Adjustment of spindle bias during movement.
Logical way to maintain or increase spindle input (to alpha motonurones) is to drive Sb2 fibre via static gamma system. This biasses Ia output. Db1 fibre biassing action too small for use except in small slow movements. Appropriate action is therefore co-activation of alpha and gamma with the gamma recieving a modulated pattern based on the alpha pattern. As muscle shortens Ia discharge is maintained. This explains the lack of change in Ia output seen in human movement.Compensation for spindle unloading could also come from group II pathway by increasing static fusimotor output to chain fibres with secondary ending. Powerful biassing action in this context.
Note that the above conditions keep the spindle responsive to unexpected length changes resulting from fatigue, change in load or obstruction of movement.
This can be demonstrated to occur during low velocity normal unobstructed movements. At higher velocities the length sensitivity if the primary ending is apparent, perhaps due to the input from Db1 fibres during rapid length changes dominating the Ia discharge.
1. A message to the spindle signalling the desired trajectory of movement.
2. A signal to extrafusal muscle that takes into account any load and fatigue. If the load proved unexpectedly great then the spindle would suffer a smaller degree of unloading, afferent discharge would increase and movement would be reflexly assisted. The discharge may also cause a central command to increase motor outflow. This is basically the alpha - gamma coactivation principle.Abolition of spindle length sensitivity during movement.
This is quite different from the above. Static fusimotor activation of chain fibres (up to 75Hz) makes then insensitive to length changes. Remember that some static gamma axons go to Sb2 and to chain fibres. Action on Sb2 potentiates the driving action of chain fibres on the primary ending. So.. tonic static fusimotor drive could hold Ia afferent discharge more or less constant during muscle shortening without the need for modulation based on alpha drive. What can be observed is a rather irregular group Ia afferent discharge that is largely unaffected by muscle length. 
Modulation of spindle length sensitivity during movement.
Dynamic intrafusal system enhances the dynamic sensitivity of the primary ending during movement. A high sensitivity to length changes is precisely what is required during movement if deviations from a desired trajectory are to be rapidly corrected. During imposed movements in conscious cats very hight Ia afferent discharges occur. This probably reflects a combination of dynamic and static fusimotor activity. The Db1 effect directly increases dynamic length sensitivity, and the Sb2 can also increase the dynamic responsiveness of the spindle either directly by increasing the dynamic sensitivity of the secondary endings (group II afferent input), or indirectly by biasing group II discharge, so causing reflex facilitation of dynamic fusimotor neurons.
Modelling of planned movements.
2 messages required.
Muscle tone and rigidity.
Low frequency spontaneous acivity in Sb2 fibres applies a modest bias to Ia afferent discharge which in turn leads reflexly to a modest contraction - aiding muscle tone. An increase in static g output to Sb2 fibres would result in increased muscle tone via group Ia pathway. An increase in static g output to the chain fibres would increase muscle tone via Ia and II pathways. Hypertonicity could be the result of over-activity in either of these static intrafusal systems. We do not at present know the relative extent of their involvement or interdependence.
Conscious appreciation of position.
Spindles contribute to propriception (sense of postion) and possibly kinaesthesia (sense of movement). Vibration, which selectively excites primaries causes large position errors in a blindfolded subject. Sense of position is not lost, but the length signal is wrongly calibrated. The secondary sensory endings provide a much better length signal than the primary ending.
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