Analysis of interaction of spinal and supraspinal reflex pathways involved in physiological tremor

A model of feedback type in which physiological tremor are produced by both muscular skeletal system and reflex action is described. Analysis of this model shows that the interaction between the spinal and the supraspinal reflex pathways is important in responsible for low and high frequency oscillations of physiological tremor. Particularly the effect of gain ratio of the two pathways is studied in order to examine the role for the two reflex pathways in controlling neuromuscular oscillations. The existence of a critical point of the gain ratio at which one oscillating frequency transitions to two ones is predicted theoretically. A shift of the critical point with variations of weight load is found and suggests a changeable correlation relation between the spinal and the supraspinal pathways due to loading conditions. Our computations for physiological tremor demonstrate the results that the high frequency component of about 25 Hz is produced by the muscle-spinal reflex loop, and the low frequency component of about 10 Hz originates from the central nervous system or from supraspinal reflex loop. Several relations derived in this study are described, and they can be compared with experimental observations. Our model sheds considerable light on the details of the possible mechanism for physiological tremor. In addition, a possibility arising from our study is that the tremulous oscillation associated with some pathological states, say Parkinson's disease, may arise from modified gains in one or more of the reflex pathways.