Conduction velocity along the cardiac fiber is directly related to the action potential amplitude (i.e., the voltage difference between the fully depolarized and the fully polarized regions) and the rate of change of potential (i.e., the rate of rise of phase 0 of the action potential [dV/dt]). By continuing you agree to the use of cookies. unidirectional upwards in xylem). In Arabidopsis, AtPUP1 and AtPUP2 have been shown to transport adenine and nucleobase CKs whereas translocation of CK ribosides appears to be mediated by ENTs. 1990, 151, 157–161. The high conductance Cx43 channel is found only in peripheral pacemaker cells, while central SAN cells express more of the low-conductance Cx45 channel, resulting in a functional transition in coupling and conduction velocity moving from the SAN center out to the surrounding atrial myocardium. In previous sections, one-dimensional models were used to study the properties and ionic mechanisms of AP propagation in cardiac tissue. David C. Coleman, ... Paul F. Hendrix, in Fundamentals of Soil Ecology (Second Edition), 2004. The root exclusion method estimates root respiration indirectly by subtracting the soil respiration without roots from the soil respiration with roots (Anderson, 1973; Edwards, 1991). During action potential propagation, an excited cell serves as a source of electrical charge for depolarizing neighboring unexcited cells. Depending on the species, minimum DO requirements can range from 4 mg/L to 1 mg/L ¹³. The coupling resistance is mainly determined by resistance of the gap junctions. 2.7). 1988). The concept was a major breakthrough in the understanding of the mechanisms of reentrant excitation. A large P. triticina EST database generated from cDNA libraries representing five distinct morphological phases in Stage II, has revealed homologs of some of the genes discussed here and others that will expand our insights. 28.3C and D). Tissues with high concentration of Na+ channels, such as Purkinje fibers, which contain up to 1 million Na+ channels per cell, have a large, fast inward current. Alternatively, if the sink overwhelms the source, propagation will fail.27, The current provided by the source must reach the sink. It should be noted that control of senescence by sugar signaling is most likely affected by other factors, such as nitrogen status and the developmental stage (Ono et al., 2001; Paul and Foyer, 2001; Yoshida, 2003). The large INa spreads quickly within and between cells to support rapid conduction (approximately 4 m/sec). In plant 92.94% dry matter comes from photosynthesis and 6 to 8% comes from mineral matter. General relationships between morphological parameters linking proximal and distal segments in the source-to -sink system. Similar to other electrophysiological properties, coupling of SAN cells is heterogeneous throughout the pacemaker, and is determined largely by variable expression of connexin isoforms important for forming gap junctions between adjacent cells. In contrast, at a less negative resting Em, Na+ channel availability is limited because of the inactivation of a portion of the channels; hence, INa amplitude is attenuated, and the upstroke velocity is slowed. Sphere Edge Tree. Insect attack and other forms of mechanical wounding activate plant defense responses both locally at the site of the wounding and systemically in undamaged parts throughout the plant. Because the maximal velocity of a convex rotating wavefront can never exceed the velocity of a planar wavefront and the period of rotation remains constant in a stable rotating wave, the velocity has to decrease from the periphery (where the highest value corresponds to linear velocity) to the center of a rotating wave. Transport between root and shoot can be demonstrated by grafting experiments with hormone biosynthesis mutants and by biochemical methods using isotopically labeled compounds. This can exhibit complex reentry patterns in which multiple meandering spiral waves divide to form daughter waves while others are simultaneously annihilated, resulting in disorganized electrical activity.28,30,34, Because the slow activation by a rotor is not dependent on conduction in relatively refractory myocardium, an excitable gap can exist, despite the functional nature of reentry. A reduction in INa, leading to a reduction in the rate or amplitude of depolarization during phase 0 of the action potential, can decrease axial current flow (and therefore capacitive current) and slow conduction or produce conduction block. Understanding the molecular basis of nonhost resistance, widely thought to underpin more durable resistance, should open new avenues for crop protection. As already mentioned, the current study looked at the relationship between fruit-set and source–sink ratio a posteriori, i.e. 28.3C, the atrial breakthrough site is about 7.5 mm away from the leading pacemaker in the human SAN,12 but the distance between the SAN leading pacemaker and atrial breakthroughs could vary from 3 to 25 mm because of multiple SACPs8,19 (Fig. Figure 51. 2.7). Electrotonic influence from the surrounding atrial myocardium at a resting potential of about −85 mV could hyperpolarize SAN pacemaker cells and inhibit automaticity28; however, this is prevented by a structural and functional insulation in the form of fibrosis and fat surrounding the SAN complex.15 Moreover, the specialized SAN structure and its electrical coupling with the atria plays a crucial role in the synchronization of pacemaker clusters inside of the SAN (mutual entrainment), allowing for stability of SR, a relationship shown by earlier microelectrode and computer simulation studies conducted by the Jalife group.29. This can exhibit complex reentry patterns in which multiple meandering spiral waves divide to form daughter waves while others are simultaneously annihilated, resulting in disorganized electrical activity.28,30,34, Because the slow activation by a rotor is not dependent on conduction in relatively refractory myocardium, an excitable gap can exist, despite the functional nature of reentry. 