INITIATION OF NERVE IMPULSES INTHE FROG MUSCLE SPINDLE

1. Propagated action potentials were recorded across the paraffin gap situatedin the vicinity of the sensory endings in an isolated frog muscle spindle.The recorded triphasic potentials were considered to consist of two boundarycomponents which depend upon the position of the frontal and real bounda-ries between Ringer's and paraffin pools.2. The intervals between peaks of the frontal and the real boundary com-ponents decreased with decreasing length in the paraffin pool. An increasein the distance from the capsule of the isolated spindle to the paraffin poolresulted in decrease of the amplitude of an initial part (first component) inthe frontal boundary component, but produced an increase in the amplitudeof the second part (second component) until it attained a constant amplitude.3. When the node N2 (second proximal to the bifurcation node NO) wascrushed, the second component disappeared but the first component remainedas a monophasic action potential. This result indicates that the first com-ponent is originated from N1 or more distal nodes and the second is due toa propagated impulse initiated at nodes proximal to N2. It is assumed thatthe first component may be a sum of propagated impulses elicited synchronously along the branches of the parent axon in a spindle capsule.4. When temperature was raised or the spindle receptor was stimulatedstrongly, modification of configuration of the triphasic potential was observed.It is discussed that the modification may be due to a propagated impulseinitiated at more proximal nodes.