Wiederfund von Najas marina L. ssp. marina im Schollener See (Elbe-Havel-Winkel, Sachsen-Anhalt)

Das Große Nixkraut (Najas marina L. s.l.) ist in der Roten Liste der Farn- und Blütenpflanzen Sachsen-Anhalts als stark bedrohte Art (Kategorie 1) verzeichnet (FRANK et al. 2004). Ein seit längerem bekannter Standort dieser Art in Sachsen-Anhalt ist der Schollener See an der unteren Havel, 12 km nördlich von Rathenow (BENKERT et al. 1996). Der jüngste Bericht von Najas marina ssp. marina im Schollener See stammt von HILBIG & REICHHOFF (1974) aus dem Jahre 1973. Seither wurde sie nicht wieder gefunden und ihr Vorkommen war in der Folgezeit auch höchst unwahrscheinlich, da im Zuge der extremen Eutrophierung der Fluss- und Auenseen der mittleren und unteren Havel und der damit verbundenen Verringerung der Sichttiefen auf 2–3 dm die gesamte Submersflora verschwand. Etwa seit der zweiten Hälfte der 90er Jahre findet im Havelgebiet je nach Gewässertrübung eine kontinuierliche Rückkehr der submersen Makrophyten statt. Es handelt sich dabei zunächst nur um sehr eutraphente Arten, wie Ceratophyllum demersum, Potamogeton pectinatus und Myriophyllum spicatum. Im Zuge dieser Entwicklung wäre ein Wiederauftauchen von Najas marina durchaus denkbar, zumal sich im Gebiet meso- bis schwach eutraphente Arten langsam wieder etablieren, wenn auch zunächst mehr in den Kleingewässern und Entwässerungsgräben (TÄUSCHER 1994, 1996, 1998). Das Jahrhunderthochwasser in der Elbe im August 2002, bei dem die Havelniederung als Entlastungsraum diente, löste im Schollener See im Jahr 2004 offensichtlich die Ausbildung eines ganzjährigen Klarwasserzustandes aus (leider keine Information über das Jahr 2003). Sofort erschienen größere Bestände submerser Makrophyten und u. a. auch von Najas marina ssp. marina. Primäres Ziel dieses Beitrages ist es, das Wiedererscheinen von Najas m. möglichst umfassend zu dokumentieren. Darüber hinaus erlauben die Umstände des Wiederauftauchens einige Einblicke in die Autökologie dieser Art. Schließlich zeigten sich bei der Bestimmung der Subspezies einige Widersprüche, die es zu klären galt. So traf keine der im ROTHMALER (Bd. 4, 2002) angegebenen Merkmalskombinationen für die Subspezies N. m. marina und N. m. intermedia eindeutig für das vorgefundene Material zu

Modelling the effect of ephaptic coupling on spike propagation in peripheral nerve fibres

Experimental and theoretical studies have shown that ephaptic coupling leads to the synchronisation and slowing down of spikes propagating along the axons within peripheral nerve bundles. However, the main focus thus far has been on a small number of identical axons, whereas realistic peripheral nerve bundles contain numerous axons with different diameters. Here, we present a computationally efficient spike propagation model, which captures the essential features of propagating spikes and their ephaptic interaction, and facilitates the theoretical investigation of spike volleys in large, heterogeneous fibre bundles. We first lay out the theoretical basis to describe how the spike in an active axon changes the membrane potential of a passive axon. These insights are then incorporated into the spike propagation model, which is calibrated with a biophysically realistic model based on Hodgkin-Huxley dynamics. The fully calibrated model is then applied to fibre bundles with a large number of axons and different types of axon diameter distributions. One key insight of this study is that the heterogeneity of the axonal diameters has a dispersive effect, and that a higher level of heterogeneity requires stronger ephaptic coupling to achieve full synchronisation between spikes

The Role of Long-Range Connectivity for the Characterization of the Functional–Anatomical Organization of the Cortex

