Excitation of soleus/gastrocnemius gamma-motoneurones 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 fibres in the posterior articular nerve (PAN) of the cat knee joint facilitated gamma-motoneurone firing in filaments of the lateral gastrocnemius/soleus (LGS) nerve. Repetitive (0.5-2.0 Hz) electrical stimulation of the PAN, produced a short latency increase of gamma-motoneurone firing with a latency ranging from 8 to 35 ms. The shortest of these delays suggests a segmental link between group II knee joint afferents and lateral gastrocnemius/soleus gamma-motoneurones. This facilitation was diminished or abolished if the first stimulus was preceded by a conditioning stimulus which excited the group IV afferents in the PAN. This depression of facilitation was dependent on the interval between the test and conditioning stimuli and the interval which produced the greatest 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 that it was not due to direct inhibition of the motoneurones.