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1

Significance of the neuronal morphology for network dynamics

Meyrand P.

Biocybernetics and Biomedical Engineering

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2006

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Vol. 26, no. 2

15-23

EN

One of the striking features of the Nervous System is the complexity and diversity of the neuronal shapes. Extensive morphological studies associated with modeling approaches have emphasized a close relationship between the neuron structure and its bioelectrical properties. However, structural implications of the synaptic connectivity (spatial distribution, number of synapses) in the context of the network operation are far less studied. To explore this issue, the chemical inhibitory synapses between identified motoneurones belonging to the stomatogastric nervous system of the lobster Homarus gammarus were examined. Using multiple intracelIular dyes injected in both pre- and post synaptic motoneurones, the sites of the appositions between these two celIs were localized under the laser-scanning confocal microscopy, and then the synaptic release sites were visualized under the electron microscopy. Such analysis indicates that only one or two zones of apposition exist between the two celIs studied, and only one release site exists per zone. Moreover, these synaptic sites are always located in the same position within the dendritic arbor. Modeling studies suggest that such a specific localization may play an important role in the operation of the network.

2

The tetanic depression in unfused tetani of fast motor units in mammalian muscle

Celichowski J.

Biocybernetics and Biomedical Engineering

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2005

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Vol. 25, no. 4

27-35

EN

Tetanic depression is a phenomenon that limits the force of unfused tetani in contractions evoked at unstable stimulation frequencies. In our experiments this phenomenon was visible in tetani evoked at two frequencies of stimulation: the lower immediately followed by the higher one. The force at the higher stimulation frequency was evidently depressed in relation to the force achieved during the control constant-frequency stimulations. This depression concerned the fast motor units and it was not observed in the slow units. The tetanic depression was initially found in the rat medial gastrocnemius muscle. However, it is not clear whether this phenomenon is specific only to the rat muscle or it concerns all mammals. Therefore, in the subsequent series of experiments, the tetanic depression was investigated in the motor units of the feline medial gastrocnemius, and the effect was also observed in all fast units. The maximum evoked depression exceeded 50% of the control values. Moreover, the analysis of the relationship between the amplitude of the tetanic depression and the degree of the tetanic fusion revealed that the strongest amplitude of depression was observed in the middle-fused tetani (with the fusion index in the range 0.5-0.7). The third experimental series proved that the tetanic depression could be visible in tetani evoked at a progressively increasing frequency of stimulation. Moreover, it was found that the effect of the force decrease could result from the prolongation of only one, the first interpulse interval. The amplitude of this decrease exceeded 20% of the control values. Summarizing, the tetanic depression is a phenomenon influencing the force of unfused tetani of the fast motor units at an increasing stimulation frequency. The depressive effect is present in various species of mammals, probably, including human. The amplitude of the depression depends on the fusion of the tetanic contraction. The physiological significance of tetanic depression is closely related to reduction of the importance of the motoneuronal firing rate.

3

Progressive die-back and death of motoneurons in a transgenic mouse model of amyotrophic lateral sclerosis

Gordon T., Tam S. L., Sharp P. S., Hegedus. J.

Biocybernetics and Biomedical Engineering

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2005

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Vol. 25, no. 4

17-26

EN

Fischer et al. [1] commented that the longest and largest nerve fibers with the highest metabolic demand appear to be the most susceptible to "dying-back" in a wide variety of degenerative and toxic conditions of the central and peripheral nervous systems. In the high copy number G93AmSOD1 transgenic mouse model of familial amyotrophic lateral sclerosis (ALS) we tested the hypothesis that the largest motoneurons which have the most terminal connections are the most susceptible to the disease. In a time course study of motoneuron, muscle and motor unit properties in fast - and slow-twitch hindlimb muscles of G93A and wild type mice, we found a rapid decline in numbers of functional motor units and motoneurons that progresses from birth to a plateau after 90-100 days of life with surprisingly Iittle compensatory axonal sprouting. Fast motoneurons are the most susceptible, contrasting with the slow motoneurons. Preliminary evidence of loss of S100 reactive perisynaptic Schwann cells at the denervated endplate regions of the affected muscles indicate loss paralIeIs rapid progression of disease with consequent decline in muscle force and motor unit numbers, folIowed closely in time by motoneuron death.

