SUMMARY1. The behaviour of nuclear bag and nuclear chain intrafusal fibres in isolated cat muscle spindles with a blood supply, during stimulation of dynamic y axons, dynamic f axons, or static y axons in ventral root filaments was observed and recorded on still and moving film.2. Most spindles were controlled by one dynamic y axon (sometimes a f axon) and three static y axons, one of which was often non-selective in distribution. A large majority of fusimotor axons controlled one pole of the spindle only.3. Dynamic y and ft axons produced focal contraction in only one of the two nuclear bag fibres in any spindle and this fibre was never activated by static y axons. Maximal tetanic contraction was attained slowly and the primary sensory spiral on this fibre was stretched by a small amount only. This fibre has been named the 'dynamic nuclear bag fibre'.4. Static y axons produced either: (a) focal contraction in the second of the two nuclear bag fibres only; (b) local contraction in the bundle of nuclear chain fibres only; or (c) contraction in one nuclear bag fibre and the nuclear chain fibres together. Maximum tetanic contraction of this nuclear bag fibre stretched its primary sensory spiral considerably and the time to plateau was relatively short. This fibre has been named the 'static nuclear bag fibre'.5. 'Driving' of the Ia afferent discharge could always be produced by non-selective static y axons, frequently by static y axons controlling nuclear chain fibres alone, and was probably due to mechanical oscillation in nuclear chain fibres. It was never produced by dynamic y axons and on one occasion only by a static y axon controlling a nuclear bag fibre alone.6. The conduction velocities of dynamic y and static y axons overlapped extensively, though dynamic y axons were absent from the lower end, and I. A. BOYD AND OTHERS static y axons innervating nuclear chain fibres only were absent from the upper end, of the range of velocities.7. The observations are correlated with spindle structure and histochemistry. Dynamic and static nuclear bag fibres are shown to correspond with 'bag1 fibres' and 'bag2 fibres', respectively (Ovalle & Smith, 1972).8. The possible origin of the dynamic and static actions of fusimotor axons and the role of the dynamic and static intrafusal systems in motor control are discussed.

No abstract

4. The muscle membrane beneath both ma and mb plates was smooth, or had a few wide, shallow folds; me plates usually had wide, shallow subjunctional folds; numerous deep, narrow folds were characteristic of the md plate. The length of unmyelinated pre-terminal axon or the number of sole plate nuclei were not useful diagnostic features.5. Obvious foci of sarcomere convergence in the capsular sleeve region of dynamic bag1 and static bag2 fibres coincided with the location of motor plates. Additional contraction foci were observed in the extracapsular region of dynamic bag1 fibres where there was no motor innervation; contraction occurs principally in the outer half of these fibres. No foci of contraction or motor plates were observed in the extracapsular region of static bag2 fibres; contraction in these fibres is typically mid-polar. 6. In some poles local contraction of chain fibres centred on the location of me plates. In others, very localized contraction occurred distal to the sites of ma plates.