Morphological identification and intrafusal distribution of the endings of static fusimotor axons in 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: 99%

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

Bessou¹,

Pagès²

1975

“…8; also Boyd, 1976c). Further a single static y motoneurone may have selective connexions in some of the spindles it supplies and non-selective ones in others, as was first demonstrated histologically by Barker et al (1973).…”

mentioning

confidence: 99%

“…As has been shown, non-selective branches of static y axons can all produce driving, those to nuclear chain fibres alone frequently do so, while those to the static nuclear bag fibre alone rarely do so. Barker et al (1973) showed histologically, also in the tenuissimus muscle, that all six static y axons they studied had a non-selective connexion to nuclear chain fibres and one nuclear bag fibre in at least two of the spindles they innervated. This supports our conclusion that a very high proportion of static y motoneurones can produce driving through their non-selective connexions to static nuclear bag fibres and nuclear chain fibres in some spindles at all lengths of the muscle and through their connexions to nuclear chain fibres alone in other spindles if the muscle is stretched to some degree.…”

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.

“…These include silver staining of the terminals and branches of motor axons in normal and de-afferented spindles (Barker, Stacey & Adal, 1970); silver staining to demonstrate the distribution of single static y axons to spindles in which all other motor axons had degenerated (Barker et al 1973); intracellular injection of Procion Yellow to identify intrafusal muscle fibres activated by single y axons (Barker, Bessou, Jankowska, Pages & Stacey, 1972; cinematographic analysis of contractions elicited by the stimulation of single fusimotor axons Bessou & Pages, 1975); and mapping of the zones of glycogen depletion produced in intrafusal muscle fibres by prolonged tetanic stimulation of single y axons (Brown & Butler, 1973.…”

Section: Introductionmentioning

confidence: 99%

Distribution of fusimotor axons to intrafusal muscle fibres in cat tenuissimus spindles as determined by the glycogen‐depletion method.

Barker¹,

Emonet-Dénand²,

Harker³

et al. 1976

SUMMARY1. The distribution of fusimotor axons to bag1, bag2 and chain muscle fibres in cat tenuissimus spindles has been studied using a modification of the glycogen-depletion technique of Edstr6m & Kugelberg (1968). Single fusimotor axons were stimulated intermittently at 40-100/sec for long periods (30-90 sec) during blood occlusion. Portions of muscle containing the activated spindles were quick-frozen, fixed in absolute ethanol during freeze-substitution, and then embedded in paraffin wax. Serial transverse sections were stained for glycogen using the periodic acid-Schiff method, and examined for depletion.2. Dynamic y axons (i.e. those that increase the dynamic index of primary-ending responses to ramp stretches of large amplitude) depleted bag1 fibres almost exclusively.3. Static y axons (i.e. those that reduce or abolish the dynamic index) depleted both bag and chain fibres. Bag1 and bag2 fibres were depleted about equally. 4. A single static y axon may activate both bag and chain fibres in one spindle (the most common pattern), chain fibres only in another, and bag fibres only in a third spindle.5. Static y axons with conduction velocities less than 25 m/sec also had a non-selective distribution, but no depletion was observed in bag2 fibres.6. The zones of depletion produced by dynamic y axrons were distributed more or less equally in the intra-and extracapsular parts of spindle poles, whereas those produced by static y axons were mainly intracapsular.7. The results are compared with the glycogen-depletion studies of Brown & Butler (1973 and our own study of the distribution of