Further evidence that the Golgi tendon organ monitors the activity of a discrete set of motor units within a muscle

Summary

The model of tendon organ activation proposed by Houk and Henneman (1967) has been tested by considering two of its implicit predictions: (1) that only a select group of motor units within a muscle can provide an adequate stimulus to a given tendon organ; and (2) that due to the presence of motor units that can “unload” a given tendon organ, its response to whole muscle contraction can be less vigorous than that to contraction of just the excitatory motor units alone. Tendon organ afferents from the soleus muscle of the cat were functionally isolated from dorsal root filaments. The L₇ and S₁ ventral roots were split into 25 to 50 “natural” subdivisions, each of which was stimulated at 50 to 100 Hz for 1.5 s. The subdivided filaments were segregated into two groups; those which when stimulated elicited discharge from the Ib afferent (“excitatory” filaments) and those which did not (“non-excitatory” filaments). In ten of eleven experiments, it was found that when all the excitatory filaments were separated out, concurrent stimulation of all the non-excitatory filaments failed to excite the tendon organ, even when these filaments generated over 90% of the muscle's peak tetanic tension. The response of a tendon organ to stimulation of just its excitatory filaments often exceeded its response to whole muscle stimulation. However, several exceptions to this finding were observed, indicating that some tendon organs are affected by “off-line” forces. In general, the present results lend strong support to the Houk and Henneman model.