[Reciprocal inhibition of human motor neurons at different firing rates]. / Retsiproknoe tormozhenie motoneironov cheloveka pri raznoi chastote ikh impul'satsii.

Reciprocal inhibition (RI) has been studied for its effect on repetitive firing of single motoneurons in the extensor carpi ulnaris and soleus muscles of healthy subjects. The peristimulus time histograms (PSTHs) were plotted for different mean firing frequency of motoneurons. Changes in the interspike interval duration were analyzed. It was shown that the "effectiveness" of reciprocal volleys (as it was previously shown for other types of inhibition) depended on the firing rate of a motoneurons. It was found out that RI duration derived from the PSTH, duration of the "effective part" of the interspike interval, rate of the interspike interval lengthening depended on the motoneuron firing frequency. It was also found that the inhibitory effect of the motoneuron firing was pronounced mostly at its low-frequency range. The dependence of "effectiveness" of an inhibitory volley on the moment of its arrival within the interspike interval was analyzed as well.

[The action of a short inhibitory volley on human firing motoneurons (an experiment and a model)]. / Deistvie korotkoi tormoziashchei posylki na impul'siruiushchie motoneirony cheloveka (éksperiment i model').

The influence of a short inhibitory volley on firing motoneurons of soleus and ext. carpi uln. was studied in human experiments and the results obtained were used as a basis for simulation. An increase of the interspike interval (ISI) depended on the timing of inhibitory volley arrival and silent period (SP) duration was studied. The ability of the individual motoneuron to respond to inhibitory volley by prolongation of ISI influenced the SP duration. Two parameters were crucial: the duration of the part of ISI where the inhibition was efficient in increasing the ISI (d) and the maximal increase of ISI (delta x max). The latter corresponded to the inhibitory volley arrival at the very end of ISI. If d less than delta x max the SP = d; if d greater than delta x max the SP = delta x max. In the case of variable ISIs those relations should be applied to the smallest ISI among the ISI distribution. Therefore the SP duration depended not only on the mean ISI of a motoneuron, but on ISI variability as well.

Testing excitability of human motoneurones capable of firing double discharges.

Single motor unit (MU) action potentials were recorded from the flexor carpi ulnaris during its weak voluntary contraction in 6 healthy subjects. At low frequencies of motoneuronal firing (the average interspike interval ranged from 70 +/- 8 to 200 +/- 22 msec), double discharges with interspike intervals from 5 to 20 msec were seen from about one-fourth of all MUs (62 out of 249). Monosynaptic testing of the excitability of firing motoneurones was carried out to study the probable mechanism of doublets. By stimulating the ulnar nerve against the background of firing MUs an H-reflex was elicited. The excitability of the motoneurones was evaluated by a firing index whose changes within an interspike interval were analysed (the step width was 10 msec). The testing of 47 MUs has shown that motoneurones capable of firing double discharges during voluntary muscle contraction have a higher excitability at the beginning of an interspike interval (the first 20 msec) as compared to those incapable of firing double discharges. Our data confirm that the leading part in the formation of double discharges is played by delayed depolarization occurring in a firing motoneurone after its successive discharge. The correlation between the characteristics of doublets and the properties of delayed depolarization are discussed. It is proposed that motoneurones with a delayed depolarization possess certain transduction properties as they produce a maximal instantaneous increase in discharge frequency on a minimal increase in synaptic input. This study has revealed a phase of increased excitability in the motoneurone axon during the first 15 msec after a regular rhythmic discharge has been propagated.(ABSTRACT TRUNCATED AT 250 WORDS)

[Correlation of the discharge frequency of a firing motor neuron and the efficiency of an excitatory volley of Ia-afferents in man]. / Sootnoshenie chastoty razriada impul'siruiushchego motoneirona i éffektivnosti vozbuzhdaiushchei posylki Ia-afferentov u cheloveka.

Single motor unit potentials of the flexor carpi ulnaris muscle were recorded during voluntary contraction and H-reflex was evoked. The afferent volley effectiveness was estimated by the firing index of a motoneuron. The background firing rate of motor units ranged from 3.3 to 16.0 imp/s. Afferent volley effectiveness depended on the background firing rate of fast muscle units under study like slow soleus muscle units investigated earlier. The effectiveness of afferent volley decreased with the motor units firing rate growth. At all range of firing rate the efficiency of afferent volley was higher for motor units of fast muscle as compared to slow muscle. It is suggested that the motoneuron excitability changes within the interspike interval underlay the relationship between the motoneuron firing rate and effectiveness of the excitatory afferent volley.

[Relation between the toxic killing of mouse small intestine enterocytes and the interval between the administration of the S-specific agent hydroxyurea]. / Zavisimost' toksicheskoi gibeli énterotsitov tonkoi kishki myshei ot intervala mezhdu vvedeniiami S-fazovo-spetsificheskogo agenta-oksimocheviny.

The dependence of small intestinal lesions on the interval between repeated regular injections of hydroxyurea was studied. The doses of 1 g/kg and 0.2 g/kg were used. To activate the enterocyte proliferation the mice were pre-irradiated at 2 Gy. 3-4 hours after the last hydroxyurea injection the two distinct minimum points of epithelial lesions were observed at interinjection intervals of 9 and 16.5 hours. The result was dose-independent.

[Optimization of the exposure of normal and transformed cell populations to phase-specific agents]. / Optimizatsiia vozdeistviia na normal'nye i transformirovannye populiatsii kletok fazospetsificheskimi agentami.

Dynamic behavior of stem cells population of the "critical" tissue (normal population) and tumor cell population under periodic treatment with a phase-specific cytotoxic agent was considered. The results were used for optimization of anticancer chemotherapy. The schedules of treatment were found which provide a maximum rate of tumor-cell elimination for any given rate of the normal population size decrease. If the mean generation times of normal and tumor populations differ (which was stated for many tumors), usage of the optimal period markedly increases the selectivity of therapy, while application of other periods can result in selective elimination of the normal population. Problems concerned with practical realization of the proposed regimes are discussed.

[Relation of the survival of mouse hematopoietic stem cells to the interval between periodic injections of hydroxyurea]. / Zavisimost' vyzhivaniia stvolovykh krovetvornykh kletok mysh ot intervala mezhdu periodicheskimi vvedeniiami oksimocheviny.

The dependence was studied between the survival of proliferating hemopoietic stem cells (CFUs) on the interval between regular multiple injections of hydroxyurea (HU). Before HU injection mice were irradiated in a dose of 200 rad to trigger the proliferation of CFUs. The dependence obtained is resonance in character, with a maximum being attained during injections made with a 12-hour interval. Comparison with the mathematical model indicates that interval corresponding with the maximal survival should correlate with the mean time of the generation of proliferating CFUs.

[Increase of heterogeneities at the interface of decrement conduction in Hodgkin-Huxley model]. / Rost neodnorodnostei na granitse oblasti dekrementnogo provedeniia v modeli Hodgkin-Huxley.

Spatial heterogeneity was simulated by a 2-fold increase in either of gNa, gK, g1 in the central part of the fibre. The interval-dependent conduction block was not observed in the cable with normal gNa, while the uniform decrease in gNa along the fibre resulted were the block appearance. Difference in the refractory period of these parts of the fibre were the higher, the lower was gNa. The mechanism of this phenomenon was investigated. The results obtained can be used for understanding the mechanism of post infarction arrhythmias.