全文获取类型
收费全文 | 2434篇 |
免费 | 100篇 |
国内免费 | 59篇 |
出版年
2023年 | 21篇 |
2022年 | 30篇 |
2021年 | 28篇 |
2020年 | 58篇 |
2019年 | 60篇 |
2018年 | 73篇 |
2017年 | 58篇 |
2016年 | 88篇 |
2015年 | 50篇 |
2014年 | 41篇 |
2013年 | 659篇 |
2012年 | 43篇 |
2011年 | 53篇 |
2010年 | 40篇 |
2009年 | 66篇 |
2008年 | 61篇 |
2007年 | 97篇 |
2006年 | 88篇 |
2005年 | 66篇 |
2004年 | 88篇 |
2003年 | 68篇 |
2002年 | 64篇 |
2001年 | 45篇 |
2000年 | 34篇 |
1999年 | 47篇 |
1998年 | 31篇 |
1997年 | 27篇 |
1996年 | 52篇 |
1995年 | 51篇 |
1994年 | 28篇 |
1993年 | 26篇 |
1992年 | 40篇 |
1991年 | 34篇 |
1990年 | 30篇 |
1989年 | 34篇 |
1988年 | 21篇 |
1987年 | 22篇 |
1986年 | 26篇 |
1985年 | 27篇 |
1984年 | 33篇 |
1983年 | 8篇 |
1982年 | 10篇 |
1981年 | 19篇 |
1980年 | 9篇 |
1979年 | 13篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1976年 | 8篇 |
1974年 | 2篇 |
1972年 | 2篇 |
排序方式: 共有2593条查询结果,搜索用时 15 毫秒
1.
The lateral leaflets of Desmodium motorium exhibit rhythmic upward and downward movements with a period in the minute range. Apoplasmic K+ and H+ activities were monitored in situ in the abaxial part of the pulvini with ion-selective microelectrodes. An extracellular electric potential was recorded simultaneously. The apoplasmic H+ activity of all pulvini exhibiting a regular rhythm of the extracellular electric potential oscillated with the same period between about 10 and 20 mM. The apoplasmic K+ activity was high when the membrane potential of the motor cells was depolarized (about 36 mV) and the cells were shrunken. In contrast, the apoplasmic K+ activity was low in the swollen state of the motor cells, when the membrane potential was hyperpolarized (about -136 mV). The volatile anesthetic enflurane suppressed reversibly the movement of the leaflets. The same treatment also arrested spontaneous oscillations in the apoplasmic K+ activity in the pulvinus. The apoplasmic K+ activity oscillated roughly in phase with the K+ activity between pH 6.6 and 6.0. Application of white light disturbed the rhythm and increased the extracellular pH. Our results indicate that the physiological mechanism that drives the lateral leaflet movements of Desmodium motorium is closely related to the osmotic motors mediating the leaf movements of Mimosa, Samanea and Phaseolus.Abbreviations Em
membrane potential
- Eex
extracellular electric potential
- Hex
extracellular H+ activity
- Kex
extracellular K+ activity
- Rex
extracellular electrical resistance
B. Antkowiak was supported by the Stiftung Volkswagenwerk. 相似文献
2.
3.
K. Lüning 《Plant biology (Stuttgart, Germany)》1991,104(2):157-162
A circannual rhythm was found in the kelp Pterygophora californica which forms a new blade with a free running period of 7 ? 8 months under constant conditions. Individual plants exposed to cycles of daylength with T = 12, 6, or 3 months performed 1, 2, or 4 growth cycles, respectively, in one calendar year showing the entrainment of the endogenous circannual rhythm. The annual growth cycle also followed a phase shift of the annual cycle of daylength (T = 12 months) by 3 or 6 months. 相似文献
4.
《Current biology : CB》2020,30(18):3544-3555.e6
Media player
5.
