FUNCTIONAL PROPERTIES OF TANDEM MUSCLE SPINDLES IN COMPARISON WITH THOSE OF SINGLE MUSCLE SPINDLES IN THE FROG

抄録

Functional properties of afferent discharges from compound, tandem and single muscle spindles in the ventral and dorsal parts of the semitendinosus muscle of the frog were compared. Six dorsal root filaments supplied intramuscular branches to two capsules which were arranged either in series along a bundle of intrafusal muscle fibers (tandem spindle) or in parallel along intrafusal bundles (compound spindle) and discharged spontaneously at frequencies from 1.1 to 4.7 impulses/sec at the in situ (+0mm) length of the muscle. The static sensitivity of the fibers during stretch of the muscle by 2mm at velocities from 1 to 10mm/sec was 12.0 impulses/sec/mm in the mean. The dynamic index increased steeply with increase of the initial length of the muscle, and also with increase in the velocity of stretch. Ten filaments supplying one capsule in tandem or compound spindles discharged spontaneously at 0.7 to 3.2 impulses/sec at +0mm length. The mean static sensitivity was 10.2 impulses/sec/mm. The dynamic indices depended upon stretch velocity and also upon initial length, but the rate of the dependence was lower than that of the fibers supplying two capsules. Twelve filaments supplying single spindles discharged spontaneously at 0.1-0.65 impulses/sec at +0mm. The mean static sensitivity was 5.1 impulses/sec/mm, and the dynamic indices were the smallest. These results suggest that the role of tandem and compound spindles may be to respond sensitively to minute changes of the muscle length.