The Response of Fast and Slow Nuclear Bag Fibres and Nuclear Chain Fibres in Isolated Cat Muscle Spindles to Fusimotor Stimulation, and the Effect of Intrafusal Contraction on the Sensory Endings

Abstract

CONTENTS2. Mammalian intrafusal muscle fibres are of three functional types. Most spindles contain one slow nuclear bag fibre, one fast nuclear bag fibre, and four or five nuclear chain fibres. 3. Contraction in slow nuclear bag fibres is characterized by a long latency and very slow initial velocity, whereas the latency for the other intrafusal fibres is short and the initial velocity rapid. The mean time for maximum contraction (at 75 Hz to 100 Hz) and relaxation is significantly longer for slow nuclear bag fibres (0-8 s) than for other intrafusal fibres (0 5 s). The contraction time of fast nuclear bag fibres is sometimes longer than that of nuclear chain fibres but the mean values are not significantly different; a difference in the time to attain 90% contraction is more obvious. 4. At low stimulation frequencies (10 Hz) contraction in slow nuclear bag fibres and in most fast nuclear bag fibres is smooth whereas nuclear chain fibres exhibit marked oscillations. Single stimuli elicit small local twitches in nuclear chain fibres and occasionally in fast nuclear bag fibres but produce no visible effect in slow nuclear bag fibres. 5. Maximum contraction of slow and fast nuclear bag fibres at body temperature is attained at a stimulation frequency of 75 Hz to 100 Hz, whereas a frequency of 150 Hz or more is required for maximum contraction of nuclear chain fibres. At 50 Hz at body temperature contraction in nuclear bag fibres is at least half the maximum, whereas in many spindles nuclear chain fibres show only a very small contraction at this frequency. 6. Contraction in slow nuclear bag fibres occurs at one or two discrete foci, most of which lie in the intracapsular region beyond the end of the fiuid space. Weak contraction extends the primary sensory spiral by a small amount (2 %-8 %) at a low velocity (5 %-10% s-1). When the fibre is passively stretched the spiral opens and then creeps back to about 75 % of the extension at the end of the stretch due to yielding in the poles of the fibre; creep is complete in 0-5 s to 2-5 s. 7. Contraction in fast nuclear bag fibres also occurs at one or two discrete foci, most of which lie in the intracapsular region beyond the end of the fluid space. Shortening of sarcomeres at the foci and extension of the sensory spiral are, however, up to eight times greater (up to 25 %) than in slow nuclear bag fibres, and the velocity of stretch of the spiral is three to eight times greater (25 %-40 % s-1). Fast nuclear bag fibres exhibit little or no creep following passive stretch. 8. Contraction in the nuclear chain fibre bundle is localized to the intracapsular region, centred on a point in the intracapsular region between 0 9 mm and 1-6 mm from the spindle equator. Maximal contraction stretches primary and secondary sensory endings by 15 % to 20%, at 30% to 40 % s-l. Nuclear chain fibres follow length changes of the spindle faithfully and rarely exhibit creep following passive stretch. 9. The resting sarcomere length of nuclear bag fibres is significantly greater in the contractin...

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

“…This is consistent with the few observations that during contraction of chain fibres, some parts of these fibres lengthen, whereas others shorten (Boyd, 1976;Poppele & Quick, 1985 The number of chain fibres activated by a static axon is probably not a significant factor since it has been observed that, even when an axon activated all chain fibres in a particular spindle, the variability of the responses of the secondary ending could still be very low (Jami, Lan-Couton & Petit, 1980).…”

“…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).…”

“…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.…”

“…The mechanism by which secondary endings, which lie on contractile regions of chain fibres, are excited when these fibres are activated is still uncertain largely because experimental data on the secondary terminals and on the properties of the regions of muscle fibres on which they lie are scarce (Boyd, 1976;Poppele & Quick, 1985 (Edman & Reggiani, 1984;Burton, Zagotta & Baskin, 31 EPH 79 1989). This is consistent with the few observations that during contraction of chain fibres, some parts of these fibres lengthen, whereas others shorten (Boyd, 1976;Poppele & Quick, 1985 The number of chain fibres activated by a static axon is probably not a significant factor since it has been observed that, even when an axon activated all chain fibres in a particular spindle, the variability of the responses of the secondary ending could still be very low (Jami, Lan-Couton & Petit, 1980).…”

Proceedings of the Physiological Society, 23‐25 September 1992, Cambridge Meeting: Poster Communications

“…It is also demonstrated that the responses can be seen at stimulus intensities which do not elicit a subjective sensation of pain and that responses are more readily evoked by the presentation of paired stimuli. (Boyd, 1976). The oscillations of chain fibres which twitch are responsible for 'driving', timelocking the discharge of the primary sensory ending to the fusimotor input frequency.…”