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MULLINS Lj 《The Journal of general physiology》1959,42(4):817-829
The influx of Na+, K+, Rb+, and Cs+ into frog sartorius muscle has been followed. The results show that a maximum rate is found for K+, while Na+ and Cs+ penetrate much more slowly. Similar measurements with Ca++, Ba++, and Ra++ show that Ba++ penetrates at a rate somewhat greater than that of either Ca++ or Ra++. All these divalent cations, however, penetrate at rates much slower than do the alkali cations. The results obtained are discussed with reference to a model that has been developed to explain the different penetration rates for the alkali cations. 相似文献
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Squid axons impaled with a microelectrode have been treated with concentrations of xylene and benzene such that there is no change in threshold or resting potential at 20°C., while the spike height declines about 10 mv. A decrease in ambient temperature results in large, reversible, increases in threshold. While neither low temperature nor the added blocking agent induces repetitive firing from a single stimulus, the two treatments when combined do yield repetitive responses which commence at a sharply defined temperature. The alteration in the membrane responsible for the effects observed can be described by saying that there has been a large increase in the inductance of the equivalent electric circuit, and the temperature coefficient of the apparent membrane inductance has a Q10 = 5. 相似文献
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JOEL B. THOMPSON HENRY T. MULLINS CATHRYN R. NEWTON THOMAS L. VERCOUTERE 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1985,18(2):167-179
An abundant calcareous fauna has been discovered in the oxygen minimum zone (OMZ; < 0.5 ml/l O2 ) off central California at oxygen concentrations considerably less than those predicted by previous ecological models. Analysis of box core samples and bottom photographs has revealed a distinct depth zonation of echinoderms. Asteroids and ophiuroids are most abundant along the upper and lower edges of the OMZ, respectively, whereas echinoids are found near the core of this zone where oxygen levels are as low as 0.3 ml/l O2 . As these heart urchins are very abundant (14/m2 ), they represent a potentially significant component of the fossil record of OMZ's. Locally these urchins are capable of disturbing over 90% of near surface sediment. The core of the OMZ is inhabited by hermit crabs that actively transport and recycle gastropod shells. This 'biotransport' results in an accumulation of potentially preservable, non-endemic. hard-bodied organisms which may lead to misinterpretation of paleo-oxygenation conditions. We propose an alternative to the Rhoads & Morse (1971, Lethaia 4 ) biofacies model for open-ocean, dysaerobic environments which consists of: (1) a zone devoid of maeroinvertebrates. characterized by laminated sediments (<0.1 ml/l O2 ); (2) a zone dominated by small (1–2 mm) soft-bodied infauca which exhibits moderate disturbance of laminae due to bioturbation (0.1–0.3 ml/l O2 ); and (3) a zone inhabited by an abundant calcareous fauna characterized by highly bioturbated sediments (>0.3 ml/l O2 ). 相似文献
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DAVID A. MORRISON GEOFFREY J. CARY STUART M. PENGELLY DAVID G. ROSS BRUCE J. MULLINS COLLETTE R. THOMAS TIMOTHY S. ANDERSON 《Austral ecology》1995,20(2):239-247
Abstract Fire frequency is the number of fires experienced by a particular community within a given time period. This concept can potentially be resolved into a number of interacting variables, including: time since the most recent fire, the length of the inter-fire intervals, and the variability of the length of the inter-fire intervals. We estimated the effects of these three variables on the floristic composition of 65 samples from dry sclerophyll vegetation with different fire histories in Brisbane Water, Ku-ring-gai Chase and Royal National Parks near Sydney. Our analyses suggest that fire frequency may account for about 60% of the floristic variation among our samples. They confirm the hypothesis that the recent (<30 years) fire frequency produces effects on floristic composition of fire-prone communities that can recognizably be attributed both to the time since the most recent fire and to the length of the intervals between fires. These effects are equal in magnitude but are different in the nature of the floristic variation they are associated with. Increasing time-since-fire is associated with a decline in the evenness of fire-tolerant species, indicating that fewer of these species come to dominate the community in the prolonged absence of fire. Herbs and small shrubs decrease in abundance, while larger shrubs increase in abundance. Inter-fire intervals of decreasing length are associated with a decrease in the evenness of the fire-sensitive species, particularly those large Proteaceae shrubs that often dominate the community biomass in dry sclerophyll shrublands of southeastern Australia. Furthermore, the variation associated with inter-fire intervals is not necessarily solely related to the shortest inter-fire interval, but is related to combinations of inter-fire intervals through time. Thus, increasing variability of the length of the inter-fire intervals is associated with an increase in the species richness of both fire-sensitive and fire-tolerant species, implying that it may be variation of the inter-fire intervals through time that is primarily responsible for maintaining the presence of a wide variety of plant species in a particular community. Our results also suggest that the floristic variation associated with different inter-fire intervals decreases with increasing time-since-fire. 相似文献
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Two-node explants from Sweet Orange cv. St Ives Valencia orangeshoots produced prolific callus and formed secondary abscissionzones within internodes when cultured in vitro with abscisicacid (ABA, 5 µM) or -naphthaleneacetic acid (NAA, 5 µM).Benzyladenine (BA, 1 µm) induced callus but had littleeffect on abscission. Secondary abscission zone formation wasassociated with ABA-induced and auxin-induced ethylene formation.Treatment of explants with inhibitors of ethylene synthesis[aminoethoxyvinyl glycine (AVG), Co2+, PO43] preventedformation of secondary abscission zones but had variable effectson callus formation. Newly made explants contained high concentrationsof endogenous ABA (up to 6000 ng g1 f.wt), as measuredby GC/MS/SIM. Long-term subculture of explants (two years) inmedia containing BA (1 µm) led to a reduction in endogenousABA level (40 ng g1 f. wt) and to loss of capacity toform extensive callus and secondary abscission zones. Citrus sinensis (L.) Osbeck cv. St Ives Valencia, sweet orange, secondary abscission zones, in vitro, ethylene, endogenous ABA, endogenous IAA 相似文献
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MULLINS LJ 《The Journal of general physiology》1959,42(5):1013-1035
The membrane of the squid axon is considered on the basis of a pore model in which the distribution of the pore sizes strongly favors K+ transfer when there is no potential. Electrical asymmetry causes non-penetrating ions on the membrane capacitor to exert a mechanical force on both membrane surfaces and this force results in a deformation of the membrane pore system such that it assumes a distribution of sizes favoring the ions exerting mechanical force. The ions involved appear to be Ca++ on the outside of the membrane and isethionate-, (i-) on the inside; as Ca++ is equivalent in size to Na+, the charged membrane is potentially able to transfer Na+, when the ions deforming the membrane pore distribution are removed. A depolarization of the membrane leads to an opening of pores that will allow Na+ penetration and a release of the membrane from deformation. The pores revert to the zero-potential pore size distribution hence the Na permeability change is a transient. Calculation shows that the potassium conductance vs. displacement of membrane potential curve for the squid axon and the "inactivation" function, h, can be obtained directly from the assumed membrane distortion without the introduction of arbitrary parameters. The sodium conductance, because it is a transient, requires assumptions about the time constants with which ions unblock pores at the outside and the inside of the membrane. 相似文献