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

1. The functional organization of heterogenic reflexes produced by act ivation of extensor force receptors (Golgi tendon organs) was studied in intact cats during stationary stance. Intramuscular stimulation (20 0 Hz, 20 ms) of hindlimb extensor muscles via chronically implanted el ectrodes was used to evoke weak muscle contractions and naturally acti vate Golgi tendon organ Ib afferents while cats stood unrestrained wit h each paw on a moveable triaxial force plate. 2. Intramuscular stimul ation of every hindlimb extensor muscle tested in this study evoked ex citatory responses that were widely distributed among hindlimb extenso r muscles. Source and target specializations in the functional organiz ation of this positive force feedback system were also observed. For e xample, stimulation of ankle extensors typically excited extensors and flexors at the ankle and hip (but not knee), whereas stimulation of h ip 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 e xtensors at the ankle and more proximal joints. 3. The electromyograph ic (EMG) reflex responses described above are attributed to the natura l (via muscle contraction) activation of extensor group Ib afferents. Direct activation of intramuscular afferents by the stimulus was unlik ely because there was no evidence that Ia afferents, which have the lo west electrical thresholds, were activated. Both the observed inhibiti on of the synergist, soleus, and the excitation of the antagonist, tib ialis anterior, produced by gastrocnemius stimulation are opposite to the reflex effects that would be produced at the ankle by activation o f gastrocnemius Ia afferents. In addition, the onsets of EMG responses occurred 25-30 ms after the onset of stimulation; EMG responses produ ced by direct activation of Ia afferents would have latencies less tha n or equal to 10 ms in hindlimb muscles. The present results complemen t evidence from locomotion studies that extensor Golgi tendon organ af ferents are an important source of the positive force feedback signal during stance. 4. The pattern of reflex responses produced by intramus cular stimulation of the left LG or MG were similar to muscle response s evoked in the implanted left hindlimb when it was rapidly loaded as a result of dropping (40 mm) the platform under either the right hindl imb or the left forelimb: widespread excitation of hindlimb extensors, with the exception of soleus, and coactivation of extensors and flexo rs at the hip and ankle. These results suggest that inputs from extens or Ib afferents contribute to the organization of postural responses e voked by perturbations of the support surface as well as a reflex rein forcement of weight support during stance.