Effects of triceps stretch by ankle flexion on intact afferents and efferents of gastrocnemius in the decerebrate cat.

We describe the effects on group I and II afferents as well as alpha- and gamma-efferents of gastrocnemius muscles elicited by stretch and release of the triceps surae muscle performed by a dorsiflexion movement, a maintained dorsiflexion and release of the ankle joint. The experiments were made in decerebrated cats in which the neural loops between the muscles and the spinal cord were intact. Multi-unit discharges of each kind of fibre were obtained, electronically on-line, from two monopolar electrodes 4 mm apart on a small branch of the gastrocnemius lateralis or medialis nerve. Sensory and motor impulses were separated according to the opposite directions of their propagation, then both groups were further subdivided according to conduction velocity. The stretch reflex induced by ankle flexion was associated in eighteen out of nineteen experiments with reflex changes in the activity of the gamma-axon population of the gastrocnemius nerve branch. Facilitatory, depressor and biphasic (facilitatory-depressor) effects were observed, the particular type seeming to depend on the level of gamma tone and gamma-motoneurone size. Flexion release led to cessation of the alpha stretch reflex and to a rebound firing of the gamma-axon population. The lengthening reaction and the clasp-knife phenomenon occurred in certain preparations and were associated with reflex inhibition of the gamma-motoneurones. The over-all responses of group I fibres to triceps stretch showed dynamic and static components, whereas those of the group II fibres were, except in two examples, almost devoid of dynamic sensitivity. During release of stretch and as a result of the simultaneous rebound in gamma activity, the afferent discharges showed a very short pause, no pause, or even a rebound. The origins of the discharges of groups I and II muscle afferents and the factors (muscle length, active and passive muscle tension, reflex changes in fusimotor drive) influencing the activity of the receptors involved are considered as far as possible. Some aspects of the mechanisms of the fusimotor reflex are discussed.