PLATEAU PATTERN OF AFFERENT DISCHARGE RATE FROM FROG-MUSCLE SPINDLES

1. A characteristic plateau pattern was observed in the rate of affere nt discharges during ramp-and-hold stretch of spindles isolated from s emitendinosus muscles of frogs. The plateau pattern was more frequent in summer frogs (84%) than winter frogs (11%). 2. The plateau pattern consisted of a discharge rate around 120 imp/s at the end of dynamic s tretch, followed by second and third steps of plateau rates around 60 and 40 imp/s, respectively. The intervals of impulses in lower steps w ere approximately n times those of the top step. 3. The plateau patter n was not sensitive to cutting extracapsular myelinated branches or lo wering temperature from 23 to 12-degrees-C. However, the number of the plateau was reduced in both cases. 4. Application of depolarizing cur rent to the sensory terminal abolished the plateau pattern. In contras t, in spindles that did not show a plateau pattern, hyperpolarizing cu rrent induced such a pattern. 5. Calcium channel blockers and protein kinase C inhibitors abolished the plateau pattern. The plateau pattern could be established in quiescent spindles by drugs eliciting Ca2+ en try, raising cytosolic-free Ca2+, and activating protein kinase C. 6. The most striking aspect of the present study is the stability in the discharge rate at each step of the plateau, irrespective of different experimental conditions. This suggests that the spindle sensory termin al possesses a stable intrinsic rhythm generator in excitation, of whi ch maximum frequency is 120 imp/s. The generator seems to be triggered by stretch stimulus and to be regulated by cytoplasmic Ca2+ and prote in kinase C.