Dependence of dynamic response of spindle receptors on muscle length and velocity

The dynamic responsiveness of primary and secondary muscle spindle receptors was studied using large (10 mm) ramp stretches applied to the cat soleus muscle at velocities in the range 0.4-100 mm/s. The authors used decerebrate animals with substantially intact spinal roots in order to preserve spontaneous skeletomotor and fusimotor activity and reflex function; only a few fine filaments of dorsal root were dissected to obtain simultaneous recordings from one to four single afferents. Control observations on deefferented receptors were made after ventral root transection. Length dependence was assessed from dynamic trajectories (plots of average discharge rate versus muscle length during stretch at constant velocity) and velocity dependence from plots of increment in discharge versus velocity at a given stretch amplitude. Superimposed dynamic trajectories starting from different initial lengths (same velocity) were found to converge after the decay of the initial burst, indicating that the postburst response can be characterized in terms of instantaneous length and velocity. Both pre- and postburst portions of the dynamic response were larger at longer initial lengths. The postburn portions of dynamic trajectories of the majority of receptors were reasonably fitted by straight lines. The lines had positive slopes and positive intercepts along the rate axis. At the usual initial length of -15 mm (relative to maximal physiological length) and standard velocity of 1 mm/s, primary endings were distinguished from secondaries by much larger intercepts (six-fold) and somewhat greater slopes (two-fold). The increment in discharge rate associated with stretch to a given muscle length was found to be proportional to approximately the 0.3 power of velocity. This relation applied throughout the 250-fold range of velocities tested in the case of innervated primary endings and throughout most of this range in the case of denervated primary and innervated and denervated secondary endings. Departures occurred at low velocities, whenever discharge fell close to the estimated value of quasi-static response. The slopes of dynamic trajectories were also proportional to the 0.3 power of velocity. Straight-line fits to primary ending dynamic trajectories at different velocities tended to intersect at a common point, the average location of which was -22 mm and 19 impulses/s. The various findings are approximately accounted for by a product relationship between muscle length and the 0.3 power of velocity. For the majority of receptors the length-dependent term can be approximated by an offset straight-line relation. Increased levels of fusimotor activity magnified the dynamic response (probably a γ-dynamic effect) and increased initial discharge rate, without altering the fractional power dependence on velocity. The bulk of these effects were manifested below skeletomotor threshold, with presumed γ-dynamic effects becoming prominent at lower threshold than presumed γ-static effects. The low fractional power dependence on velocity means that a 10-fold increase in velocity results in only a two-fold increase in response, a rather weak dependence on velocity. It is suggested that the large dynamic responses of primary endings are more suited for motion detection than for signaling the precise velocity of stretch. Subtraction of secondary-ending responses would not be expected to yield any greater dependence of velocity.