Studies on the site of termination of static and dynamic fusimotor fibres within muscle spindles of the tenuissimus muscle of the cat

SUMMARY1. Muscle spindles with an intact blood supply and uninterrupted connexions with ventral and dorsal spinal roots (Bessou & Pages, 1967 have been prepared in cat's tenuissimus muscles with the aim of cinephotographically recording intrafusal movements induced by the stimulation ofsingle static or dynamic y axons; the time course ofthese movements and the morphological kind of activated intrafusal muscle fibres have been established.2. Displacements of spindle guiding marks in the equatorial region elicited by stimulating single static y axons are 4-20 times greater in amplitude than the ones elicited by stimulating dynamic y axons at the same frequency.3. The dynamic y axons induced a contraction only in nuclear bag fibres which, in addition, never received any static y innervation. The static y axons evoked contractions either in nuclear bag fibres alone, or in nuclear chain fibres alone, or in both types of intrafusal fibres. Two thirds of static y axons supplied nuclear bag fibres. For various reasons, one half only of static y axons innervating nuclear bag fibres could be shown to simultaneously innervate nuclear chain fibres. Consequently, about one third of static y axons supplied both nuclear bag fibres and nuclear chain fibres, but it is highly probable that this latter figure is an underestimate. One third of static y axons produced contraction in nuclear chain fibres only. In this work, the distribution of fusimotor axons has been established in only one muscle spindle of the cluster of muscle spindles that each fusimotor axon is generally innervating.4. Generally speaking, a static y axon elicits contraction of several intrafusal fibres whereas a dynamic y axon innervates only one intrafusal fibre and frequently only one pole of the fibre. P. BESSOU AND B. PAGES 5. One third of static y axons evoked contractions in nuclear chain fibres that seemed to involve the whole pole. The other static y axons and all dynamic y axons produced, in the intrafusal fibres that they supplied, one or several foci of localized contractions.6. The nuclear chain fibres contract and relax faster than nuclear bag fibres. The contractions of nuclear bag fibres supplied by static y axons are stronger and faster than those of nuclear bag fibres innervated by dynamic y axons. Nearly all nuclear bag fibres innervated by static y axons, like the nuclear chain fibres, show transient contractions at each pulse of a stimulation at low frequency (2-20/sec).7. The results are discussed taking into account the available anatomical and physiological data on the muscle spindle. Their consequences with regard to intrafusal working are briefly considered.

Section: Considerations Of the Functional Implications Of Fusimotor Amentioning

confidence: 98%

Section: Considerations Of the Functional Implications Of Fusimotor Amentioning

confidence: 99%

See 1 more Smart Citation

Cinematographic analysis of contractile events produced in intrafusal muscle fibres by stimulation of static and dynamic fusimotor axons.

Bessou¹,

Pagès²

1975

“…Further, the motor control of spindles has been studied using the depletion of glycogen in particular intrafusal fibres following prolonged stimulation of their axons as the indicator (Brown & Butler, 1973, 1975 Barker, Emonet-D6nand, Harker, Jami & Laporte, 1975). These investigations are referred to in the Discussion.…”

mentioning

confidence: 99%

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

Boyd¹,

Gladden²,

McWilliam³

et al. 1977

189

132

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.

“…This is likely to be substantially longer for fibres activated by static axons (chains and bag 2; cf. Brown & Butler, 1973Bessou & Pages, 1975;Boyd, 1976a, b;Barker, Emonet-Denand, Harker, Jami & Laporte, 1976) than for dynamic activated fibres (bag 1), since statics but not dynamics tend to provoke propagated action potentials and occasionally even full-length activation of intrafusal fibres (Bessou fibres is smaller than the length of the bag 1 fibres (Barker, 1974;Barker et al 1976). Indeed, from the published data of Bessou & Pages (1975) it would appear that the speed of contraction induced by static fibres, measured by its time constant, exceeded dynamic action only by a factor of about 2.…”

Section: Speed Offusimotor Actionmentioning

confidence: 99%

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

Hulliger¹

1979

SUMMARY1. Single fusimotor fibres to de-efferented soleus of the cat were stimulated to investigate the size and time course of the responses elicited in single primary spindle afferents. The muscle was kept at constant length close to the physiological maximum. Constant and alternating rates of fusimotor stimulation were used: (a) repetitive stimulation at constant rate (maintained stimulation); (b) modulated stimulation with the rate of activation alternating between two constant levels at repeat frequencies between 009 and 2 Hz (rectangular stimulation). The responses were averaged and displayed as post-stimulus time (pst) histograms (a) or as cycle histograms (b).2. During static fusimotor stimulation the pst histograms could be clearly modulated over a range of rates of stimulation. However, histogram modulation was not a prerequisite of static action since with different fibres the degree of modulation could range from deeply modulated to completely non-modulated.3. Dynamic fusimotor stimulation was almost always accompanied by nonmodulated pst histograms.4. Primary spindle afferents responded to rectangular stimulation of either kind of fusimotor fibre with an approximately rectangular modulation of the rate of discharge. At the repeat frequencies studied the size of the responses was appreciably larger with static than with dynamic activation. It was assessed as 'fusimotor ratesensitivity during alternating stimulation' by the response/stimulus ratio which is defined as change in firing/change in alternating rate of stimulation, in impulses/ stimuli. The mean values of rate-sensitivity were 1-35 impulses/stimuli (statics) and 029 (dynamics), with a static/dynamic ratio of 4-7.5. The afferents' 'fusimotor rate-sensitivity during steady stimulation' (change in firing/change in maintained rate of stimulation) was also determined. The mean values were 078 (static) and 037 (dynamics), with a static/dynamic ratio of 2 1.6. The time course of the responses to rectangular stimulation was of the same order of magnitude for static and dynamic fibres. It was assessed by fitting a single exponential to the rising and falling phase of cycle histograms. The mean values of the time constants for static fibres were 58 msec (rising phase) and 59 msec (falling phase), and for dynamic fibres 34 msec (rising phase) and 49 msec (falling phase).