Changes in fusimotor outflow during vibration-induced contraction of triceps surae muscles in decerebrate cats

Changes in fusimotor outflow in medial gastrocnemius and lateral gastrocnemius plus soleus nerves were investigated during vibration-induced contraction of triceps surae muscles in decerebrate cats. A sustained increase in spike occurrence appeared in 33 of 58 investigated neurons, and a decrease in 13. Transient changes of an opposite sign occurred in 19 excited and 6 inhibited fusimotor neurons during the rising phase of reflex muscle contraction. The later changes appeared coincidently with silence in the electromyogram. Thus the discharge of one-third of the examined fusimotor population directed to the triceps surae muscles was modulated parallelly with that of the skeletomotor neurons. It is supposed that the changes in fusimotor spike occurrence during the rising phase of reflex muscle tension are induced mainly by changes in afferent input from muscle spindle primary endings and/or Golgi tendon organs. We could not ascertain whether the different responses (excitation vs inhibition) are related to fusimotor type or to changes in transmission through the reflex pathways. The possible influence of the changes in fusimotor outflow on muscle spindle pause and the silent period in skeletomotor discharge at the beginning of reflex muscle contraction induced by vibration is considered.