Control of dynamic and static nuclear bag fibres and nuclear chain fibres by gamma and beta axons in isolated cat muscle spindels.

Abstract

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.

Mathematical Models of Proprioceptors. I. Control and Transduction in the Muscle Spindle

“…This allows each intrafusal fiber model to capture the saturation effects that take place at high fusimotor stimulation frequencies as observed in isolated, identified fibers (bag 1 : freq bag 1 ϭ 100 pps; bag 2 : freq bag 2 ϭ 100 pps; chain: freq chain ϭ 150 pps; Boyd 1976). In addition, the model incorporates the different temporal properties of intrafusal fiber responses that were measured previously for individual intrafusal fibers in response to step changes in fusimotor activation (Boyd et al 1977). These different temporal properties are thought to arise from differences in the spread of activation in twitch muscle fibers that propagate action potentials (including chain fibers) versus tonic muscle fibers where synaptic depolarization spreads electrotonically (including bag fibers; Boyd 1976), as well as being related to differences in calcium kinematics.…”

“…By combining these two values, the "G" term for bag 1 fiber was estimated [G(for bag 1 ) ϭ 40 pps/0.002L 0 Ϸ 20,000 pps/L 0 ]. Similarly, we obtained the "G" value for the combined bag 2 and chain intrafusal fiber model, where maximal observed stretch during maximal static fusimotor stimulation of bag 2 and chain was 12-30 and 15-20%, respectively (average 19% of sensory region rest length ϭ 0.19 ϫ 0.04L 0 ϭ 0.0076L 0 ) (Boyd 1976;Boyd et al 1977), and primary afferent firing about 150 pps (Boyd 1986) [G(for bag 2 &chain, primary) ϭ 150 pps/0.0076L 0 Ϸ 20,000 pps/L 0 ]. In the case of the secondary afferent endings, the maximal observed stretch during maximal static fusimotor stimulation of bag 2 and chain fibers was comparable to the primary afferent case (Boyd 1976;Boyd et al 1977), whereas the secondary firing observed at this stretch was about 110 -115 pps (Boyd 1986) [G(for bag 2 &chain, secondary) ϭ 110 pps/0.0076L 0 Ϸ 14,500 pps/L 0 ].…”

“…"G," the term that relates the stretch of the intrafusal fiber's sensory endings to primary afferent firing, was estimated based on experimental data on cat tenuissimus muscle (Boyd 1976;Boyd et al 1977) in the presence of maximal dynamic fusimotor stimulation; the bag 1 sensory endings stretch by some 2-8% (scaling it to units of L 0 : 5% of primary sensory ending rest length ϭ 0.05 ϫ 0.04L 0 ϭ 0.002L 0 ) while generating about 40 pps of the primary afferent firing. By combining these two values, the "G" term for bag 1 fiber was estimated [G(for bag 1 ) ϭ 40 pps/0.002L 0 Ϸ 20,000 pps/L 0 ].…”

“…1A) (Boyd et al 1977). Typically, one bag 1 , one bag 2 , and about four to 11 chain fibers lie in parallel within a spindle (Boyd and Smith 1984).…”

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

“…This could be shown when successive responses to individual fusimotor stimuli were superimposed and displayed as frequencygrams (Bessou, Laporte & Pages, 1968;Bessou & Pages, 1969;Emonet-Denand & Laporte, 1969). A number of studies have emphasized this difference in the speed of fusimotor action (Brown, 1971, for frog spindles; Goodwin, 1972;Bessou & Pages, 1975;Boyd, 1976a, b;Boyd, Gladden, McWilliam & Ward, 1977). These investigations were based on an analysis of the time course of the rate of afferent discharge or of the shortening of intrafusal muscle fibres at the onset of tetanic fusimotor stimulation.…”