《Journal of electromyography and kinesiology》2014,24(1):65-71
The shoulder is complex and comprised of many moving parts. Accurately measuring shoulder rhythm is difficult. To classify shoulder rhythm and identify pathological movement, static measures have been the preferred method. However, dynamic measures are also used and can be less burdensome to obtain. The purpose of this paper was to determine how closely dynamic measures represent static measures using the same acromion marker cluster scapular tracking technique. Five shoulder angles were assessed for 24 participants using dynamic and static tracking techniques during humeral elevation in three planes (frontal, scapular, sagittal). ANOVAs were used to identify where significant differences existed for the factors of plane, elevation angle, and tracking technique (static, dynamic raising, dynamic lowering). All factors were significantly different for all shoulder angles (p < 0.001), except for elevation plane in scapulothoracic protraction/retraction (p = 0.955). Tracking techniques were influential (p < 0.001), but the grouped mean differences fell below a clinically relevant 5° benchmark. There was large variation in mean differences of the techniques across individuals. While population averages are similar, individual static and dynamic shoulder assessments may be different. Caution should be taken when dynamic shoulder assessments are performed on individuals, as they may not reflect those obtained in static scapular motion tracking. 相似文献
6.
Steven W. Sanders M. Arturo Ballersteros Daniel L. Hogan Michael A. Koss Jon I. Isenberg 《Chronobiology international》1991,8(3):186-193
Twelve patients with inactive ulcer disease were administered placebo and ranitidine via bolus and continuous intravenous infusions, at doses ranging from 50 every 8 h, to 12.5 mg/h for 24 h. Gastric acid was collected for 20 min each h for 24 h, and ranitidine serum concentrations were measured ± every 2 h, during each of the six study periods. Cosinor analysis of gastric acid secretion during placebo treatment revealed a significant circadian rhythm in all subjects. Mesor acid output ranged from 1.7 to 11.6 mmol/h (mean 5.6 ± 2.8 mmol/h) and the amplitude ranged from 0.7 to 6.5 mmol/h (mean 2.8 ±1.6 mmol/h). Peak acid output (acrophase) occurred at 10 p.m. ± 3 h. A pharmacodynamic model, relating ranitidine serum concentration to hourly acid secretion, was derived, which incorporated the circadian change in basal acid output. Data for this fractional response model included basal acid secretion-as determined by time of day, measured acid secretion, and associated serum ranitidine concentration. The 50% inhibitory concentration (IC50) for ranitidine ranged from 10-75 ng/ml, with a mean of 44 ng/ml. The variation in IC50 and in basal acid secretion combined to produce a wide variation in the pharmacodynamic response to ranitidine. The model-predicted serum concentrations, required to maintain acid secretion at 0.1 mmol/h, ranged from 250 to 1550 ng/ml, at the time of peak evening acid secretion. Despite a constant degree of acid inhibition by ranitidine during the day, higher serum concentrations are required during times of peak acid output to maintain adequate suppression of hydrogen ion secretion. 相似文献
7.
Niki Rensen Lindsay M. H. Steur Noa Wijnen Eus J. W. van Someren Gertjan J. L. Kaspers Raphaële R. L. van Litsenburg 《Chronobiology international》2020,37(5):660-672
ABSTRACT Sleep and the sleep-wake rhythm are essential for children’s health and well-being, yet reference values are lacking. This study therefore aimed to assess actigraphic estimates of sleep and the 24-h sleep-wake rhythm, as well as 6-sulfatoxymelatonin (aMT6s) levels in healthy children of different age groups. Additionally, relationships between the outcomes and sex, highest parental educational level (as an indication of socioeconomic status (SES)), and body-mass-index (BMI) were explored. In this cross-sectional study, healthy Dutch children (2–18 years) wore an actigraph (GT3x) for 7 consecutive days, collected first-morning void urine and completed a sleep log and sociodemographic questionnaire. Actigraphically estimated sleep variables were sleep onset latency (SOL), sleep efficiency (SE), total sleep time (TST), and wake after sleep onset (WASO). Non-parametric sleep-wake rhythm variables were intradaily variability (IV); interdaily stability (IS); the activity counts and timing of the least active 5-h period (L5counts and midpoint) and of the most active 10-h period (M10 counts and midpoint); and the relative amplitude (RA), i.e. the ratio of the difference and the sum of M10 and L5 counts. Finally, creatinine-corrected aMT6s levels were obtained by isotope dilution mass spectrometry. Effects of age group (preschool 2–5 years/school-aged 6–12 years/teenager 13–18 years), sex, highest parental educational level and BMI (Z-scores) were explored. Ninety-four children participated, equally divided across age groups (53% boys). Teenagers slept less, but more efficiently, than younger children, while their 24 h sleep-wake rhythm was the least stable and most fragmented (likely due to fragmentation of daytime activity). Additionally, aMT6s levels significantly declined over the age groups. Children from highly educated parents had lower sleep efficiency, but a more stable sleep-wake rhythm. Finally, sex or increase in BMI was not associated with any of the outcomes in this study. In conclusion, this study provides reference values of healthy children across different age groups and different sociodemographic factors. In the future, this information may help to better interpret outcomes in clinical populations. 相似文献
8.
