SUMMATION OF RESPONSES OF CAT MUSCLE-SPINDLES TO COMBINED STATIC AND DYNAMIC FUSIMOTOR STIMULATION

This is a study of the process of interaction between the responses of muscle spindles to stimulation of two fusimotor fibres. Combined stim ulation of a static and a dynamic fusimotor fibre supplying the same m uscle spindle in the soleus muscle of the anaesthetised cat gave a res ponse which was larger than from stimulating each fibre separately, bu t less than their sum. A similar summation process was observed with p airs of static fusimotor fibres. The mean summation coefficient for th e responses to stimulation of 14 pairs of static fusimotor fibres was 0.29 (range 0.14-0.52; S.D. 0.09), while for 42 static:dynamic pairs i t was 0.30 (range 0.07-0.89; S.D. 0.20). Mechanisms considered for the summation process were probabilistic mixing of impulse traffic from t wo or more impulse generators within the terminals of the primary endi ng of the spindle, the spread of generator current from one encoding s ite to another and mechanical interactions between contracting intrafu sal fibres. In an experiment where single static and dynamic fusimotor fibres were stimulated together, and then stimulation of the static f ibre stopped, the size of the continuing dynamic response was larger t han when the dynamic fibre had been stimulated alone. This finding sug gested some kind of mechanical interaction between the contracting int rafusal fibres and implies that static and dynamic fusimotor effects w ithin a spindle cannot be considered to be entirely independent of one another. (C) 1998 Elsevier Science B.V. All rights reserved.