28.4). For example, xylem sap contains CKs comprising predominantly trans-zeatin ribosides, whereas phloem sap contains cis-zeatin ribosides and isopentenyl adenine, consistent with the idea that they are transported in opposite directions between shoot and root (Bishopp et al., 2011; Kudo et al., 2010). Isotope methods have a significant advantage over the component integration and root exclusion methods because they permit researchers to partition carbon between rhizosphere respiration and SOM decomposition in situ, thus avoiding the effects of soil disturbance. Because the curvature increases from the periphery to the center of a rotating wave, the corresponding velocity decreases and a spiral shape is formed. A wealth of evidence indicates that systemin and JA work together in the same signaling pathway to promote systemic defense responses. In contrast, a polymorphic VT, such as the torsades de pointes encountered with LQTS, is thought to be caused by a meandering or drifting spiral wave. A notable phenotype found in the transgenic plants is accelerated leaf senescence, supporting the theory that reduced photosynthetic activity may be related to premature leaf senescence (Dai et al., 1999; Jang et al., 1997). Yield of many crops rarely meets its maximum potential for production. Source-sink relationships reflect the interplay between the main factors influencing source current (the rate of rise of the upstroke and amplitude of the action potential) and those that influence the current requirements of the sink (the membrane resistance, the difference between the resting and threshold potentials, cell-to-cell coupling, and tissue geometry). Clearly, ρ cannot be infinite, implying that conduction cannot occur with a radius of curvature smaller than some critical minimum value rm. This review focuses on the current understanding of how the source-sink relationship in crop plants influences rates of yield development and the resilience of yield and nutritional quality. If the edge of this spiral wave encounters unexcitable tissue, then it will break, giving rise to daughter spirals. With progressive reduction of excitability, less Na+ source current is generated, and conduction velocity and the safety factor decrease monotonically. Another important consideration in the normal function of the SAN is the source–sink relationship between the SAN leading pacemaker and atria. Similar to other electrophysiological properties, coupling of SAN cells is heterogeneous throughout the pacemaker, and is determined largely by variable expression of connexin isoforms important for forming gap junctions between adjacent cells. A developing leaf is a sink, but when fully grown it becomes a potential source. This depends on the electrical (e.g., wavelength) and structural (e.g., fibrosis) properties of the tissue in which they occur.28, In the heart, both two-dimensional spiral waves and three-dimensional scroll waves have been implicated in the mechanisms of reentry in atrial and VT and fibrillation.19 Monomorphic VT results when the spiral wave is anchored within the ventricular myocardium and cannot drift away from its origin. The polar distribution of PIN proteins is a self-reinforcing process which establishes a file of cells for the delivery of auxin from apex to base. As a consequence, any freely rotating wave in an excitation-diffusion system has to assume a spiral shape.19, The curvature of the rotating spiral waves progressively increases (and the conduction velocity progressively decreases) toward the tip near the spiral center (the core). Source-sink relationships differ among early-, mid-, and late-ripening varieties, as evidenced, for example, by the diverse nutrient content of peach fruits. Transport between root and shoot can be demonstrated by grafting experiments with hormone biosynthesis mutants and by biochemical methods using isotopically labeled compounds. This will likely favor the expansion of temperate forests well north into the boreal zone. The degree of electrical coupling within the node and to the surrounding atrial myocardium is a critical determinant of pacemaker function. As stated above, the velocity of a rotating wave depends on its (convex) curvature. This apparent source–sink mismatch may be overcome by the specialized branching myofiber structure of SACPs, as was originally suggested for atrioventricular (AV) nodal conduction by the Rudy group.30 Slow conduction through branching myofiber tracts of SACPs19 gives the SAN enough time to build up a sufficient charge to excite the large atria (see Fig. A convex wavefront, as might be observed after point stimulation, has a smaller source and a larger sink because the local excitatory current supplied by the cells in the front of a convex wave diverges into a larger membrane area downstream. (a)–(d) Core trajectory as a function of increasing wavelength λ (numbers below trajectories). The safety factor for conduction predicts the success of action potential propagation and is based on the source-sink relationship. It should be noted