Tendon organs of cat medial gastrocnemius: responses to active and passive forces as a function of muscle length.

The responses of 13 Golgi tendon organs to graded force development of 29 motor units in medial gastrocnemius of the cat have been studied in five experiments. Of the 13 tendon organs, 11 were responsive to passive stretch within the physiological range of muscle length and 5 were "spontaneously" active at very short lengths where no passive tension could be recorded. The relationship between passive force and the firing rates of the various afferents ranged from a linear one to a power relation (Y = Axb + c) with b, a widely varying exponent. Results support the general conclusion that although many Ib afferents respond to passive force within the physiological range of muscle stretch, this form of stimulus is not a particularly effective one. The statis responses of Golgi tendon organs to active force development produced by single motor units was studied at different muscle lengths. In all cases the apparent sensitivity (change in firing rate per active force developed) decreased as muscle length approached Lo. The static responses of Golgi tendon organs to force developed by single motor units were also studied during fatiguing contractions. The data suggest a sigmoid relationship between force developed at the tendon and the Ib response. The collective response of all 13 tendon organs to active and passive forces at different muscle lengths was also examined. This analysis offered further support for the viewpoint that active motor unit contractions provide themost significant excitatory input to tendon organs and that changes in passive force during muscle stretch have comparatively little effect on the collective tendon organ response. The interaction between active and passive force inputs to the Golgi tendon organs is discussed in relation to the complicated nature of the relationship between forces measured at the tendon and those acting within the receptor capsule. When these complications were taken into account it was possible to explain the differences in responsiveness of a given tendon organ to active contraction of several motor units and to passive force in terms of a single force-firing rate curve for the receptor. It is concluded that changes in the force of contraction of single motor units result in relatively small changes in Ib afferent firing and that during normal muscle contractions, changes in the number of motor units acting on a single receptor must produce far more significant changes in firing rate than changes in the amount of force developed by any single unit. Changes in dynamic Ib sensitivity to single motor unit contractions are also shown to depend on length and in a similar way to the changes in static Ib sensitivity. During fatiguing contractions, a sigmoid relation was found between the dynamic Ib response and the rate of force development by single motor units.