The Journal of Neurophysiology Vol. 80 No. 1 July 1998, pp. 130-142
Copyright ©1998 The American Physiological Society

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

A. Taylor, P. H. Ellaway, and R. Durbaba

Department of Sensorimotor Systems, Division of Neuroscience and Psychological Medicine, Imperial College School of Medicine, Charing Cross Hospital, London W6 8RF, United Kingdom

Taylor, A., P. H. Ellaway, and R. Durbaba. Physiological signs of the activation of bag₂ and chain intrafusal muscle fibers of gastrocnemius muscle spindles in the cat. J. Neurophysiol. 80: 130-142, 1998. A method is described for identifying the effect of single gamma static (_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 _s 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.

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