EVIDENCE OF POSITIVE FORCE FEEDBACK AMONG HINDLIMB EXTENSORS IN THE INTACT STANDING CAT

1. The functional organization of heterogenic reflexes produced by activation of extensor force receptors (Golgi tendon organs) was studied in intact cats during stationary stance. Intramuscular stimulation (200 Hz, 20 ms) of hindlimb extensor muscles via chronically implanted electrodes was used to evoke weak muscle contractions and naturally activate Golgi tendon organ Ib afferents while cats stood unrestrained with each paw on a moveable triaxial force plate. 2. Intramuscular stimulation of every hindlimb extensor muscle tested in this study evoked excitatory responses that were widely distributed among hindlimb extensor muscles. Source and target specializations in the functional organization of this positive force feedback system were also observed. For example, stimulation of ankle extensors typically excited extensors and flexors at the ankle and hip (but not knee), whereas stimulation of hip extensors typically excited only extensors at all three joints. In addition, intramuscular stimulation of either lateral (LG) or medial (MG) gastrocnemius consistently inhibited soleus while exciting other extensors at the ankle and more proximal joints. 3. The electromyographic (EMG) reflex responses described above are attributed to the natural (via muscle contraction) activation of extensor group Ib afferents. Direct activation of intramuscular afferents by the stimulus was unlikely because there was no evidence that Ia afferents, which have the lowest electrical thresholds, were activated. Both the observed inhibition of the synergist, soleus, and the excitation of the antagonist, tibialis anterior, produced by gastrocnemius stimulation are opposite to the reflex effects that would be produced at the ankle by activation of gastrocnemius Ia afferents. In addition, the onsets of EMG responses occurred 25-30 ms after the onset of stimulation; EMG responses produced by direct activation of Ia afferents would have latencies less than or equal to 10 ms in hindlimb muscles. The present results complement evidence from locomotion studies that extensor Golgi tendon organ afferents are an important source of the positive force feedback signal during stance. 4. The pattern of reflex responses produced by intramuscular stimulation of the left LG or MG were similar to muscle responses evoked in the implanted left hindlimb when it was rapidly loaded as a result of dropping (40 mm) the platform under either the right hindlimb or the left forelimb: widespread excitation of hindlimb extensors, with the exception of soleus, and coactivation of extensors and flexors at the hip and ankle. These results suggest that inputs from extensor Ib afferents contribute to the organization of postural responses evoked by perturbations of the support surface as well as a reflex reinforcement of weight support during stance.