Drug-Induced Modulation of the Positive Afterpotential Following the Propagated Impulse and of the Discharge Pattern in the Frog Muscle Spindle

Abstract

A previous work (1) has shown that an individual propagated impulse recorded at the sensory nerve terminal of the frog muscle spindle was always followed by a long-lasting positivity of up to 0.1 sec in duration and of 0.1-0.3 mV in the maximal amplitude, during which the appearance of propagated and abortive spikes was suppressed. Time course of the positive after-potential was similar to that of the after-hyperpolarization following spike discharge of amphibian motoneuron, during which excitatory processes were de pressed (2). Matthews (3) has suggested that similarity in the electrical responses of the motoneuron and the sensory ending may result from similar properties of polarized surfaces.<BR>Objectives of the present experiments are to determine whether or not the positive after potential is selectively modulated by certain ions or drugs and to clarify the root of the selective modification from the effects of the drugs on the sensory terminal. MET1IODsThirty-three experiments were carried out with isolated muscle spindles of the frogs (Rana nigronaaculata). The single parent axon of a spindle receptor was isolated along its intramuscular course until the capsule of the spindle receptor was cleared, but the capsule and intrafusal muscle bundle were left attached to the remaining musculature.The excised preparation was placed in a Ringer's pool (RA) in a perspex box, and the isolated nerve passed into another Ringer's pool (RB) through a liquid paraffin pool of 2 mm length. The paraffin pool was situated in a slit of 1 mm at the center of a par tition between the two Ringer's pools. The paraffin gap method has been described in detail elsewhere (1). A pair of calomel electrodes were inserted into subsidiary Ringer's pools, each of which was connected with RA and RB by means of two Ringer's agar bridges.

The Positive After-Potential Following the Orthodromic and Antidromic Propagated Impulses in the Frog Muscle Spindle

“…The present experiment has demonstrated a decrease in the amplitude of the positive after-potential by shortening of the interval between the propagated spike followed by the after-potential and its preceding spike. Such a relation between the frequency of spike discharge and the amplitude of the positive after-potential has been observed also in the pharmacological experiments by ITO and KURODA (1972). Application of 10 mm tetramethylammonium chloride gives rise to a decrease in the overall discharge rate and to an increase in the amplitude of the positive after-potential, while that of 5 mm EGTA or 2 mm EDTA results in high frequency discharge of propagated spike and removes positive after-potential following each spike.…”