The Golgi Tendon Organ

Figure 1: Tendon diagram

Overview
The Golgi tendon organ (GTO) is the next most prominent proprioceptor after the muscle spindle.  Each GTO receptor responds nonlinearly to the active tension produced by approximately eleven muscle fibers, each from a separate motor unit (MU) whose muscle fiber type and tension producing capabilities may be quite different from adjacent fibers.  Based on the arrangement of collagen fibers within the GTO capsule, a single GTO receptor can be expected to produce activity that is monotonically but nonlinearly related to the tension of the inserting muscle fibers.  The activity appears to depend on the order in which inserting muscle fibers are recruited (Gregory and Proske, J Physiol 295:251-262, 1979).  This dependency manifests itself as "self-adaptation" and "cross-adaptation" in which the dynamic response of GTO to single fiber activation is decreased as the result of activation of same or different fiber (belonging to the same GTO) shortly beforehand.

The information available to the CNS is derived from the aggregate activity of about 100 GTO receptors in a typical muscle.  While the individual GTOs may be strongly influenced by the vagaries of muscle fiber sampling and recruitment, the aggregate activity may be a more reliable indicator of total muscle force.  That will depend on factors such as the distributions of GTOs and MU types within the muscle (often highly heterogeneous) and the orderliness of MU recruitment (usually quite stable).  To quantify these effects, we created a statistical model of the distributions of muscle fibers belonging to individual MUs and of GTO receptors in the medial gastrocnemius muscle of a cat.  We incorporated a realistic muscle model to calculate the tension generated by individual type-specific muscle fibers responding to recruitment rules based on MU size in order to determine the amount of force each GTO would be experiencing and the afferent activity it would be contributing.  We are now using the model to explore the relationship between aggregate GTO activity and total muscle force, as well as how that relationship may change by various distributions of receptors, MUs and recruitment patterns.

Questions/Comments
For questions or comments regarding the model of Golgi tendon organs, please contact Gerald E. Loeb (gloeb@usc.edu).


Alfred Mann Institute University of Southern California