensory Discrimination

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Science  25 Aug 2006:
Vol. 313, Issue 5790, pp. 1020-1021
DOI: 10.1126/science.313.5790.1020d

An attractive (or noxious) signal might come from any direction, so how can a single cell remain on the lookout in all directions? Escherichia coli are known to cope by interspersing periods of directional swimming with tumbling, which reorients them randomly. The single-celled slime mold Dictyostelium lives socially and has taken advantage of this lifestyle to apportion the community's detectors to cover all points of the compass.

Samadani et al. show that single Dictyostelium cells respond reproducibly (as assessed by the angular location of a cAMP-sensing component) to 10 trials of a fixed pulse of cAMP, yet this angle varies over more than 180° when measured across 40 cells. Nevertheless, the population response, summed over orientation and magnitude, yields a peak unerringly directed at the origin of the pulse. Each cell's innate inclination can be modeled as a function of (i) the overall mobilization of sensor components, (ii) the degree to which the components are distributed asymmetrically, and (iii) the orientation of the peak of asymmetry. Each cell's response can then be calculated as the vector sum of the intrinsic properties and the stimulus, and this captures the observed behaviors of the population, behaviors that are similar to the orientation selectivity of visual cortex neurons. This analogy leads one to wonder how the intrinsic orientations are divvied up and maintained in a Dictyostelium community. — GJC

Proc. Natl. Acad. Sci. U.S.A. 103, 11549 (2006).

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