EXCITATION OF SOLEUS GASTROCNEMIUS GAMMA-MOTONEURONS BY GROUP-II KNEE-JOINT AFFERENTS IS SUPPRESSED BY GROUP-IV JOINT AFFERENTS IN THE DECEREBRATE, SPINALIZED CAT/

Electrical stimulation sufficient to excite group II/III afferent fibr es in the posterior articular nerve (PAN) of the cat knee joint facili tated gamma-motoneurone firing in filaments of the lateral gastrocnemi us/soleus (LGS) nerve. Repetitive (0.5-2.0 Hz) electrical stimulation of the PAN, produced a short latency increase of gamma-motoneurone fir ing with a latency ranging from 8 to 35 ms. The shortest of these dela ys suggests a segmental link between group II knee joint afferents and lateral gastrocnemius/soleus gamma-motoneurones. This facilitation wa s diminished or abolished if the first stimulus was preceded by a cond itioning stimulus which excited the group IV afferents in the PAN. Thi s depression of facilitation was dependent on the interval between the test and conditioning stimuli and the interval which produced the gre atest depression was consistent with the effect being due to group IV afferents. Inhibition of the facilitation did not significantly alter the on-going discharge rate of the gamma-motoneurones. This suggests t hat it was not due to direct inhibition of the motoneurones.