Some Relations between Action Potential and Resting Potential of the Lobster Giant Axon

Experiments were performed to determine the quantitative relation existing between action potential and resting potential of the lobster giant axon. Resting potential changes were induced by either increasing the external potassium concentration or by reducing the external calcium concentration. For either treatment the action potential amplitude is proportional to the logarithm of the resting potential minus a constant. This constant is equivalent to the minimum resting potential at which a propagated spike is possible, and is larger for depolarization in low calcium than in high potassium. Thus the change in action potential per unit change in resting potential is greater in low external calcium than in high external potassium. Analog computer solutions to the Hodgkin-Huxley equations for squid axon membrane potentials show that, if the initial conditions are properly specified, the action potential is proportional to the logarithm of the potassium potential minus a constant. The experimental results and the analog computations suggest that reducing external calcium produces changes in the invertebrate axon that cannot be accounted for solely on the basis of alterations in the potassium potential.     

Electrical response of beef-heart submitochondrial particles bound to phospholipid-impregnated millipore filters during ATP hydrolysis

1. Beef heart submitochondrial particles bound to asolectin impregnated Millipore filter, according to the method described earlier (Drachev et al. (1974) Nature 249, 321--324), are able to generate, upon addition of ATP, an electrical potential which can be directly measured. 2. The transmembrane electrical potential generated by ATP hydrolysis reaches values up to 80 mV. The half-time required to attain the plateau of potential is paradoxically long (5 to 10 min at room temperature) and is temperature-dependent. Among different phospholipid species which have been used to impregnate the Millipore filter, phosphatidylethanolamine was found to be the most effective for generation of electrical potential. 3. The potential generated by ATP hydrolysis is inhibited by inhibitors of mitochondrial ATPase, by the uncoupler FCCP and by reagents collapsing the membrane potential. 4. Addition of inhibitors of mitochondrial ATPase, when the plateau of potential is attained, results in a decay of potential. This decay of potential is as slow as the generation of potential induced by ATP hydrolysis. 5. The initial rise in electrical potential is proportional to the ATPase activity.     

A model of the effects of water stress on seed advancement and germination

A model of seed germination is proposed which uses a variable with the units of an osmotic potential (virtual osmotic potential) to integrate the effect of a constant or a varying water potential. This differs from existing models that describe the effects of fixed water potentials on germination, or the effects of fixed priming water potentials on the subsequent germination at a fixed water potential. When a seed is sown, the virtual osmotic potential is assumed to fall at a rate that depends on the ambient water potential, and on the difference between its current and a minimum value. Radicle growth is assumed to initiate when the difference between the ambient water potential and the virtual osmotic potential exceeds a threshold. The germination of carrot and onion seeds at various fixed potentials below 0 MPa was well described by the virtual osmotic potential model. The model was also used to simulate the results of experiments in which seeds were given a single step change in water potential.     

Electrical signals in higher plants: Mechanisms of generation and propagation

Local stimulation induces generation and propagation of electric signals in higher plants. Noninvasive stimulus induces an action potential and damaging influences lead to the variation potential. The mechanism of the generation of an action potential is rather complex in nature and is associated with both activation of ion channels (Ca²⁺, Cl^–, and K⁺) and transient change in the activity of the plasma membrane H⁺-ATPase. Generation of the variation potential, the duration of which is considerably longer than that of the action potential, is based on transient inactivation of the electrogenic pump; however, passive ion fluxes also contribute to such process, which causes qualitative similarity of the mechanisms of action potential and variation potential generation. Propagation of electrical signals mainly occurs in conducting bundles; thus, transfer of an action potential is associated with vascular parenchyma and sieve elements, while the variation potential is connected to the xylem vessels. The mechanism of the distribution the action potential is similar to nerve impulse transmission, while generation of the variation potential is induced by transfer of a chemical substance, whose propagation is accelerated by a hydraulic wave.     

