Static and dynamic fusimotor action on the response of spindle primary endings to sinusoidal stretches in the cat

Abstract

In anaesthetized cats, the discharge of single spindle primary endings of flexor and extensor muscles was studied during sinusoidal stretches of the muscle (3–4 mm peak-to-peak, 1–5 Hz). In the response to each stretch, two main components could be distinguished; they were related respectively to the speed and to the amplitude of lengthening. By analogy with the usual definitions of the ‘dynamic index’ (d) and ‘position sensitivity’ (s) during ramp stretches of the same amplitude at a velocity of 35–60 mm/sec, a ‘sinusoidal dynamic index’ (D) and a ‘sinusoidal position sensitivity’ (S) were defined. Stimulation of dynamic fusimotor fibres increased D and d, S and s, while stimulation of static fusimotor fibres decreased D and d and had a less consistent effect on S and s. The total frequency variation during a sinusoidal stretch was always increased by a dynamic fusimotor action while it could be either decreased or increased by a static fusimotor action, depending on the stretch frequency and on the rate of fusimotor stimulation. These results were obtained in various muscles, without any significant difference related to their flexor or extensor function. It is concluded that in order to identify a fusimotor effect as static or dynamic during sinusoidal muscle stretch, it is necessary to use, as here, stretch parameters which elicit a dynamic component in the primary afferent response.