The dependence of the response of cat spindle Ia afferents to sinusoidal stretch on the velocity of concomitant movement.

1. The responses of de‐efferented soleus muscle spindle primary afferents to 1 Hz sinusoidal stretches, which were superimposed on triangular background movements of intermediate amplitude (1.2 mm, half peak‐to‐peak) and widely varying speed, were recorded in anaesthetized cats. 2. Compared with control responses to the same sinusoids applied at fixed mean muscle length, the sensitivity to small (50 and 100 microns, half peak‐to‐peak), but not to large (1000 microns), sinusoidal movements was dramatically reduced during concomitant stretching, unless the background movements were extremely slow (well below 0.005 resting lengths per second). 3. For small stretches (50 and 100 microns) the reduction of sensitivity caused by background movement depended on the speed of this movement. For the highest velocity studied (1.6 mm s‐1, corresponding to about 0.03 resting lengths per second) sensitivity dropped to below 10% of the control values. 4. The functional implication is that the sensitivity of spindle Ia afferents to small irregularities of voluntary movements (of any but the slowest speeds) may well be very much lower than it has hitherto been inferred from the striking sensitivity to minute disturbances at fixed mean muscle length. The present finding clearly puts extra demands on the gain of any spinal or central reflex actions of the sensory feedback from spindle afferents. 5. The effect is interpreted in terms of the widely accepted cross‐bridge hypothesis of spindle small‐movement sensitivity. The result suggests that in de‐efferented intrafusal muscle fibres, which are subjected to imposed stretches, connected cross‐bridges (conveying a friction‐like property to the contractile fibre poles) may exist not only in a state of permanent attachment, but also in a dynamic equilibrium between stretch‐induced detachment and reattachment. Indirect evidence further suggests that the duration of this disruption and reattachment cycle is of the order of 1 s.