Synaptic potential in the motor giant axon of the crayfish

Some electrical properties of the synapses between central giant axons (presynaptic) and the motor giant axon (postsynaptic) of the crayfish abdominal nerve cord have been investigated. Postsynaptic potential change in response to presynaptic volleys contains two components: a spike potential and a synaptic potential of very long time course. Amplitude of the synaptic potential is graded according to the number of active presynaptic axons. Conductance increase in the synaptic membrane endures over most of the period of potential change, and it is this rather than the "electrical time constant" of the membrane that in large measure determines the form of the synaptic potential. Temporal summation of synaptic potential occurs during repetitive presynaptic stimulation, and after such stimulation the rate of decay of synaptic potential is greatly slowed.     

The propagation potential. An axonal response with implications for scalp-recorded EEG

An electrophysiological response of axons, referred to as the "propagation potential," was investigated. The propagation potential is a sustained voltage that lasts as long as an action potential propagates between two widely spaced electrodes. The sign of the potential depends on the direction of action potential propagation. The electrode towards which the action potential is propagating is positive with respect to the electrode from which it is receding. For normal frog sciatic nerves the magnitude of the propagation potential was 17% of the peak of the extracellular action potential; TEA increased it to 32%. For normal earthworm median or lateral giant fibers it was 30%. A ripple pattern on the propagation potential was attributed to variation in resistance along the length of the worm. Cooling increased the duration of the propagation potential and attenuated the higher frequency components of the ripple pattern. Differential records from two widely spaced intracellular microelectrodes in the same axon differed from the propagation potential. The amplitude of the plateau relative to the peak was smaller, it decreased as the action potential propagated from one electrode site to the other, and the potential did not return to zero as rapidly as for extracellular records. When propagation was blocked by heat, the propagation potential slowly decayed. There was no ripple pattern during the decay. In a volume conductor, electrodes contacting the worm did not show the typical propagation potential, but electrodes located a few centimeters away from the worm did. Simple core-conductor models based on classical action potential theory did not reproduce the propagation potential. More complex, modified core-conductor models were needed to accurately simulate it. The results suggest that long, slowly conducting fibers can contribute to the scalp-recorded EEG.     

Fiber Interaction in a Nerve Trunk

This paper is concerned with the nature of the transmembrane potential induced in an inactive nerve fiber lying in an impressed potential field within a nerve trunk. The impressed potential field is assumed to be produced by the synchronous activity of other fibers within the trunk; an expression for the induced transmembrane potential is obtained utilizing the principles of electromagnetic field theory. The results strongly indicate that membrane capacitance is the main determinant of the induced transmembrane potential wave form.     

玉米生育期土壤-植物-大气连续体水流阻力与水势的分布

根据玉米生育期的田间试验资料分析了土壤-植物-大气连续体中水势和水流阻力的分布,结果表明土壤与植物叶片之间的水势差在玉米抽雄期前达0.8—1.0MPa,到抽雄期以后达1.0—1.5MPa,叶片与大气之间的水势差则在抽雄期前后分别达80—120MPa和60—80MPa;连续体内的水流阻力主要在叶片与大气之间.建立了连续体中玉米叶片水势的动态模拟公式,模拟叶水势具有较高的精度.最后,揭示了叶片蒸腾速率与叶-气系统水势差和水流阻力的关系,当叶片与大气之间的水势差达90—100MPa之后,蒸腾速率随叶-气间水势差增加而减小.     

The control of membrane ionic currents by the membrane potential of muscle

Comparisons between electrotronic potentials and certain predicted curves allow the identification of the membrane potential at which the sodium and potassium currents are switched on in frog sartorius. The activation potentials (the membrane potentials at which the ionic currents are great enough to be resolved by the method) are functions of the resting potential and time but not of ionic concentration. In the normal fiber, the activation potential for sodium lies nearer the resting potential and depolarizations set off sodium currents and action potentials. Below a resting potential of 55 to 60 mv. sodium activation is lost and conduction is impossible. A tenfold increase of calcium concentration lowers (moves further from the resting potential) the sodium activation potential by 20 to 25 mv. whereas the potassium activation potential is lowered by only 15 mv. Certain consequences of this are seen in the behavior of the muscle cell when it is stimulated with long duration shock.     

