Different fusimotor reflexes from the ipsi- and contralateral hind limbs of the cat assessed in the same primary muscle spindle afferents

Experiments were performed in forty-five cats anaesthetized with alpha-chloralose. The aim of the study was to investigate a sample of primary muscle spindle afferents from triceps muscle with respect to their fusimotor reflex control from ipsi- as well as contralateral hind limb. Primary muscle spindle afferents of the triceps surae muscle were recorded from the mean rate of firing and the modulation of the afferent response to sinusoidal stretching of the triceps surae muscle was determined. Test measurements were made during tonic stretch of the ipsilateral PBSt, contralateral PBSt, contralateral triceps muscle or during extension of the intact contralateral hind limb. Control measurements were made with ipsi- and contralateral PBSt as well as contralateral triceps muscles relaxed and with contralateral hind limb in resting position. The occurrence and types of fusimotor effects were assessed by comparing test to control responses. The main finding of the present investigation was the great variability in type and size of the fusimotor effects evoked by different ipsi- and contralateral reflex stimuli. Both ipsi- and contralateral stimulations gave rise to predominantly dynamic, predominantly static or mixed static and dynamic fusimotor reflexes. In the same preparation, a given reflex stimulus often caused different reflex responses in different triceps surae primary spindle afferents. In the same afferent unit, different reflex stimuli usually produced fusimotor effects which differed from each other in type and/or size. In general, contralateral whole limb extension and stretch of contralateral PBSt muscles were more potent as reflex stimuli than stretch of the ipsilateral PBSt muscle. Stretch of the contralateral triceps surae muscle was, but for a few afferent units, ineffective as reflexogenic stimulus. It is concluded that the individualized receptive profiles of the primary muscle spindle afferents, which have been postulated in earlier investigations where the effects of different stimuli have been investigated on different cell populations, still seems to hold good when the stimuli are tested on the same units. The individuality of the receptive profiles of gamma-motoneurones is discussed in relation to different motor control hypotheses.