PHYSIOLOGICAL SIGNS OF THE ACTIVATION OF BAG(2) AND CHAIN INTRAFUSAL MUSCLE-FIBERS OF GASTROCNEMIUS-MUSCLE SPINDLES IN THE CAT

A method is described for identifying the effect of single gamma stati c (gamma(s)) axons on bag(2) 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 la rge, simple, brief response was taken to indicate pure chain fiber act ivation and a small, prolonged response to indicate pure bag(2) activa tion. A compound response with brief and prolonged components was take n to be a sign of mixed innervation. The correlogram components could be well fitted with lognormal curves. They could also be transformed i nto curves of gain as a function of frequency, which were convenient f or estimating the strength of the effects. In 68 effects of gamma(s) a xons on Ia afferents, 16 were pure chain, 17 pure bag(2), and 35 mixed . This distribution was significantly different (P < 0.05) from that e xpected from chance nonspecific innervation of chain and bag(2) fibers . Making use of the estimates of the strength of chain and bag(2) effe cts derived from the gain curves, the classification was modified by t reating mixed responses that had one effect more than five times stron ger than the other as belonging to the dominant type. The distribution was then as follows: chain 16, bag(2) 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 a nd bag(2) fibers is discussed in relation to previous work and with re gard to the ways in which the two fiber types might be used in natural movements.