Electrical changes in isolated rat jejunum induced by hypertonicity

Mucosal hypertonicity produces a drop of transepithelial potential difference in isolated rat jejunum with a half time of about 15 s. The same effect is obtained when the osmorality of both bathing solutions is raised simultaneously. Serosal hypertonicity produces an increase of transepithelial potential difference an order of magnitude lower than the drop produced by mucosal hypertonicity. The change in the short circuit current parallels the one in potential difference.When the transepithelial potential difference is varied by adding different concentrations of glucose to the bathing media, the potential drop induced by mucosal hypertonicity is linearly related to the magnitude of the transepithelial potential difference just before the increase in osmolarity.The drop of potential can be explained by a decrease of the electrical resistance of the extracellular shunt pathway due to opening of the tight junctions. The results can be accounted for in terms of an equivalent electrical circuit proposed for small intestine. Using this equivalent circuit model it is possible to obtain estimates of the values of the diffusion potential and the salt gradient across the tight junction.     

Vagal and sympathetic effects on the pacemaker fibers in the sinus venosus of the heart

1. Action potentials from sinus venosus and auricle fibers of spontaneously beating frog hearts have been recorded with intracellular electrodes. 2. Sinus fibers show a slow depolarization, the pacemaker potential, during diastole. The amplitude of this potential varies in different parts of the sinus. In some fibers the membrane potential falls by 11 to 15 mv. during diastole and the transition to the upstroke of the action potential is comparatively gradual. In other regions the depolarization develops more slowly and the action potential takes off more abruptly from a higher membrane potential. It is proposed that the fibers showing the largest fall in membrane potential during diastole are the pacemaker fibers of the heart, and that the rest of the preparation is excited by conduction. In auricle fibers the membrane potential is constant during diastole. 3. The maximum diastolic membrane potential and the overshoot of the action potential vary inversely with the amplitude of the pacemaker potential. The highest values were measured in auricle fibers. 4. Stimulation of vagi suppresses the pacemaker potentials. While the heart is arrested the membrane potential of the sinus fibers rises to a level above the maximum diastolic value reached in previous beats. In 28 experiments vagal stimulation increased the membrane potential from an average maximal diastolic value of 55 mv. to a "resting" level of 65.4 mv. The biggest vagal polarization was 23 mv. 5. In contrast to the sinus fibers vagal inhibition does not change the diastolic membrane potential of frog auricle fibers. 6. Vagal stimulation greatly accelerates the repolarization of the action potential and reduces its amplitude. These changes were seen both in the sinus and in auricle fibers stimulated by direct shocks during vagal arrest. 7. The conduction velocity in the sinus venosus of the tortoise is reduced by vagal stimulation. Block of conduction often occurs. 8. In the frog sinus venosus sympathetic stimulation increases the rate of rise of the pacemaker potential, accelerating the beat. The threshold remains unchanged. The rate of rise of the upstroke and the amplitude of the overshoot are increased. 9. The analogies between the vagal inhibition of the heart and the nervous inhibition of other preparations are discussed.     

On the steady-state electrical potential difference across the thylakoid membranes of chloroplasts in illuminated plant cells.

The potential defference across the thyladoid membranes under steady-state saturating light conditions, measured with microcapillary glass electrodes, was found to be small as compared to the potential initially generated at the onset of illunimation. This result is discussed to be in agreement with quantitative estimates on the approximate magnitudes of the potential generating electron flux through the photo-synthetic electron transport chain and of the potential dissipating ion fluxes across the thylakoid membrane under steady-state conditions. It is concluded that a pH gradient of approx. 3-3.4 units is built up in the light across the membrane. The negative diffusion potential associated with this gradient is suggested to cause the transient negative potential observed in the dark after illumination.     

