Aftereffects of static or dynamic fusimotor activation on primary afferent discharge.

An analysis is presented of the effects of previous static or dynamic fusimotor activity on various phases of primary afferent discharge before and during ramp stretch of the soleus muscle of anesthetized cat. Short term effects (under 1 sec) following termination of fusimotor activity are generally opposite for static and dynamic fusimotor fibers. Compared to control levels, there is a pronounced depression of primary afferent discharge shortly after termination of static fusimotor activation, and a facilitation following termination of dynamic fusimotor activation. Facilitatory and depressive effects on afferent discharge at periods beyond 1 sec following cessation of fusimotor activation are similar for static and dynamic fusimotor fibers. Possible mechanisms of these changes are discussed in terms of changes in muscle spindle actin-myosin crossbridges, receptor adaptation, and the innervation of nuclear bag and nuclear chain spindle fibers.