Fusimotor activity: its possible significance in muscle hypertonia.

Apart from the direct influences of muscle spindle sensory endings on skeletomotor neurones a great deal of their reflex drive is provided indirectly via the gamma-loop. Coactivity of skeletomotor and fusimotor neurones provides, on that occasion, for the servoassistance of muscular movement. Fusimotor neurones modulate their discharge rate in parallel with that of the skeletomotor neurons during reflex muscle contraction in decerebrated cats. These modulations take part during the rising phase of the reflex muscle tension as well as during the maintained reflex muscle contraction when small tension oscillations are present in a muscle. Fusimotor firing does not provide the main stimulus for the contractile activity of the muscle in the decerebrated cats but serves to reflect or to reproduce (within the spindle) the extent and the time course of tension changes in a muscle throughout the whole course of its reflex contraction.