毛乌素沙地4种植物叶水势变化及其影响因素分析

对毛乌素沙地旱柳( Salix matsudana Koidz.)、小叶杨( Populus simonii Carr.)、北沙柳( Salix psammophila C. Wang et Chang Y. Yang)和沙蒿(Artemisia desertorum Spreng.)4种植物叶水势的月变化和日变化规律以及不同冠层叶水势的差异进行了研究,并分析了4种植物叶水势与气候因子及树干液流速度的相关性。结果表明：在5月份至8月份,旱柳、小叶杨和北沙柳清晨(5：00)的叶水势波动不明显;但3种植物正午(13：00)的叶水势有一定差异,其中旱柳和小叶杨正午的叶水势差异不明显,而北沙柳7月份和8月份的叶水势则明显小于其5月份和6月份;总体上看,各月份3种植物正午的叶水势均明显小于清晨叶水势。供试4种植物的叶水势日变化曲线均以“单峰型”为主,最高值出现在5：00至6：00,最低值出现在12：00至16：00;日间北沙柳的平均叶水势最低,而其他3种植物的平均叶水势差异不明显,5月份和8月份,北沙柳的平均叶水势分别为-11.9和-17.1 MPa,而旱柳、小叶杨和沙蒿的平均叶水势分别为-6.2和-7.1、-7.7和-7.3、-6.8和-8.4 MPa;但夜间4种植物的平均叶水势无明显变化。4种植物日间均表现出树冠下层叶水势高、上层叶水势低的趋势,而夜间则树冠上层和下层的叶水势几乎一致。相关性分析结果表明：4种植物的叶水势与气温、净辐射量以及树干液流速度负相关、与空气相对湿度正相关,其中,不同月份4种植物的叶水势与净辐射量和树干液流速度均极显著相关(P<0.01)。研究结果显示：毛乌素沙地不同植物叶水势的变化因种类、季节、气候条件的不同而异,但总体上看净辐射量是影响4种植物叶水势变化的最主要气象因子。     

The causal theory of the resting potential of cells

In this pedagogical article the causal theory of the resting potential of cells is presented, which for given extracellular ion concentrations predicts the intracellular ones simultaneously with the resting potential. In addition to the Na, K-pump, fixed charges on the membrane surfaces are taken into account. The equation determining the resting potential in the causal theory suggests a new explanation of the genesis of the resting potential. The usual criterion for an ion pump to be electrogenic is not relevant for the whole of the resting potential, and may therefore be misleading. The physical meaning of the Goldman-Hodgkin-Katz formula for the membrane potential as a diffusion potential is also explained and tested with numbers for the giant axon of the squid. A significant discrepancy between theory and experiment is found which calls for an experimental re-examination of the constitutive equations for passive potassium and sodium currents.     

Skin surface electric potential induced by ion-flux through epidermal cell layers

The skin surface electric potential has been widely used for psychological studies because it is sensitive to emotional conditions. We measured the electric potential on the surface of hairless mice skin in organ culture with several physiological factors. Disruption of mitochondrial function and inhibition of ATPase reduced the skin surface potential 50-70%. Calcium, potassium, and sodium channel blockers also reduced the potential. A calcium-specific and potassium ionophore reduced the potential, but the calcium and magnesium ionophore increased it. EDTA decreased the potential but EGTA had no effect. Skin surface barrier disruption reduced the potential and calcium and potassium channel blockers partially prevented the decrease. Substance P and corticotropin-releasing factor decreased the potential, and antagonists blocked the decreases. These results suggest that the ion flux in the nucleated layer of the epidermis induce the skin surface potential and it is influenced by environmental and neuroendocrinological factors.     

Donnan potential and surface potential of a charged membrane.