Eliana Roveda Jacopo Vitale Angela Montaruli Letizia Galasso Franca Carandente Andrea Caumo 《Chronobiology international》2017,34(5):551-562
Actigraphy is the reference objective method to measure circadian rhythmicity. One simpler subjective approach to assess the circadian typology is the Morningness–Eveningness Questionnaire (MEQ) by Horne and Ostberg. In this study, we compared the MEQ score against the actigraphy-based circadian parameters MESOR, amplitude and acrophase in a sample of 54 students of the University of Milan in Northern Italy. MEQ and the acrophase resulted strongly and inversely associated (r = ?0.84, p < 0.0001), and their relationship exhibited a clear-cut linear trend. We thus used linear regression to develop an equation enabling us to predict the value of the acrophase from the MEQ score. The parameters of the regression model were precisely estimated, with the slope of the regression line being significantly different from 0 (p < 0.0001). The best-fit linear equation was: acrophase (min) = 1238.7–5.49·MEQ, indicating that each additional point in the MEQ score corresponded to a shortening of the acrophase of approximately 5 min. The coefficient of determination, R2, was 0.70. The residuals were evenly distributed and did not show any systematic pattern, thus indicating that the linear model yielded a good, balanced prediction of the acrophase throughout the range of the MEQ score. In particular, the model was able to accurately predict the mean values of the acrophase in the three chronotypes (Morning-, Neither-, and Evening-types) in which the study subjects were categorized. Both the confidence and prediction limits associated to the regression line were calculated, thus providing an assessment of the uncertainty associated with the prediction of the model. In particular, the size of the two-sided prediction limits for the acrophase was about ±100 min in the midrange of the MEQ score. Finally, k-fold cross-validation showed that both the model’s predictive ability on new data and the model’s stability to changes in the data set used for parameter estimation were good. In conclusion, the actigraphy-based acrophase can be predicted using the MEQ score in a population of college students of North Italy. 相似文献
9.
10.
W. Puchalski G. R. Lynch 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1991,169(2):185-189
Summary Djungarian hamsters (Phodopus sungorus), were exposed to constant light with increasing intensities (20, 60, 350 lux), and wheel running activity was recorded. With increasing light intensity the percentage of hamsters showing a split in their daily activity pattern increased and the free running period was lengthened for both the unsplit and the split state. The fact that the free running period of both states depended on the light intensity together with the observation that the highest incidence of acircadian activity occurred under 350 lux, provoked the idea that the emergence of splitting or acircadian rhythmicity is a direct consequence of the light induced lengthening of the free running period. However, analysis of the data failed to support the idea that emergence of a split or acircadian activity is a threshold phenomenon with respect to the free running period.Due to differences in circadian function some Djungarian hamsters do not exhibit photoinduction following short day exposure. In these individuals splitting also occurred but required exposure to a higher light intensity than in photo-responsive hamsters. This observation is in accordance with the idea that the two phenotypes differ in the interaction of the two component oscillators underlying circadian rhythmicity.Abbreviations
LD
long day photoperiod
-
LL
constant light
-
SD
short day photoperiod
-
free running period 相似文献