Responses of muscle spindles to stretch after a period of fusimotor activity compared in freely moving and anaesthetized cats

Recordings of responses of muscle spindles in the soleus muscle of freely moving unanaesthetized cats show that the onset of receptor discharge during the portion of the step cycle in which the muscle is stretched is earlier when compared with a similar stretch applied in the anaesthetized animal. In anaesthetized animals the response to stretch also commences earlier if the movement is preceded by fusimotor activity. It is suggested that the relatively long time-course of intrafusal relaxation, as well as an associated increase in compliance of intrafusal fibres, can account for the early onset in the response to stretch.