Fusimotor after-effects on responses of primary endings to test dynamic stimuli in cat muscle spindles.

Conditioning stimulation of individual dynamic fusimotor axons, either gamma or beta, leaves after-effects which enhance the responses of primary endings to test stimulation of the same axon applied during slow ramp stretch. These after-effects have a long duration, persisting well over 5 min, but are abolished by stretch of large amplitude. The dynamic after-effects also enhance frequencygrams elicited by low-frequency repetitive stimulation during slow ramp stretch, causing single stimuli to become much more effective. When several dynamic axons to the same spindle are isolated, conditioning stimulation of one leaves an after-effect to test stimulation of itself and of all other dynamic axons. When two dynamic axons are used for conditioning stimulation, facilitation or occlusion can be demonstrated in their interaction, indicating that they converge on the same intrafusal element. Dynamic after-effects persist during background static fusimotor activity of considerable amplitude, suggesting that static and dynamic actions are quite independent. Dynamic after-effects appear to result from residual changes in the bag fibre, probably from a persistent increase in the number of cross-bridges between thick and thin filaments. These after-effects produce a large increase in the response of primary endings to dynamic fusimotor activity and probably have an important functional role.