Movement reorganization to compensate for fatigue during sawing

Peripheral (muscle) aspects of fatigue are well documented. However, little is known about the central aspects of fatigue that could influence, in particular, multijoint coordination. To investigate the central aspects of fatigue, we compared the multijoint kinematics of non-fatigued and fatigued individuals while sawing. Muscle fatigue was associated with decreases in sawing force and movement amplitude at the elbow whereas the basic characteristics of the saw trajectory, including the movement direction, extent and duration, remained invariant. This invariance was maintained by increasing the movement amplitude at the wrist, shoulder and trunk. The system thus takes advantage of the redundancy of the motor apparatus to maintain the endpoint trajectory despite fatigue.     

Dopa and dopamine levels in the hypothalamus and brain stem of rats with experimental gastric ulcer treated by acupuncture

Department of Normal Physiology, Ivano-Frankovsk Medical Institute. Laboratory of Experimental and Clinical Biochemistry, Central Research Institute of Reflex Therapy, Ministry of Health of the USSR, Moscow. (Presented by Academician of the Accademy of Medical Sciences of the USSR K. V. Sudakov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 107, No. 3, pp. 274–279, March, 1989.     

Hand trajectory invariance in reaching movements involving the trunk

Movements of different body segments may be combined in different ways to achieve the same motor goal. How this is accomplished by the nervous system was investigated by having subjects make fast pointing movements with the arm in combination with a forward bending of the trunk that was unexpectedly blocked in some trials. Subjects moved their hand above the surface of a table without vision from an initial position near the midline of the chest to remembered targets placed within the reach of the arm in either the ipsi- or contralateral workspace. In experiment 1, subjects were instructed to make fast arm movements to the target without corrections whether or not the trunk was arrested. Only minor changes were found in the hand trajectory and velocity profile in response to the trunk arrest, and these changes were seen only late in the movement. In contrast, the patterns of the interjoint coordination substantially changed in response to the trunk arrest, suggesting the presence of compensatory arm-trunk coordination minimizing the deflections from the hand trajectory regardless of whether the trunk is recruited or mechanically blocked. Changes in the arm interjoint coordination in response to the trunk arrest could be detected kinematically at a minimal latency of 50 ms. This finding suggests a rapid reflex compensatory mechanism driven by vestibular and/or proprioceptive afferent signals. In experiment 2, subjects were required, as soon as they perceived the trunk arrest, to change the hand motion to the same direction as that of the trunk. Under this instruction, subjects were able to initiate corrections only after the hand approached or reached the final position. Thus, centrally mediated compensatory corrections triggered in response to the trunk arrest were likely to occur too late to maintain the observed invariant hand trajectory in experiment 1. In experiment 3, subjects produced similar pointing movements, but to a target that moved together with the trunk. In these body-oriented pointing movements, the hand trajectories from trials in which the trunk was moving or arrested were substantially different. The same trajectories represented in a relative frame of reference moving with the trunk were virtually identical. We conclude that hand trajectory invariance can be produced in an external spatial (experiment 1) or an internal trunk-centered (experiment 3) frame of reference. The invariance in the external frame of reference is accomplished by active compensatory changes in the arm joint angles nullifying the influence of the trunk motion on the hand trajectory. We suggest that to make a transition to the internal frame of reference, control systems suppress this compensation. One of the hypotheses opened to further experimental testing is that the integration of additional (trunk) degrees of freedom into movement is based on afferent (proprioceptive, vestibular) signals stemming from the trunk motion and transmitted to the arm muscles.     

Organization of the nervous tissue (hippocampus and septum) developing in the anterior eye chamber. II. Neuronal perikarya and dendritic processes

Rat's embryonal tissue of septum and hippocampus was transplanted into anterior eye chamber (AEC) of the adult rats. Ultrastructure of septal (SGs) and hippocampal (HGs) grafts was analyzed after three-four month of survival in AEC. Special attention was directed to deviations of ultrastructure of the grafted neurons from standard characteristics. Neuronal perikarya and dendritic processes basically had normal structure typical of highly differentiated, mature neurons. Organotypic features of neuronal ultrastructure were well preserved by the grafted tissue. At the same time certain anomalies were observed: presence of multilaminated bodies, vacuolization of some areas in mitochondria, cisternae of Golgi apparatus and of endoplasmic reticulum, slight increase of electron opacity of cytoplasm in some neurons. Dendrites had irregular contour due to abundance of various microprocess with and without postsynaptic specializations. Some dendritic spines with abnormal configurations were observed (with long, thin stalks, with arrow-shaped heads). The microprocesses and spines were present also upon the surface of perikarya. Extensive appositions of plasmalems of adjacent neurons with gap junctions were usual. Synaptic vesicles and disaggregation of microtubules were observed in some dendrites. The neurons and large dendritic processes covered by multilayer myelin-like sheaths were encountered in the grafted tissue. Active micropinocytosis and exchange by cytoplasmic fragments (phagocytosis) was observed between the neurons as well as between the nervous and glial cells. It is suggested that some unusual features of ultrastructure of the grafted tissue result of deficit of extrinsic afferentation, while others may be regarded as consequences of excessive excitability of the grafted tissue.