A model is presented for the electrical potential distribution across a charged biological membrane that is in equilibrium with an electrolyte solution. We assume that a membrane has charged surface layers of thickness d on both surfaces of the membrane, where the fixed charges are distributed at a uniform density N within the layers, and that these charged layers are permeable to electrolyte ions. This model unites two different concepts, that is, the Donnan potential and the surface potential (or the Gouy-Chapman double-layer potential). Namely, the present model leads to the Donnan potential when d much greater than 1/k' (k' is the Debye-Hückel parameter of the surface charge layer) and to the surface potential as d----0, keeping the product Nd constant. The potential distribution depends significantly on the thickness d of the surface charge layer when d less than or approximately equal to 1/k'.     

Fixed charges of the heart muscle interstitium.

The interstitium of the heart muscle is primarily composed of ground substance. The glycoproteins and proteoglycans that formed the ground substance bore negative charges at neutral pH like the glycosaminoglycans and proteoglycans of the water-rich phase of the interstitium. Microelectrodes were used to look for the existence of an electrical potential between the interstitium of the rabbit papillary muscle and an ambient medium. Evidence is presented for the existence of such an electrical potential. When the ambient solution was a Krebs solution, this potential was evaluated at -5.7 mV. This electrical potential is dependent on the filling solution of the microelectrodes and on the ambient medium; in rabbit serum, the electrical potential diminished to -0.6 mV. Assuming that this potential is a measure of the Donnan potential, the Cl and Na activities in the interstitium were evaluated to 76 and 135 mM when the rabbit papillary muscle was superfused with a Kreb's solution.     

Effects of Some Inhibitors on the Temperature-Dependent Component of Resting Potential in Lobster Axon

The resting membrane potential of the lobster axon becomes 5–8 mv more negative when the temperature of the perfusion solution is increased 10°C. This potential change is about twice that predicted if the axon membrane potential followed that expected for a potassium ion electrode potential. When the inhibitors, 2, 4-dinitrophenol, sodium cyanide, and sodium azide, were added separately to the perfusion medium the potential change was reduced to about 1.4 times that predicted for a potassium ion electrode potential. Assays of axons exposed to these inhibitors showed that ATP levels were reduced to about one-fourth that obtained for control axons. Ouabain added to the perfusion medium reduced the potential change to that expected for a potassium ion electrode potential. These results suggest that the resting potential changes with temperature as a result of the activity of an electrogenic ion pump.     

Simulation studies on bacteriorhodopsin bundle of transmembrane α segments

Bacteriorhodopsin (BR) is a membrane protein which pumps protons through the plasma membrane. Seven transmembrane BR helical segments are subjected to simulation studies in order to investigate the packing process of transmembrane helices. A Monte Carlo simulated annealing protocol is employed to optimize the helix bundle system. Helix packing is optimized according to a semi-empirical potential mainly composed of six components: a bilayer potential, a crossing angle potential, a helix dipole potential, a helix-helix distance potential, a helix orientation potential and a helix-helix distance restraint potential (a loop potential). Necessary parameters are derived from theoretical studies and statistical analysis of experimentally determined protein structures. The structures from the simulations are compared with the experimentally determined structures in terms of geometry. The structures generated show similar shapes to the experimentally suggested structure even without the helix-helix distance restraint potential. However, the relative locations of individual helices were reproduced only when the helix-helix distance restraint potential was used with restraint conditions. Our results suggest that transmembrane helix bundles resembling those observed experimentally may be generated by simulations using simple potentials. Received: 19 April 1999 / Revised version: 6 September 1999 / Accepted: 17 September 1999     

新型平均势函数在蛋白质反向折叠中的应用

利用蛋白质主链的极性分数及主链二面角为参量,构建了一种基于蛋白质结构数据库的势函数。将该势函数应用于蛋白质反向折叠研究中,发现该函数可成功地将蛋白质分子的天然构象从构建的构象库中识别出来;将一目标序列与构象库的每一可能的构象匹配,并用该势函数计算相应的能量,结果表明对绝大多数蛋白质分子来说,天然的构象的能量值总是最低。此外,该函数还将一些序列相似性较低,而结构相似性较高的蛋白质分子识别出来。我们认