The Phasic and Tonic Contraction in the Longitudinal Muscle of the Earthworm

Abstract

ABSTRACT In extension of preceding studies on the mechanical properties of the longitudinal muscle fibres of the earthworm, the phasic and tonic contractions were analysed under various conditions. The phasic contractions appeared on the tonic contraction which was maintained at a steady level by repetitive stimulation, and the tonic contraction decayed very slowly after cessation of stimulation. The length-tension curve indicated that the phasic and tonic contractions were both affected similarly by changing the muscle length. The mechanical ‘active state’ was investigated during the phasic and tonic contractions by means of the quick-release method. During the phasic contraction the ability to redevelop the tension was observed. During the early phase of the tonic contraction the muscle was in the ‘active state‘, but the tonic contraction continued even after the tension redevelopment had nearly ceased. The tonic contraction was absent when the temperature was raised to about 30 °C. 5-Hydroxytryptamine (2·5 × 10−6 M) decreased the tonic more than the phasic contraction. The tonic contraction disappeared completely in the presence of acetylcholine (5·5 × 10−4 M), while the phasic contraction was reduced but not abolished, λ-aminobutyric acid (10−8 to 5 × 10−4 M) had no effect on the contractions. Transition-metal ions suppressed mainly the phasic contraction in the order of Mn2+> Co2+ > Ni2+. The decrease in the phasic contraction was proportional to the logarithm of concentration of added divalent cations. The tonic contraction was also reduced but at higher concentration than the phasic contraction. It is postulated that, by depolarization produced by electrical stimulation or by acetylcholine, calcium ions are released from the plasma membrane and also from the sarcoplasmic reticulum. The phasic contraction might be generated mainly by calcium ions released from the membrane, and the tonic contraction might be caused mainly by calcium ions released from the sarcoplasmic reticulum.