The discharge of cat tendon organs during unloading contractions

Mathematical models of proprioceptors. II. Structure and function of the Golgi tendon organ

We developed a physiologically realistic mathematical model of the Golgi tendon organ (GTO) whose elements correspond to anatomical features of the biological receptor. The mechanical interactions of these elements enable it to capture all salient aspects of GTO afferent behavior reported in the literature. The model accurately describes the GTO's static and dynamic responses to activation of single motor units whose muscle fibers insert into the GTO, including the different static and dynamic sensitivities that exist for different types of muscle fibers (S, FR, and FF). Furthermore, it captures the phenomena of self- and cross-adaptation wherein the GTO dynamic response during motor unit activation is reduced by prior activation of the same or a different motor unit, respectively. The model demonstrates various degrees of nonlinear summation of GTO responses resulting from simultaneous activation of multiple motor units. Similarly to the biological GTO, the model suggests that the activation of every additional motor unit to already active motor units that influence the receptor will have a progressively weaker incremental effect on the GTO afferent activity. Finally, the proportional relationship between the cross-adaptation and summation recorded for various pairs of motor units was captured by the model, but only by incorporating a particular type of occlusion between multiple transduction regions that were previously suggested. This occlusion mechanism is consistent with the anatomy of the afferent innervation and its arrangement with respect to the collagen strands inserting into the GTO.

Effect of eccentric muscle contractions on Golgi tendon organ responses to passive and active tension in the cat

To investigate the possibility of a peripheral contribution to the perturbations of force sensation reported to occur after eccentric exercise, responses to passive and active tension were recorded from Golgi tendon organs in the medial gastrocnemius muscle of the anaesthetised cat, before and after a series of eccentric contractions. After the eccentric contractions, nearly all tendon organs commenced firing at a shorter muscle length during slow passive stretch than before, probably because of a rise in whole muscle passive tension. There was a small drop in the sensitivity to incremental tension, but no mean change in tension threshold. Following the eccentric contractions, there was a small, but not significant, increase in tendon organ sensitivity to active tension, which was graded using a method of optimised, distributed stimulation of divided ventral roots. Sensitivity was estimated as the mean response over a range of tensions and as the change in discharge rate in response to incremental tension. The experiments provided the opportunity of comparing tendon organ sensitivities to graded passive and active whole muscle tension. In agreement with previous work in which whole muscle nerve stimulation was employed, little difference was found. It was concluded that the peripheral contribution to perturbations of force perception after eccentric exercise is likely to be small and that the centrally derived sense of effort plays the dominant role. Tendon organs appear to be remarkably reliable in signalling whole muscle tension, whether passive or active, and even after the muscle's force production has been disturbed by fatigue or eccentric exercise.

Unloading of tendon organ discharges by in-series motor units in cat peroneal muscles

1. The discharges from individual Golgi tendon organs of peroneus tertius and brevis muscles were recorded in anaesthetized cats during the isometric contractions of single motor units. Upon combined contractions of several motor units, two sorts of unloading effects were observed. 2. First, the contraction of a motor unit which, by itself, was without action on a tendon organ could produce a reduction in the response of the receptor to one of its activating motor units. Unloading effects exerted by such in-parallel motor units could effectively interfere with the actions of in-series motor units on the receptor. 3. Second, the contraction of a motor unit activating a tendon organ could reduce the response of this tendon organ to the contraction of another of its activating units. This new type of unloading effect, exerted by in-series motor units, was demonstrated by the fact that the simultaneous contraction of both units elicited less discharge from the receptor than the contraction of a single unit. 4. Unfused contractions of a fast-type motor unit eliciting a response in which the tendon organ discharge was driven 1:1 at the frequency of stimulation of the motor unit, could exert unloading actions on the response of the receptor to another motor unit eliciting a higher discharge frequency. 5. In-series unloading actions were exerted not only by fast-type motor units developing large forces, but also by relatively small slow-type motor units. 6. The high incidence of in-parallel and in-series unloading effects suggests that their consequences may be functionally significant. When large numbers of motor units are being recruited in a muscle, unloading effects might result in a limitation of the Ib afferent discharges from this muscle, preventing an excessive increase of autogenetic inhibition.