A study of the encoder properties of muscle-spindle primary afferent fibers by a random noise disturbance of the steady stretch response.

Muscle spindle stretch responses (cat gastrocnemius muscles) were studied when both steady stretch and small near-threshold random stretch determined the Ia impulse sequence. Statistical properties of the inter-impulse-intervals gave some insight into the Ia encoder mechanism. Superimposed random stretch of mean velocities sigma vel below 5 mm s-1 did not change the mean discharge rate, but the width of the Ia interspike interval distribution was clearly increased. Raising the stretch velocity further (sigma vel greater than 5 mm s-1) led to an additional increase in the distribution width, finally reaching values of 0.6 for the coefficient of variation. The shapes of the impulse interval histograms changed from symmetrical to positively skewed ones. The 1st order serial correlation coefficient of the interval sequence shifted to slightly more negative values with increasing sigma vel; on the average, the r1,2-value varied between zero and -0.2. The data were discussed in relation to current ideas on the mechanism of impulse initiation in the Ia terminal ending. They provide evidence that a combination of multiple encoder sites located in the myelinated terminal branches and a separate pathway for large static and small-amplitude dynamic stretch is not very likely. A model is proposed as to how the whole tree of myelinated axons functions as a single encoder site.