Physiology and Ultrastructure of Phasic and Tonic Skeletal Muscle Fibres in the Locust, Schistocerca Gregaria

Abstract

Insect muscle fibres can be classed as either phasic or tonic according to their response to potassium depolarization. The phasic fibres contract only transiently during prolonged potassium depolarization, whereas the tonic fibres give a sustained contracture. The extensor tibiae muscle in the metathoracic leg of the locust contains both tonic (T/et) and phasic (P/et) fibres; the electrical, mechanical and ultrastructural properties of these fibres have been compared with those of phasic fibres from the retractor unguis muscle (P/ru) in the same leg. A broad correlation has been established between the mechanical response and the amount of sarcoplasmic reticulum (SR) in the fibres. At maximal body length the rise time to peak twitch tension for the T/et fibres was found to be 790±60 ms, for the P/et fibres 59±2.5 ms and for the P/ru fibres, 30±1.1 ms. The half-decay times for the isometric twitch contractions were 2950±88 ms for the T/et fibres, 119±4.2 ms for the P/et, and 35±2.3 ms for the P/ru. The P/et and P/ru gave brief isometric contractures during potassium depolarization; under the same treatment the T/et fibres remained contracted throughout the treatment period. The major structural differences between the 3 types lies in the SR. Expressed as percentages of total fibre volume, the SR represents in the T/et 1.1%, in the P/et 6.8%, and in the P/ru 19%. The surface area of the SR, in terms of µm2/µm3 of fibre volume is 1.0±0.1 in the T/et, 2.9±0.2 in the P/et and 11.9±1.0 in the P/ru. Microtubules, often associated with elements of the SR, are sparsely distributed amongst the contractile elements in the T/et fibres. All 3 muscle types have a well developed T-system which forms dyadic associations with the SR. Larger-diameter Z-invaginations which conduct tracheoles into the muscles also give rise to ‘longitudinal T-tubules’, particularly in the T/et fibres. Dyads arise by association of cisternae of the SR: (i) with T-tubules sensu strictu, (ii) with Z-invaginations and T-tubule-like extensions from them, and (iii) directly with the plasma membrane at the surface of the fibre.