Physiological Signs of the Activation of Bag₂ and Chain Intrafusal Muscle Fibers of Gastrocnemius Muscle Spindles in the Cat

Abstract

A method is described for identifying the effect of single gamma static (gamma(s)) axons on bag2 or chain intrafusal fibers using random (Poisson-distributed) stimuli. The cross-correlogram of the stimuli with the firing of spindle primary afferents took one of three forms. A large, simple, brief response was taken to indicate pure chain fiber activation and a small, prolonged response to indicate pure bag2 activation. A compound response with brief and prolonged components was taken to be a sign of mixed innervation. The correlogram components could be well fitted with lognormal curves. They could also be transformed into curves of gain as a function of frequency, which were convenient for estimating the strength of the effects. In 68 effects of gammas axons on Ia afferents, 16 were pure chain, 17 pure bag2, and 35 mixed. This distribution was significantly different (P < 0. 05) from that expected from chance nonspecific innervation of chain and bag2 fibers. Making use of the estimates of the strength of chain and bag2 effects derived from the gain curves, the classification was modified by treating mixed responses that had one effect more than five times stronger than the other as belonging to the dominant type. The distribution was then as follows: chain 16, bag2 28, and mixed 24. This differed very significantly from the prediction of chance distribution (P < 0.001). This evidence for some degree of specific innervation of chain and bag2 fibers is discussed in relation to previous work and with regard to the ways in which the two fiber types might be used in natural movements.

Changes in Human Muscle Spindle Sensitivity during a Proprioceptive Attention Task

“…First, the decreased depth of modulation of discharge associated with increased activity during muscle shortenings and increased variability of the instantaneous frequency of discharge, which might also be concomitant with altered spontaneous activities, seem to be related to a static fusimotor drive (for review, see Hulliger, 1984;Prochazka, 1996). The effects observed, in particular the biasing effect, were neither as frequent nor as powerful as expected from animal data (Celichowski et al, 1994;Taylor et al, 1998;Durbaba et al, 2001). This difference may be explained by the natural condition of ␥ driving in the present study compared with the electrical stimulation used in animal experiments.…”

The Influence of Bag2 and Chain Intrafusal Muscle Fibres on Secondary Spindle Afferents in the Cat

“…It has been shown that a substantial part of the γ s motor output during locomotion in the cat fluctuates with a smoothly modulated pattern that resembles the profile of muscle shortening (Murphy et al . 1984; Taylor et al . 2000 a , b ) and acts to oppose the unloading effect of shortening.…”

“…2000 a , b ) and acts to oppose the unloading effect of shortening. For this reason it may be better to regard the γ s pattern as a signal that is algebraically summed with muscle length to determine the afferent firing, rather than as a means of controlling the sensitivity of the spindle to stretch (see Taylor et al . 2000 a,b ).…”

“…With this in mind, the primary afferents have been studied recently by stimulating γ s axons with sinusoidally frequency‐modulated pulse trains with the muscle held at a constant length (Durbaba et al . 2001 b ) after identifying which intrafusal fibres were activated by random stimulation (Taylor et al . 1998).…”

A method for the study of the effects of combining multiple pseudorandom fusimotor stimulation on the responses of muscle-spindle primary-ending afferents

“…Both groups also used random length inputs, and confined their attention to single static fusimotor fibres. The only remaining instances of random, or pseudorandom, fusimotor stimulation that have come to our attention, once again using single static fibres but with the muscle held at constant length, were concerned with the problem of identifying the type, or types, of intrafusal muscle fibre activated in each case (Taylor et al, 1998;Petit et al, 1999).…”