Role of the fusimotor system in locomotion.

The contribution of muscle spindles to the control of locomotion depends on the patterns of discharge that occur in static and dynamic gamma motoneurones (gammaS and gammaD). Discharges of gamma-axons to the MG muscle were studied during treadmill locomotion in pre-mammillary, decerebrated cats. All gammaS-efferents increased their rate of discharge at onset of locomotion and were modulated with movement. Type-1 increased their rate with muscle shortening whereas type-2 gammaS efferents increased their discharge rate during muscle lengthening. The type-1 gammaS pattern appears to be a temporal template of the intended movement. The type-2 gammaS pattern may be appropriate for afferent biasing through bag2 intrafusal fibres. The gammaD axons showed an interrupted discharge pattern with sudden onset of firing at the start of muscle shortening and falling quiet shortly after the beginning of lengthening. The gammaD discharge would prepare primary endings to respond with high sensitivity at the start of muscle lengthening.