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 el bow 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 muscl es [A.K. Wise, J.E. Gregory, U. Proske, J. Physiol., 508 (1998) 325-330]. H ere we report observations on the responses of muscle spindles of the soleu s muscle of the anaesthetised cat to determine their ability to signal smal l length changes in the passive muscle and during a contraction, under cond itions resembling those of the human experiments. After appropriate conditi oning of the muscle to control for history effects, primary endings of musc le spindles showed thresholds to ramp stretch at 20 mu m s(-1) of between l ess than 5 mu m and 15 mu m, which translates to 0.05 degrees-0.15 degrees of human elbow joint rotation. Thresholds were much higher following condit ioning to introduce slack in the muscle. Since during a voluntary contracti on there is likely to be alpha:gamma co-activation, responses of spindles w ere also recorded during slow stretches (100 mu m at 20 mu m s(-1)) during static fusimotor stimulation, dynamic fusimotor stimulation, combined fusim otor stimulation and fusimotor plus skeletomotor stimulation. Invariably, r esponses to passive stretch were larger than during motor stimulation. It i s concluded that spindles are sensitive enough to signal fractions of a deg ree of elbow joint rotation and that the rise in threshold observed during a voluntary contraction may be accounted for by the actions of fusimotor an d skeletomotor axons on spindle stretch responses. (C) 1999 Elsevier Scienc e B.V. All rights reserved.