4

Computer system for identification and analysis of motor unit potential trains

Mazurkiewicz Ł., Piotrkiewicz M.

Biocybernetics and Biomedical Engineering

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2004

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Vol. 24, no. 3

15-23

EN

The paper presents a computerized system for identification and analysis of electromyographic signals recorded from muscle by needle electrodes. The system provides information on human motoneuron activity as time sequences of identified single motor unit potentials. This information can afterwards be statistically processed to obtain data on motoneuron properties in healthy subjects and in some neuromuscular diseases. A special attention has been paid to the description of the methods of spike identification, including extraction of spikes from the signal and automatic classification, which is based on the statistical pattern recognition approach. The system described is also an example of adaptation of modern computer technology for biomedical research.

5

Computer system for analysis of recurrent inhibition between human motoneurons

Jakubiec M., Piotrkiewicz M.

Biocybernetics and Biomedical Engineering

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2004

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Vol. 24, no. 3

3-14

EN

Despite years of extensive research, the physiological role of recurrent inhibition (RI) is still far from being clear. We decided to study RI between human motoneurons (MNs) during isometric constant force contractions, which has never been systematically analyzed before. The paper presents a description of a system for detection of RI. In the system several methods for analysis of correlations between single motor units (MUs) were implemented. The software was developed in environment DELPHI 6. The results of evaluation of the system on the basis of the data obtained in the computer simulations are presented. The system detects recurrent IPSPs of amplitudes as low as 50 /mi V.

6

Modeling of motoneuronal rhythmic activity

Piotrkiewicz M.

Biocybernetics and Biomedical Engineering

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2000

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Vol. 20, no. 4

35-43

EN

Computer simulations were applied to establish the relationship between properties of a motoneuron and the characteristics of its rhythmic activity. Sequences of motoneuronal spikes were obtained in the model at the moments of threshold crossings by a sum of an afterhyperpolarization (AHP) curve and steady level representing synaptic inflow with a variable component (noise). Standard deviation, ?, and mean interspike interval, T(m), were computed thereafter. Two functions describing AHP shape were applied as well as two kinds of inflow with different noise time structure and few different values of noise amplitude. The results show that the break point of ..(Tm) is not dependent neither on noise structure, nor on its amplitude, but onIy on the shape of AHP. The linear correIation between the AHP duration and the break point was observed, with AHP corresponding to Ionger intervals than the break point and the correlation coefficient dependent on the AHP shape. The curves for shorter AHP duration were not onIy shifted towards shorter interspike intervals (ISIs) but also had lower values of ? at their short-interval parts.

PL

W celu zbadania zależności między właściwościami motoneuronu oraz charakterystykami statystycznymi jego rytmicznej impulsacji zastosowano metodę symulacji komputerowych. Potencjały modelu motoneuronu otrzymywano w momentach przecięcia krzywej, uzyskanej ze zsumowania krzywej reprezentującej potencjał hiperpolaryzacyjny następczy (AHP) oraz stacjonarnego poziomu reprezentującego sumę wpływów synaptycznych i zawierającego składową zmienną (szum). Z tak uzyskanych ciągów potencjałów obliczano wartość średnią T(m) oraz odchylenie standardowe [?] odstępów czasu między kolejnymi potencjałami. W modelu zastosowano dwie różne krzywe opisujące AHP oraz dwa rodzaje szumu o zdecydowanie różnej strukturze czasowej i kilku różnych amplitudach. Wyniki wskazują na to, iż charakterystyka [?(Tm)] nie zależy ani od struktury, ani od amplitudy szumu, ale wyłącznie od kształtu AHP. Stwierdzono występowanie korelacji między czasem trwania AHP a punktem złamania charakterystyki [?(Tm)], przy czym punkt złamania przypadal na czas zdecydowanie krótszy niż czas trwania AHP, a współczynnik korelacji zależał od kształtu AHP. Krzywe dla krótszych AHP przypadały nie tylko na krótsze interwały międzyimpulsowe, ale również na niższe wartości [?] w części krzywej odpowiadającej wysokim częstościom motoneuronu.