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

Abstract

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.

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

“…This effect, observed in extrafusal fibers, is believed to result from the influence of myofilament lattice spacing on cross-bridge kinetics (Brown et al 1999). Under most physiological conditions the sarcomere length of the intrafusal fiber's polar region tends to follow that of extrafusal fibers (extrafusal sarcomere: Scott et al 1996;intrafusal sarcomere: Barker 1974;Bessou et al 1975;Boyd 1976;Poppele and Quick 1981), so the extrafusal fiber measurements were used to estimate the length (R) of the polar region of intrafusal fibers for this effect (assuming that length changes in sensory region are minor comparing to those in the polar region; see Implementation of the spindle model and parameter determination).…”

Primary and secondary afferent discharges from the same spindle during chain fibre contraction in cat tenuissimus muscle

“…This is probably due to the fact that, at that low frequency, the sustained contraction of bag2 fibres is already a large fraction of the maximal contraction observed at 60-70 stimuli/s. This is not so for chain fibres which, at this frequency, give unfused oscillations and whose maximal contraction is obtained only for much higher frequencies (Bessou & Pages, 1975;Boyd, 1976). That difference was already evoked to explain that the ratio of the increase in firing frequency of primary and secondary endings lying in the same spindle elicited by stimulation at 30 stimuli/s to that observed at 100 stimuli/s was greater when bag2 fibres were involved (Celichowski, Emonet-Denand, Gladden, Laporte & Petit, 1993).…”

“…At 30 stimuli/s, the contraction of bag2 fibres is nearly fused (Bessou & Pages, 1975) and can be expected to elicit a sustained and regular increase in the discharge frequency of primary endings. On the other hand, the contraction of chain fibres presents large oscillations (Bessou & Pages, 1975;Boyd, 1976) capable of eliciting either a 1: 1 driven response or an increase in the frequency of discharge of the primary endings with large variations in instantaneous frequency.…”

“…On the other hand, the contraction of chain fibres presents large oscillations (Bessou & Pages, 1975;Boyd, 1976) capable of eliciting either a 1: 1 driven response or an increase in the frequency of discharge of the primary endings with large variations in instantaneous frequency. As the ending quickly repolarizes (Hunt & Ottoson, 1977) after each oscillation, the minimal frequency of discharge will fall to a level close to that of the frequency of stimulation.…”

Distribution of muscle spindles in a simply structured muscle: Integrated total sensory representation

“…A bag fiber is subjected to traction at its distal terminal, and the tapering pole leading up to it slips freely through the capsule collar and then places stress on the sensory ending adherent to its surface. The annulospiral wrappings of the Ia afferent are principally affected (Bessou and Pages, 1975;Boyd and Ward, 1975). Most chain fibers do not emerge from the capsule and are pulled on through attachments to the fusiform capsule and its inner continuation around the axial bundle (Bridgman et al, 1969;Sahgal et al, 1987), that is, they lengthen secondary to deformation of the capsule.…”