that the theoretical principles above are based on observations in homogeneous and uniform tissue. Potential SAR mobile signals include methyl salicylic acid (Fu and Dong, 2013). Brian J. Hansen, ... Vadim V. Fedorov, in Cardiac Electrophysiology: From Cell to Bedside (Seventh Edition), 2018, The specific structural and molecular features of the human SAN are extremely important for its successful function as the leading pacemaker of the heart.15,19 As described earlier, normal SAN automaticity is dependent on a relatively depolarized state (about −60 mV). Hence, slow conduction (approximately 0.1 to 0.2 m/sec) and prolonged refractoriness are characteristic features of nodal regions. Action potentials with reduced upstroke velocity resulting from partial inactivation of Na+ channels are called depressed fast responses.35. The terms "sources" and "sinks" are used by plant physiologists to designate the depar­ ture and arrival point of needed nutrients and carbo­ hydrates. If the length of the tip trajectory is longer than the excitation wavelength λ, then head-tail interaction is minimal and the tip rotates around a stable circular core of nonexcited (but intrinsically excitable) tissue. Transport of hormones between root and shoot establishes communication between organs and regulates source-sink relationships. The degree of electrical coupling within the node and to the surrounding atrial myocardium is a critical determinant of pacemaker function. This is convincing proof that the combined belowground system—roots, microbes, soil, and fauna—is governed by source-sink relationships, just as are intact plants (i.e., roots and shoots). Long-distance transport also provides systemic signaling systems such as during pest attack or symbiosis (Jin et al., 2017; Song et al., 2017). Eng. The pathway between the source and the sink includes intracellular resistance (provided by the cytoplasm) and intercellular resistance (provided by the gap junctions). VF seems to be the most complex representation of rotating spiral waves in the heart. However, the senescence phenotype observed in this case was not thoroughly examined and needs to be analyzed in more detail to support the idea. Copyright © 2021 Elsevier B.V. or its licensors or contributors. While additional septal and inferior lateral SACPs were observed, the superior lateral SACP and corresponding atrial breakthrough were the primary excitation patterns during normal SR, which is consistent with previous clinical and experimental observations.8,42, Moreover, cardiac disease and drug-induced structural and molecular remodeling can significantly alter pacemaker cell excitability and intracellular coupling in the SAN, which may lead to variations from the classical hierarchy of head–tail leading pacemakers25,43 as well as the emergence of latent atrial pacemakers.23,44,45, Both epi-7 and endocardial43,46,47 surface electrode mapping studies have demonstrated anatomically widespread sites of earliest atrial activation during SR that may even excite simultaneously. The source – sink architectural manipulation treatments are (i) Sources manipulation at heading stage, flag leaf from each ear-bearing tiller removed surgically (ii) Sink manipulation two days after anthesis, 1/3rd of each spike from the top was removed and (iii) Source – sink manipulation as in (1) and (2). And for the situation where λ is much smaller than the trajectory, the movement is along a stable ‘Z-shaped’ line (Figure 51).196 This third condition holds in cardiac tissue at a normal state of excitability. The comparison of global gene expression profiles using microarrays harboring large sets of genes representing several life cycle stages of P. triticina (Bakkeren lab in progress) might shed light on such questions. If the edge of this spiral wave encounters unexcitable tissue, then it will break, giving rise to daughter spirals. Rotors can be stationary, continuously drift or migrate away from their origin, or anchored, initially drifting and then becoming stationary by anchoring to an anatomical obstacle. Conduction in a one-dimensional fiber or a ring model of reentry involves a planar wavefront. Extensive amounts and complexities of carbon compounds are elaborated in the rhizosphere (Rovira et al., 1979; Kilbertus, 1980; Foster et al., 1983; Foster, 1988; Lee and Foster, 1991; Cheng et al., 1993). Dynamics of a spiral wave core. The first important steps in the investigation of SAN excitation and conduction were made by microelectrode studies of small mammal hearts.32 In 1965, Sano and Yamagashi33 and then Bleeker and associates18 performed the systematic mapping of rabbit SAN conduction using microelectrodes and observed a highly anisotropic spread of activation (Fig. There are two models available in literature to scrutinize the effect of heat source/sink on the body. Because ICaL has lower amplitude and slower activation kinetics than INa, slow response action potentials exhibit lower amplitudes and upstroke velocities. PREDICTIONS IN ANCIENT SOURCE-TO-SINK SYSTEMSThe relationships shown for modern source-to-sink systems in Figs 4, 11, 12 and 13 allow for the development of Fig. Such transport is highly specific. However, it became evident that these considerations alone do not fully describe wave rotation in two- and three-dimensional cardiac tissue.10,33, Spiral waves typically describe reentry in two dimensions. Experiments in a rabbit perfused ventricular epicardial layer. In addition, low prey density increases the mortality of its predator because it has more difficulty locating its food source. “Genomics”, the large-scale analysis of the genome, its genes and their expression, is in its infancy for the rusts but is expected to provide breakthroughs for these hard to manipulate obligate parasites. 