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Persistent Na-channels: Origin and function
A review János Salánki memory lecture
Voltage-dependent sodium channels have a decisive role in the generation of action potentials (AP) in many types of cells. In addition to the fast inactivating Na-current, associated with AP generation, the Na-channel can give rise to a noninactivating or persistent Na-current. The latter current generally comprises up to 5% of the transient current having important physiological consequences. It was established that persistent Na-currents have functional significance in setting the membrane potential in a subthreshold range regulating by this way dendritic depolarisations, repetitive firing and enhancing synaptic transmission. Voltage dependent sodium channel genes have been identified in a variety of invertebrates, as well as mammalian and nonmammalian vertebrates. It has been established that the biophysical properties, pharmacology and gene organization of invertebrate sodium channels are largely similar to the vertebrate ones, supporting the view that the ancestral sodium channel was established before the evolutionary separation of the invertebrates from the vertebrates. Although different isoforms of voltage sensitive Na-channels have now been identified the mechanism for persistent current remains controversial. An important yet unanswered question is whether persistent and fast inactivating Na-currents arise from different sets of sodium channels or whether the persistent Na-current results from different gating of the same channel type. The aim of the present review is to discuss the origin and the function of the persistent current, focusing on data derived from an invertebrate animal.
Studies addressing the question of how communities develop reported contrasting temporal patterns of species associations during succession. Several hypotheses were formulated about succession, but a general explanation of community assembly is missing. We analysed trends of species associations during old-field succession in two contrasting habitats: the first with chernozemic brown forest soil and temperate climate, and the second with sand soil and dryer climate. Significant pair-wise associations were calculated across a range of spatial scales. Comparing the two succession seres, one under harsh and the other under favourable environment, we attempted to make generalisations about species relation patterns. We found no trend but fluctuation in the level of community organization during succession. None of the existing succession models explained our results about changes in spatial structure of grassland communities during succession. Fluctuation in the number of significant associations was more intense and took longer under less favourable environmental conditions. Our results suggest that the stressed habitat type posed stronger constraints on species coexistence during succession than the favourable habitat did, but validating this hypothesis needs further investigations.
Disturbance related vegetation dynamics differs with Azorean mountain forest communities, where each tree species has its own regeneration strategy. Knowledge of the spatial distribution of tree species may help us to generate hypotheses on the relation between disturbance, regeneration and spatial organization and on the possible underlying ecological mechanisms. In view of this, we asked the following questions regarding the spatial pattern of tree species: What is the spatial distribution of saplings and adults? Are there any spatial associations or exclusions between saplings and adults of the same and different species? To what extent do the disturbance regimes and regeneration strategies of each species explain its spatial pattern? Six 225 m2 (15 × 15 m) plots were placed in each of three different forest types in three Islands (Pico, Terceira and Flores). Patterns of tree individuals were analysed through Morisita’s index of dispersion (I δ) and Iwao ω index. With the exception of Laurus azorica and Frangula azorica, saplings are in most cases aggregated. Erica azorica is the only species whose adults are aggregated at short distances. Spatial distribution is mostly random for the other species. At short distances, few strong associations or exclusions were detected. Pioneer species such as Juniperus brevifolia tend to be more aggregated due to their dependence on gaps to germinate and recruit new individuals. In fact, increasing disturbance and gap size enhances the regeneration of J. brevifolia. Primary species tend to be randomly distributed in part due to their strategy of forming seedling-sapling banks. Spatial pattern of tree species is largely explained by disturbance regimes and regeneration strategies of each species. However, factors such as habitat related patchiness, competition and dispersion limitation may also explain many of the observed patterns.
The appearance and development of the GABA-immunoreactive nervous elements in the central nervous system of Eisenia fetida were studied by immunocytochemistry. The nervous system originates from the neuroectoderm situated on the ventral side of the embryo. The organization of the circumpharyngeal ring starts earlier than that of the ventral cord. In the elementary ring the first GABA-immunopositive neurons can be observed (E1 stage) around the mouth. Later the cell number gradually increases and parallel to this process the elementary ring is separeted into a superficial and a deeper portion. The brain and the subesophageal ganglion will be organized from the superficial ring, while the nervous elements of the deeper ring will give rise for the first GABA-immunoreactive elements of the stomatogastric nervous system. In the early stages of the embryogenesis the immunoreactive cells of the developing brain appear solitary, while from the stage E4 they gradually are observed in groups. According to their position, these cell groups are similar to those observed in the brain of the adult earthworms. During embryogenesis the level of the ventral cord ganglia depends on their position in the ectodermal germ bands. It means, that the more organized ganglia are near the circumpharyngeal ring, mean while less developed ganglia are situated caudally from them. By the end of the embryogenesis all ganglia of the ventral cord will be equally well organized. The nerve tracts of the ganglia are built up from contra- and ipsilateral by projected fibres. From E3 stage the medial tracts, mean while from the E4 stage the lateral tracts begin to be formed. During the next stages, more and more fibres connect to the both tracts. At hatching, the development of the central nervous system of Eisenia fetida is not completed, the process is continued during the postembryonic development.
The spatial distribution of living trees of different size-classes was examined on two sites with different fire histories in a mature Pinus sylvestris L. dominated forest in eastern Finland. The study area was a 4-ha square plot, which was divided into two parts based on fire history analysis. In one part, the last fire event was a stand-replacing fire in the early 19thcentury, after which the whole stand regenerated, while the other part of the study plot was subsequently burnt by a surface fire in 1906. For the analyses, the trees were divided into five height-classes, reflecting competitive hierarchy. Spatial patterns of different tree height-classes were analysed using formulae for spatial autocorrelation with categorical data. In both areas, saplings showed an aggregated and overstorey trees a random spatial pattern at all scales studied. The other tree height-classes showed aggregated or random spatial patterns depending on the area and distance class. Generally, similar tree height-classes tended to be randomly or positively associated (attraction), except for the co-dominant and dominant, and overstorey trees, which were negatively associated (repulsion). The results suggest that the three dimensional spatial organization of the tree population was likely driven by two main interacting processes: (1) Low severity disturbances such as surface fires and treefalls, which create favourable regeneration patches, leading to positive association in smaller tree height-classes and (2) strong neighborhood competition among the largest trees and the competitive effect of these large trees on the small understorey trees, leading to negative association among these components of the tree population. The results also indicate that canopy replacement by trees currently in the understorey is unlikely to occur without a sufficient opening in the forest.
31 226 Strausfeld, N. J. (2002) Organization of the honey bee mushroom body: Representation of the calyx within the vertical and gamma lobes. J. Comp. Neurol. 450 , 4
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Hyphomycetes in the Trophic Structure of Streams. The Fungal Community: Its Organization and Role in the Ecosystem , Marcel Dekker, New York. Kendrick B Role of
References Allen , L. , de Benoist , B. , Dary , O. , Hurrell , R. 2006 . Guidelines on Food Fortification with Micronutrients . World Health Organisation
Strausfeld, N. J. (2002) Organization of the honey bee mushroom body: representation of the calyx within the vertical and gamma lobes. J. Comp. Neurol. 450 , 4–33. Strausfeld N. J
