The effects on primary and secondary spindle afferents of fusimotor reflex during ramp and sinusoidal muscle stretch

Summary Tetanic stimulation of ipsilateral Triceps surae afferents leads to a reflex increase in the initial burst from the primary spindle afferents during ramp stretch in both intercollicular decerebrate and spinal preparations. The dynamic index does not change significantly.Slow, low-amplitude sinusoidal muscle stretches (1.6 Hz, 0.55 mm peak-peak) disclose an increased dynamic sensitivity of primary afferents during reflex activation. The same activation in the statically adapted muscle results in a great many cases in little or no increase in spindle discharge rate.In the frequency-stretch relation, the discharge rates of primary afferents in the spinal cat and of secondaries in decerebrate animals are greatly increased by combined fusimotor activation and low-amplitude sinusoidal stretch.We conclude that both static and dynamic motoneurons are implied in this reflex.