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1

The role of computer simulations in the investigation of mechanisms underlying rhythmic firing of human motoneuron

Piotrkiewicz M.

Biocybernetics and Biomedical Engineering

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2021

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Vol. 41, no. 4

1406--1417

EN

Over the past few decades, a great deal of information on the function of mammalian motor neurons (MNs) has been obtained from intracellular recordings collected in acute animal experiments. Nowadays, it is becoming increasingly clear that human experiments in which MNs are tested under physiological conditions are equally important for further MN research. Investigation of human MNs is possible by recording the potentials of single motor units (MUs), which respond to action potentials from their MNs in the one-to-one fashion. Thus the analysis of MU firing patterns, based on basic knowledge of the MN physiology obtained from animal experiments and verified by computer simulations, allows the evaluation of the biophysical properties of human MNs. The MN models can be roughly classified as threshold-crossing and compartmental. Threshold-crossing models allowed verification of the methods for assessing the shape of synaptic volleys by analyzing the stimulus-correlated MN discharge patterns. They also helped in the development of methods for estimation of afterhyperpolarization of human MNs. The earliest compartmental models did not take into account the effects of persistent inward currents (PICs), which are now considered to be one of the most important factors in shaping human MN discharge patterns. Recent MN models increasingly focus on PICs and their interaction with synaptic inputs. It has been shown that different combinations of the two can produce various MU discharge patterns, including those mimicking the lack of effect of neuromodulators. This review shows how computer simulations support scientists in obtaining information from human experiments.

2

Assessment of human motoneuron afterhyperpolarization duration in health and disease

Piotrkiewicz M., Kudina L., Jason Chen J. J., Hausmanowa-Petrusewicz I.

Biocybernetics and Biomedical Engineering

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2012

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Vol. 32, no. 3

43-61

EN

The results of the investigatigation of afterhyperpolarization (AHP) duration in normal aging and selected neuromuscular disorders are presented. This investigation yielded unexpected results: the AHP shortening in myogenic disease (DMD) and no significant difference from control values in neurogenic disease (ALS). However, introduction of age factor revealed novel aspects of the human ALS, which can be interpreted on the basis of the results obtained in a SOD1 mice, thus confirming usefulness of this animal model of ALS. In spastic patients the AHP was prolonged and the difference from the control AHP duration decreased with age and disease duration. Our results suggest that the match between temporal characteristics of the AHP of MN and of the twitch of its muscle unit is preserved during normal aging and in spasticity, but not in the DMD.

3

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.