STATIC FUSIMOTOR FIBRES AND POSITION SENSITIVITY OF MUSCLE SPINDLE RECEPTORS

The position sensitivity (the slope of the approximately linear relation between firing frequency and muscle length) of spindle afferents in the soleus muscle of anaesthetised cats has been determined during stimulation of single static fusimotor fibres. These sometimes did, and sometimes did not, increase the position sensitivity of both primary and secondary afferent endings. There was no bimodality in the range of effects. The effects of stimulation of a single fusimotor fibre on the position sensitivity of two different endings, both of which were influenced by it, were not usually closely similar. Simultaneous stimulation of several static fusimotor fibres usually increased the position sensitivity of both primary and secondary endings. These results were found both under static conditions and during the course of a single slow stretch (either a 'ramp' or a 'triangle'). It is concluded that individual static fusimotor fibres cannot be sub-classified into those which do, and those which do not, produce a change in position sensitivity of afferent endings. But the effects of static fusimotor fibres on the position sensitivity of many endings is so definite that its functional significance cannot be ignored, particularly in discussing the secondary endings. © 1969.