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

Abstract

SUMMARY1. 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 (1P2 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 ,um, half peak-to-peak), but not to large (1000 jtm), 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,um) the reduction of sensitivity caused by background movement depended on the speed of this movement. For the highest velocity studied (1P6 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 I a 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 crossbridges (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

Coding of pulsatile motor output by human muscle afferents during slow finger movements.

“…It may be questioned whether the responses to discontinuities found in the present study were dependent on the small-range high sensitivity as studied in the cat. The average step size of the 8-10 Hz discontinuities in the present study would be of the order of 0-13 mm in terms of tendon excursion (Edin & Vallbo, 1990a) (Baumann & Hulliger, 1991). Hence it is far from clear that the responses to the discontinuities of the present study are dependent on the high sensitivity range of the muscle spindles.…”

Temporal Development of Anticipatory Reflex Modulation to Dynamical Interactions During Arm Movement

“…The peripheral effects of movement speed and corresponding afferent state, such as muscle spindle discharge regulated by the fusimotor system (Prochazka 1989), are also unlikely to account for the observed reflex modulation. This is because 1) we observed reflex modulation in the temporal set but not in the spatial set regardless of the identical changes in movement speeds in both sets, and 2) several studies have shown that as the speed of voluntary contraction increases, muscle spindle discharge rather decreases (Baumann and Hulliger 1991;Burke et al 1978). Second, the unchanged reflex amplitudes against the SPD (Fig.…”

A numerical simulation of muscle spindle ensemble encoding during planar movement of the human arm: correlation sensitivity and parameter dependence

“…For example, Baumann and Hulliger (1991) report data indicating that the phenomenon of gain-compression is greatly reduced for movements which fail to satisfy highly restrictive length-and velocity-related criteria. On the basis of current evidence, therefore, we conclude that while such nonlinearities could, conceivably, alter the correlations we report it is unlikely that they will lead to significant reductions in the magnitudes of the observed correlations.…”