Reflex activation of dynamic fusimotor neurons by natural stimulation of muscle and joint receptor afferent units

Summary

The reflex control of fusimotor neurons to triceps surae of the cat has been investigated by natural stimulation of ipsilateral posterior biceps-semitendinosus (PBSt) by tonic stretch, or of the contralateral hindlimb by full extension of its principal joints, or by a combination of these two forms of stimulation. The induced fusimotor activity was indirectly recorded by monitoring changes in responses, during sinusoidal stretching at 1 Hz 1 mm, of muscle spindle primary afferent units from triceps. A method is described which permits, first, the quantitative assessment of the size of the reflex and, second, the characterisation of the induced activity as involving dynamic, static or both types of fusimotor neurons (mixed).

Fusimotor reflexes could be induced in non-spinalised or spinalised preparations with both ipsilateral and contralateral stimulation. Stimulation of the contralateral hindlimb evoked fusimotor reflexes which usually were purely dynamic or mixed. Also, both joint and muscle afferents could elicit such reflex activity when they were selectively activated. Tonic stretch of ipsilateral PBSt provoked fusimotor reflexes to triceps which, however, were less potent and less uniform: in two thirds of the cases they were dynamic or mixed, and in one third they were static. Simultaneous ipsilateral and contralateral stimulation revealed two patterns of reflex interaction: tonic stretch of PBSt could enhance or inhibit the reflex excitation from the contralateral hindlimb. The reflex activation of fusimotor neurons occurred without concomitant activation of skeletomotor units. Thus the concept of rigid linkage of skeletomotor and fusimotor activity seems not to be a general rule as far as dynamic fusimotor neurons are concerned.