The responses of primary spindle afferents to fusimotor stimulation at constant and abruptly changing rates.

SUMMARY1. Simultaneous recordings were made from fusimotor axons in the central ends offilaments ofthe masseter nerve, and from masseter and temporalis spindle afferents in the mesencephalic nucleus of the fifth cranial nerve in lightly anaesthetized cats.2. Fusimotor and a-motor units in the masseter nerve were differentiated on the basis of their response to passive ramp and hold stretches applied to the jaw. Spindle afferents were identified as primary or secondary according to their dynamic index after administration of suxamethonium.3. The activity of a given fusimotor unit during reflex movements of the jaw followed one oftwo distinct patterns: so-called 'tonic' units showed a general increase in activity during a movement, without detailed relation to lengthening or shortening, while 'modulated' units displayed a striking modulation of their activity with shortening, and were usually silent during subsequent lengthening.4. Comparison of the simultaneously recorded fusimotor and spindle afferent activity suggests that modulated units may be representative ofa population of static fusimotor neurones, and tonic units of a population of dynamic fusimotor neurones.5. In these lightly anaesthetized animals, both primary and secondary spindle afferents showed increased firing during muscle shortening as well as during lengthening. This increase during shortening is not usually seen in conscious animals and reasons are given for the view that it is due to greater depression of a-motor activity than of static fusimotor activity during anaesthesia.6. The results are discussed in relation to the theories of 'a-y co-activation' and of 'servo-assistance'; and it is suggested that static fusimotor neurones provide a 'temporal template' of the intended movement, while dynamic fusimotor neurones set the required dynamic sensitivity to deviations from the intended movement pattern.

Section: Discussionmentioning

confidence: 99%

Interpretation of fusimotor activity in cat masseter nerve during reflex jaw movements.

Gottlieb¹,

Taylor²

1983

“…Normally static action should reduce and dynamic action should increase the afferent's dynamic sensitivity for length changes, even if fusimotor activity varies with time. Further, the direct excitatory action of static fibres is known to be considerably stronger than that of dynamic fibres (Hulliger, 1979). Hence, when static action prevails, the spindle is likely to act as a summer of reflex-shaped fusimotor and external length signals, with more 290 weight being given to the neural input.…”

Section: B Appelberg and Othersmentioning

confidence: 99%

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

Appelberg¹,

Hulliger²,

Johansson³

et al. 1983

Self Cite

172

SUMMARY1. The reflex actions evoked by electrical stimulation of group III muscle afferent fibres were investigated with micro-electrode recordings from ninety-three ymotoneurones projecting to hind-limb muscles of cats anaesthetized with chloralose. For seventy-eight of the ninety-three y-cells the frequency of occurrence and types of effects mediated via group II and group III muscle fibres were compared.2. Seventy-seven of the cells tested at intensities which excited group III and seventy-five of the cells tested at intensities which excited both group II and group III afferent fibres were classified as either static or dynamic, using the method of mesencephalic stimulation (Appelberg, 1981).3. The responsiveness of the whole sample of y-motoneurones to inputs from group III muscle fibres was high and comparable to that found with group II fibres.4. It was found that group III muscle fibres acted preferentially on static y-motoneurones. In contrast, group II fibres acted preferentially on dynamic y-motoneurones.5. Both excitatory and inhibitory effects were provoked by stimulation of group III fibres. Generally excitation was more frequent than inhibition.6. A strong dominance of excitation over inhibition was found in flexor muscles, and a weaker prevalence of excitation was also encountered in extensor muscles. This prevalence of excitation in extensor y-motoneurones is in contrast to the striking predominance ofgroup III-evoked inhibition ofextensor a-motoneurones as described by the flexion reflex afferents concept. 7. A comparative survey is also given of the patterns of responses elicited in individual posterior biceps-semitendinosus and gastrocnemius-soleus y-cells by stimulation of group II and group III fibres. These data further corroborate the view that reflexes from high-threshold muscle afferent fibres to y-motoneurones are organized differently from those to a-motoneurones.8. The functional implications of these findings are discussed. It is proposed that the pools of y-motoneurones should be considered as integrative systems intercalated

“…The contribution of random noise to the error term is limited by the amount of averaging and by histogram resolution (cut-off at repeat frequency x No. of bins/2), and for recordings from I a afferents it tends to slightly increase with repeat frequency (reflecting the power distribution of noise in these recordings; for documentation see Hulliger, 1979, on a closely related application of r.m.s. error estimates).…”

Section: Data Collection and Analysismentioning

confidence: 99%

“…For a given response profile the r.m.s. error is proportional to the residual harmonic power contained in all Fourier coefficients beyond the fundamental (fitted sine; Hulliger et al 1977 a;Hulliger, 1979). The r.m.s.…”

Section: Data Collection and Analysismentioning

confidence: 99%

The dependence of the response of cat spindle Ia afferents to sinusoidal stretch on the velocity of concomitant movement.

Baumann

Hulliger

1991

Self Cite

SUMMARY1. The responses of de-efferented soleus muscle spindle primary afferents to 1 Hz sinusoidal stretches, which were superimposed on triangular background movements of intermediate amplitude (1P2 mm, half peak-to-peak) and widely varying speed, were recorded in anaesthetized cats.2. Compared with control responses to the same sinusoids applied at fixed mean muscle length, the sensitivity to small (50 and 100 ,um, half peak-to-peak), but not to large (1000 jtm), sinusoidal movements was dramatically reduced during concomitant stretching, unless the background movements were extremely slow (well below 0 005 resting lengths per second).3. For small stretches (50 and 100,um) the reduction of sensitivity caused by background movement depended on the speed of this movement. For the highest velocity studied (1P6 mm s-1, corresponding to about 0 03 resting lengths per second) sensitivity dropped to below 10% of the control values.4. The functional implication is that the sensitivity of spindle I a afferents to small irregularities of voluntary movements (of any but the slowest speeds) may well be very much lower than it has hitherto been inferred from the striking sensitivity to minute disturbances at fixed mean muscle length. The present finding clearly puts extra demands on the gain of any spinal or central reflex actions of the sensory feedback from spindle afferents.5. The effect is interpreted in terms of the widely accepted cross-bridge hypothesis of spindle small-movement sensitivity. The result suggests that in de-efferented intrafusal muscle fibres, which are subjected to imposed stretches, connected crossbridges (conveying a friction-like property to the contractile fibre poles) may exist not only in a state of permanent attachment, but also in a dynamic equilibrium