Fusimotor reflexes in triceps surae elicited by natural stimulation of muscle afferents from the cat ipsilateral hind limb

Abstract

SUMMARY1. Experiments were performed in forty-one cats anaesthetized with chloralose. 2. The aim of the study was to investigate whether activity in stretch-sensitive muscle receptors may cause reflex effects in fusimotor neurones.3. Activity in fusimotor neurones was studied indirectly by recording from primary and secondary muscle spindle afferents ofthe triceps surae muscle. The mean rate of firing of the afferents as well as either dynamic index (during ramp extension) or modulation (during sinusoidal extension) was determined. This was done under control conditions, with the posterior biceps-semitendinosus muscles relaxed, and under test conditions, with the same muscles extended.4. All together, seventy-one primary afferents were studied quantitatively. Pure or predominantly dynamic effects were observed in twenty-two, pure or predominantly static effects in nine and no statistically significant effects in forty of the units. Amongst seven secondary afferents studied, two showed weak fusimotor activation, the other five were not influenced.5. Electrical stimulation of the posterior biceps-semitendinosus or medial gastrocnemius nerves at group II strength was observed to cause dynamic fusimotor reflexes on a number of occasions.6. The reflex effects observed were, on many occasions, recorded in spinalized preparations.7. The reflex effects were not accompanied by any detectable e.m.g. activity in triceps, as judged from surface e.m.g. recordings. The reflex effects observed are

Recurrent actions on gamma‐motoneurones mediated via large and small ventral root fibres in the cat.

“…However, recent findings ofconsiderable differences between a-and y-motoneurones with regard to reflexes from muscle, skin and joint inputs (Appelberg et al , 1982(Appelberg et al , 1983a make it less likely that a parallelism in a-and y-activity is a general rule. Therefore, if the recurrent inhibitory system acts as a gain regulator, it seems likely that the recurrent control ofy-motoneurones should, at least in part, be specific.…”

“…Thus there seem to be differences between the recurrent control of z-and y-motoneurones, which add to the arguments against the concept of stereotyped a-y linkage (see Appelberg, Hulliger, Johansson & Sojka, 1981, 1982, 1983Johansson, 1981 (Wilson, 1959;Wilson, Diecke & Talbot, 1960;Wilson & Burgess, 1962a, b) is caused by Renshaw cells feeding inhibition to group Ia inhibitory interneurones, resulting in disinhibition of a-cells (Hultborn et al 1971). Since group Ia inhibitory effects seem to be rare or absent in y-motoneurones (Appelberg et al 1983a), the findings of recurrent facilitatory actions must be interpreted in another way.…”

Actions on gamma‐motoneurones elicited by electrical stimulation of group II muscle afferent fibres in the hind limb of the cat.

“…The finding of a prevalence of excitation from group II muscle afferents in dynamic y-motoneurones is also supported by other experimental evidence. Following our initial preliminary account on this aspect of y-cell reflex organization (Appelberg et al 1977), Noth & Thilmann (1980) and Noth (1981) (Appelberg, Hulliger, Johansson & Sojka, 1982).…”

Actions on gamma‐motoneurones elicited by electrical stimulation of group III muscle afferent fibres in the hind limb of the cat.

“…Using natural stimulation of muscle receptor afferents Appelberg, Hulliger, Johansson & Sojka (1979, 1982b have shown that the fusimotor reflexes which so far have only been demonstrated by electrical stimulation, can also operate under more natural conditions. Particularly relevant in the present context is the finding that the reflex activation of triceps fusimotor neurones was sufficiently powerful to drastically alter the responses of triceps primary afferent units to stretching of the parent muscle.…”