Gorkovenko A. The study of nonlinear dynamics of signal transmission in the system of motoneurons-muscle

Thesis is devoted to study of nonlinear effects of signal transmission in the system of motoneurons-muscle. In experiments on animals we studied subthreshold activation of spinal motoneurons. Analysis of amplitude distribution of separate EPSPs on motoneuron membrane during natural activation of muscle proprioceptors in the system of stretch-reflex allowed us to create mathematical model of integration of separate EPSPs. This revealed two-component structure of evoked depolarization. First component reproduced the process of low-frequency filtration of excitatory postsynaptic currents in synapses located on the somatic membrane and the nearest dendrites. Second one, where one could not separate single EPSPs, showed integral synaptic activity of distal dendrites. We also studied the dependence of the spike generation threshold on the frequency of motoneuron activity in the stretch-reflex. Majority of cells demonstrated rise of the threshold with frequency but decrease of the level was observed also. During cyclic change of frequency of evoked activity in its separate stages we noticed both positive and negative correlation of the spike threshold to the frequency. In experiments with humans we studied voluntary two-joint movement of the arm. We concluded that dynamic component of EMG activity during transition from one point of operational space to another could be decomposed on separate components related to single-joint movements. Central commands of motor control could be formed by principle of superposition when the command could be composed from more simple commands related to movements in single joints. Thereby the motor control could be constructed on the basis of hierarchic model of superposition of basic programs of movement. The investigation of the experimental model of interaction of antagonistic muscles showed that study of central mechanism of motor control is impossible without detailed information about hysteretic effects in the system of antagonistic muscles. In the experiments on anaesthetized cats we made quantitative analysis reciprocal and coactivation patterns of activation of antagonistic muscles of experimental model of joint. We concluded that reciprocal activation was the most appropriate for the control of both amplitude and velocity of movement in the join. Coactivation of muscles was the most suitable for reduction of aftereffects of preceding movement in the joint and prehistory of muscle activation.