Fusimotor reflexes in triceps surae muscle elicited by stretch of muscles in the contralateral hind limb of the cat.

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Abstract
Experiments were performed on twenty-one cats anaesthetized with .alpha.-chloralose. The aim of this study was to investigate the reflex effects on triceps surae and plantaris fusimotor neurones elicited by tonic stretch of the contralateral posterior biceps and semitendinosus (p.b.s.t.) and the contralateral triceps surae and plantaris muscles, to compare these effects with the effects evoked by flexion or extension of the intact contralateral hind limb (Appelberg, Hulliger, Johansson and Sojka, 1984) anfd to clarify the interactions between the reflexes from contralateral and ipsilateral muscles. With stretch of the contralateral p.b.s.t. ten out of eighty-four primary afferents (11.9%) showed predominantly dynamic reflexes (six out of forty-one in spinalized preparations: 14.6%), twenty-two (26.2%) showed mixed or predominantly static effects (one spinalized:2.4%) and fifty-two units (61.9%) showed no effect (thirty-four spinalized:81.0%). The reflex could be reproduced by electrical stimulation of the cut contralateral p.b.s.t. nerve either at group II or at group III strength. With stretch of the contralateral triceps and plantaris muscles seventy out of seventy-six (92.1%) primary muscle spindle afferents showed no effect and six (7.9%) mixed or predominantly static reflex effects. In general, the reflex effects were not accompanied by detectable electromyographic (e.m.g.) activity in the ipsilateral triceps and plantaris (recorded with surface or needle electrodes), indicating that the reflexes mainly involved .gamma.-motoneurones. The difference in efficacy between contralateral flexor (p.b.s.t.) and extensor (triceps and plantaris) muscles seems to be in accordance with the response pattern found with extension or flexion of the intact contralateral hind limb (Appelberg et al. 1984). A comparison was made between the reflexes elicited by extension of the contralateral hind limb (Appelberg et al. 1984) and stretch of the contralateral p.b.s.t. muscles, and it was concluded that the higher incidence of dynamic reflex effects with the whole limb extension was due to stimulus-correlated activation of receptors (skin, joint and muscle) which were not stimulated in the present study. Facilitatory, summatory and inhibitory interactions were found between fusimotor reflexes elicited by stretch of the ipsilateral p.b.s.t., the contralateral p.b.s.t. and the contralateral triceps and plantaris muscles. Excitatory and inhibitory reflex pathways from the two contralateral muscles seemed to interact independently of each other with the reflexes from the ipsilateral muscle. Also, in many of the interactions a shift in balance between dynamic and static action was seen, indicating a degree of independence between pathways to dynamic and static fusimotor neurones.