Evidence of Positive Force Feedback Among Hindlimb Extensors in the Intact
Standing Cat.
Pratt, Carol A.
Department of Neurology and Robert S. Dow Neurological Sciences Institute
Good Samaritan Hospital and Medical Center Portland, OR 97209.
APStracts 2:0020N, 1995.
SUMMARY AND CONCLUSIONS
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 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
Received 17 March 1994; accepted in final form 23 February 1995.
APS Manuscript Number J125-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 3 April 1995.