Knee movements cause changes in the firing behaviour of muscle spindles located within the mono-articular ankle extensor soleus in the rat

We recently showed that within an intact muscle compartment, changing the length of one muscle affects the firing behaviour of muscle spindles located within a neighbouring muscle. The conditions tested, however, involved muscle lengths and relative positions that were beyond physiological ranges. The aim of the present study was to investigate the effects of simulated knee movements on the firing behaviour of muscle spindles located within rat soleus (SO) muscle. Firing from single muscle spindle afferents in SO was measured intra-axonally for different lengths (static) and during lengthening (dynamic) of the lateral gastrocnemius and plantaris muscles. Also, the location of the spindle within the muscle was assessed. Changing the length of synergistic ankle plantar flexors (simulating different static knee positions, between 45 and 130°) affected the force threshold, but not the length threshold, of SO muscle spindles. The effects on type II afferents were substantially (four times) higher than those on type IA afferents. Triangular stretch-shortening of synergistic muscles (simulating dynamic knee joint rotations of 15°) caused sudden changes in the firing rate of SO type IA and II afferents. Lengthening decreased and shortening increased the firing rate, independent of spindle location. This supports our prediction that the major point of application of forces exerted by connections between adjacent muscles is at the distal end of SO. We conclude that muscle spindles provide the CNS with information about the condition of adjacent joints that the muscle does not span.