Temporal sampling, resetting, and adaptation orchestrate gradient sensing in sperm

Sperm, navigating in a chemical gradient, are exposed to a periodic stream of chemoattractant molecules. The periodic stimulation entrains Ca2+ oscillations that control looping steering responses. It is not known how sperm sample chemoattractant molecules during periodic stimulation and adjust their sensitivity. We report that sea urchin sperm sampled molecules for 0.2–0.6 s before a Ca2+ response was produced. Additional molecules delivered during a Ca2+ response reset the cell by causing a pronounced Ca2+ drop that terminated the response; this reset was followed by a new Ca2+ rise. After stimulation, sperm adapted their sensitivity following the Weber–Fechner law. Taking into account the single-molecule sensitivity, we estimate that sperm can register a minimal gradient of 0.8 fM/µm and be attracted from as far away as 4.7 mm. Many microorganisms sense stimulus gradients along periodic paths to translate a spatial distribution of the stimulus into a temporal pattern of the cell response. Orchestration of temporal sampling, resetting, and adaptation might control gradient sensing in such organisms as well.

Coordinated and circumlocutory semantic naming errors are related to anterolateral temporal lobes in mild AD, amnestic mild cognitive impairment, and normal aging

Naming difficulties are characteristic of Alzheimer’s disease (AD) and, to a lesser extent, of amnestic mild cognitive impairment (aMCI) patients. The association of naming impairment with anterior temporal lobe (ATL) atrophy in Semantic Dementia (SD) could be a tip of the iceberg effect, in which case the atrophy is a marker of more generalized temporal lobe pathology. Alternatively, it could reflect the existence of a functional gradient within the temporal lobes, wherein more anterior regions provide the basis for greater specificity of representation. We tested these two hypotheses in a study of 15 subjects with mild AD, 17 with aMCI, and 16 aged control subjects and showed that coordinate and circumlocutory semantic error production on the Boston Naming Test was weakly correlated with ATL gray matter density, as determined by voxel-based morphometry. Additionally, we investigated whether these errors were benefited by phonemic cues, and similarly to SD, our AD patients had small improvement. Because there is minimal gradient of temporal lobe atrophy in AD or MCI, and, therefore, no basis for a tip of the iceberg effect, these findings support the theory of a modest functional gradient in the temporal lobes, with the ATLs being involved in the naming of more specific objects. (JINS, 2010, 16, 1099–1107.)