This review focuses on the role of long-range connectivity as one element of brain structure that is of key importance for the functional–anatomical organization of the cortex. In this context, we discuss the putative guiding principles for mapping brain function and structure onto the cortical surface. Such mappings reveal a high degree of functional–anatomical segregation. Given that brain regions frequently maintain characteristic connectivity profiles and the functional repertoire of a cortical area is closely related to its anatomical connections, long-range connectivity may be used to define segregated cortical areas. This methodology is called connectivity-based parcellation. Within this framework, we investigate different techniques to estimate connectivity profiles with emphasis given to non-invasive methods based on diffusion magnetic resonance imaging (dMRI) and diffusion tractography. Cortical parcellation is then defined based on similarity between diffusion tractograms, and different clustering approaches are discussed. We conclude that the use of non-invasively acquired connectivity estimates to characterize the functional–anatomical organization of the brain is a valid, relevant, and necessary endeavor. Current and future developments in dMRI technology, tractography algorithms, and models of the similarity structure hold great potential for a substantial improvement and enrichment of the results of the technique

Action potential propagation and synchronisation in myelinated axons

With the advent of advanced MRI techniques it has become possible to study axonal white matter non-invasively and in great detail. Measuring the various parameters of the longrange connections of the brain opens up the possibility to build and refine detailed models of large-scale neuronal activity. One particular challenge is to find a mathematical description of action potential propagation that is sufficiently simple, yet still biologically plausible to model signal transmission across entire axonal fibre bundles. We develop a mathematical framework in which we replace the Hodgkin-Huxley dynamics by a spike-diffuse-spike model with passive sub-threshold dynamics and explicit, threshold-activated ion channel currents. This allows us to study in detail the influence of the various model parameters on the action potential velocity and on the entrainment of action potentials between ephaptically coupled fibres without having to recur to numerical simulations. Specifically, we recover known results regarding the influence of axon diameter, node of Ranvier length and internode length on the velocity of action potentials. Additionally, we find that the velocity depends more strongly on the thickness of the myelin sheath than was suggested by previous theoretical studies. We further explain the slowing down and synchronisation of action potentials in ephaptically coupled fibres by their dynamic interaction. In summary, this study presents a solution to incorporate detailed axonal parameters into a whole-brain modelling framework

Mean-field approximations of networks of spiking neurons with short-term synaptic plasticity

Low-dimensional descriptions of neural network dynamics are an effective tool for bridging different scales of organization of brain structure and function. Recent advances in deriving mean-field descriptions for networks of coupled oscillators have sparked the development of a new generation of neural mass models. Of notable interest are mean-field descriptions of all-to-all coupled quadratic integrate-and-fire (QIF) neurons, which have already seen numerous extensions and applications. These extensions include different forms of short-term adaptation (STA) considered to play an important role in generating and sustaining dynamic regimes of interest in the brain. It is an open question, however, whether the incorporation of pre-synaptic forms of synaptic plasticity driven by single neuron activity would still permit the derivation of mean-field equations using the same method. Here, we discuss this problem using an established model of short-term synaptic plasticity at the single neuron level, for which we present two different approaches for the derivation of the mean-field equations. We compare these models with a recently proposed mean-field approximation that assumes stochastic spike timings. In general, the latter fails to accurately reproduce the macroscopic activity in networks of deterministic QIF neurons with distributed parameters. We show that the mean-field models we propose provide a more accurate description of the network dynamics, although they are mathematically more involved. Using bifurcation analysis, we find that QIF networks with pre-synaptic short-term plasticity can express regimes of periodic bursting activity as well as bi-stable regimes. Together, we provide novel insight into the macroscopic effects of short-term synaptic plasticity in spiking neural networks, as well as two different mean-field descriptions for future investigations of such networks

The perception of musical phrase structure: A cross-cultural ERP study

Electroencephalography (EEG) was used in a cross-cultural music study investigating phrase boundary perception. Chinese and German musicians performed a cultural categorization task under Chinese and Western music listening conditions. Western music was the major subject for both groups of musicians, while Chinese music was familiar to Chinese subjects only. By manipulating the presence of pauses between two phrases in the biphrasal melodies, EEG correlates for the perception of phrase boundaries were found in both groups under both music listening conditions. Between 450 and 600 ms, the music CPS (closure positive shift), which had been found in earlier studies with a false tone detection task, was replicated for the more global categorization task and for all combinations of subject group and musical style. At short latencies (100 and 450 ms post phrase boundary offset), EEG correlates varied as a function of musical styles and subject group. Both bottom–up (style properties of the music) and top–down (acculturation of the subjects) information interacted during this early processing stage