The responses of muscle spindles to small, slow movements in passive muscle and during fusimotor activity

We have previously shown that movement detection thresholds at the human elbow joint were less than a degree of joint rotation in the passive limb but were higher if they were measured while subjects co-contracted elbow muscles [A.K. Wise, J.E. Gregory, U. Proske, J. Physiol., 508 (1998) 325-330]. Here we report observations on the responses of muscle spindles of the soleus muscle of the anaesthetised cat to determine their ability to signal small length changes in the passive muscle and during a contraction, under conditions resembling those of the human experiments. After appropriate conditioning of the muscle to control for history effects, primary endings of muscle spindles showed thresholds to ramp stretch at 20 micrometers s-1 of between less than 5 micrometers and 15 micrometers, which translates to 0.05 degrees -0.15 degrees of human elbow joint rotation. Thresholds were much higher following conditioning to introduce slack in the muscle. Since during a voluntary contraction there is likely to be alpha:gamma co-activation, responses of spindles were also recorded during slow stretches (100 micrometers at 20 micrometers s-1) during static fusimotor stimulation, dynamic fusimotor stimulation, combined fusimotor stimulation and fusimotor plus skeletomotor stimulation. Invariably, responses to passive stretch were larger than during motor stimulation. It is concluded that spindles are sensitive enough to signal fractions of a degree of elbow joint rotation and that the rise in threshold observed during a voluntary contraction may be accounted for by the actions of fusimotor and skeletomotor axons on spindle stretch responses.