2.5 and 2.6). 28.4C shows optical mapping examples of the leading pacemaker shift from the SAN center to the head. The hexokinase is a glucose sensor and has a central role in modulating multiple signaling pathways (Rolland et al., 2002). Pioneering atrial epicardial multielectrode mapping studies in canine and human hearts7 by the Schuessler and Boineau group have led to the suggestion of a widely distributed “atrial pacemaker complex.” Atrial breakthroughs were reported to arise over a region of 7.5 cm7,46 along the CT. That region is significantly larger than the length of the human anatomical SAN body (10–20 mm).3,4 In vivo mapping studies have also suggested the presence of a pacemaker hierarchy within the atrial pacemaker complex, with the fastest pacemakers preferentially located at the superior part and the slowest ones located at the inferior part of the CT in normal canine48 and human hearts.7,47. The safety factor for conduction predicts the success of action potential propagation and is based on the source-sink relationship. The three-dimensional representation of the spiral wave is called a “scroll wave.” The center of the spiral wave is called the core, and the distribution of the core in three dimensions is referred to as the filament, which is formed by the revolving trajectory of the spiral tip. 1.6). For example, this is realized in the sinus node, where a small current source (sinus node) meets a large sink (atrium). It should be noted that the theoretical principles above are based on observations in homogeneous and uniform tissue. Copyright © 2021 Elsevier B.V. or its licensors or contributors. (a)–(d) Core trajectory as a function of increasing wavelength λ (numbers below trajectories). As the spinning rate increases, the turbulence and spiral wave fractionation around the rotor increase. The term rotor initially described the rotating source, and the spiral wave defined the shape (i.e., curvature) of the wave emerging from the rotating source. On the other hand, PUPs are primarily expressed in the phloem, and it is suggested that they participate in the uptake of CKs from xylem sap during long-distance transport. The pathway between the source and the sink includes intracellular resistance (provided by the cytoplasm) and intercellular resistance (provided by the gap junctions). These root-tip capsule components include high molecular weight (MW) mucilage secreted by the root cap, as well as cell-wall breakdown products resulting from the separation of thousands of border cells from each other and the root cap (see Fig. 13. Thus in Lotus japonicus CLE-RS2 produced in roots is found in xylem sap collected from shoots, implying that CLE-RSs may provide a long-distance mobile signal in the regulation of the initial step of nodulation (Okamoto et al., 2013). The large INa spreads quickly within and between cells to support rapid conduction (approximately 4 m/sec). This property has important consequences to the formation and sustenance of functional reentrant waves (called spiral waves or rotors) that underlie many cardiac arrhythmias. Determining the Relationship between Floral Initiation and Source–Sink ... source to regulate flower initiation but also acts as a direct energy resource for photo-synthetic accumulation (Kinet, 1977). The SAN activation maps demonstrated that these two different atrial activation patterns corresponded to atrial activations by the SAN excitation waves exiting through superior and inferior SACPs rather than two different leading pacemakers. Most source–sink research has focused on leaf–boll relationships, with little study of vegetative storage reserves. As curvature of a wavefront increases, conduction velocity decreases; with sufficient curvature, propagation fails. In particular, spatial gradients of membrane excitability or repolarization can cause the spiral wave to drift, while structural inhomogeneities can cause anchoring of the spiral wave, rendering it stationary. The requirements of adjacent resting cells to reach the threshold Em constitute an electrical sink (load) for the excited cell. When the safety factor falls to less than 1, conduction can no longer be sustained, and failure (conduction block) occurs. Long-distance transport is achieved by phloem and xylem tissues, and hormones can be found in xylem and phloem sap. In the core of the rotating wave, the tip of the spiral moves along a trajectory that depends on the excitability (degree of refractoriness) of the surrounding tissue. Potential SAR mobile signals include methyl salicylic acid (Fu and Dong, 2013). Figure 50. In the heart, heterogeneities of membrane properties and tissue structure affect spiral wave characteristics. Because ICaL has lower amplitude and slower activation kinetics than INa, slow response action potentials exhibit lower amplitudes and upstroke velocities. Long-distance transport of hormones coordinates growth such as in the imposition of apical dominance by auxin transported from the SAM, and in tropic responses such as phototropism and gravitropism. Thus in Lotus japonicus CLE-RS2 produced in roots is found in xylem sap collected from shoots, implying that CLE-RSs may provide a long-distance mobile signal in the regulation of the initial step of nodulation (Okamoto et al., 2013).
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