AA Fenton  RU Muller 《Canadian Metallurgical Quarterly》1998,95(6):3182-3187

The idea that the rat hippocampus stores a map of space is based on the existence of \"place cells\" that show \"location-specific\" firing. The discharge of place cells is confined with remarkable precision to a cell-specific part of the environment called the cell's \"firing field.\" We demonstrate here that firing is not nearly as reliable in the time domain as in the positional domain. Discharge during passes through the firing field was compared with a model with Poisson variance of the location-specific firing determined by the time-averaged positional firing rate distribution. Place cells characteristically fire too little or too much compared with expectations from the random model. This fundamental property of place cells is referred to as \"excess firing variance\" and has three main implications: (i) Place cell discharge is not only driven by the summation of many small, asynchronous excitatory synaptic inputs. (ii) Place cell discharge may encode a signal in addition to the current head location. (iii) The excess firing variance helps explain why the errors in computing the rat's position from the simultaneous activity of many place cells are large.  相似文献   

    

K Then Bergh  AA Brakhage 《Canadian Metallurgical Quarterly》1998,64(3):843-849

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AA Zamani  T Moriarty  L Hsu  CS Winalski  JL Schaffer  H Isbister  JF Schenck  KW Rohling  F Jolesz 《Canadian Metallurgical Quarterly》1998,8(6):1329-1333

High-level penicillin resistance in pneumococci is due to alterations in penicillin-binding proteins (PBPs) 2X, 2B, and 1A. We have sequenced the penicillin-binding domain of PBP 1A from penicillin-resistant South African pneumococcal isolates and have identified amino acid substitutions which are common to all the resistant isolates analyzed. Site-directed mutagenesis was then used to determine whether particular amino acid substitutions at specific positions in PBP 1A mediate penicillin resistance. PCR was used to isolate PBP 2X, 2B, and 1A genes from clinical isolate 8303 (penicillin MIC, 4 micrograms/ml). These wild-type PBP genes were cloned into pGEM-3Zf and were used as the transforming DNA. Susceptible strain R6 (MIC, 0.015 microgram/ml) was first transformed with PBP 2X and 2B DNA, resulting in PBP 2X/2B-R6 transformants for which MICs were 0.25 microgram/ml. When further transformed with PBP 1A DNA, 2X/2B/1A-R6 transformants for which MICs were 1.5 micrograms/ml were obtained. Site-directed mutagenesis of the PBP 1A gene from isolate 8303 was then used to reverse particular amino acid substitutions, followed by transformation of PBP 2X/2B-R6 transformants with the mutagenized PBP 1A DNA. For PBP 2X/2B/1A-R6 transformants, the introduction of the reversal of Thr-371 by Ser or Ala in PBP 1A decreased the MIC from 1.5 to 0.5 micrograms/ml, whereas the reversal of four consecutive amino acid substitutions (Thr-574 by Asn, Ser-575 by Thr, Gln-576 by Gly, and Phe-577 by Tyr) decreased the MIC from 1.5 to 0.375 micrograms/ml. These data reveal that amino acid residue 371 and residues 574 to 577 of PBP 1A are important positions in PBP 1A with respect to the interaction with penicillin and the development of resistance.  相似文献   

    

WF Lam  HA Gielkens  SY de Boer  CB Lamers  AA Masclee 《Canadian Metallurgical Quarterly》1998,65(3):505-511

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AA Golubov  GY Logvenov 《Canadian Metallurgical Quarterly》1995,51(6):3696-3700

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GM Fox  AA McBeath  JP Heiner 《Canadian Metallurgical Quarterly》1994,76(2):195-201

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J Douglass  AA McKinzie  KM Pollock 《Canadian Metallurgical Quarterly》1994,8(3):333-344

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VIu Amiiants  GV Gromova  AA Veres  ZN Bidzhieva  MA Kazarian  VG Tolmachev 《Canadian Metallurgical Quarterly》1996,(6):7-9

Left ventricular myocardial contractility was found essential in dynamics of clinical IHD picture in patients on rehabilitation treatment at low mountain resorts. All the patients have previously undergone surgical myocardial revascularization. Patients with normal left ventricular myocardial contractility benefited most of all.  相似文献   

    

AA Tulapurkar  P Das  SN Mishra  RG Pillay  JA Sheikh 《Canadian Metallurgical Quarterly》1996,54(6):2904-2909

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CS Barr  AA Naas  M Fenwick  AD Struthers 《Canadian Metallurgical Quarterly》1997,79(3):328-333

Intranasal administration of protein antigen is an efficient way to induce mucosal tolerance. Suppressive mechanisms that might be involved in this phenomenon include down-regulation of T-helper type-1 (Th1)-mediated processes by Th2 cells. However, since Th2 responses can also be subjected to mucosal tolerance, we wanted to investigate whether suppression of a typical Th1 response, such as a delayed-type hypersensitivity (DTH) reaction by intranasal tolerance induction, was causally related to up-regulation of Th2 responses. We therefore treated mice either systemically or locally with anti-interleukin-4 (IL-4) or anti-IL-10 antibodies before intranasal tolerance induction or before sensitization for DTH to see whether we could prevent or abrogate tolerance. Although the up-regulation of antigen-specific IgE levels in tolerant mice could be prevented by anti-IL-4 treatment, the extent of tolerance as measured by suppression of DTH was not affected. We therefore conclude that up-regulation of Th2 responses observed after intranasal tolerance induction is an additional or consequential rather than a necessary reaction.  相似文献