Evidence from the use of vibration during procaine nerve block that the spindle group II fibres contribute excitation to the tonic stretch reflex of the decerebrate cat.

1. Experiments have been performed to test the hypothesis that the group II fibres from the secondary endings of the muscle spindle provide an excitatory contribution to the tonic stretch reflex of the decerebrate cat. They have consisted of studying the effect of fusimotor paralysis by procaine, applied to the muscle nerve, on the reflex response to the combined stimuli of stretch (5-9 mm at 5 mm/sec) and of high-frequency vibration (100-150 Hz, 150 mum).2. The reflex response to the combined stimuli was found to be paralysed in two distinct stages which paralleled those of the ordinary stretch reflex described earlier. The two phases of paralysis may be attributed to an early paralysis of the gamma efferents followed by a later paralysis of the Ia afferents and alpha motor fibres. However, the Ia discharges elicited by the combined stimuli, unlike those elicited by simple stretch, should have remained unchanged on gamma efferent paralysis since the Ia firing frequency may be presumed to have been clamped at the vibration frequency by the occurrence of one-to-one ;driving'. The early reduction of the response to the combined stimuli may thus be attributed to the removal of a stretchevoked autogenetic excitatory input other than that long known to be provided by the Ia pathway. This supports the view that the spindle group II fibres have such an action, since their firing will be appropriately reduced on gamma efferent paralysis by removal of their pre-existing fusimotor bias; there is no evidence for the existence of any other group of fibres with the right properties.3. Recording of compound action potentials and of single units confirmed the great sensitivity of the gamma efferents to procaine but showed that the group II fibres were nearly as resistant as the Ia fibres and alpha motor fibres.4. The reliability of one-to-one driving of the Ia discharges by the vibration was tested in control experiments in which the reflex was elicited by an asymmetrical vibratory waveform with a rapid rising phase (1.5 or 1.9 msec at 140 Hz) and a slower falling phase. Recordings from single units showed that the use of this wave form greatly diminished any tendency to double driving (2 spikes/cycle of vibration) during the dynamic phase of stretch and never elicited it during the static phase of stretch when the reflex measurements were made. These ;pulsed' vibrations elicited reflex contractions which were of the same general size and which were paralysed in the same two phases by procaine as those elicited by sinusoidal vibrations. This eliminates the possibility that the early phase of paralysis might have been due to conversion of the pattern of Ia firing from double to single driving on gamma efferent paralysis.5. Wedensky inhibition of the afferent fibres could not be held responsible for the early phase of paralysis.6. The results are taken to strengthen the hypothesis that the spindle group II fibres contribute excitation rather than inhibition to the stretch reflex. The particular support derived from the present experiments is that all measurements of the size of the reflex at various times were made with the muscle at the same length so that the findings cannot be attributed to the tension-length properties of muscle. The detailed mechanism of the excitation, however, remains to be established and certain of the present findings suggest that it may not be a direct one.