Cellular networks underlying human spatial navigation

Place cells of the rodent hippocampus constitute one of the most striking examples of a correlation between neuronal activity and complex behaviour in mammals. These cells increase their firing rates when the animal traverses specific regions of its surroundings, providing a context-dependent map of the environment. Neuroimaging studies implicate the hippocampus and the parahippocampal region in human navigation. However, these regions also respond selectively to visual stimuli. It thus remains unclear whether rodent place coding has a homologue in humans or whether human navigation is driven by a different, visually based neural mechanism. We directly recorded from 317 neurons in the human medial temporal and frontal lobes while subjects explored and navigated a virtual town. Here we present evidence for a neural code of human spatial navigation based on cells that respond at specific spatial locations and cells that respond to views of landmarks. The former are present primarily in the hippocampus, and the latter in the parahippocampal region. Cells throughout the frontal and temporal lobes responded to the subjects' navigational goals and to conjunctions of place, goal and view.     

The nucleolar chromosome in embryos of Rana pipiens

Summary Karyotype analyses of prometaphases from medullary plate cells derived from mid-neurulae of Rana pipiens have led to the identification of the nucleolar chromosome and nucleolar organizing region, which is located on the longer arm of a small sub-metacentric chromosome (No. 10).Supported in part by research grant GB19631X from the Natioal Science Foundation. This study constituted a portion of a dissertation submitted by G. R. N. to the Medical College of Pennsylvania in partial fulfillment of requirements for a Ph. D. degree.Supported by NDEA and subsequently NSF predoctoral traineeships.To whom reprint requests should be sent.     

A distance to the galaxy NGC4258 from observations of Cepheid variable stars

Cepheid variable stars pulsate in a way that is correlated with their intrinsic luminosity, making them useful as 'standard candles' for determining distances to galaxies; the potential systematic uncertainties in the resulting distances have been estimated to be only 8-10%. They have played a crucial role in establishing the extragalactic distance scale and hence the value of the Hubble constant. Here we report observations of Cepheids in the nearby galaxy NGC4258; the distance calculated from the Cepheids is 8.1 +/- 0.4 Mpc, where the uncertainty does not include possible systematic errors. There is an independently determined geometric distance to this galaxy of 7.2 +/- 0.5 Mpc, based on the observed proper motions of water masers orbiting the central black hole; the distances differ by 1.3sigma. If the maser-based distance is adopted and the Cepheid distance scale revised accordingly, the derived value of the Hubble constant would increase by 12 +/- 9%, while the expansion age of the Universe would decrease by the same amount.     

Extracellular electron transfer

Results from several laboratories indicate that extracellular electron transfer may be a general mechanism whereby microoorganisms generate energy for cell growth and/or maintenance. Specifically, bacteria can use redox-active organic small molecules, generated outside or inside the cells, to shuttle electrons between reduced and oxidized compounds. Electron shuttling has now been reported for several different bacterial species, and exchanges of shuttling compounds may even syntrophically link diverse organisms in nature. Biofilm systems in both geological and clinical settings are likely to be important environments for metabolisms that employ extracellular electron transfer. Both structural and functional analyses suggest that electron shuttles and some virulence factors may be related to one another. Received 21 March 2001; received after revision 10 May 2001; accepted 11 May 2001